Soldering Characteristics and Mechanical Properties of Sn-1.0Ag-0.5Cu Solder with Minor Aluminum Addition

materials

Article Soldering Characteristics and Mechanical Properties of Sn-1.0Ag-0.5Cu Solder with Minor Aluminum Addition

Yee Mei Leong and A.S.M.A. Haseeb *

Centre for Advanced Materials, Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia; [email protected] * Correspondence: [email protected]; Tel.: +60-3-79674492

Academic Editor: Geminiano Mancusi Received: 26 May 2016; Accepted: 16 June 2016; Published: 28 June 2016

Abstract: Driven by the trends towards miniaturization in lead free electronic products, researchers are putting immense efforts to improve the properties and reliabilities of Sn based solders. Recently, much interest has been shown on low silver (Ag) content solder SAC105 (Sn-1.0Ag-0.5Cu) because of economic reasons and improvement of impact resistance as compared to SAC305 (Sn-3.0Ag-0.5Cu. The present work investigates the effect of minor aluminum (Al) addition (0.1–0.5 wt.%) to SAC105 on the interfacial structure between solder and copper substrate during reflow. The addition of minor Al promoted formation of small, equiaxed Cu-Al particle, which are identified as Cu3Al2. Cu3Al2 resided at the near surface/edges of the solder and exhibited higher hardness and modulus. Results show that the minor addition of Al does not alter the morphology of the interfacial intermetallic compounds, but they substantially suppress the growth of the interfacial Cu6Sn5 intermetallic compound (IMC) after reflow. During isothermal aging, minor alloying Al has reduced the thickness of interfacial Cu6Sn5 IMC but has no significant effect on the thickness of Cu3Sn. It is suggested that of atoms of Al exert their influence by hindering the flow of reacting species at the interface.

Keywords: intermetallic; mechanical properties; nanoindentation; interfacial reaction; microstructure; differential scanning calorimetry; scanning electron microscopy; minor Al addition

1. Introduction Restriction on the use of lead based solder in the electronic industry has led to extensive developments in lead free solders. Driven by the necessity to improve the reliability of lead free electronic products and by the trend towards miniaturization, researchers are putting intense efforts into improving the properties of Sn based solders. Sn-Ag-Cu (SAC solder) alloys have been a favored ˝ replacement for Sn-Pb solders. However, the currently used ternary eutectic (Tm = 217 C) or near eutectic SAC solders have some drawbacks. These SAC solders were reported to have relatively high amount of undercooling (10–30 ˝C), as β-Sn requires large undercooling to nucleate and solidify. This large undercooling promotes the formation of large, plate-like Ag3Sn structures that have been reported to cause joint embrittlement and reliability problem [1]. Research has been done on solders with lower silver content of, e.g., Sn-1.0Ag-0.5Cu (SAC105), in an attempt to inhibit the formation of Ag3Sn and also to reduce the solder cost, as the price of Ag has increased dramatically in recent years [2–5]. Although it has been reported that SAC105 performs better in drop tests, it has a higher liquidus temperature, which requires a higher reﬂow proﬁle than the eutectic Sn-3.8Ag-0.7Cu (SAC387) alloy [5]. It also performs poorly in thermal cycling tests [6]. These drawbacks have prompted researchers to modify the solder content by adding small amounts of other alloying elements to further improve its reliability and lower its melting temperature.

Materials 2016, 9, 522; doi:10.3390/ma9070522 www.mdpi.com/journal/materials Materials 2016, 9, 522 2 of 17

Fourth minor alloying elements (0.05–0.1 wt.%) that have been added into SAC solder include Ni, Co, Fe, Mn, Zn, Ti, Ce, In and Al [7–10]. The effects of the fourth alloying element on the microstructure and mechanical properties have been investigated. Improvements in mechanical properties such as shear strength [10], tensile strength [7], impact resistance [5], and creep resistance [7] have been reported. It has been observed that the addition of fourth alloying elements imparts their influence on the mechanical properties by modifying the microstructure of the solder. It was observed that the fourth alloying element modify the microstructure, by: (i) refining the microstructure of the solder matrix; (ii) suppressing brittle intermetallic compound (IMC) formation; and (iii) changing the morphology of the IMC in solder matrix/interface [11]. The addition of aluminum has attracted much interest in recent years. Research on Al addition as nanoparticles or minor alloying element has been reported [12–18]. Amagai [12] observed that the addition of 0.05% Al nanoparticles did not have any significant effect on the interfacial IMC. As for the addition of aluminum as minor alloying element, a few works on the effect of Al on the mechanical properties, microstructure and solder/Cu substrate interfacial reaction have been reported [2,9,11,14–18]. Previous studies have shown that minor addition of Al gives promising results in suppressing undercooling of β-Sn and reduced formation of brittle phase Ag3Sn in SAC solder matrix [14]. It was found that Al addition (0.05 wt.%) results in excellent shear strength retention after thermal aging at 150 ˝C for up to 1000 h. Faizul et al. have investigated the effect of Al (0.1–2 wt.%) on the tensile strength of SAC105 [12,19]. They found that with 0.1 wt.% Al addition, Al reduced the yield strength and ultimate tensile strength (UTS) and promoted ductile fracture. When Al addition was more than 0.1 wt.%, the yield strength and ultimate tensile strength of the solder increased as a function of Al content and brittle fracture modes were seen. A couple of studies dealt with the effects of Al on the bulk microstructure and interfacial IMC between solder and copper substrate [15,16,18]. With the addition of Al into Sn-Ag (SA) solder, Al2Cu IMC and ambiguous Al-Cu IMC were found in the bulk microstructure of SA solder on copper substrate after reflow [16,18]. On the other hand, only ambiguous Al-Cu IMC (Anderson et al. reported it as δ-Cu33Al17) was found in the bulk microstructure of SAC [14,19]. Though the reported effect of Al on the bulk microstructure seen by other researchers seems consistent, there are contradictory reports on the effects of Al addition on the interfacial solder/Cu reaction. With addition of 0.5 wt.% of Al into SA solder, Xia et al. observed a spalled layer of Al2Cu compound layer near the interface and the suppression of the growth of the Cu6Sn5 layer. When Al was added up to 1 wt.%, a layer of Al2Cu compound was formed at the solder/Cu interface, which completely replaced Cu6Sn5 layer [15]. On the other hand, Kotadia et al. reported that spherical Al-Cu IMC spalled away from the solder matrix when Al was depleted in the solder and there was no suppression of the growth of interfacial Cu6Sn5 layer when 0.5 to 2.0 wt.% of Al was added in to SA solder [18]. Li et al. have reported that with the addition of 1 wt.% Al into SAC387 solder, layered δ-Al2Cu3 formed at the interface after 10 min reflow and the growth of the Cu6Sn5 layer was reduced [16]. Dhaffer et al. have investigated the effect of 0.1.wt.% Al addition on the interfacial reaction by dipping method, they reported that 0.1% Al addition have suppressed the growth of Cu6Sn5 layer [20]. Until now, no detailed study has been done for the effect of minor Al addition (<1 wt.%) on interfacial reaction of SAC/Cu. Nanoindentation as a mechanical testing method has attracted a great deal of attraction in many fields of research as it has the ability to measure the properties of sample in extremely small scales such as thin films and coating in nanometer range [21]. This technique is well suited to investigate the mechanical properties of IMCs, which have thicknesses of only several nanometers/micrometers. Cu-Al IMC was reported to form in SAC even with addition of as low as 0.05 wt.% [22]. Besides, it has been reported that Al is likely to substitute into Cu6Sn5 as well [14] Previous studies with addition such as Ni, and Mn have shown that solubility of minor alloying element in Cu6Sn5 could alter the nanomechanical properties of Cu6Sn5 [14]. Thus, understanding the mechanical properties of Cu-Al IMC and Cu6Sn5 is essential in the understanding of deformation behavior and failure mechanisms in lead-free solder joints. The present work investigates the effect of Al on the interfacial IMC between SAC105 solder alloy and copper substrate. This work concentrates on the effect of the lower percentage of Al (0.1–0.5 wt.%) Materials 2016, 9, 522 3 of 17 Materials 2016, 9, 522 3 of 17 on the Cu–SnThe present reaction work during investigates reflow andthe isothermaleffect of Al aging.on the Asinterfacial has been IMC noted between earlier SAC105 [15,16 ],solder absence of Cualloy in and the copper solder ledsubstrate. to the This formation work concentrates of Al2Cu and on the Al-Cu effect IMC of the near/at lower the percentage interface of during Al reflow,(0.1–0.5 and wt.%) the IMCs on the tended Cu–Sn toreaction spall awayduring from reflow the and solder isothermal matrix aging. when As Al has was been depleted noted earlier in solder, thus[15,16], complicating absence theof Cu situation in the so atlder the led interface. to the formation With the of presenceAl2Cu and of Al-Cu Cu in IMC SAC near/at solder, the it interface is expected thatduring Al will reflow, react and with the Cu IMCs in the tended bulk to and spall thus away provide from the a more solder simplified matrix when scenario Al was at depleted the interface, in solder, thus complicating the situation at the interface. With the presence of Cu in SAC solder, it is which may lead to a better understanding of the effects of Al. This study concentrates on the lower expected that Al will react with Cu in the bulk and thus provide a more simplified scenario at the percentage of Al (0.1%–0.5%), as higher percentages may lead to formation of Al Cu and other IMCs interface, which may lead to a better understanding of the effects of Al. This study2 concentrates on at the interface. Furthermore, this study investigates the nanomechanical properties of Cu-Al IMC the lower percentage of Al (0.1%–0.5%), as higher percentages may lead to formation of Al2Cu and particlesother IMCs and alsoat the examines interface. whetherFurthermore, addition this st ofudy Al investigates would affect the thenanomechanical nanomechanical properties properties of of CuCu-Al6Sn5 .IMC particles and also examines whether addition of Al would affect the nanomechanical properties of Cu6Sn5. 2. Results 2. Results 2.1. Differential Scanning Calorimetry 2.1.Figure Differential1 shows Scanning the Differential Calorimetry Scanning Calorimetry (DSC) curves for SAC105, SAC105 + 0.1Al, SAC105Figure + 0.3Al 1 shows and the SAC105 Differential + 0.5Al. Scanning During Calorimetry heating, (DSC) SAC105 curves for and SAC105,SAC105 SAC105 + 0.1Al + 0.1Al,solder ˝ ˝ showSAC105 an onset + 0.3Al melting and SAC105 temperature + 0.5Al. (T Duringm) of 216.84heating, CSAC105˘ 0.50 and and SAC105 216.43 + 0.1AlC ˘ solder0.38, respectively.show an ˝ Thisonset onset melting temperature temperature corresponds (Tm) of to216.84 the ternary°C ± 0.50 eutectic and 216.43 reaction °C ± (T 0.38,m = 217respectively.C) of Sn-Ag-Cu This onset alloy, ˝ ˝ L ÑtemperatureAg3Sn + Cu corresponds6Sn5 + Sn [ 23to ]the. Two ternary prominent eutectic peaksreaction were (Tm = seen 217 at°C) ~221 of Sn-Ag-CuC and ~228alloy, LC → in Ag SAC1053Sn and+SAC105 Cu6Sn5 + + Sn 0.1Al [23]. (FigureTwo prominent1a,b) . These peaks two were peaks seen wereat ~221 associated °C and ~228 with °C the in SAC105 eutectic and temperature SAC105 + for 0.1Al (Figure 1a,b). These two peaks were associated with the eutectic temperature for Sn-Ag Sn-Ag (L Ñ Ag3Sn + Sn) and Sn-Cu (L Ñ Cu6Sn5 + Sn), respectively. As the amount of Al was added up to(L 0.3→ Ag wt.%,3Sn + there Sn) and is a Sn-Cu change (L in→the Cu6 DSCSn5 + curveSn), respectively. (Figure1c). As The the onset amount temperature of Al was added was shifted up to to 221.140.3 wt.%,˝C ˘ there0.36 andis a change 221.65 in˝C the˘ DSC1.02, curve respectively (Figure 1c). for The SAC105 onset+ temperature 0.3Al and SAC105was shifted +0.5Al to 221.14 which °C ± 0.36 and 221.65 °C ± 1.02, respectively for SAC105˝ + 0.3Al and SAC105 + 0.5Al which corresponded to Sn-Ag eutectic temperature (Tm = 221 C). For SAC105 + 0.3Al, a first small peak of corresponded to Sn-Ag eutectic temperature (Tm = 221 °C). For SAC105 + 0.3Al, a first small peak of heat absorption appears at 224 ˝C, followed by a second larger peak of heat absorption at ~231 ˝C heat absorption appears at 224 °C, followed by a second larger peak of heat absorption at ~231 °C (Figure1c). For SAC105 + 0.5Al, only a large peak of heat absorption appears at ~231 ˝C (Figure1d). (Figure 1c). For SAC105 + 0.5Al, only a large peak of heat absorption appears at ~231 °C (Figure 1d). The peak at ~231 ˝C corresponds to the melting temperature of Sn (T = 231 ˝C). The peak at ~231 °C corresponds to the melting temperature of Sn (Tm m= 231 °C). DuringDuring cooling, cooling, the the nucleation nucleation temperature temperature was determined by by the the onset onset solidification solidification of the of the ˝ exothermicexothermic peak peak (Figure (Figure1b). 1b). SAC105 SAC105 hashas an onset solidification solidification at at200.12 200.12 ± 0.64˘ 0.64°C. AdditionC. Addition of of aluminumaluminum to SAC105 shifted shifted the the exothermic exothermic peak peak to to the the right, right, where where the the onset onset solidification solidification temperaturetemperature is 215.84is 215.84˝C °C˘ ±1.39, 1.39, 217.74217.74 ˝°CC ˘± 0.83 and and 219.17 219.17 °C˝C ± ˘0.210.21 for for SAC105 SAC105 + 0.1Al, + 0.1Al, SAC105SAC105 + 0.3Al + 0.3Al and and SAC105 SAC105 + + 0.5Al, 0.5Al, respectively.respectively.

(a)

Figure 1. Cont. Materials 2016, 9, 522 4 of 17 Materials 2016, 9, 522 4 of 17

(b)

Figure 1. DSC curves for as cast SAC105, SAC105 + 0.1Al, SAC105 + 0.3Al and SAC105 + 0.5Al alloys: Figure 1. DSC curves for as cast SAC105, SAC105 + 0.1Al, SAC105 + 0.3Al and SAC105 + 0.5Al alloys: (a) heating; and (b) cooling. (a) heating; and (b) cooling. The degree of undercooling, ∆T, was defined by the difference of two onset temperatures in coolingThe degree and heating of undercooling, curve. In Table∆T, 1, was it can defined be seen by that the SAC105 difference has ofthe two largest onset undercooling temperatures of in cooling~17 °C. and The heating addition curve. of Al into In TableSAC1051, itsolder can bereduced seen thatthe degree SAC105 of undercooling has the largest significantly undercooling in all of ˝ ~17casesC.The where addition Al was of added. Al into SAC105 solder reduced the degree of undercooling significantly in all cases where Al was added. Table 1. Onset of melting, onset of solidification and undercooling of SAC105, SAC105 + 0.1Al, TableSAC105 1. Onset + 0.3Al of and melting, SAC105 onset + 0.5Al. of solidification and undercooling of SAC105, SAC105 + 0.1Al, SAC105 + 0.3Al and SAC105 + 0.5Al. Solder Onset of Melting (°C) Onset of Solidification (°C) Undercooling (°C) SAC105Solder Onset 216.84 of Melting ± 0.50 (˝C) Onset of 200.12 Solidification ± 0.64 (˝C) Undercooling ~17 (˝C) SAC105 + 0.1Al 216.43 ± 0.38 215.84 ± 1.39 ~0.6 SAC105 216.84 ˘ 0.50 200.12 ˘ 0.64 ~17 SAC105 + 0.3Al 221.14 ± 0.36 217.74 ± 0.83 ~3.4 SAC105 + 0.1Al 216.43 ˘ 0.38 215.84 ˘ 1.39 ~0.6 SAC105SAC105 + + 0.5Al 0.3Al 221.65 221.14 ±˘ 1.020.36 219.17 217.74 ±˘ 0.210.83 ~2.48~3.4 SAC105 + 0.5Al 221.65 ˘ 1.02 219.17 ˘ 0.21 ~2.48 2.2. Microstructure 2.2. MicrostructureFigure 2a–h shows the cross sectional Scanning Electron Microscopy (SEM) images of as- received solder alloys and reflowed solder joints prepared on copper substrates. Cross sectional imagesFigure of2 a–has-received shows the SAC105 cross samples sectional show Scanning primary Electron Sn phase Microscopy having a (SEM)lighterimages contrast of while as-received the solderdarker alloys contrast and reflowedphase represents solder jointsCu6Sn5 prepared. The Cu6Sn on5 formed copper a substrates. continuous Crossnetwork sectional in the as-cast images of as-receivedSAC105 solder SAC105 and samples fine Ag show3Sn particles primary are Sn phasealso seen having in the a lighter as-cast contrast solder. whileWith the darkeraddition contrast of phase0.1 wt.% represents Al, Cu6 CuSn56 phaseSn5. Theand particles Cu6Sn5 offormed another a new continuous darker phase network are seen in thedistributed as-cast in SAC105 the lighter solder andcontrast fine Ag Sn3Sn phase. particles The darker are also particles seen in are the seen as-cast to agglomerate solder. With in the Sn addition phase (Figure of 0.1 wt.% 2b). With Al,Cu the6 Sn5 phaseaddition and particles of 0.3 wt.% of and another 0.5 wt.% new Al, darker elongated phase sh areapes seen of new distributed darker IMC in thephase lighter are seen contrast distributed Sn phase. Thein darker the lighter particles contrast are Sn seen phase. to agglomerate It can be seen in that the the Sn phaseSn grain (Figure size decreases2b). With with the Al addition addition of in 0.3 the wt.% andas-received 0.5 wt.% Al, alloys. elongated After reflow, shapes the of new bulk darker microstr IMCucture phase of arethe seenreflowed distributed solder was in the different lighter from contrast Snthat phase. of the It can as-received be seen solder. that the From Sn grain Figure size 2e,f decreases it can be seen with that Al additionCu6Sn5 and in theAg3Sn as-received particles are alloys. Afterlarger reflow, after the 1 × bulkreflow microstructure as compared to of Figure the reflowed 2a,b. After solder 1 × reflow, was different with thefrom addition that of of 0.3 the wt.% as-received and 0.5 wt.% Al, equiaxed darker IMC was seen distributed in Sn phase instead of elongated shape of solder. From Figure2e,f it can be seen that Cu 6Sn5 and Ag3Sn particles are larger after 1 ˆ reflow as new darker IMC in the as-received sample. Sn grain size reduction as a function of Al content still compared to Figure2a,b. After 1 ˆ reflow, with the addition of 0.3 wt.% and 0.5 wt.% Al, equiaxed persists in reflowed sample. darker IMC was seen distributed in Sn phase instead of elongated shape of new darker IMC in the as-received sample. Sn grain size reduction as a function of Al content still persists in reflowed sample.

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Figure 2. SEM images of cross sectional samples of bulk microstructure of as cast (a–d) and after 1 × FigureFigure 2. 2. SEMSEM images images of of cross cross sectional sectional samples samples of ofbulk bulk microstructure microstructure of as of cast as cast (a–d ()a –andd) andafter after 1 × reflow (e–h) of SAC105, SAC105 + 0.1Al, SAC105 + 0.3Al and SAC105 + 0.5Al. reflow1 ˆ reflow (e–h) ( eof–h SAC105,) of SAC105, SAC105 SAC105 + 0.1Al, + 0.1Al, SAC105 SAC105 + 0.3Al + 0.3Al and SAC105 and SAC105 + 0.5Al. + 0.5Al. Figure 3a shows the optical microscope cross-sectional images of SAC105 + 0.5Al near top FigureFigure 33aa showsshows the the optical optical microscope microscope cross-sectional cross-sectional images images of SAC105 of SAC105 + 0.5Al + near0.5Al top near surface top surface of solder after 1 × reflow. It can be seen that the new darker phase equiaxed IMC agglomerated surfaceof solder of aftersolder 1 afterˆ reflow. 1 × reflow. It can It be can seen be thatseen thethat new the new darker darker phase phase equiaxed equiaxed IMC IMC agglomerated agglomerated and and was mostly found near the top surface of the solder joint. Agglomeration of the new darker phase andwas was mostly mostly found found near near the the top top surface surface of of the the solder solder joint. joint. Agglomeration Agglomeration ofof thethe new darker phase phase near the top surface of solder is also found SAC105 + 0.3Al. Figure 3b shows the new darker phase nearnear the the top top surface surface of of solder solder is is also also found found SAC105 SAC105 + + 0.3Al. 0.3Al. Figure Figure 3b3b shows the new darker phase equiaxed IMC under high magnification. It can be seen that the new faceted IMC particles have varied equiaxedequiaxed IMC IMC under under high high magnification. magnification. It It can can be be se seenen that that the the new new faceted faceted IMC IMC particles particles have have varied varied sizes, ranging from 1 to 5 μm. sizes,sizes, ranging ranging from from 1 1 to to 5 5 μµm.m.

(a) (b) (a) (b) Figure 3. (a) Optical microscope cross sectional images SAC105 + 0.5Al near top surface of the solder Figure 3. (a) Optical microscope cross sectional images SAC105 + 0.5Al near top surface of the solder Figureafter 1 3.× reflow(a) Optical and microscope(b) FESEM crossimages sectional of equiaxed images but SAC105 faceted + 0.5AlIMCs near found top at surface cross-sectioned of the solder of after 1 × reflow and (b) FESEM images of equiaxed but faceted IMCs found at cross-sectioned of afterSAC105 1 ˆ + reflow0.5Al. and (b) FESEM images of equiaxed but faceted IMCs found at cross-sectioned of SAC105SAC105 + + 0.5Al. 0.5Al. Energy-dispersive X-ray Spectroscopic (EDS) analysis was conducted on the new IMC phase Energy-dispersive X-ray Spectroscopic (EDS) analysis was conducted on the new IMC phase foundEnergy-dispersive in SAC105 + 0.1Al, X-ray SAC105 Spectroscopic + 0.3Al and (EDS) SAC1 analysis05 + 0.5Al. was conductedAnalysis results on the indicated new IMC that phase the found in SAC105 + 0.1Al, SAC105 + 0.3Al and SAC105 + 0.5Al. Analysis results indicated that the foundcomposition in SAC105 of the + 0.1Al,new phase SAC105 was + 60–65 0.3Al andat.% SAC105 Cu, 35–40 + 0.5Al. at.% AnalysisAl. EDS resultsLine scan indicated and elemental that the composition of the new phase was 60–65 at.% Cu, 35–40 at.% Al. EDS Line scan and elemental compositionmapping analysis of the newwere phase also wasconducted 60–65 at.% on Cu,the 35–40new at.%IMC Al.phase EDS (Figures Line scan 4 andand elemental 5). Both mappinganalysis mapping analysis were also conducted on the new IMC phase (Figures 4 and 5). Both analysis analysisconfirmed were that also this conducted darker IMC on phase the new consists IMC phase of only (Figures Al and4 andCu. 5Based). Both on analysis the ratio confirmed of Al and that Cu confirmed that this darker IMC phase consists of only Al and Cu. Based on the ratio of Al and Cu thiscontent, darker possible IMC phase identification consists of onlythis darker Al and IMC Cu. Basedphase onis Cu the3Al ratio2 (δ) of or Al Cu and9Al Cu4 (γ content,1), both of possible which content, possible identification of this darker IMC phase is Cu3Al2 (δ) or Cu9Al4 (γ1), both of which identificationcould exist in ofthe this temperature darker IMC range phase below is Cu 3003Al °C2 ( δ[24].) or Cu9Al4 (γ1), both of which could exist in the couldtemperature exist in rangethe temperature below 300 ˝rangeC[24 ].below 300 °C [24].

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3 µm 3 µm

Figure 4. EDS line scan across equiaxed IMCs in SAC105 + 0.5Al. FigureFigure 4. EDS line scan across equi equiaxedaxed IMCs IMCs in in SAC105 SAC105 + + 0.5Al. 0.5Al.

Figure 5. (a) Cross-sectioned image of SAC105 + 0.5Al after 1 × reflow. Elemental maps for the Figure 5. (a) Cross-sectioned image of SAC105 + 0.5Al after 1 × reflow. Elemental maps for the Figureconstituent 5. (a )elements: Cross-sectioned (b) Al; (c image) Cu; (d of) Sn; SAC105 and (e) + Ag. 0.5Al after 1 ˆ reflow. Elemental maps for the constituentconstituent elements:elements: (b) Al; (c) Cu; ( d) Sn; and and ( (ee)) Ag. Ag. In the previous studies on minor addition of Al, many researchers attempted to identify Cu-Al In the previous studies on minor addition of Al, many researchers attempted to identify Cu-Al IMCIn by the EDS previous analysis. studies There onare minora few Cu-Al addition binary of Al, phases many that researchers have compositions attempted close to identify to each other, Cu-Al IMC by EDS analysis. There are a few Cu-Al binary phases that have compositions close to each other, IMCsuch by as EDS γ1-Cu analysis.9Al4, ζ2-Cu There4Al3, are and a fewδ-Cu Cu-Al3Al2. This binary makes phases it difficult that have for compositions researchers to close accurately to each such as γ1-Cu9Al4, ζ2-Cu4Al3, and δ-Cu3Al2. This makes it difficult for researchers to accurately other,identify such the as exactγ -Cu Cu-AlAl , phaseζ -Cu formAl , [2,17,20]. and δ-Cu Similarly,Al . This in makes this study, it difficult EDX for analysis researchers could tonot accurately provide identify the exact1 Cu-Al9 4 phase2 form4 3 [2,17,20]. Similarly,3 2 in this study, EDX analysis could not provide identifyclear identification the exact Cu-Al for the phase Cu-Al form compound. [2,17,20]. Anderson Similarly, et in al. this who study, studied EDX the analysis effect couldof Al addition not provide on clear identification for the Cu-Al compound. Anderson et al. who studied the effect of Al addition on clearthe microstructure identification forof solder the Cu-Al have compound.identified the Anderson Cu-Al IMC et al.as δ who-Cu33 studiedAl17. They the did effect so by of Alcasting addition out the microstructure of solder have identified the Cu-Al IMC as δ-Cu33Al17. They did so by casting out ona Cu-Al the microstructure block which has of solderthe similar have composition identified the with Cu-Al the Cu-Al IMC asIMCδ-Cu theyAl obtained. They in did the sobulk by solder, casting a Cu-Al block which has the similar composition with the Cu-Al IMC they33 obtained17 in the bulk solder, outand a analyzed Cu-Al block it using which X-ray has diffraction the similar (XRD) composition [14]. Though with theAnderson Cu-Al IMCet al. theyhave obtainedchosen to in refer the to bulk it and analyzed it using X-ray diffraction (XRD) [14]. Though Anderson et al. have chosen to refer to it solder,as δ-Cu and33Al analyzed17, the general it using nomenclature X-ray diffraction for these (XRD) IMC is [ 14also]. known Though as Anderson δ-Cu3Al2 [24]. et al. By have comparing chosen as δ-Cu33Al17, the general nomenclature for these IMC is also known as δ-Cu3Al2 [24]. By comparing tothe refer morphology to it as δ-Cu of Cu-Al33Al17 IMC, the (Figure general 3a) nomenclature in this study for to δ these-Cu33Al IMC17 [14], is also it can known be seen as thatδ-Cu they3Al 2both[24]. the morphology of Cu-Al IMC (Figure 3a) in this study to δ-Cu33Al17 [14], it can be seen that they both exhibit a similar morphology (equiaxed and faceted dark color appearanceδ under Field-emission Byexhibit comparing a similar the morphology morphology (equiaxed of Cu-Al and IMC facet (Figureed dark3a) incolor this appearance study to under-Cu33 AlField-emission17 [14], it can scanning electron microscopy (FESEM) and size (size varied from 1 to 5 μm)). Thus, it would be bescanning seen that electron they bothmicroscopy exhibit a(FESEM) similar morphologyand size (size (equiaxed varied from and 1 facetedto 5 μm)). dark Thus, color it appearancewould be underlogical Field-emission to assume both scanning IMC are electronof the same microscopy kind, and (FESEM) it will beand referred size (sizeto as variedCu3Al2 fromin the 1 following to 5 µm)). logical to assume both IMC are of the same kind, and it will be referred to as Cu3Al2 in the following discussion. Thus,discussion. it would be logical to assume both IMC are of the same kind, and it will be referred to as Cu3Al2 in the following discussion. 2.3. Interfacial Reaction after Reflow 2.3. Interfacial Reaction after Reflow 2.3. Interfacial Reaction after Reflow Figure 6 shows cross-sectional FESEM micrographs at the solder/Cu interface after 1 × reflow. A Figure 6 shows cross-sectional FESEM micrographs at the solder/Cu interface after 1 × reflow. A typicalFigure scallop6 shows type cross-sectionalCu6Sn5 layer forms FESEM at the micrographs SAC105/Cu atinterface the solder/Cu after reflow interface (Figure after 6a). 1 Uponˆ reflow. the typical scallop type Cu6Sn5 layer forms at the SAC105/Cu interface after reflow (Figure 6a). Upon the addition of aluminum, the scallop morphology is still seen, but the IMC becomes more flat as Al is Aaddition typical scallopof aluminum, type Cu the6Sn scallop5 layer morphology forms at the is SAC105/Cu still seen, but interface the IMC after becomes reflow more (Figure flat6a). as UponAl is added up to 0.5 wt.% (Figure 6c). The IMC height is reduced with the addition of Al. The average theadded addition up to of 0.5 aluminum, wt.% (Figure the scallop6c). The morphology IMC height is is still reduced seen, butwith the the IMC addition becomes of Al. more The flat average as Al is

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addedMaterials up 2016 to 0.5, 9, 522 wt.% (Figure6c). The IMC height is reduced with the addition of Al. The7 averageof 17 thicknessMaterials of 2016 the, 9, total 522 IMC layer is plotted as a function of Al content in Figure7. The inﬂuence7 of 17 of Al additionthickness on of the the interfacial total IMC IMClayer thicknessis plotted as is a obvious. function of The Al thicknesscontent in Figure of the interfacial7. The influence IMC of does Al not additionthickness on of the the interfacial total IMC IMClayer thickness is plotted is as obviou a functions. The of thicknessAl content of in the Figure interfacial 7. The IMCinfluence does ofnot Al seem to vary with the Al content of the solder in the range 0.1–0.5 wt.%. This may indicate that the seemaddition to vary on withthe interfacial the Al content IMC thicknessof the solder is obviou in the s.range The thickness0.1–0.5 wt.%. of the This interfacial may indicate IMC thatdoes the not addition of Al beyond a certain percentage does not bring additional beneﬁt in terms of suppression of additionseem to ofvary Al beyondwith the a Alcertain content percentage of the solder does inno thet bring range additional 0.1–0.5 wt.%. benefit This in terms may indicateof suppression that the IMC growth. Cu6Sn5 was the only IMC found at the interface, and no trace of Al in Cu6Sn5 and Al-Cu ofaddition IMC growth. of Al beyondCu6Sn5 wasa certain the only percentage IMC found does at no thet bring interface, additional and no benefit trace of in Al terms in Cu of6 Snsuppression5 and Al- compoundCuof compoundIMC growth. could could be Cu detected6 Snbe5 detectedwas atthe the onlyat interfacethe IMC interface found of allof at all aluminum-addedthe aluminum-added interface, and no solder. solder.trace of Al in Cu6Sn5 and Al- Cu compound could be detected at the interface of all aluminum-added solder.

(a) (b) (a) (b)

(c) (d) (c) (d) Figure 6. Cross sectional FESEM micrographs of: (a) SAC105; (b) SAC105 + 0.1Al; (c) SAC105 + 0.3Al; Figure 6. Cross sectional FESEM micrographs of: (a) SAC105; (b) SAC105 + 0.1Al; (c) SAC105 + 0.3Al; andFigure (d) SAC105 6. Cross + sectional 0.5Al after FESEM 1 × reflow. micrographs of: (a) SAC105; (b) SAC105 + 0.1Al; (c) SAC105 + 0.3Al; andand (d) SAC105(d) SAC105 + 0.5Al + 0.5Al after after 1 1ˆ ×reﬂow. reflow.

Figure 7. Variation of Cu6Sn5 thickness with Al content during 1 × reflow. Figure 7. Variation of Cu6Sn5 thickness with Al content during 1 × reflow. Figure 7. Variation of Cu6Sn5 thickness with Al content during 1 ˆ reﬂow.

Materials 2016, 9, 522 8 of 17

2.4. TopMaterials View 2016 of Cu, 9,6 522Sn 5 Solder after 1 ˆ Reflow 8 of 17 Figure8a shows top view image of deeply etched SAC105 + 0.5Al after 1 ˆ reflow. Prior to 2.4. Top View of Cu6Sn5 Solder after 1 × Reflow FESEM imaging, both samples were deeply etched in a mixture of 93% CH3OH, 5% HNO3 and 2% Figure 8a shows top view image of deeply etched SAC105 + 0.5Al after 1 × reflow. Prior to FESEM HCl to remove the solder matrix and thereby expose the interfacial IMC. The IMC grains found on the imaging, both samples were deeply etched in a mixture of 93% CH3OH, 5% HNO3 and 2% HCl to interface of SAC105 and SAC105 + 0.5Al were identified as Cu Sn . It can be seen from the images remove the solder matrix and thereby expose the interfacial IMC.6 5 The IMC grains found on the that SAC105 + 0.5Al shows somewhat faceted Cu Sn grains. Under high magnification in the high interface of SAC105 and SAC105 + 0.5Al were identified6 5 as Cu6Sn5. It can be seen from the images Al concentrationthat SAC105 region,+ 0.5Al smallshows andsomewhat agglomerated faceted Cu particles6Sn5 grains. are Under seen (encircledhigh magnification in yellow in dottedthe high line). EDS analysisAl concentration was done region, on the small high and Al agglomerated concentration particles particle. are Figureseen (encircled8b–d shows in yellow the EDSdotted elemental line). maps.EDS Under analysis high was resolution done on imagingthe high Al and concentrat elementalion mapping,particle. Figure Al,Ag 8b–d and shows Cu werethe EDS detected elemental on the surfacemaps. of the Under exposed high IMC.resolution In Figure imaging8b, and the elemen presencetal mapping, of Al is indicated Al, Ag and in Cu green. were The detected region on of the high concentrationsurface of of the Al exposed is encircled IMC. In in Figure yellow. 8b, Itthe can presence be seen of thatAl is theindicated region in with green. high The Alregion concentration of high correspondsconcentration to that of of Al Cu is (compareencircled in Figure yellow.8b,c). It can The be agglomerated seen that the region particles with are high identical Al concentration to the Cu-Al corresponds to that of Cu (compare Figure 8b,c). The agglomerated particles are identical to the IMC that was found in the bulk microstructure. The elongated and plate-like particles (right corner of Cu-Al IMC that was found in the bulk microstructure. The elongated and plate-like particles (right Figure8d) on the Cu 6Sn5 IMC grains are identified as Ag3Sn. corner of Figure 8d) on the Cu6Sn5 IMC grains are identified as Ag3Sn.

FigureFigure 8. (a) Top8. (a) view Top view image image of deeply of deeply etched etched SAC105 SAC105 + 0.5Al + 0.5Al after after 1 ˆ1 ×reflow. reflow. Elemental Elemental mapsmaps for the the constituent elements: (b) Al; (c) Cu; and (d) Ag. constituent elements: (b) Al; (c) Cu; and (d) Ag. 2.5. Interfacial Reaction After Isothermal Aging 2.5. Interfacial Reaction After Isothermal Aging After 1 × reflow, some of the solder samples were also subjected to isothermal aging at 150 °C ˝ Afterfor up 1 toˆ 720reflow, h in order some to of study the solder the effects samples of Al wereaddition also on subjected the solid state to isothermal reaction between aging solder at 150 C for upand to copper 720 h insubstrate. order toFigure study 9 shows the effects the cross of Al sectional addition images on the of isothermally solid state reactionaged SAC105, between solderSAC105 and copper + 0.1Al, substrate. SAC105 + Figure0.3Al and9 shows SAC105 the + 0.5Al. cross sectionalAfter thermal images aging of for isothermally 720 h, another aged SAC105,intermetallicSAC105 +layer 0.1Al (darker, SAC105 layer) + 0.3Alformed and in betweenSAC105 the +0.5Al first intermetallic. After thermal layer aging and Cu for substrate 720 h, another in intermetallicboth SAC layer and (darkerSAC + Al layer) solders. formed EDS inwas between used to thedetermine first intermetallic the composition layer of and each Cu layer. substrate It is in both SACconfirmed and SACby elemental + Al solders. ratio by EDS EDS wasanalysis used that to the determine outer layer the is composition Cu6Sn5 and the of inner each layer layer. is It is Cu3Sn. Within the resolution of EDS, there was no aluminum detected in both Cu6Sn5 and Cu3Sn after confirmed by elemental ratio by EDS analysis that the outer layer is Cu6Sn5 and the inner layer is thermal aging. The morphology of Cu6Sn5 and Cu3Sn are seen to be similar in SAC105 and Cu Sn. Within the resolution of EDS, there was no aluminum detected in both Cu Sn and Cu Sn 3 SAC105 + Al solders. 6 5 3 after thermal aging. The morphology of Cu6Sn5 and Cu3Sn are seen to be similar in SAC105 and SAC105 + Al solders.

Materials 2016, 9, 522 9 of 17 Materials 2016, 9, 522 9 of 17 Materials 2016, 9, 522 9 of 17

(a(a) ) (b()b)

(c(c) ) (d()d) Figure 9. Cross sectional images of: (a) SAC105; (b) SAC105 + 0.1Al; (c) SAC105 + 0.3Al; and Figure 9.9. Cross sectional images of: ( aa)) SAC105; ( b) SAC105 + 0.1Al; (c) SAC105SAC105 ++ 0.3Al;0.3Al; andand (d) SAC105 + 0.5Al after aging at 150 ˝°C for 720 h. ((d)) SAC105SAC105 ++ 0.5Al after aging at 150 °CC for 720 h. From Figure 9, it is seen that the addition of Al up to 0.5 wt.% has slowed down the growth of From FigureFigure9 ,9, it it is is seen seen that that the the addition addition of of Al Al up up to 0.5to 0.5 wt.% wt.% has has slowed slowed down down the growththe growth of the of the total interfacial IMCs. Figure 10 shows the thickness of Cu6Sn5 and Cu3Sn plotted as a function of totalthe total interfacial interfacial IMCs. IMCs. Figure Figure 10 shows 10 shows the thicknessthe thickness of Cu of6 CuSn56Snand5 and Cu 3CuSn3Sn plotted plotted as aas function a function of Alof Al content. It can be seen that Cu6Sn5 is reduced as Al is added, up to 0.5 wt.%. However, the thickness content.Al content. It can It can be be seen seen that that Cu CuSn6Snis5 is reduced reduced as as Al Al is is added, added, up up to to 0.50.5 wt.%.wt.%. However,However, thethe thickness of Cu3Sn is almost the same for6 all5 samples. of Cu3Sn is almost the same for all samples.

Figure 10. Variation of thickness of IMC with Al content after aging at 150 °C for 720 h. ˝ 2.6. MechanicalFigure Properties 10. Variation of thickness of IMC withwith AlAl contentcontent afterafter agingaging atat 150150 °CC for 720 h.

2.6. Mechanical Indentation Properties testing was carried out on bulk solder and the intermetallic phase in the solders, e.g., Cu6Sn5 and Cu3Al2. The hardness of the bulk solder was determined by Vickers hardness test whileIndentation the hardness testing of individual was carried intermetallic out on bulk phase solder was determinedand the intermetallic by nanoindentation. phase in Figurethe solders, 11 e.g.,shows Cu66 Snthe5 andhardnessand Cu 33Al Alof2 2.solder .The The hardness hardnesswith varying of of the the Al bulk bulkcontent. solder solder It can was was be determined determinedseen that the by byhardness Vickers of hardnesshardness the solder testtest whileincreases thethe hardnesshardness as a function ofof individualindividual of Al content. intermetallicintermetallic SAC105 phaseexhibited was hardness determined of 9.78 by HV nanoindentation. while SAC105 +Figure 0.5Al 11 showsexhibited the hardness the highest of solderhardness with value varying of 14.12 Al HV. content. The resultIt can of be SAC105 seen that is thein good hardness agreement of the with solder increasesother studies asas aa functionfunction [25,26]. ofof AlAl content.content. SAC105SAC105 exhibitedexhibited hardnesshardness ofof 9.789.78 HVHV whilewhile SAC105SAC105 ++ 0.5Al0.5Al exhibited the highest hardness value of 14.12 HV. TheThe resultresult ofof SAC105SAC105 is inin goodgood agreementagreement with other studies [[25,26].25,26].

Materials 2016, 9, 522 10 of 17 Materials 2016, 9, 522 10 of 17 Materials 2016, 9, 522 10 of 17

FigureFigure 11. 11.Variation Variation of of Vickers Vickers hardnesshardness with Al content for for as as received received samples. samples. Figure 11. Variation of Vickers hardness with Al content for as received samples. FigureFigure 12 12shows shows typical typical load load displacement displacement data data obtained obtained through through indentations indentations performed performed on on Sn, Figure 12 shows typical load displacement data obtained through indentations performed on CuSn,6Sn Cu5 and6Sn Cu5 and3Al Cu2 of3Al a2 maximum of a maximum load of load 500 ofµN. 500 For μN. a test For of a thetest same of the maximum same maximum load, the load, maximum the Sn, Cu6Sn5 and Cu3Al2 of a maximum load of 500 μN. For a test of the same maximum load, the penetrationmaximum of penetration the indenter of for the Sn-Ag-Cu indenter solder for Sn-Ag-Cu is approximately solder is 4.5 approximately times of that measured 4.5 times for of Cuthat6Sn 5 maximum penetration of the indenter for Sn-Ag-Cu solder is approximately 4.5 times of that andmeasured Al3Cu2. for The Cu solder6Sn5 and matrix, Al3Cu as2 expected,. The solder is verymatrix, soft, as withexpected, a hardness is very of soft, 0.23–0.3 with GPa.a hardness The matrix of measured for Cu6Sn5 and Al3Cu2. The solder matrix, as expected, is very soft, with a hardness of exhibited0.23–0.3 significant GPa. The matrix plasticity. exhibited Upon significant unloading, plas theticity. solder Upon recovers unloading, only approximately the solder recovers 10 nm only of the 0.23–0.3 GPa. The matrix exhibited significant plasticity. Upon unloading, the solder recovers only 230approximately nm that the indenter10 nm of penetrated. the 230 nm Inthat contrast the indenter to the penetrated. solder, both In intermetallics contrast to the are solder, significantly both intermetallicsapproximately are 10 significantly nm of the 230harder: nm Cuthat6Sn the5 (~6.2 indenter GPa) andpenetrated. Al3Cu2 (~10.50In contrast GPa). to The the intermetallics solder, both harder: Cu6Sn5 (~6.2 GPa) and Al3Cu2 (~10.50 GPa). The intermetallics typically recover around intermetallics are significantly harder: Cu6Sn5 (~6.2 GPa) and Al3Cu2 (~10.50 GPa). The intermetallics 40%typically of the maximumrecover around penetration 40% of ofthe the maximum indenter penetration upon unloading. of the indenter From this, upon the deformationunloading. From of the this,typically the deformation recover around of the 40% intermetallic of the maximum phases penetrationwas found to of be the both indenter elastic upon and plastic,unloading. while From the intermetallic phases was found to be both elastic and plastic, while the deformation of the solder is deformationthis, the deformation of the solder of the is found intermetallic to be primaril phasesy wasplastic found [26]. toScanning be both pr elasticobe microscopy and plastic, (SPM) while was the found to be primarily plastic [26]. Scanning probe microscopy (SPM) was used to accurately perform useddeformation to accurately of the solderperform is foundindentation to be primaril on specify plasticic IMC [26]. phases. Scanning It was probe also microscopy used to observe (SPM) wasthe indentation on specific IMC phases. It was also used to observe the residual indents, as shown in residualused to indents,accurately as shownperform in indentationFigure 13. As on expect specifedic from IMC the phases. nanoindentation It was also dataused in to Figure observe 12, thethe Figure 13. As expected from the nanoindentation data in Figure 12, the residual indents in the solder residualresidual indentsindents, in as the shown solder in wereFigure much 13. As larger expect thaned fromin intermetallics the nanoindentation (Figure 13). data All in of Figure the residual 12, the wereindentsresidual much observed indents larger in thanfor the the insolder intermetallics intermetallics were much exhibited (Figurelarger thana 13smooth). in All intermetallics profile of the with residual (Figureno det indentsected 13). Allpile-up observed of the or residual sink-in for the intermetallicsofindents material, observed while exhibited forsofter the a materials smooth intermetallics profile like Sn exhibited with exhibit no a detected pile-upsmooth behavior. pile-upprofile with or sink-in no detected of material, pile-up whileor sink-in softer materialsof material, like Snwhile exhibit softer a materials pile-up behavior. like Sn exhibit a pile-up behavior.

Figure 12. Load displacement data obtained for 500 μN maximum load indentations performed on Figure 12. Load displacement data obtained for 500 μN maximum load indentations performed on FigureSn, Cu 12.6SnLoad5 and displacement Cu3Sn. data obtained for 500 µN maximum load indentations performed on Sn, CuSn,Sn Cuand6Sn5 Cu andSn. Cu3Sn. 6 5 3

Materials 2016, 9, 522 11 of 17 MaterialsMaterials 2016 2016, ,9 9, ,522 522 1111 of of 17 17

((aa)) ((bb))

((cc)) ((dd))

Figure 13. Scanning probe microscopy image of: (a) Sn and Cu3Al2 before indentation; (b) Sn and Figure 13. ScanningScanning probe probe microscopy microscopy image image of: of: (a) ( aSn) Sn and and Cu Cu3Al32Al before2 before indentation; indentation; (b) Sn (b )and Sn Cu3Al2 after indentation; (c) Cu6Sn5 before indentation; and (d) Cu6Sn5 after indentation of SAC105 + andCu3Al Cu2 after3Al2 indentation;after indentation; (c) Cu6 (Snc)5 Cu before6Sn5 indentation;before indentation; and (d) andCu6Sn (d5) after Cu6 Snindentation5 after indentation of SAC105 of + 0.1Al.SAC1050.1Al. + 0.1Al.

HysitronHysitron nanoDMA nanoDMA with with CMX CMX correct correctioncorrectionion and and maximum maximum load load of of 1000 1000 μµμNN was was used used to to perform performperform continuous nanoscale dynamic mechanical measurement of Cu3Al2. Hardness and elastic modulus of continuous nanoscale dynamic dynamic mechanical mechanical measurement measurement of of Cu Cu3Al3Al2. 2Hardness. Hardness and and elastic elastic modulus modulus of Cu3Al2 could be obtained as a function of indentation depth. Figure 14 shows the hardness and elastic ofCu Cu3Al32Al could2 could be obtained be obtained as a asfunction a function of indentation of indentation depth. depth. Figure Figure 14 shows 14 shows the hardness the hardness and elastic and modulus of Cu3Al2 plotted against indent displacement. The hardness and elastic modulus of Cu3Al2 elasticmodulus modulus of Cu3Al of2Cu plotted3Al2 plottedagainst againstindent displacement. indent displacement. The hardness The hardness and elastic and modulus elastic modulus of Cu3Al of2 was lower near the surface but was fairly constant at depths greater than ~15 nm. The results are Cuwas3Al lower2 was near lower the near surface the surface but was but fairly was fairlyconsta constantnt at depths at depths greater greater than than~15 nm. ~15 nm.The Theresults results are similararesimilar similar toto thatthat to that obtainedobtained obtained fromfrom from quasi-staticquasi-static quasi-static measurementmeasurement measurement withwith with maximummaximum maximum loadload load ofof 500500 of 500 μμN.N.µ N.

FigureFigure 14.14. HardnessHardness andand ElasticElastic ModulusModulus ofof CuCu33AlAl22 plottedplotted againstagainst indentindent displacement.displacement. Figure 14. Hardness and Elastic Modulus of Cu3Al2 plotted against indent displacement. As seen in Table 2, Sn has hardness and elastic modulus of 0.22–0.36 GPa and ~50 GPa, As seenseen in in Table Table2, Sn 2, has Sn hardness has hardness and elastic and moduluselastic modulus of 0.22–0.36 of GPa0.22–0.36 and ~50 GPa GPa, and respectively, ~50 GPa, respectively,respectively, whilewhile CuCu66SnSn55 hashas hardnesshardness andand elasticelastic modulusmodulus ofof ~6.2~6.2 GPaGPa andand ~92–110~92–110 GPa,GPa, while Cu6Sn5 has hardness and elastic modulus of ~6.2 GPa and ~92–110 GPa, respectively. The results respectively.respectively. TheThe resultsresults areare inin goodgood agreementagreement wiwithth otherother studiesstudies [27–29].[27–29]. OnOn thethe otherother hand,hand, CuCu33AlAl22 exhibitsexhibits aa muchmuch higherhigher hardnesshardness andand elasticelastic modulusmodulus asas comparedcompared toto SnSn andand CuCu66SnSn55,, withwith aa hardnesshardness ofof ~10.50~10.50 GPaGPa andand anan elasticelastic modulusmodulus ofof ~170.08~170.08 GPa.GPa.

Materials 2016, 9, 522 12 of 17

are in good agreement with other studies [27–29]. On the other hand, Cu3Al2 exhibits a much higher hardness and elastic modulus as compared to Sn and Cu6Sn5, with a hardness of ~10.50 GPa and an elastic modulus of ~170.08 GPa.

Table 2. Hardness and modulus of Pure Sn, Cu6Sn5, and Cu3Al2.

SAC105 SAC + 0.1Al Element/Compound E (GPa) H (GPa) E (GPa) H (GPa) Sn 49.43 ˘ 7.93 0.29 ˘ 0.07 50.43 ˘ 4.86 0.23 ˘ 0.04 Cu6Sn5 92.25 ˘ 16.24 6.20 ˘ 0.59 100.99 ˘ 10.09 6.26 ˘ 1.07 Cu3Al2 – – 170.08 ˘ 12.92 10.52 ˘ 1.71

3. Discussion From the DSC curves (Figure1), it is found that when 0.1 wt.% of Al was added to the solder, the endothermic curve remains similar to that of SAC105. Both SAC105 and SAC105 + 0.1Al exhibit twin peaks endothermic curve. The onset melting temperature of SAC105 + 0.1Al is the same as that of SAC105, which is ~217 ˝C. This corresponds to Sn-Ag-Cu ternary eutectic temperature. When the percentage of Al is increased to 0.3 wt.% and 0.5 wt.%, the onset of melting temperature shifts to ~221 ˝C (Sn-Ag eutectic temperature) [30]. The DSC curves of SAC105 + 0.3Al and SAC + 0.5Al solders are similar to the DSC curve of non-eutectic Sn-Ag solder [31]. This shows that free Cu atoms are not available in SAC105 + 0.3Al and SAC + 0.5Al solders as Sn-Ag-Cu solder alloy always shows the ˝ onset melting at ~217 C[32]. One possible explanation for this is the formation of Cu3Al2 compound, which has reduced the available Cu in the bulk solder to react with Sn and Ag. The Cu3Al2 compound already formed in the as-received samples, which were supplied in the as-cast condition. As seen in Figure2b–d, Cu 3Al2 is formed in the as-cast solder. Cu3Al2 has a higher melting temperature of ˝ ˝ 960 C[33]. Thus, during the melting of solders for up to 300 C, the stable Cu3Al2 IMC did not react and this lowers the activity of Cu in solder melts. The lack of free Cu atom in the solder is further indicated by the shift of a prominent peak in the DSC curves of SAC105 and SAC105 + 0.1Al. The ~228 ˝C (Sn-Cu eutectic temperature) peak in SAC105 and SAC105 + 0.5Al has shifted to ~231 ˝C ˝ (Tm of Sn = 232 C) in SAC105 + 0.3Al and SAC105 + 0.5Al. This shift indicates that the deficiency of Cu atom in SAC105 + 0.3Al and SAC105 + 0.5Al has reduced the strong eutectic reaction of Sn-Cu, L Ñ Cu6Sn5 + Sn. With Al addition, the peak indicating Sn melting is seen, as fractions of un-melted Sn remaining in the solder melted when temperature increased to 232 ˝C[30]. Table1 shows that SAC105 has the largest undercooling of ~17 ˝C. This is well within range of undercooling values, 10–30 ˝C reported for Sn-Ag-Cu solder [1]. This is because β-Sn requires large undercooling to induce nucleation and solidification [1]. The presence of Al in SAC105 has reduced the undercooling significantly. It is seen that with addition of Al up to 0.5%, undercooling has reduced to 1–5 ˝C. Kotadia et al. and Anderson et al. have observed the effect of Al in reducing undercooling of Sn-3.5Ag and Sn-3.5Ag-0.95Cu solder to 7 ˝C and 4 ˝C respectively [14,34]. The addition of minor alloying element into solder has been one of the effective ways of reducing undercooling by promoting nucleation of β-Sn. Minor alloying atoms which have a much higher melting temperature can exist in molten Sn and provide heterogeneous site for β-Sn nucleation. In the case of Al as minor alloying, Cu3Al2 compound is formed even when the addition of Al is as low as 0.1%. During exothermic reaction in DSC, the existing Al-Cu intermetallic compounds act as a preferential site to promote heterogeneous nucleation of β-Sn, and thus lowering the undercooling of SAC105. Segregated Cu3Al2 IMC was found near the top surface of the sample with addition of Al after 1 ˆ reflow. One of the possible reasons could be that, during reflow, solders are melted on the Cu ˝ substrates at 250 C, the Cu3Al2 particles are not wet by the molten solder, thus they are not drawn into the melt as the molten solder particles solidify. This causes rejection of the Cu3Al2, which remain near the surface/edges of the solder after reflow. Another possible reason for the segregation of Materials 2016, 9, 522 13 of 17

Cu3Al2 particles at the edge/surface could be attributed to the buoyancy effects. The density of Cu3Al2 particles is 6.278 g/cm3, which is lower than the density of liquid Sn, 6.99 g/cm3 [35]. Thus, during reflow, the less dense Cu3Al2 particles migrate to the surface/edges. Kotadia et al. have reported the segregation of Al2Cu in SA solder. They suggested that the segregation of Al rich phase and Al-Cu compound is caused by Stokes and Marangoi motion, which is due to large stable liquid miscibility gap in binary Sn-Al and ternary Sn-Ag-Al [18]. Minor alloying elements which could affect the growth of Cu-Sn compound are divided into two categories: (i) elements that show marked solubility in either one or both of the Cu-Sn IMCs; and (ii) elements that do not significantly dissolve in either of the Cu–Sn IMCs [11]. The effect of elements in category 1 on IMC growth could be explained by using thermodynamic argument. These elements stabilize Cu6Sn5 and decrease the growth of Cu3Sn. The elements in category 2 do not have a prominent effect on IMC as they only affect the growth of IMC layers indirectly. It can be seen from Figures4 and5 that the addition of Al has reduced the growth of Cu6Sn5. Though Anderson et al. have suggested Al has some solubility in Cu6Sn5, no trace of Al could be found in Cu6Sn5 in this study. In this study, the addition of up to 0.5 wt.% Al did not alter the scallop morphology of Cu6Sn5 [14]. Scallop type IMC formation is promoted by higher value of the IMC/liquid solder interfacial energy. There is very limited amount of information available on the influence of Al on the Cu–Sn reaction. Li et al. have reported the suppression of Cu6Sn5 IMC growth with addition of 1% Al. They suggested that the suppression is due to the formation of an Al-Cu IMC layer at the interface, which acts as a barrier for Cu and Sn diffusion [16]. However, Al-Cu IMC layer is not found at the interface in this study as the amount of Al added is less (ď5%). Dhaffer et al. have also reported suppression of Cu6Sn5 IMC growth with addition of 0.1% Al during dip soldering and solid state reaction [20]. The Cu3Al2 IMC found at the Cu6Sn5/Sn interface (Figure8) could account for the suppression of Cu 6Sn5. This is seen in previous work where Zn found at the Cu6Sn5/Sn interface, hindered the flow of copper atoms to the solder thereby slowing down the IMC growth [36]. Thus, the segregation of Al atoms at the IMC/Sn interface may have similar effect on the growth of IMC, by hindering the flow of Cu or Sn atom. With minor Al addition, most of the Al reacts with Cu in the bulk solder to form Cu3Al2. Hence, Al does not form a layer of compound at the interface. Reduction of free Cu atom in the bulk solder could also be attributed to the retardation of Cu6Sn5 growth. When the amount of Al in the solder increased (ě0.5 wt.% Al in SA, ě1 wt.% Al in SAC), it formed a layer of Cu-Al compound at the solder/Cu interface [15,16]. By their presence at the interface, Cu3Al2 IMC hinders the flow of Cu or Sn atom to the solder thereby retarding IMC growth during reflow. During isothermal aging, the Cu6Sn5 IMC layer grows by interdiffusion of Cu and Sn and reaction with each other, while the Cu3Sn IMC forms and grows by reactions between the Cu substrate and Cu6Sn5 IMC layer, as given in the equation below [11]:

Cu6Sn5 ` 9Cu Ñ 5Cu3Sn (1)

The presence of Cu3Al2 IMC at the Cu6Sn5/Sn interface hinders the flow of Cu or Sn atom to the solder, however it does not affect the reaction in Equation (1). Cu3Sn grows by consuming Cu6Sn5 that is formed during reflow. Thus, the thickness of Cu3Sn was not significantly affected by the addition of Al in solder. On the other hand, with slower interdiffusion of Cu and Sn at the interface (due to presence of Cu3Al2) and Cu3Sn formation by consumption of Cu6Sn5, the thickness of Cu6Sn5 was reduced in SAC105 + Al solder. The addition of Al has increased the hardness of bulk solder. This could be due to the grain refinement of Sn, which can be seen in Figure3. Kim et al. has also reported that addition of Al as low as 0.01 wt.% could refine the Sn grain of Sn-Cu solder [8]. From nanoindentation, Cu3Al2 that are found in all SAC + Al solder exhibits higher hardness than other elements in SAC105 + Al. This seems promising in strengthening the solder joints. However, its high elastic modulus should be considered as well, as high elastic modulus could be detrimental for impact testing. Thus, further testing need to be done to further verify the effects of strengthening effects of Cu3Al2. Materials 2016, 9, 522 14 of 17

In spite of the incorporation by only a small fraction, Al is clearly seen to have inﬂuenced decisively on the hardness, undercooling of solder and the interfacial characteristics during reﬂow. However, the amount of Al must be kept below 0.3 wt.% in order to avoid large Cu3Al2 agglomeration, which could affect the performance of solder.

4. Materials and Methods SAC105-xAl (where x = 0, 0.1, 0.3, 0.5 wt.%) were fabricated by Beijing Compo Advanced Technology Co. Ltd. (Beijing, China). All the solder alloys were prepared in rod shape (0.7 cm diameter, 15 cm length). The solder alloy was then cut into thin disks with 1 mm thickness by using electric discharge machining (EDM, A500W, Sodick, Schaumburg, IL, USA). As-received solder was prepared for micro-examination by standard metallographic technique which included, cutting, mounting, grinding (up to 3000 grit paper) and polishing (diamond paste with size 9 µm, 6 µm, 3 µm, 1 µm and colloidal silica with size 0.2 µm). The microstructure of the as-received SAC105-Al solders was characterized by Field emission scanning electron microscope (FESEM, FEI-FEG450, FEI, Houston, TX, USA) and intermetallic compound (IMC) composition was investigated by energy dispersive X-ray spectroscope (EDS, EDAX-Genesis Utilities, EDAX, Mahwah, NJ, USA). Differential scanning calorimeter, DSC (DSC Q20, TA Instruments, New Castle, DE, USA) was used to evaluate the effect of the addition of Al on the onset melting and onset solidification temperature of the SAC105 solder. Each of the samples was weighed to approximately 10 mg and placed on an aluminum pan and covered with a lid. It was then heated from 25 ˝C to 300 ˝C and then cooled down to room temperature at a rate of 10 ˝C/min. For each composition, DSC test was repeated 3 times to ensure the reproducibility of the DSC results. For reflow, commercial grade polycrystalline copper (Cu) sheets (30 mm ˆ 30 mm ˆ 0.3 mm) were used as substrates. Before soldering, Cu sheets were polished with 2000 Grit silica carbide paper, washed with detergent and soaked in 10 vol.% H2SO4 solution for 15–30 min to remove any oxide film present. These were then rinsed with distilled water followed by drying with acetone. Sparkle Flux WF-6317 (Senju Metal Industry, Tokyo, Japan) was then evenly placed on the Cu substrate. SAC-xAl thin disc (diameter = 6.5 mm, thickness = 1.24 mm) were placed in the middle of copper substrates which had been covered with Flux WF-6317. Reflow was carried out in an oven at 250 ˝C for 60 s. After the reflow process, the residual flux was cleaned and removed by rinsing the sample under running distilled water. After that, the reflowed samples were prepared by standard metallographic specimen preparation for microstructural investigation. The microstructure of the bulk solder and the morphology of the IMCs formed at the solder/substrate interface were investigated by FESEM and the composition of the IMCs was investigated by EDS. IMC thickness was calculated by dividing the IMC area by the length of the IMC using a built-in image analyzer software in an Olympus SZX10 (Olympus, Tokyo, Japan) stereoscope. For each experimental condition, thickness values were measured on 5 micrographs and the average IMC thickness is reported. For the top view observation of the interfacial IMCs, the solders were etched in a mixture of 93% CH3OH, 5% HNO3, and 2% HCl to remove the solder matrix and expose the interfacial intermetallic compound. Vickers hardness measurement was performed to investigate the relationship between microstructure and microhardness. The Vickers hardness values were obtained as [36]: HV “ 2Psinϕ{2d (2) where ϕ is the indenter apex angle, P is the applied load and d is the average length of diagonals. The applied load and loading period are 1 kgf and 5 s, respectively. For each specimen, five points were tested and the mean values were obtained. Nanoindentation testing was done using Hysitron Ubi-750 (Hysitron, Minneapolis, MI, USA). Two modes of indentation were conducted on the samples: Quasi-static and Continuous Dynamic Measurement. Quasi-static indentation was conducted with a maximum load of 500 µN, holding time of 2 s, loading rate and unloading rate of 16.67 µN/s and continuous dynamic measurement test Materials 2016, 9, 522 15 of 17 was conducted with a maximum of 1000 µN, holding time of 2 s and a loading rate and unloading rate of 16.67 µN/s. For each element, 5 points were tested and the mean values were obtained. The load–displacement data obtained were analyzed using the method proposed by Oliver and Pharr [37] to determine the hardness and elastic modulus as functions of the displacement of the indenter. For quasi static nanoindentation testing, hardness (H) and reduced modulus (E) can be obtained. The elastic modulus E of the material being indented is related to the reduced modulus using the following equation [38]:

1 ´ v2 E “ (3) 1´v2 1 ´ i Er Ei where v is the Poisson’s ratio of the indented material (usually assumed to be 0.3 if unknown), and vi and Ei are the Poisson’s ratio and elastic modulus of the indenter tip material, respectively. The elastic modulus Ei and Poisson’s ratio vi of the Berkovich indenter used in this study are 1141 GPa and 0.07 respectively. For continuous dynamic measurement, results for the hardness and complex modulus as a function of indentation depth can be obtained. Complex modulus, which is also known as dynamic modulus, is a ratio of stress strain under vibratory conditions. It could be deﬁned by the equation below [39]:

E˚ “ E1 ` iE2 (4) where E* is the reduced complex modulus, E1 is the reduced storage modulus (or elastic modulus), E” is the loss modulus and i is the imaginary unit.

5. Conclusions The following conclusions can be drawn from this study: ‚ Minor addition of Al has reduced the undercooling of SAC105 solder signiﬁcantly.

‚ With addition of 0.1–0.5 wt.% Al to SAC105, Cu3Al2 IMC was found. ‚ Cu3Al2 IMC segregated near the edge of solder upon reflow as Al was added up to 0.3 wt.%. ‚ Minor alloying Al has reduced the thickness of interfacial Cu6Sn5 IMC significantly but do not alter the morphology of Cu6Sn5 IMC. ‚ Minor alloying Al has reduced the thickness of interfacial Cu6Sn5 IMC but has no significant effect on the thickness of Cu3Sn during isothermal aging. ‚ It is suggested that the influence of Al exert its influence on the interfacial reaction by hindering the flow of reacting species at the interface during reflow.

‚ Cu3Al2 IMC has higher hardness and elastic modulus than other microstructure in SAC105 + Al microstructures.

Acknowledgments: The authors would like to acknowledge the financial support from High Impact Research (HIR) Grant, University of Malaya (Project No. UM.C/625/1/HIR/MOHE/ENG/26). Author Contributions: Y.M.L. and A.S.M.A.H. conceived and designed the experiments; Y.M.L. performed the experiments; Y.M.L. and A.S.M.A.H. analyzed the data; A.S.M.A.H. contributed reagents/materials/analysis tools; and Y.M.L. and A.S.M.A.H. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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