Some Fundamental Issues in the Use of Zn-Containing Lead-Free Solders for Electronic Packaging

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Materials Transactions, Vol. 45, No. 3 (2004) pp. 630 to 636 Special Issue on Lead-Free Soldering in Electronics #2004 The Japan Institute of Metals

Some Fundamental Issues in the Use of Zn-Containing Lead-Free Solders for Electronic Packaging

S. Vaynman, G. Ghosh* and M. E. Fine

Department of Materials Science and Engineering, Robert R. McCormick School of Engineering and Applied Science, Northwestern University, 2225 N. Campus Dr., Evanston, IL 60208-3108, USA

A synergistic approach is applied to address the major concerns about the use of Zn-containing lead-free solders for electronic packaging. Using computational thermodynamics as a predictive tool, the phase stability of the Ag-Al-Cu-In-Sn-Zn system is examined to design a Zn- containing lead-free solder with melting characteristics similar to near-eutectic Pb-Sn solder. Theoretically, it is found that a Sn-0.3 mass%Al- 4.2 mass%In-7.8 mass%Zn solder has a melting point (liquidus temperature) of 185C and a solidification range of 10C. It is demonstrated that environmentally benign fluxes containing tin-organometallics significantly improve the wetting behavior compared to rosin fluxes used for lead- tin solders. For the Sn-Zn eutectic solder on a Cu substrate at 260C, it is found that the contact angle is reduced from 150 to about 25 when tin- organometallic fluxes are used instead of rosin flux. Severe accelerated tests (85% relative humidity at 85C) for up to six weeks show that the mechanical properties of Sn-Zn eutectic solder interconnects are not affected adversely by the environment.

(Received July 25, 2003; Accepted September 26, 2003) Keywords: computational thermodynamics, ﬂuxes, lead-free solders, mechanical properties, wetting angle, zinc-containing solders

1. Introduction

Environmental considerations have stimulated efforts to design lead-free solders for interconnects for electronic applications to replace all lead-containing solders. While considerable research has been focused in the development of lead-free solders to replace eutectic or near-eutectic lead-tin solder that melts at 183C, no such satisfactory solder has been developed. In the near term, Sn-Ag,1) Sn-Cu1) or Sn-Ag- Cu2–5) eutectic solders with the melting temperatures of 221, 227, and 217C, respectively are being used. In the long term it is very likely that Sn-Zn based solders will find extensive applications in lead-free electronic components. Sn-Zn based solders are much less costly than Sn-Ag solders and they melt at a 20 to 29C lower Fig. 1 Isothermal fatigue life of different solders at 25C. Ramp time 1 s. temperature than Sn-Ag or Sn-Cu eutectic based solders. No hold time (Ref. 6)). The melting point of the Sn-Zn eutectic is 198C. Upon further alloying it is possible to design Zn-containing lead- free solders with melting/solidification characteristics sim- In in Sn-Zn solder lowers melting point and also improves ilar to near-eutectic Pb-Sn solders. This may help adoption of wetting behavior. the existing manufacturing infrastructure in device fabrica- One manufacturing barrier to using Zn-containing lead- tion such as printed circuit boards and make it easier to find a free solders is the lack of an appropriate flux. Fluxes suitable interconnect material for the chip to chip carrier commonly used for Sn-Pb solders (typically rosin-based) application. result in unacceptably high contact angles when used with Previously6) we reported that bulk Sn-Zn eutectic solder Sn-Zn based solders on substrates such as Cu. A novel has superior isothermal fatigue properties compared to Pb-Sn approach for flux development to improve wetting of copper eutectic solder over total strain ranges from 0.2 to 2% (Fig. 1). surfaces by tin-zinc eutectic solder is detailed in this paper: This is another reason why Sn-Zn eutectic solder is of interest tin containing organic compounds, which decompose at to the electronics industry. soldering temperatures and produce metallic tin on surfaces At present, it not common to use zinc-containing lead-free to be soldered were added to several specially formulated solders due to some drawbacks, such as oxidation, poor fluxes. This process improves wetting of copper surfaces by wetting, lack of mechanical properties and reliability data- molten tin-zinc eutectic solder.9,10) base. To address these issues a synergistic approach is Long-term environmental mechanical reliability of Sn-Zn needed. These include lowering of melting point of Sn-Zn eutectic solder-based joints is an issue that has to be answered eutectic solder by alloying to match Pb-Sn near-eutectic before this solder finds widespread use in interconnects. The solder. In fact, MacCormack et al.7,8) reported that addition of main concern is possible degradation of the solder interconnect due to zinc oxidation. Some of these issues are the *Corresponding author, E-mail: [email protected] subject of this paper. Some Fundamental Issues in the Use of Zn-Containing Lead-Free Solders for Electronic Packaging 631

2. Design of Zn-Containing Lead-Free Solders 2.1 Al-Sn-Zn system For thermodynamic modeling of this ternary system, we It is well established that computational thermodynamics have adopted the constituent binary assessments from and kinetics play crucial roles in modern approaches to alloy following sources: Al-Sn,16) An-Zn,17) and Sn-Zn.18) Earlier, design. Computational thermodynamics and kinetics based a thermodynamic assessment of the ternary system was on the CALPHAD (CALculation of PHAse Diagrams) reported by Fries et al.19) However, in view of recent framework11) have become widely used as the basis for experimental thermodynamic data of the liquid phase20) in modeling phase stability and phase-transformation kinetics in ternary alloys we have re-evaluated the contributions of complex multicomponent alloy systems. Of course, the ternary interaction parameters to the excess Gibss energy of accuracy of the predictions derived from these methods liquid phase as depends critically upon the thermodynamic models that form xs liq G ¼ xAlxSnxZn½36447:41xAl the basis for calculations of phase stability and phase- ð5Þ 7495:28x þ 20538:86x transformation driving forces. Sn Zn In the CALPHAD formalism, the Gibbs energies of the Detailed phase stability calculations show that a ternary phases of interest are expressed as simple analytical eutectic reaction Liquid , (Al)+(Sn)+(Zn) takes place at functions. As an example, the Gibbs energy of a non- 195C and at the liquid composition of Sn-0.42 mass%Al- magnetic solution phase (), such as liquid, fcc, bcc, hcp etc., 8.35 mass%Zn. These are in good agreement both with can be expressed as experimental data,21) and also previous CALPHAD calcu- 19) lation. However, these results offers no significant advant- SER ¼ ref þ id þ xs ð Þ Gm H G G G 1 age over binary Sn-Zn eutectic. X ref ¼ ½ ref SERð Þ ð Þ G Gi Hi 298:15 :xi 2a 2.2 Ag-Sn-Zn system i Ohtani et al.18) have carried out detailed phase stability SER 2 modeling and phase equilibria calculations. In the Sn-corner, Gf ðTÞH ð298:15Þ¼A þ BT þ CT ln T þ DT f they reported the occurrence of seven invariant reactions, 1 7 9 þ ET þ IT þ JT ð2bÞ but only one of them is eutectic: Liquid , (Sn) + (AgZn) + "# Ag Sn at 216.4C. Other invariant reactions are ‘‘trans- X 3 id formed’’ or U-type. G ¼ RT xi ln xi ð3Þ i X 2.3 In-Sn-Zn system xs 0 1 G ¼ xixj½Lij þðxi xjÞLij Cui et al.22) reported the results of phase stability modeling i6¼j and phase equilibria calculations. Three invariant reactions 2 2 þðxi xjÞ Lij þ ... are reported to occur in this system, and one is eutectic: , þ x x x ½L0 x þ L1 x þ L2 x ð4Þ Liquid (Zn)+(InSn)+(InSn) at 107 C. The other two i j k ijk i ijk j ijk k invanriant reactions are U-type, and occur at 120 and 178C. where refG is the Gibbs energy of the reference state, idG is MacCormack et al.7,8) reported that addition of In in Sn-Zn the ideal Gibbs energy of mixing, and xsG is the excess solder lowers melting point and also improves wetting Gibbs energy of mixing which is expressed by a Redlich- behavior. Their results were based on trial and error. To Kister-Muggianu polynomial.12,13) For pure elements, the investigate the effect of In and Zn on the melting/solid- lattice stability equations (2b) are given in the form of ification behavior and solid-solid phase transformation, we SER Gf ðTÞHf ð298:15 K), where the stability of the phase is have carried out a systematic study of phase stability. Figures described relative to the stable element reference (SER) at 2(a) to (d) show the calculated vertical sections of the In-Sn- 14) 298.15 K. Both the binary interaction parameters Lij and Zn system at a constant Zn-contents of 9, 8, 7.5 and the ternary interaction parameters Lijk may be temperature 6.5 mass% respectively. These figures clearly demonstrate dependent. Experience has shown that up to ternary inter- the importance of phase stability on the design of Zn- action terms are sufficient to describe the thermodynamic containing solder as a function of alloy composition and properties of multicomponent systems. Using experimental temperature. It is obvious that to minimize melting temper- thermodynamic and phase equilibria data, the optimum ature and solidification range, the Zn-content should be values of the unknown parameters, Lij and Lijk, may be 8 mass% or less. While addition of 5 to 11 mass% In determined using the PARROT module within Thermo-Calc decreases the melting temperature to about 178C, it also software systems.15) introduces InSn phase in the solder which is known to be For the design of Zn-containing lead-free solders, the brittle. Another disadvantage of adding excessive In is the phase stability of Ag-Al-Cu-In-Sn-Zn is of great interest. An solid-solid phase transformation which is highly undesirable accurate thermodynamic database can predict the melting when the interconnects undergo thermomechanical cycling. temperature, solidification range, solid-solid phase trans- For example, a solder containing 6.5 to 9 mass% Zn and formation, if any, in the composition and temperature of 5 mass% In will undergo (Sn)+(Zn) to (Sn)+(Zn)+ InSn interest. Within the above multicomponent system, first we transformation during heating from room temperature to will examine some of the ternary sub-systems as potential 125C, and vice versa during cooling. Similarly, a solder candidates for the design of lead-free solders, and then containing 6.5 to 9 mass% Zn and 10 mass% In will undergo explore the multicomponent system. (Sn)+(Zn)+ InSn to (Zn)+ InSn transformation during 632 S. Vaynman, G. Ghosh and M. E. Fine

210 Liquid Liquid 200 Liquid+(Zn) 180 Liquid+(Zn)

150 150

(Zn)+γ_InSn (Zn)+γ_InSn 120 (Sn)+(Zn) 100 (Sn)+(Zn) 90 Temperature, C Temperature, C 60 50 (Sn)+(Zn)+γ_InSn (Sn)+(Zn)+γ_InSn 30

0 0 0 0.05 0.10 0.15 0 0.05 0.10 0.15 Weight Fraction In Weight Fraction In (a) (c)

210 210 Liquid Liquid Liquid+(Zn) 180 180 Liquid+(Zn)+γ_InSn

150 150

γ 120 (Zn)+ _InSn 120 (Zn)+γ_InSn (Sn)+(Zn) (Sn)+(Zn) 90 90 Temperature, C Temperature, C 60 60 (Sn)+(Zn)+γ_InSn (Sn)+(Zn)+γ_InSn 30 30

0 0 0 0.05 0.10 0.15 0 0.05 0.10 0.15 Weight Fraction In Weight Fraction In (b) (d)

Fig. 2 Calculated vertical sections of the In-Sn-Zn system: (a) at a constant Zn-content of 9 mass%, (b) at a constant Zn-content of 8 mass%, (c) at a constant Zn-content of 7.5 mass%, and (d) at a constant Zn-content of 6.5 mass%. heating from room temperature to 125C, and vice versa was explored further to decrease the melting point without during cooling. In addition to thermomechanical stress, such affecting the solidification range. The relevant thermody- transformations may impart further stresses and induce namic and phase stability data are taken from the solid transformation defects which may otherwise affect reliability solution (SSOL) database in Thermo-Calc software sys- of the interconnect. tems.23) Al is an attractive candidate as it may help improve Considering the brittle nature of InSn phase and the the oxidation resistance of the alloy by forming a protective abovementioned factors into account, optimum solder com- oxide coating subsequent to soldering. For example, in hot- position may be determined by minimizing melting temper- dip galvanizing process a small amount (0:2 mass%) of Al ature and solidification range while avoiding InSn phase. is added to reduce the oxidation rate of the Zn-bath. After In other words, the microstructure in the solid-state remains extensive calculations, an optimum solder composition is (Sn)+(Zn) during thermal cycling. Then, the optimum solder found to be Sn-0.3 mass%Al-4.2 mass%In-7.8 mass%Zn. composition is found to contain about 8 mass% Zn and about Figure 3 shows the calculated phase fractions of this alloy 4 mass% In. as a function of temperature defining melting point as 185C and a solidification range of 10C. The absence of InSn 2.4 Al-In-Sn-Zn system phase may also be noted in this alloy. Taking the In-Sn-Zn system as basis, the role of Ag and Al Addition of minor amounts (less than 0.5 mass%) of Ag Some Fundamental Issues in the Use of Zn-Containing Lead-Free Solders for Electronic Packaging 633

1.0 185 showed that SnCl2 promotes good spreading of solders on Liquid → copper not only because it is a very good activator but also 0.9 (Sn) because it reacts with copper to give tin or tin-copper 9,10) 0.8 intermetallics on the copper surface. As this paper demonstrates, tin containing organic compounds that decom- 0.7 pose at soldering temperatures to produce metallic tin on 0.6 surfaces to be soldered can be added to the ﬂux. This addition improves wetting of the copper surface by molten tin-zinc 0.5 eutectic solder. 0.4 The ﬂuxes were prepared by mixing together vehicles,

Phase Fraction activators, and additives. The polymers (from Aldridge 0.3 Chemical Company) listed in Table 1 were used as vehicles. Thermal stability of the polymer at the soldering temperature 0.2 (Zn) and non-reactivity toward other components of the flux were 0.1 the main criteria used for polymer selection. Poly(vinyl (Al) acetate) (PVA) easily dissolves in either methyl alcohol or 0 175 185 0 50 100 150 200 isopropyl alcohol, the other two polymers, poly(methyl Temperature, C methacrylate-co-methacrylic acid) (PMM-MA) and poly- (methyl methacrylate) (PMM), used in this study dissolve Fig. 3 Calculated phase fractions of a Sn-0.3 mass%Al-4.2 mass%In- only in isopropyl alcohol. 7.8 mass%Zn defining melting temperature of 185C and a solidifcation Amines are commonly used as activators in fluxes. Three range of 10 C. Furthermore, there is no InSn phase. amines were used in the present research: ethanol amine, di- ethanol amine and tri-ethanol amine (abbreviated in text as EA, DEA and TEA respectively). and Cu offers no significant advantage in lowering the Tin(II) 2-ethylhexanoate (from Aldridge Chemical Com- melting point and/or reducing the solidification range. pany) was selected as the additive that decomposes to coat However, such minor additions are known to improve the copper surface with tin to promote wetting of copper by tin- microstructural stability and creep properties due to the zinc eutectic. This organometallic was selected because when 8) formation of precipitates. it decomposes, it releases only CO2 and water vapors. The fluxes were evaluated in spread tests by measuring the 3. Design of Fluxes for Zn-Containing Lead-Free Sol- contact angles between Sn-Zn eutectic solder and copper. ders The copper plates were cleaned prior to spread testing by immersion for 10 seconds in 50% nitric acid and then for 10 The functions of the flux are to clean the substrate, protect seconds in 10% sulfuric acid aqueous solutions. They were the molten solder and substrate from oxidation and to then rinsed in deionized water and methanol and dried. promote solder spreading. Usually fluxes contain activators Solder disks (3 mm in diameter and 1 mm thick) were that clean the substrate, vehicles that protect the surfaces of degreased with acetone, placed on the fluxed copper surface the substrate and molten solder from oxidation during (two drops of flux applied from an eyedropper were used), soldering and improve heat-transfer. Also, fluxes may and covered with another drop of flux. The copper plates with contain some additives such as surfactants that improves solder discs on them were placed on a hot plate pre-heated to their wetting performance. 260C. After the solder had melted, the copper plate was kept It is well known that pre-tinning of surfaces promotes on the hot plate for another two minutes, removed, cooled in wetting of the substrate by the solder. At the present time tin air and cross-sectioned for examination. The contact angles or tin-lead solders are used for pre-tinning. This procedure were measured and recorded. Each point in the figures that adds to the production cost. Also Sn-Pb coatings are follow is an average of 2 or 3 measurements. Reproducibility unacceptable for a lead-free soldering system. Our research was within 10 to 15%.

Table 1 Polymers vehicles used in ﬂux formulations.

Polymer mess% of the polymer in: Polymer abbreviation used in Methyl Isopropyl text alcohol alcohol Poly(vinyl acetate) PVA 10% 1.3% (m.w.ave=83,000) Poly(methyl methacrylate-co- methacrylic acid) PMM-MA * 2.4%

(m.w.ave=34,000) Poly(methyl methacrylate) PMM * 2.0% (m.w.ave=999,000) * Very low solubility 634 S. Vaynman, G. Ghosh and M. E. Fine

Fig. 4 Eﬀect of tin(II) 2-ethylhexanoate concentration on wetting of copper by tin-zinc eutectic when poly(vinyl acetate) solution in isopropyl Fig. 6 Eﬀect of tin(II) 2-ethylhexanoate concentration on wetting of alcohol is used. Polymer solution to amine ratio is 1:1. Polymer solution + copper by tin-zinc eutectic when poly(methyl methacrylate) solution in amine + SnII equals 100%. isopropyl alcohol is used. Polymer solution to amine ratio is 1:1. Polymer solution + amine + SnII equals 100%.

Fig. 5 Eﬀect of tin(II) 2-ethylhexanoate concentration on wetting of Fig. 7 Eﬀect of tin(II) 2-ethylhexanoate concentration on wetting of copper by tin-zinc eutectic when poly(vinyl acetate) solution in methanol copper by tin-zinc eutectic when poly(methyl methacrylate co-methacrylic is used. Polymer solution to amine ratio is 1:1. Polymer solution + amine acid) solution in isopropyl alcohol is used. Polymer solution to amine ratio + SnII equals 100%. is 1:1. Polymer solution + amine + SnII equals 100%.

When a mildly-activated rosin flux (Kester #197) was used dropped rapidly and 24% gave the lowest contact angles, 25 at 260C reflow temperature, the wetting angle of pure Sn on (see Fig. 4). a copper surface was 37; however, the wetting angle for Sn- Figure 5 shows the same information for PVA with Zn eutectic on copper was 89, too large for any practical methanol as the solvent. With this solvent TEA gives the application. When fluxes containing the polymers listed in lowest contact angle in the absence of the metalo-organic. Table 1 as vehicles, one of the amines as an activator and When EA and DEA are used Sn-Zn eutectic does not wet the tin(II) 2-ethylhexanoate as an additive were used much lower copper surface. For all three amines the contact angle contact angles were obtained (Figs. 4–9). This tin-containing decreases with increasing SnII concentration in the flux. The organic compound was shown by X-ray diffraction to effect of metallo-organic on the contact angle of Sn-Zn decompose with formation of pure tin on the copper surface. eutectic on copper again saturates at about 15% of metallo- Figure 4 shows the effect of Sn(II)2-ethylhexanoate organic in flux. The lowest contact angle obtained with this (referred to as SnII in text) concentration in a PVA/IPA flux system was 40. with the three different amine activators on the contact angle The data for PMM in IPA is shown in Fig. 6. Again the of Sn-Zn eutectic solder on copper surfaces. The total of PVA ratio of PMM solution to amine was one to one and the PMM solution in the solvent, amine and SnII compound equals solution+amine+SnII equals 100%. In the absence of SnII 100%. The ratio of amine to PVA solution was kept at one. the TEA is the most effective of the three in reducing the From this figure DEA is obviously the most effective amine contact angle. The beneficial effect of SnII is also seen, but of the three in this formulation and by itself reduced the the lowest contact angle observed was over 40. contact angle to about 48. Addition of SnII reduced the In Fig. 7, PMM-MA is the polymer and IPA is the solvent. contact angle further to about 35 at 16% of SnII. With the Wetting is observed for all three amines in the absence of other two amines the effect of SnII on the contact angle is SnII compound, but again the metallo-organic reduces the even more pronounced. The effect is more or less saturated at contact angle; the lowest value is about 25 with 24% of DEA about 15% of the SnII. When EA was used as the activator, in as the activator. With EA there is little effect of adding SnII. the absence of the metallo-organic there was non-wetting, but The effects of varying the concentrations of TEA and SnII when the metallo-organic was added the contact angle in PVA-methanol solution and PMM-IPA solution are shown Some Fundamental Issues in the Use of Zn-Containing Lead-Free Solders for Electronic Packaging 635

4. Environmental Performance of Sn-Zn Eutectic Solder

The solder joints were produced as described below. Copper plates coated with Ni/Pd/Au were received from Texas Instruments, the details of which are described elesewhere.24) The plates were cut into 12-mm by 12-mm squares and then coated with the following ﬂux: polyvinyl acetate (1.3% solution) in isopropyl alcohol 20% ethanol amine 20% Sn(II) 2-ethylhexanoate 20%

Fig. 8 Effect of tri-ethanol amine concentration on wetting of copper by isopropyl alcohol 40% tin-zinc eutectic when poly(vinyl acetate) solution in methanol is used. The Sn-Zn eutectic solder was received from Alfa-Aesar Polymer solution + amine + SnII equals 100%. as 3 mm diameter rod and was rolled into 0.3 mm thick strip. Solder joint pre-forms 3 mm in diameter were punched out from this strip. Four solder pre-forms previously dipped into the flux were placed on the copper plate squares and then melted in air on a hot plate heated to 245 5C for 2 minutes. Then the plates with four solder bumps were cooled down to room temperature and covered with another copper plate without solder bumps. The distance between these two plates was controlled by a shim inserted between them. A solder resist from Kester Solder Company was used as glue to keep this assembly aligned. The assembly was then placed on the hot plate at 245 5C for another 2 minutes. After cooling to room temperature it was washed with methanol to remove the excess flux and the solder resist. The schematic of the solder Fig. 9 Effect of tri-ethanol amine concentration on wetting of copper by joint is shown in Fig. 10. tin-zinc eutectic when poly(methyl methacrylate) solution in isopropyl To study the effect of humidity and elevated temperature alcohol is used. Polymer solution + amine + SnII equals 100%. on the mechanical and electrical properties of Sn-Zn eutectic solder interconnects two experimental tests were conducted. One batch of interconnects was exposed in an 85% relative in Figs. 8 and 9, respectively. There is general improvement humidity and 85C chamber. Another batch of specimens in the contact angles on increasing the TEA and SnII contents was encapsulated into evacuated glass tubes and then but the effects generally diminish as the concentrations exposed to 85C. A standard humidity controlled temperature increase. Without SnII a high concentration (up to 50%) of chamber for testing electronic assemblies was used in this TEA is needed to significantly reduce the contact angle of Sn- project. Some of the specimens were removed after each Zn eutectic on copper surface. week during a six-week period, the length of the tests. Considering Figs. 4 to 9 it is obvious that the contact angle For mechanical property measurements the joint assem- observed is a function of all of the components in the flux. It blies were attached to two copper blocks with epoxy. The is desirable to keep the concentration of the activator in the joints were tested in shear using an attachment to a Sintech flux as low as possible since the residue left after decom- testing machine. The displacement rate was 0.01 mm/s. The position of the activator can be corrosive and, therefore, load and displacement were recorded during the test. undesirable on the printed wiring boards. While high Figure 11 shows the relationships between the peak shear concentrations of amines reduce the contact angle to near 50, a much lower concentration of activator is needed if tin- containing organometallic is also added to the flux and even lower contact angles are achieved. This preliminary investigation has shown that Sn-Zn eutectic solder wets copper with sufficiently low contact angles for interconect use if appropriate flux (environmentally benign) is used. Such fluxes are achieved by adding a tin metallo-organic compound that decomposes to plate tin on the substrate. Also, it is very likely that the use of metallo- organics based fluxes in conjunction with the multicomponent Zn-base solders discussed in Section 2, there will be a further reduction of contact angle. Fig. 10 Schematic diagram of the solder joint assembly. 636 S. Vaynman, G. Ghosh and M. E. Fine

improvement is due to the decomposition of these compounds to give a tin coating on the substrate. The tin that forms on the surface of the copper leads to better spreading of solder. A flux was formulated that gave a contact angle of 25 with Sn-Zn eutectic solder on copper at 260C. Long time exposure of Sn-Zn eutectic solder interconnects to very severe accelerated test (85% relative humidity at 85C) for up to six weeks did not affect adversely their mechanical properties. The shear stress strain properties (yield stress, maximum stress and strain to failure) of Sn-Zn eutectic solder were not affected by this exposure.

Acknowledgments Fig. 11 The dependence of average peak shear stress on aging time. Financial support of National Science Foundation through stress and the aging time for solder joints that were aged in the Grant No. DMR-9201834, DMR-9523447, DMR- vacuum and humid air respectively. There is little difference 9813919; the Semiconductor Research Corporation; and among the peak stress values obtained for solder joint International Lead Zinc Research Organization is appreci- specimens that were aged for the same period of time and in ated. Advice of Professor D. Shriver in selection of polymers the same environment. In both environments the values of and tin-organometallics for present research is greatly peak stress do not depend significantly on the aging time at appreciated. 85C; there is only slight reduction of the solder joints strength at the end of the 6-week aging period in both REFERENCES environments. 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