Effect of Copper Sulfate and Sulfuric Acid on Blind Hole Filling of HDI Circuit Boards by Electroplating

materials

Article Effect of Copper Sulfate and Sulfuric Acid on Blind Hole Filling of HDI Circuit Boards by Electroplating

Pingjun Tao 1,*, Yugan Chen 1, Weitong Cai 1 and Zhaoguang Meng 2

1 School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China; [email protected] (Y.C.); [email protected] (W.C.) 2 Dongguan Wuzhu Electronic Technology Co., Ltd., Dongguan 523390, China; [email protected] * Correspondence: [email protected]

Abstract: Here, in a certain high density interconnect (HDI) printed circuit board, the effect of copper sulfate and sulfuric acid on the ﬁlling effect of a blind hole with a certain diameter and depth was

investigated by making a blind hole using a CO2 laser drilling machine, filling the blind hole via electroplating by simulating the electroplating line in a Halin cell, and observing the cross-section of a micro blind hole after polishing using metallographic microscope, as well as the effect of hole filling, are evaluated. The results show that, under the conditions of a certain plating solution formula and electroplating parameters (current density and electroplating time), the sag degree decreases with the increase in the copper sulfate concentration. When the concentration of copper sulfate increases from 210 g/L to 225 g/L, the filling effect is good and the sag degree is about 0. However, with the increase in sulfuric acid concentration, the sag increases gradually. When the sulfuric acid concentration is 25–35 g/L, both the sag and copper coating thickness are in a small range. Under appropriate electroplating conditions, a better blind hole filling effect can be obtained. The volume of blind hole has a certain effect on the diffusion and exchange of copper sulfate and sulfuric acid, as well as on the concentration distribution of additives.

Keywords: HDI printed circuit board; electroplating; blind hole ﬁlling; sag degree; copper plating layer

Citation: Tao, P.; Chen, Y.; Cai, W.; Meng, Z. Effect of Copper Sulfate and Sulfuric Acid on Blind Hole Filling of 1. Introduction HDI Circuit Boards by Electroplating. Materials 2021, 14, 85. https://dx.doi. High density interconnect (HDI) printed circuit boards were proposed in the United org/10.3390/ma14010085 States and Japan in the early 1990s. This kind of printed circuit board is also known as a “build-up multilayer” board. HDI is a fine circuit, small aperture, high density, and Received: 9 November 2020 ultra-thin printed circuit board [1–3]. Its main features are as follows: the aperture is not Accepted: 23 December 2020 more than 0.15 mm, the hole ring diameter is less than 0.25 mm, the density of welding Published: 27 December 2020 point is greater than 0.2 points/mm2 or 130 points/in2, the wiring density is greater than 4.6 mm/mm2 or 117 in/in2, and the line/space is not more than 0.076 mm/0.076 mm or Publisher’s Note: MDPI stays neu- 3 mil/3 mil (1 mil = 0.254 mm). With the development of electronic equipment moving in tral with regard to jurisdictional claims the direction of being light, thin, intelligent, and multi-functional, HDI is required to have in published maps and institutional high-density interconnection and high reliability. Blind hole electroplating, which has been affiliations. widely studied, is one of the important technologies in order to realize the high-density interconnection and high reliability of printed circuit boards [4]. The basic principle of blind hole filling in electroplating is to fill the blind hole with super filling electrodeposition

Copyright: © 2020 by the authors. Li- mode, i.e., bottom-up deposition mode. In the bottom-up deposition mode, the deposition censee MDPI, Basel, Switzerland. This rate of copper on the bottom of the blind hole is greater than that on the surface of the plate, article is an open access article distributed so as to achieve the filling effect of having no cavity, a low sag, and a low surface copper under the terms and conditions of the plating thickness [5,6]. However, although electroplating blind hole filling technology has Creative Commons Attribution (CC BY) been applied in the field of HDI printed circuit board manufacturing, how to implement license (https://creativecommons.org/ high-quality blind hole electroplating filling of an HDI printed circuit board is still a licenses/by/4.0/). research hotspot [7–10].

Materials 2021, 14, 85. https://dx.doi.org/10.3390/ma14010085 https://www.mdpi.com/journal/materials Materials 2021, 14, 85 2 of 11

A large number of studies have shown that there are many factors affecting the effect of blind hole filling, such as the formula of the electroplating solution (composition and concentration), electroplating process parameters (electroplating method, stirring mode, current density, and electroplating time), and the geometric size of blind hole (diameter and diameter depth ratio) [11–13]. However, there are not many reports about how to obtain a good blind hole filling effect for an HDI substrate by changing the content and proportion of copper sulfate and sulfuric acid in the plating solution formula, and the available references are very limited. Therefore, in order to improve the quality of blind hole filling and to improve the production process of blind hole filling in HDI printed circuit board electroplating, this paper takes a blind hole with a specific hole depth and aperture as the research object, and studies the influence of copper sulfate and sulfuric acid in the plating solution formula on the blind hole filling effect, so as to lay the foundation for optimizing the blind hole filling electroplating process and plating solution formula, as well as realizing high-quality blind hole filling electroplating.

2. Materials and Methods 2.1. Materials and Instruments

The following were used: copper sulfate CuSO4·5H2O, analytical pure; sulfuric acid (96–98%), analytical pure; hydrochloric acid (38–40%), analytical pure; electroplating solution additives (including wetting agent, leveling agent, and brightener) for blind hole filling in electroplating, provided by Dongguan Wuzhu Electronic Technology Co., Ltd., Dongguan, China; FR4 (Abbreviation of flame retardant type 4) copper clad laminate, provided by Dongguan Wuzhu Electronic Technology Co., Ltd.; and distilled water, self- made. The following equipment were used: HD600E2A CO2 dual beam dual table PCB laser drilling machine, Han’s Laser Technology Industry Group Co., Ltd., Shenzhen, China; Horizontal Orizontal Electroless Copper Line, Guangzhou Julong PCB Equipment Co., Ltd., Guangzhou, China; vertical continuous electroplating production line, Yongtian Machinery Equipment Manufacture (Shenzhen) Co., Ltd., Shenzhen, China; Haring cell, Suzhou Jiangdong Precision Instrument Co., Ltd., Suzhou, China; DC (Abbreviation of direct current) rectifier, Xiamen Songye Electric Technology Co., Ltd., Xiamen, China; JMP-2B metallographic slicing grinder, Shenzhen Jujie Instrument Equipment Co., Ltd., Shenzhen, China; and metallographic microscope, Shanghai Wumo Optical Instrument Co., Ltd., Shanghai, China.

2.2. Preparation of Hole-Filling Plate In this paper, 12 µm copper foil, 106 pp/12 µm copper foil, and 1080 pp laminate were selected, and blind holes with diameters of 75 µm, 100 µm, and 125 µm were made using a CO2 laser drilling machine on double-sided plates with a medium layer thickness of 60 µm and 80 µm, respectively, and the distance between holes was 500 µm. The thickness of the copper coating was about 5 µm using chemical deposition and ﬂash plating. The plates were cut into 100 mm × 50 mm test plates, which were degreased, washed, micro etched, washed, and pickled.

2.3. Electroplating In a 500-mL Halin cell, the experiment plates were electroplated with a simulated electroplating line using DC plating. The anode was an insoluble anode, the stirring mode was air stirring, and the distance between the anode and cathode was 0.5 cm. The bath temperature was 25 degrees centigrade. The concentration control range of each component in the electroplating solution was as follows: 200~260 g/L CuSO4·5H2O, 20~60 g/L H2SO4, 2~22 mL/L wetting agent, 0~2 mL/L sulfur containing brightener, 5~25 mL/L nitrogen containing leveling agent, and 30~70 mg/L Cl−. Materials 2021, 14, x FOR PEER REVIEW 3 of 12

Materials 2021, 14, 85 3 of 11

2.4. Characterization of Blind Hole-Filling Effect 2.4. CharacterizationThe cross section of Blind of the Hole-Filling micro blind Effect hole after polishing was observed using a metallographicThe cross section microscope. of the microThe filling blind effect hole after of the polishing hole filling was electroplating observed using is ausually metal- lographiccharacterized microscope. by the filling The filling rate, copper effect of thickn the holeess, fillingand sag electroplating degree. The ishole usually filling charac- rate = terizedB/A × 100% by the and filling the rate,sag degree copper = thickness, A − B, where and sagA is degree.the surface The thickness hole filling of rate the =copper B/A ×layer100% from and the the bottom sag degree of the = blind A − B,hole where to the A su isrface the surface of the thicknessplate, B is ofthe the thickness copper of layer the frommost theconcave bottom copper of the layer blind from hole tothe the hole surface botto ofm theof the plate, blind B is hole the thickness to the surface of the of most the concavehole, and copper C is the layer thickness from the from hole the bottom copper of foil the blindto the hole surface to the of surfacethe copper of the layer hole, on and the Cplate is the surface, thickness as fromshown the in copper Figure foil 1. toAccordin the surfaceg to of whether the copper the layerblind on hole the is plate fully surface, filled; aswhether shown there in Figure are holes,1. According cavities, to and whether other thedefects blind in hole the ishole; fully and filled; whether whether the therefilling arerate holes, meets cavities, certain andindustry other requirements, defects in the hole;the filling and whethereffect can the be filling judged. rate The meets sag certaindegree industryshould be requirements, controlled within the filling a dozen effect microns. can be judged. The thickness The sag of degree the copper should plating be controlled should withinbe as small a dozen as possible microns. on The the thickness premise of of the en coppersuring platingthat the should sag meets be as the small production as possible re- onquirements. the premise of ensuring that the sag meets the production requirements.

FigureFigure 1.1. CharacterizationCharacterization ofof thethe blindblind holehole ﬁllingfilling capacity.capacity.

3.3. ResultsResults andand DiscussionDiscussion TheThe processprocess ofof holehole fillingfilling electroplatingelectroplating mustmust maximizemaximize thethe fillingfilling capacitycapacity andand minimize the thickness of the surface copper, showing comprehensive characteristics of minimize the thickness of the surface copper, showing comprehensive characteristics of a a low sag, uniform copper surface distribution, and no cavity. The filling capacity of low sag, uniform copper surface distribution, and no cavity. The filling capacity of elec- electroplating can be characterized by the sag degree, copper plating thickness, and filling troplating can be characterized by the sag degree, copper plating thickness, and filling rate. Figure1 shows the characterization of the blind hole filling capacity. In order to meet rate. Figure 1 shows the characterization of the blind hole filling capacity. In order to the high density interconnection requirements of HDI circuit boards, the effect of blind meet the high density interconnection requirements of HDI circuit boards, the effect of hole filling by electroplating is generally evaluated from the following aspects. The first blind hole filling by electroplating is generally evaluated from the following aspects. The requirement is that the sag should not exceed 15 µm. The second requirement is that the first requirement is that the sag should not exceed 15 μm. The second requirement is that deposition thickness of the copper should not exceed 25 µm. The third requirement is the deposition thickness of the copper should not exceed 25 μm. The third requirement that the filling rate should be greater than 78%. The fourth requirement is that the copper is that the filling rate should be greater than 78%. The fourth requirement is that the coating should be even and smooth. However, the electroplating process conditions, the compositioncopper coating and should formula be of even the electroplatingand smooth. However, solution, andthe theelectroplating level of the process electroplating condi- holetions, filling the composition process will haveand formula more or lessof the influence electroplating on the filling solution, effect. and the level of the electroplatingCopper sulfate hole filling is the process main source will have of copper more or ions less ininfluence an electroplating on the filling solution. effect. A high copperCopper andsulfate low is acid the main system source is usually of copper used ions in electroplating in an electroplating hole filling. solution. When A high the concentrationcopper and low of acid copper system sulfate is usually is too low, used the in depositionelectroplating rate hole of copper filling. isWhen slow. the When con- thecentration concentration of copper of copper sulfate sulfate is too islow, too the high, deposition the crystal rate particles of copper increase is slow. and When affect thethe dispersionconcentration ability of copper of the electroplating sulfate is too solution.high, the Figure crystal2 showsparticles the increase relationship and betweenaffect the dispersion ability of the electroplating solution. Figure 2 shows the relationship between the copper sulfate concentration and the blind hole filling effect (35 g/L H2SO4, 45 mg/L Clthe− ,copper 15 mL/L sulfate leveling concentration agent, 1 mL/Land the brightener, blind hole 10 filling mL/L effect wetting (35 g/L agent, H2SO 1.54, 45 A/dm mg/L2 currentcl-, 15 mL/L density, leveling and55 agent, min electroplating1 mL/L brightener, time). 10 It mL/L can be wetting seen from agent, Figure 1.5 2A/dm that2 the current sag decreases with the increase in the copper sulfate concentration. When the CuSO4·5H2O

Materials 2021, 14, x FOR PEER REVIEW 4 of 12

Materials 2021, 14, 85 4 of 11 density, and 55 min electroplating time). It can be seen from Figure 2 that the sag decreases with the increase in the copper sulfate concentration. When the CuSO4·5H2O concentration increasesconcentration from increases 210~225 g/L, from the 210~225 filling g/L, effect the is filling good andeffect the is saggood is and about the 0. sag is about 0. When the concentrationWhen the ofconcentration CuSO4·5H2 Oof increases CuSO4·5H to2O 230 increases g/L and to above, 230 g/L the and blind above, hole the blind hole filling is prominentfilling and is prominent overfilled, and overfilled, CuSO4·5H and2O is CuSO easy4 to·5H crystallize2O is easy and to crystallize precipitate. and precipitate. On the other hand,On the thickness other hand, of the the copper thickness plating of the on thecoppe surfacer plating is almost on the independent surface is almost inde- of the concentrationpendent of theof the copper concentration sulfate, and of the the copper thickness sulfate, of copper and the is betweenthickness 19 of andcopper is between 24 µm, which indicates19 and 24 that μm, the which concentration indicates of that copper the concentration sulfate hardly of affects copper the sulfate thickness hardly affects the of the copper inthickness the experimental of the copper range. in The the experresultsimental from the range. above The experimental results fromdata the above experi- show that the blindmental hole data filling show effect that wasthe blind good whenhole filling the concentration effect was good of CuSO when4 ·5Hthe2 concentrationO of varied from 210CuSO to 2254·5H g/L.2O varied from 210 to 225 g/L.

30 Copper thickness Sag degree 25

15 μm 10

-5 180 190 200 210 220 230 240 250

CuSO4·5H2O (g/L)

FigureFigure 2. Effect 2. Effect of the of copperthe copper sulfate sulfate concentration concentration on the on sagthe degreesag degree and and copper copper thickness. thickness.

The concentrationThe ofconcentration sulfuric acid of is sulfuric one of theacid most is one important of the most factors important affecting factors the affecting the effect of blind holeeffect filling. of blind The hole increase filling. in The sulfuric increase acid in concentration sulfuric acid canconcentration improve thecan improve the conductivity andconductivity dispersion and of the dispersion electroplating of the solution.electroplating However, solution. if the However, sulfuric acidif the sulfuric acid concentration isconcentration too high, the is leveling too high, agent the leveling will be protonated agent will be rapidly, protonated which rapidly, will affect which will affect the electroplatingthe effect,electroplating and the effect, ductility and of the the ductility copper coatingof the copper will be coating reduced. will When be reduced. When the sulfuric acidthe concentration sulfuric acid isconcentra too lowtion (<25 is g/L), too low the (< blind 25 g/L), hole the filling blind mode hole is filling of an mode is of an equiangular deposition,equiangular which deposition, is not conducive which is not to blind conducive hole filling. to blind The hole relationship filling. The relationship between the sulfuricbetween acid the concentration sulfuric acid andconcentration blind hole and filling blind effect hole is filling shown effect in Figure is shown3 in Figure 3 (210 g/L CuSO ·5H O, 45 mg/L Cl−, 15 mL/L leveling agent, 1 mL/L brightener, 10 mL/L 4(210 2g/L CuSO4·5H2O, 45 mg/L Cl−, 15 mL/L leveling agent, 1 mL/L brightener, 10 mL/L 2 wetting agent, 1.5wetting A/dm agent,current 1.5 A/dm density,2 current and 55 density, min electroplating and 55 min ).electroplating It can be seen ). fromIt can be seen from Figure3 that whenFigure the 3 sulfuric that when acid theconcentration sulfuric acid is greater concentration than 25 g/L, is greater with the than increase 25 g/L, with the in- of sulfuric acidcrease concentration, of sulfuric the acid thickness concentration, of copper platingthe thickness layer changes of copper slightly, plating but layer changes the sag graduallyslightly increases., but the When sag gradually the concentration increases. ofWhen sulfuric the concentration acid is 25~35 of g/L, sulfuric the acid is 25~35 sag and the thicknessg/L, the of sag the and copper the coatingthickness are of small. the copper Therefore, coating the are results small. of Therefore, the above the results of experimental data show that the blind hole filling effect was good when the optimum the above experimental data show that the blind hole filling effect was good when the concentration of sulfuric acid was 25~35 g/L. optimum concentration of sulfuric acid was 25~35 g/L.

Materials 2021, 14, x FOR PEER REVIEW 5 of 12 Materials 2021, 14, 85 5 of 11

30 Copper thickness Sag degree 25

15 μm

0 25 30 35 40 45 50 55

H2SO4 (g/L)

Figure 3. EffectFigure of the 3. sulfuricEffect ofacid the sulfuric concentration acid concentration on the sag and on thickness.the sag and thickness.

Based on the aboveBased test on results,the above the test suitable results, electroplating the suitable processelectroplating and bath process formula and bath formula for a blind holefor with a ablind diameter hole with of 100 a µdiameterm and depth of 100 of 50μmµ mand are depth as follows: of 50 μm 210~225 are as g/L follows: 210~225 · − CuSO4 5H2O concentration,g/L CuSO4·5H2 25~35O concentration, g/L H2SO 425~35concentration, g/L H2SO4 45~60 concentration, mg/L Cl 45~60concen- mg/L Cl− concentration, 0.8~1.2tration, mL/L brightener0.8~1.2 mL/L concentration, brightener concentration, 10~16 mL/L leveling 10~16 mL/L agent, leveling 50~80 min agent, 50~80 min 2 electroplating time,electroplating 10~15 mL/L time, wetting 10~15 agentmL/L concentration,wetting agent andconcentration 1.4~1.7 A/dm, and 1.4~1.7current A/dm2 current density. Accordingdensity to the. According selected platingto the selected solution plating formula, solution blind formula, hole 1 (depth blind diameter hole 1 (depth diameter ratio of 1:2), withratio a diameterof 1:2), with of 100a diameterµm and of a depth100 μm of and 50 µam, depth and of blind 50 μm hole, and 2 (depth blind hole 2 (depth µ µ diameter ratio ofdiameter 1.17:1), ratio with of a hole1.17:1) diameter, with a ofhole 60 diameterm and depthof 60 μm of 70andm, depth were of each 70 μm, were each plated under differentplated under levels different of various levels factors. of various The optimum factors. concentrationThe optimum ofconcentration sulfuric of sulfuric acid was obtained by fixing the H SO at a concentration of 25 g/L, Cl− concentration of acid was obtained 2by fixing4 the H2SO4 at a concentration of 25 g/L, Cl- concentration of 45 mg/L, wetting agent concentration of 10 mL/L, brightener concentration of 1 mL/L, 45 mg/L, wetting agent concentration of2 10 mL/L, brightener concentration of 1 mL/L, leveling agent ofleveling 15 mL/L, agent current of 1 density5 mL/L, of current 1.5 A/dm density, and of electroplating 1.5 A/dm2, and time electroplating of 55 min, time of 55 and changing themin, concentration and changing of the the copper concentration sulfate. Then,of the by copper selecting sulfate. the best Then concentra-, by selecting the best tion of copper sulfate, while the other factors remain unchanged, the best concentration of concentration of copper sulfate, while the other factors remain unchanged, the best con- sulfuric acid could be obtained through experiments. The hole filling rate changes of the centration of sulfuric acid could be obtained through experiments. The hole filling rate two types of holes are shown in Figures4 and5, respectively. changes of the two types of holes are shown in Figure 4 and Figure 5, respectively. The relationships between the concentration of CuSO4·5H2O and H2SO4 and the plating solution formula for the blind hole with a diameter of 100 µm and a depth of 50 µm are shown in Figure6. For the blind hole with a diameter of 100 µm and a hole depth of 50 µm, within a certain range of plating solution formula, such as a 45~60 mg/L Cl− concentration, 0.8~1.2 mL/L brightener concentration, 10~16 mL/L leveling agent, and wetting agent concentration of 10~15 mL/L, copper sulfate and sulfuric acid had a preferred region to achieve better filling effect. In the present paper, the optimum ranges of the concentrations of copper sulfate and sulfuric acid were 210~225 g/L and 25~35 g/L, respectively. The relationships between the current density and electroplating time and the concentration of CuSO4·5H2O and H2SO4 for a blind hole with a diameter of 100 µm and a depth of 50 µm are shown in Figure7. It can be seen from Figure7 that when copper sulfate and sulfuric acid are located in the preferred regions, the process conditions for achieving a good filling effect are a current density of 1.4~1.7 A/dm2 and electroplating time of 50~80 min.

Materials 2021, 14, x FOR PEER REVIEW 6 of 12 Materials 2021, 14, 85 6 of 11

106 Blind hole 2 Blind hole 1 104

102

100

Hole Filling Hole (%) Rate Hole Filling Hole (%) Rate 94

90 180 190 200 210 220 230 240 250 CuSO ·5H O (g/L) CuSO4·5H2O (g/L)

Figure 4. ChangeFigure in4. Change the hole in ﬁlling the hole rate filling with the rate copper with the sulfate copper concentration. sulfate concentration.

104 Blind hole 2 Blind hole 1 100

Hole Filling Hole (%) Rate Hole Filling Hole (%) Rate

80 25 30 35 40 45 50 55 H SO (g/L) H2SO4 (g/L)

Figure 5. ChangeFigure in the5. Change hole ﬁlling in the rate hole with filling sulfuric rate with acid concentration.sulfuric acid concentration.

The relationships between the concentration of CuSO4·5H2O and H2SO4 and the plating solution formula for the blind hole with a diameter of 100 μm and a depth of 50 μm are shown in Figure 6. For the blind hole with a diameter of 100 μm and a hole depth of 50 μm, within a certain range of plating solution formula, such as a 45~60 mg/L Cl- concentration, 0.8~1.2 mL/L brightener concentration, 10~16 mL/L leveling agent, and

Materials 2021, 14, x FOR PEER REVIEW 7 of 12

wetting agent concentration of 10~15 mL/L, copper sulfate and sulfuric acid had a preferred region to achieve better filling effect. In the present paper, the optimum ranges of the concentrations of copper sulfate and sulfuric acid were 210~225 g/L and 25~35 g/L, respectively. The relationships between the current density and electroplating time and the concentration of CuSO4·5H2O and H2SO4 for a blind hole with a diameter of 100 μm and a depth of 50 μm are shown in Figure 7. It can be seen from Figure 7 that when copper sulfate and sulfuric acid are located in the preferred regions, the process condi- Materials 2021, 14, 85 tions for achieving a good filling effect are a current density of 1.4~1.7 A/7 ofdm 112 and electroplating time of 50~80 min.

H2SO4 (g/L) 24.5 28.0 31.5 35.0 17.5 65

Leveling agent 14.0 60

Wetting agent 10.5

(mg/L) -

7.0 Cl 50 Components(mL/L) Pore diameter: 100 μm Hole depth: 50 μm 3.5

Brightener 45

0.0 210 215 220 225 230

CuSO4·5H2O (g/L)

Materials 2021, 14,Figure x FOR PEER 6. The REVIEW relationships between the concentration4 of2 CuSO ·25H 4O and H SO and the plating 8 of 12 Figure 6. The relationships between the concentration of CuSO ·5H O and H4 SO2 and the 2plating4 solution formula for the blind holesolution with a formula diameter for of the 100 blind μm and hole a depth with a of diameter 50 μm. of 100 µm and a depth of 50 µm.

Plating time (min) 50 55 60 65 70 75 80 228 36

224 34

220

O (g/L) O

2 (g/L)

30 4

·5H SO

4 216

2 H

CuSO 28 Pore diameter: 100 μm 212 Hole depth: 50 μm

26 208

24 1.40 1.45 1.50 1.55 1.60 1.65 1.70 Current density (A/dm2)

Figure 7. TheFigure relationship 7. The relationshipss between the between current the density current and density electroplating and electroplating time and timethe concentration and the concentration of CuSO4·5H2O and H2SO4of for CuSO the blind4·5H hole2O and with H a2 SOdiameter4 for the of blind100 μm hole and with a depth a diameter of 50 μm of. 100 µm and a depth of 50 µm.

The relationshipsThe between relationship thes concentrationbetween the concentration of CuSO4·5H of2 O CuSO and4·5H H2SO2O 4andand H the2SO4 and the plating solutionplating formula solution for a formula blind hole for witha blind a diameter hole with of a diameter 60 µm and of a60 depth μm and of 70a depthµm of 70 μm are shown in Figureare shown8. For in Figure a blind 8. hole For a with blind a hole diameter with a of diameter 60 µm of and 60 aμm hole and depth a hole of depth of 70 70 µm, within aμm, certain within range a certain of the range plating of the solution plating formula, solution such formula, as a Clsu−chconcentration as a Cl- concentration of of 45~60 mg/L,45~6brightener0 mg/L, brightener concentration concentration of 0.8~1.2 of mL/L, 0.8~1.2 leveling mL/L, leveling agent of agent 10~16 of mL/L,10~16 mL/L, and and wetting agentwetting concentration agent concentration of 10~15 mL/L, of 10~1 copper5 mL/ sulfateL, copper and sulfate sulfuric and acid sulfuric also have acid aalso have a preferred region in order to achieve a better filling effect. In the present paper, the optimum ranges of the concentration of CuSO4·5H2O and H2SO4 were 210~225 g/L and 35~45 g/L, respectively. The relationships between the current density and electroplating time and the concentration of CuSO4·5H2O and H2SO4 for a blind hole with a diameter of 60 μm and a depth of 70 μm are shown in Figure 9. It can be seen from Figure 9 that when copper sulfate and sulfuric acid are located in the preferred regions, the process conditions for achieving a good filling effect are a current density of 1.2~1.6 A/dm2 and electroplating time of 55~75 min.

Materials 2021, 14, 85 8 of 11

preferred region in order to achieve a better filling effect. In the present paper, the optimum ranges of the concentration of CuSO4·5H2O and H2SO4 were 210~225 g/L and 35~45 g/L, respectively. The relationships between the current density and electroplating time and the concentration of CuSO4·5H2O and H2SO4 for a blind hole with a diameter of 60 µm and a depth of 70 µm are shown in Figure9. It can be seen from Figure9 that when copper sulfate Materials 2021, 14, x FOR PEER REVIEW 9 of 12 and sulfuric acid are located in the preferred regions, the process conditions for achieving a good filling effect are a current density of 1.2~1.6 A/dm2 and electroplating time of 55~75 min.

H2SO4 (g/L) 35.0 37.5 40.0 42.5 45.0

17.5 59.5

Leveling agent 14.0 56.0 Wetting agent

10.5

52.5

(mg/L)

- Cl 7.0

Components(mL/L) Pore diameter: 60 μm 49.0 Hole depth: 70 μm 3.5 Brightener 45.5

0.0 210 215 220 225

CuSO4·5H2O (g/L)

Figure 8. TheFigure relationship 8. The relationshipss between the between concentration the concentration of CuSO4·5H of2O CuSO and H4·25HSO24 Oand and the H 2platingSO4 and solution the plating formula for the blind holesolution with a formula diameter for of the 60 blindμm and hole a depth with of a diameter70 μm. of 60 µm and a depth of 70 µm.

The section morphology of the micro blind hole after polishing was observed using a Plating time (min) µ metallographic microscope.55 For60 a blind hole65 with a diameter70 of 100 75m and a hole depth of 50 µm, when the current density was 1.4~1.7 A/dm2 and the electroplating46 time was 50~80 min,228 the representative metallographic micrographs of the blind hole are shown in Figure 10A. For the blind hole with a diameter of 60 µm and a hole depth of 70 µm, when the current density was 1.2~1.6 A/dm2 and the electroplating time was 55~7544 min, the representative224 metallographic micrographs of the blind hole are shown in Figure 10B. As can be seen from Figure 10, it was found that there were no holes, pores, contaminants,42 and

delamination220 fractures in the blind holes, showing good ﬁlling effects.

O (g/L) O

2 (g/L)

40 4

·5H

SO 4

216 2 H

CuSO 38 Pore diameter: 60 μm 212 Hole depth: 70 μm

36 208

34 1.2 1.3 1.4 1.5 1.6 Current density (A/dm2)

Figure 9. The relationships between the current density and electroplating time and the concentration of CuSO4·5H2O and H2SO4 for the blind hole with a diameter of 60 μm and a depth of 70 μm

The section morphology of the micro blind hole after polishing was observed using a metallographic microscope. For a blind hole with a diameter of 100 μm and a hole depth of 50 μm, when the current density was 1.4~1.7 A/dm2 and the electroplating time

Materials 2021, 14, x FOR PEER REVIEW 9 of 12

H2SO4 (g/L) 35.0 37.5 40.0 42.5 45.0

17.5 59.5

Leveling agent 14.0 56.0 Wetting agent

10.5

52.5

(mg/L)

- Cl 7.0

Components(mL/L) Pore diameter: 60 μm 49.0 Hole depth: 70 μm 3.5 Brightener 45.5

0.0 210 215 220 225

CuSO4·5H2O (g/L)

Figure 8. The relationships between the concentration of CuSO4·5H2O and H2SO4 and the plating solution formula for Materials 2021, 14, 85 9 of 11 the blind hole with a diameter of 60 μm and a depth of 70 μm.

Plating time (min) 55 60 65 70 75 46 228

44 224

220

O (g/L) O

2 (g/L)

40 4

·5H

SO 4

216 2 H

CuSO 38 Pore diameter: 60 μm 212 Hole depth: 70 μm

Materials 2021, 14, x FOR PEER REVIEW 10 of 12 36

208

was 50~80 min, the representative metallographic micrographs of the blind hole are 34 1.2 1.3 1.4 1.5 1.6 shown in Figure 10A. For the blind hole with a diameter of 60 μm and a hole depth of 70 2 μm, when the current density was 1.2~1.6 A/dm2 anCurrentd the electroplatingdensity (A/dm time) was 55~75 min, the representative metallographic micrographs of the blind hole are shown in Fig- ure 10B. As can be seen from Figure 10, it was found that there were no holes, pores, Figure 9. The relationships between the current density and electroplating time and the concentration Figureconta 9. Theminants relationship, and delaminations between the fracture currents indensity the blind and electroplating holes, showing time good and filling the concentration ef- of CuSO4·5H2O and H2SO4 of CuSO ·5H O and H SO for the blind hole with a diameter of 60 µm and a depth of 70 µm. for thefect blinds. hole with a diameter4 2 of 60 μm2 and4 a depth of 70 μm

Figure 10. TheFigure representative 10. The representativesection of metallographic section micrographs of metallographic of the blind micrographs holes. (A) A of the blind holes. (A) A blind blind hole withhole a diameter with a diameter of 100 μm ofand 100 a holeµm depth and aof hole 50 μm depth, and ( ofB) 50a blindµm, hole and with (B) a a diam- blind hole with a diameter of eter of 60 μm60 andµm a hole and depth a hole of depth70 μm.of 70 µm.

The overall trend of the filling rate for the two blind holes with CuSO4·5H2O and H2SO4 concentrations is similar, but there are differences due to different thickness the diameter ratio. The hole filling rate is different under different concentrations of CuSO4·5H2O and H2SO4. Different sizes of blind holes require different concentrations of CuSO4·5H2O and H2SO4, and the electroplating conditions are also different. The volume of the blind hole has a certain effect on the diffusion exchange of CuSO4·5H2O and H2SO4 and the concentration distribution of the additives. Different sizes of blind holes require different electroplating conditions. When the size of the blind holes changes, the concentration of CuSO4·5H2O and H2SO4 in the electroplating solution should also change accordingly. Therefore, when filling a blind hole, the proper electroplating conditions

Materials 2021, 14, 85 10 of 11

The overall trend of the filling rate for the two blind holes with CuSO4·5H2O and H2SO4 concentrations is similar, but there are differences due to different thickness the diameter ratio. The hole filling rate is different under different concentrations of CuSO4·5H2O and H2SO4. Different sizes of blind holes require different concentrations of CuSO4·5H2O and H2SO4, and the electroplating conditions are also different. The volume of the blind hole has a certain effect on the diffusion exchange of CuSO4·5H2O and H2SO4 and the concentration distribution of the additives. Different sizes of blind holes require different electroplating conditions. When the size of the blind holes changes, the concentration of CuSO4·5H2O and H2SO4 in the electroplating solution should also change accordingly. Therefore, when filling a blind hole, the proper electroplating conditions should be selected according to the hole geometry size, so as to obtain an excellent filling hole.

4. Conclusions Under the conditions of a certain plating solution formula and electroplating parameters (current density and electroplating time), the sag degree decreases with the increase in the copper sulfate concentration. When the concentration of copper sulfate increases from 210 g/L to 225 g/L, the filling effect is good, and the sag degree is about 0. How- ever, with the increase of sulfuric acid concentration, the sag increases gradually. When the sulfuric acid concentration is 25–35 g/L, both the sag and copper coating thickness are in a small range. For a blind hole with pore diameter of 100 µm and hole depth of 50 µm, under the process conditions of a 1.6~1.8 A/dm2 current density and 55~75 min electroplating time, when the concentrations of CuSO4·5H2O and H2SO4 are 210~225 g/L and 25~35 g/L, respectively, a better filling effect can be obtained. For a blind hole with a pore diameter of 60 µm and hole depth of 70 µm, under the process conditions of a 1.4~1.7 A/dm2 current density and 60~75 min electroplating time, when the concentrations of CuSO4·5H2O and H2SO4 are 210~225 g/L and 35~45 g/L, respectively, a better filling effect can be obtained. The volume of the blind hole has a certain effect on the diffusion and exchange of CuSO4·5H2O and H2SO4, as well as on the concentration distribution of additives.

Author Contributions: Data curation, P.T.; Writing-original draft, P.T. and Y.C.; Supervision, W.C.; Methodology, Y.C. and W.C.; Project administration, P.T. and W.C.; Formal analysis, Z.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Applied Science and Technology Research Project of Guang- dong Province (2015B010127003), the Guangzhou Science and Technology Project (201807010012), the 2019 Guangdong Province Science and Technology Special Fund (“Big Special Project + Task List” Management Mode) Funded Project (2019dzx025) of Qingyuan City, and the National Natural Science Foundation of China (51735003). Data Availability Statement: The authors confirm that the data supporting the findings of this study are available within the article and the referenced article. Acknowledgments: The authors gratefully acknowledge the assistance from Zhiwei Xie, Xianchao Chen, and Wenwu Zhang from the School of Materials and Energy, Guangdong University of Technology. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Shen, F.Y.; Dow, W.P.; Liu, A.H.; Lin, J.Y.; Chang, P.H.; Huang, S.M. Electrochemical societyperiodic pulse reverse Cu plating for through-hole filling. ECS Electrochem. Lett. 2013, 2, D23–D25. [CrossRef] 2. Dow, W.-P.; Li, C.-C.; Su, Y.-C.; Shen, S.-P.; Huang, C.-C.; Lee, C.; Hsu, B.; Hsu, S. Microvia filling by copper electroplating using diazine black as a leveler. Electrochim. Acta 2009, 54, 5894–5901. [CrossRef] 3. Wang, J.; Wu, M.; Cui, C. Factors governing filling of blind via and through hole in electroplating. Circuit World 2014, 40, 92–102. [CrossRef] 4. Wang, C.; Zhang, J.Q.; Yang, P.X.; An, M.Z. Through-hole filling by copper electroplating using sodium thiazolinyl-dithiopropane sulfonate as the single additive. Int. J. Electrochem. Sci. 2012, 7, 10644–10651. Materials 2021, 14, 85 11 of 11

5. Nothdurft, P.; Rieß, G.; Kern, W. Copper/Epoxy Joints in Printed Circuit Boards: Manufacturing and Interfacial Failure Mechanisms. Materials 2019, 12, 550. [CrossRef][PubMed] 6. Szałatkiewicz, J. Metals Recovery from Artificial Ore in Case of Printed Circuit Boards, Using Plasmatron Plasma Reactor. Materials 2016, 9, 683. [CrossRef][PubMed] 7. Yan, J.-J.; Chang, L.-C.; Lu, C.-W.; Dow, W.-P. Effects of organic acids on through-hole filling by copper electroplating. Electrochim. Acta 2013, 109, 1–12. [CrossRef] 8. Srivastava, P.R. A cooperative approach to optimize the printed circuit boards drill routing process using intelligent water drops. Comput. Electr. Eng. 2015, 43, 270–277. [CrossRef] 9. Kim, K.-R.; Cho, J.-H.; Lee, N.-Y.; Kim, H.-J.; Cho, S.-H.; Park, H.-J.; Choi, B. High-precision and ultrafast UV laser system for next-generation flexible PCB drilling. J. Manuf. Syst. 2016, 38, 107–113. [CrossRef] 10. Dewil, R.; Küçüko˘glu,I.; Luteyn, C.; Cattrysse, D. A Critical Review of Multi-hole Drilling Path Optimization. Arch. Comput. Methods Eng. 2019, 26, 449–459. [CrossRef] 11. Zhang, W.-B.; Zhu, G.-Y. Drilling Path Optimization by Optimal Foraging Algorithm. IEEE Trans. Ind. Inform. 2018, 14, 2847–2856. [CrossRef] 12. Zhao, S.; Pang, K.; Wang, X.W.; Xiao, N. Function of Sulfhydryl (–HS) Group During Microvia Filling by Copper Plating. J. Electrochem. Soc. 2020, 167, 112502. [CrossRef] 13. Jo, Y.E.; Yu, D.Y.; Cho, S.K. Revealing the inhibition effect of quaternary ammonium cations on Cu electrodeposition. J. Appl. Electrochem. 2020, 50, 245–253. [CrossRef]