Tin-silver alloy plating solution and method of plating with said plating solution

Abstract: In the present invention, the tin-silver alloy plating solution comprising: a tin compound; a silver compound; and a complexing agent including a pyrophosphoric compound and an iodic compound. Therefore, the tin-silver alloy layer, whose composition can be optionally designed, can be formed, with high electric current efficiency, without using harmful compound: cyanide. The plating solution can resist to an air-stir; the plating solution is very stable; an external shape, adhesivility, solder-wettability of the tin-silver alloy layer are satisfactory; and the alloy is an advantageous alloy for solder plating.

Description: The present invention relates to aqueous solution for forming a metallic complex for tin-silver alloy plating solution, and a method of plating in said plating solution. A technology of forming metals into metallic complexies are an essential technology in fields of plating, analyzing, etc.. Conventionally, cyanides, ammonia, organic acids, etc. are used as complexing agents. But, there is no complexing agent which effects to all metals; and it is difficult to treat waste water including the cyanides, which are relatively effective among said agents, due to harmful for human bodies. Thus, a complexing agent, which is capable of effectively complexing many metals without damaging human health, has been required. On the other hand, pollution of underground water by lead has been taken as an environmental pollution these days, and products including lead are severely restricted, so that tin-lead solder is replaced by lead-free solder. Thus, plating layers coated with the tin-lead solder should be replaced by the lead-free solder. Tin-silver alloy will be employed instead of the tin-lead solder alloy, so Matsushita Electric Company disclosed tin-silver solder paste (see Nikken Sangyo Press, February 1, 1996). A method of forming the tin-silver solder layer by plating is required now. The tin-silver alloy has anti-sulphurizability, so it will be employed instead of silver. Difference of Electrodeposition potential between silver and tin is 900 mV or more as standard oxidation-reduction potential, the cyanide, e.g., potassium cyanide, is included in plating solution so as to codeposite tin and silver when forming the tin-silver alloy layer. With the cyanide, there are many problems of polluting waste water, safe work, etc., so the tin-silver plating solution including no cyanides is required. An object of the present invention is to provide: aqueous solution for forming metallic complexies including no cyanides for tin-silver alloy plating solution, in which composition can be optionally changed, can be formed; and a method of plating in said plating solution. The tin-silver alloy plating solution comprises at least a tin compound, a silver compound, and a complexing agent including a pyrophosphoric compound and an iodic compound as a basic composition. In the plating solution, the pyrophosphoric compound may include: pyrophosphate, e.g., potassium pyrophosphate, sodium pyrophosphate; and/or pyrophosphoric acid. The iodic compound may include: iodide, e.g., potassium iodide, sodium iodide; iodite, e.g., potassium iodite, sodium iodite; and/or iodine. The tin compound may be tin compound of inorganic acid or tin compound of organic acid, which is selected from following tin compounds: tin chloride; tin sulfate; tin pyrophosphate; tin iodide; stannic acid; potassium stannate; tin acetate; tin methanesulfonate; tin alkanolsulfonate; and tin phenolsulfonate. The silver compound may be silver compound of inorganic acid or silver compound of organic acid, which is selected from following silver compounds: silver iodide; silver chloride; silver nitrate; silver sulfate; silver pyrophosphate; silver iodate; silver acetate; silver methanesulfonate; silver alkanolsulfonate; and silver phenolsulfonate. The amount of the pyrophosphoric compound and the iodic compound may be included so as to exist tin and silver, as complex ions, in the plating solution. The method of electrolytic plating of the present invention is characterized in that plating solution comprising: a tin compound; a silver compound; and a complexing agent including a pyrophosphoric compound and an iodic compound, whereby a work is plated with tin-silver alloy. And, the method of plating of the present invention comprises the steps of: forming a resin layer on a surface of a work; forming the resin layer into a prescribed pattern as a plating mask; executing electrolytic plating on the surface of the work in plating solution comprising: a tin compound; a silver compound; and a complexing agent including a pyrophosphoric compound and an iodic compound, whereby the work is plated with tin-silver alloy. The resin layer may be a layer of photosensitive resin, and the photosensitive resin layer may be formed into the prescribed pattern by a manner of photo-lithograph. The work may be a semiconductor chip, and the connecting terminals (bumps) may be plated with the tin-silver alloy. The work may be a substrate including printed circuits, and connecting terminals (bumps), which are connected with the printed circuits, are plated with the tin-silver alloy. The method may further comprise the step of executing heat treatment to a plating part of the work, the plating part is preferably formed into a hemispheric bump. The pyrophosphoric compound may include pyrophosphate, e.g., potassium pyrophosphate, sodium pyrophosphate, and/or pyrophosphoric acid. The iodic compound may include: iodide, e.g., potassium iodide, sodium iodide; iodite, e.g., potassium iodite, sodium iodite; and/or iodine. The tin compound may be tin compound of inorganic acid or tin compound of organic acid, which is selected from following tin compounds: tin chloride; tin sulfate; tin pyrophosphate; tin iodide; stannic acid; potassium stannate; tin acetate; tin methanesulfonate; tin alkarolsulfonate; and tin phenolsulfonate. The silver compound may be silver compound of inorganic acid or silver compound of organic acid, which is selected from following silver compounds: silver iodide; silver chloride; silver nitrate; silver sulfate; silver pyrophosphate; silver iodate; silver acetate; silver methanesulfonate; silver alkanolsulfonate; and silver phenolsulfonate. In the aqueous solution for forming metallic complexies, prescribed amount of pyrophosphate is added so as to preferably make the molarity of pyrophosphoric ions, with respect to that of the metal, equivalent molarity or more, which corresponds to the coordination number of said metal, so that pyrophosphoric complex ions of said metal can be stabilized more. In the tin-silver alloy plating solution , the prescribed amount of pyrophosphate is added to the metal corresponding to the coordination number of said metal, so that pyrophosphoric complex ions of said metal can be stabilized more. The Pyrophosphate is added preferably 2-times molarity or more for tin. Pyrophosphate, e.g., potassium pyrophosphate, sodium pyrophosphate, and/or pyrophosphoric acid may be employed as a pyrophosphoric compound. In the aqueous solution for forming metallic complexies, amount of an iodic compound can be optionally changed within a range in which complex ions of the metal can be stably exist; density of iodine ions (I - ) is made 0.5 mol/l or more so as to stabilize the complex ions of iodic compound of the metal more. Preferably, the density of iodine ions (I - ) is 1.5 mol /l or more. In the plating solution, the amount of the iodic compound can be optionally changed within a range in which the complex ions of silver can be stably exist; the density of the iodine ions (I - ) ) is made 0.5 mol/l or more so as to stabilize the complex ions of the iodic compound of silver. Preferably, the density of iodine ions (I -) is 1.5 mol/l or more. Iodide, e.g., potassium iodide, sodium iodide, iodite, e.g., potassium iodite, sodium iodite, and iodine may be solely employed as the iodic compound; mixture of two or more may be employed as the iodic compound. The pH of the aqueous plating solution can be adjusted by adding an acid, e.g., pyrophosphoric acid, hydrochloric acid, and/or an alkali, e.g., potassium hydroxide, sodium hydroxide. Preferred range of the pH is 5-10, but the pH may be in a range in which the pyrophosphoric compound and the iodic compound do not deteriorate and change in quality. In the aqueous solution, the complexes of one or more metals, e.g., nickel, bismuth, copper, zinc, tin, silver,can be formed. Glycine, tartrate, oxalate, citrate, sulfite, thiosulfate, saccharin, butynediol, formaldehyde, plylethylene glycol, sodium lauryl sulfate, polyoxyethylenenonylphenylether, dicyclohexylammonuimnitrite, etc. may be solely or jointly added to the aqueous solution on the basis of purposes. In the plating solution, glycine, tartrate, oxalate, citrate, sulfite, thiosulfate, saccharin, butynediol, formaldehyde, polylethylene glycol, sodium lauryl sulfate, polyoxyethylenenonylphenylether, dicyclohexylammonuimnitrite, etc. may be solely or jointly added to the plating solution on the basis of purposes. Further, complexing agents, brightener, antioxidant, surface-active agents, etc. may be added to the solution. Oxalate, sulfite, thiosulfate, tartrate, etc. may be solely or jointly added as the complexing agents. Peptone, β-naphthol, aminoaldehyde, formaldehyde, acetaldehyde, polyethylene glycol, acrylic acid, methyl acrylate, bismuth oxide, triethanolamine, 2-diethylaminoethanol, salicylic acid, N,N-dimethylformaldehyde, hexaethylenetetraamine, malonic acid, etc. may be solely or jointly added as the brightener. Especially, L-ascorbic acid, phenol, hydroquinone, resorcin, etc. may be solely or jointly added as the antioxidant for the tin. Sodium lauryl sulfate, polyoxyethylenenonylphenylether, benzalkonium chloride, etc. may be solely or jointly added as the surface-active agents. The tin compound of the tin-silver alloy plating solution is not limited, so the tin compound of inorganic acid or the tin compound of organic acid such as tin chloride, tin chloride 2 hydrate, tin sulfate, tin pyrophosphate, stannic acid, tin methanesulfonate, may be solely employed; mixture of two or more may be employed, too. The silver compound of the tin-silver alloy plating solution is not limited, so the silver compound of inorganic acid or the silver compound of organic acid such as silver iodide, silver chloride, silver nitrate, silver sulfate, silver methanesulfonate, may be solely employed; mixture of two or more may be employed, too. Blending ratio of the silver compound and the tin compound in the tin-silver alloy plating solution may be defined optionally. In the case of forming an alloy layer which includes much silver, the ratio of the silver compound should be greater; in the case of forming an alloy layer which includes much tin, the ratio of the tin compound should be greater. In the tin-silver alloy plating solution, composition of the alloy layer can be changed by changing electric current density, etc., so many kinds of the alloy layers can be formed in the same plating solution. In the same solution, the greater the current density is increased, the less the amount of silver in the alloy layer is reduced; the composition of the alloy layer is fixed if the current density is at prescribed value or more. An ordinary electroplating can be executed in the tin-silver alloy plating solution. For example, direct current plating, pulse plating, and periodical reverse current plating can be executed in the plating solution. The plating may be executed under the following conditions: temperature of the plating solution is 20-80 ° C, for example; the solution is stirred or not stirred; galvanostatic or potentiostatic electrolysis. For example, tin, silver, tin-silver alloy, platinum, titanium plated with platinum, carbon may be used as an anode. Works to be plated are not limited, any materials which are capable of being electrically plated may be employed as the works. Embodiments of the aqueous solution for forming metallic complex bodies will be explained in Embodiments 1-4 (not according to the invention); embodiments of the tin-silver alloy plating solution and the method of plating in the plating solution will be explained in Embodiments 5-28, but the present invention is not limited to the embodiments, and composition of the aqueous solution for forming metallic complexies and composition and plating conditions of the tin-silver alloy plating solution may be optionally changed according to purposes. Embodiment 1 K 4 P 2 O 7 165 g/l KI 166 g/l Aqueous solution for forming metallic complexies including above compounds has been prepared. 53 g of NiSO 4 · 6H 2 O and 2.4 g of AgNO 3 are added to the aqueous solution, then all the metals are dissolved therein as complexions; the aqueous solution is transparent and yellow solution. The pH is 9.1. An external appearance of the aqueous solution has been kept for more than one month. Embodiment 2 K 4 P 2 O 7 165 g/l KI 250 g/l Aqueous solution for forming metallic complexies including above compounds has been prepared. 12.5 g of CuSO 4 · 5H 2 O and 4.7 g of AgI are added to the aqueous solution,then all the metals are dissolved therein as complex ions; the aqueous solution is transparent and deep blue solution. The pH is 9.2. An external appearance of the aqueous solution has been kept for more than one month. Embodiment 3 K 4 P 2 O 7 165 g/l KI 330 g/l Aqueous solution for forming metallic complexies including above compounds has been prepared. The aqueous solution is colorless and transparent solution, and an external appearance has been kept for more than three months. The pH is 8.9. 42 g of SnP 2 O 7 , 0.5 g of CuP 2 O 7 and 1.2 g of AgI are added to the aqueous solution, then all the metals are dissolved therein as complex ions; the aqueous solution becomes transparent and light blue solution. The external appearance of the aqueous solution has been kept for more than three months without deposition, etc.. Embodiment 4 K 4 P 2 O 7 180 g/l KI 350 g/l Aqueous solution for forming metallic complexies including above compounds has been prepared. The aqueous solution is colorless and transparent solution, and an external appearance has been kept for more than three months. The pH is 9.0. 42 g of SnP 2 O 7 , 1.5 g of ZnSO 4 · 7H 2 O and 0.5 gof AgI are added to the aqueous solution, then all the metals are dissolved therein as complex ions; the aqueous solution is colorless and transparent solution. The external appearance of the aqueous solution has been kept for more than three months without deposition, etc.. Embodiment 5 SnCl 2 · 2H 2 O 45 g/l K 4 P 2 O 7 200 g/l AgI 1.2 g/l KI 330 g/l Tin-silver alloy plating solution including above compounds has been prepared. The plating solution is colorless and transparent solution, and an external appearance has been kept for more than three months without deposition, etc.. The pH is 8.9. Pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 25°C ; no stir; and cathodic current density 0.1-1 A/dm 2 . Amount of silver (WT%) in a tin-silver alloy layer, current efficiency and external appearance of the alloy layer with respect to the current density are shown in TABLE 1. According to the TABLE, the tin-silver alloy layer, which includes 5-19 % of silver, can be formed with high current efficiency in the plating solution. Current Density 0.1 0.3 0.5 0.7 0.9 (A/dm 2 ) Amount of Silver (%) 19 10 7 6 5 Current Efficiency (%) 82 90 85 88 84 External appearance half bright half bright dull dull dull Embodiment 6 SnCl 2 · 2H 2 O 45 g/l K 4 P 2 O 7 200 g/l AgI 4.7 g/l KI 330 g/l Tin-silver alloy plating solution including above compounds has been prepared. The plating solution is colorless and transparent solution, and an external appearance has been kept for more than three months without deposition, etc.. The pH is 8.9. Pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 25°C; no stir; and cathodic current density 0.1-1 A/dm 2 . Amount of silver (WT%) in a tin-silver alloy layer, current efficiency and external appearance of the alloy layer with respect to the current density are shown in TABLE 2. According to the TABLE, the tin-silver alloy layer, which includes 20-75 % of silver, can be formed with high current efficiency in the plating solution. Current Density (A/dm 2 ) 0.1 0.3 0.5 0.7 0.9 Amount of Silver (%) 75 39 25 20 18 Current Efficiency (%) 60 82 86 90 89 External appearance dull dull half bright half bright half bright Embodiment 7 Sn 2 P 2 O 7 74 g/l K 4 P 2 O 7 120 g/l AgI 4.7 g/l KI 330 g/l Tin-silver alloy plating solution including above compounds has been prepared. The plating solution is colorless and transparent solution, the pH is 7.0, and an external appearance has been kept for more than one month without deposition, etc.. Pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 25 °C; no stir; and cathodic current density 0.1-1 A/dm 2 . Amount of silver (WT%) in a tin-silver alloy layer, current efficiency and external appearance of the alloy layer with respect to the current density are shown in TABLE 3. According to the TABLE, the tin-silver alloy layer, which includes 20-75 % of silver, can be formed with high current efficiency in the plating solution. Current Density (A/dm 2 ) 0.1 0.3 0.5 0.7 0.9 Amount of Silver (%) 72 46 26 20 18 Current Efficiency (%) 83 94 95 92 93 External appearance dull dull half bright half bright half bright Embodiment 8 Sn 2 P 2 O 7 103 g/l K 4 P 2 O 7 330 g/l AgI 1.2 g/l KI 330 g/l Tin-silver alloy plating solution including above compounds has been prepared. The plating solution is colorless and transparent solution, the pH is 8.9, and an external appearance has been kept for more than one month without deposition, etc.. Pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 25 °C; no stir; and cathodic current density 0.2-2.2 A/dm 2 . Amount of silver (WT%) in a tin-silver alloy layer, current efficiency and external appearance of the alloy layer with respect to the current density are shown in TABLE 4. According to the TABLE, the tin-silver alloy layer, which includes 2-18 % of silver, can be formed with high current efficiency in the plating solution. Especially, in tin-silver solder, the preferable amount of the silver is about 3.5 % due to melting point; in the present embodiment, the tin-silver alloy layer including 2-5 % of silver can be formed in a range of the current density 0.8-2.2 A/dm 2 . Current Density (A/dm 2 ) 0.2 0.8 1.0 1.5 2.2 Amount of Silver (%) 18 3.5 3.1 2.5 2.0 Current Efficiency (%) 88 99 99 100 95 External appearance dull dull dull dull dull Embodiments 9-13 Plating solution of the embodiments 9-13 are shown in TABLE 5 (unit of amount of compounds: g/l). All the plating solutions of the embodiments are colorless and transparent solution, and an external appearances have been kept for more than one month. Embodiment 9 10 11 12 13 Sn 2 P 2 O 7 103 103 103 103 103 K 4 P 2 O 7 330 330 330 330 330 AgI 1.2 2.4 3.6 4.8 6.0 KI 332 332 332 332 332 Pure copper substrates are electroplated, with direct current, in the plating solution of each embodiment, under the conditions of: cathodic current density 0.1-2.5 A/dm 2 ; temperature 45°C; and no stir; platinum anode. Amount of silver (WT%) in tin-silver alloy layers and external appearances thereof ( Δ is gray and no glossy; ○ is white and no glossy; and o ○ is white and half glossy), with respect to each current density, are shown in TABLEs 6-9. In all embodiments, the current efficiency is 90 % or more. Current Density= 0.1 A/dm 2 9 10 11 12 13 Amount of Silver (%) 5.5 7.8 10.5 15.1 17.0 External Appearance Δ Δ Δ Δ Δ Current Density= 0.5 A/dm2 9 10 11 12 13 Amount of Silver (%) 2.0 3.3 5.5 7.9 9.9 External Appearance Δ Δ Δ Δ Δ Current Density= 1.0 A/dm 2 9 10 11 12 13 Amount of Silver (%) 1.8 3.2 5.0 7.4 9.7 External Appearance ○ ○ ○ ○ Δ Current Density= 2.0 A/dm 2 9 10 11 12 13 Amount of Silver (%) 1.7 2.7 4.9 6.7 8.4 External Appearance ○ ○ ○ ○ Δ Na 4 P 2 O 7 · 10H 2 O 224 g/l SnCl 2 · 2H 2 O 45.2 g/l AgNO 3 0.5 g/l KI 250 g/l Tin-silver alloy plating solution including above compounds has been prepared. The plating solution is colorless and transparent solution, the pH =8.2. Pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 60 °C ; no stir; and cathodic current density 0.1-2.5 A/dm 2 . Amount of silver (WT%) in a tin-silver alloy layer, current efficiency and external appearance of the alloy layer ( Δ is gray and no glossy; ○ is white and no glossy; and o ○ is white and half glossy) with respect to the current density are shown in TABLE 10. According to the TABLE, the tin-silver alloy layer, which includes 1.5-6.2 % of silver, can be formed in the plating solution. Current Density (A/dm 2 ) 0.1 0.2 1.0 2.0 2.5 Amount of Silver (%) 6.2 4.0 2.1 1.7 1.5 External Appearance ○ ○ ○ ○ ○ Embodiment 15 K 4 P 2 O 7 166 g/l SnCl 2 · 2H 2 O 45.2 g/l Ag 2 SO 4 0.42 g/l KI 250 g/l Tin-silver alloy plating solution including above compounds has been prepared. The plating solution is colorless and transparent solution, the pH =9.0. Pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 80 °C; no stir; and cathodic current density 0.5-3.0 A/dm 2 . Amount of silver (WT%) in a tin-silver alloy layer, current efficiency and external appearance of the alloy layer ( Δ is gray and no glossy; ○ is white and no glossy; and o ○ is white and half glossy) with respect to the current density are shown in TABLE 11. According to the TABLE, the tin-silver alloy layer, which includes 2.5-4.8 % of silver, can be formed in the plating solution. Current Density (A/dm 2 ) 0.5 1.0 1.5 2.0 3.0 Amount of Silver (%) 4.9 3.6 3.3 2.8 2.5 External Appearance ○ ○ ○ Δ Δ Embodiment 16 K 4 P 2 O 7 330 g/l Sn 2 P 2 O 7 103 g/l AgI 1.2 g/l KI 250 g/l Sodium lauryl sulfate 0.5 g/l Tin-silver alloy plating solution including above compounds has been prepared. The plating solution is colorless and transparent solution, the pH =9.2. Pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 80 °C; no stir; and cathodic current density 0.5-3.0 A/dm 2 . Amount of silver (WT%) in a tin-silver alloy layer, current efficiency and external appearance of the alloy layer ( Δ is gray and no glossy; ○ is white and no glossy; and o ○ is white and half glossy) with respect to the current density are shown in TABLE 12. According to the TABLE, the tin-silver alloy layer, which includes 1.4-5.2 % of silver, can be formed in the plating solution. Current Density (A/dm 2 ) 0.5 1.0 1.5 2.0 3.0 Amount of Silver (%) 5.2 2.6 1.8 1.7 1.4 External Appearance ○ ○ ○ Δ Δ Embodiment 17 K 4 P 2 O 7 250 g/l Sn 2 P 2 O 7 62 g/l AgI 0.7 g/l KI 250 g/l Salicylic acid 50 g/l Tin-silver alloy plating solution including above compounds has been prepared. The plating solution is colorless-and transparent solution, the pH =6.6. Pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 25 °C; no stir; and cathodic current density 0.1-1.5 A/dm 2 . Amount of silver (WT%) in a tin-silver alloy layer, current efficiency and external appearance of the alloy layer ( Δ is gray and no glossy; ○ is white and no glossy; and o ○ is white and half glossy) with respect to the current density are shown in TABLE 13. According to the TABLE, the tin-silver alloy layer, which is white and half glossy, can be formed in the plating solution. Current Density (A/dm 2 ) 0.1 0.2 0.5 1.0 1.5 Amount of Silver (%) 1.3 1.2 1.0 1.0 1.0 External Appearance o ○ o ○ o ○ o ○ o ○ Embodiment 18 K 4 P 2 O 7 250 g/l SnCl 2 · 2H 2 O 68 g/l AgI 0.7 g/l KI 250 g/l N,N-dimethylformamide 100 g/l Tin-silver alloy plating solution including above compounds has been prepared. The plating solution is colorless and transparent solution, the pH =8.9. Pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 25 °C ; no stir; and cathodic current density 0.1-1.0 A/dm 2 . Amount of silver (WT%) in a tin-silver alloy layer, current efficiency and external appearance of the alloy layer ( Δ is gray and no glossy; ○ is white and no glossy; o ○ is white and half glossy; and is glossy) with respect to the current density are shown in TABLE 14. According to the TABLE, the glossy tin-silver alloy layer can be formed in the plating solution. Embodiment 19 The alloy plating solution of the Embodiment 8 has been prepared, and pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 25°C; and stir by a stirer. Current density, amount of silver (WT%) in the alloy layer, current efficiency and external appearance of the alloy layer ( ○ is white and no glossy; and o ○ is white and half glossy) are shown in TABLE 15. In spite of the stir, an external appearance of the plating solution did not change. Current Density (A/dm 2 ) 2 3 4 5 6 7 8 10 11 Amount of Silver (%) 8.0 4.9 3.7 3.1 2.5 2.4 2.1 1.6 1.6 Current Efficiency(%) 97 99 98 100 100 100 100 97 92 External Appearance ○ ○ ○ ○ ○ ○ o ○ o ○ ○ Embodiment 20 Pyrophosphoric acid is added to the plating solution of the Embodiment 8 to adjust the pH from 8.9 to 7, then pure copper substrates are electroplated, with direct current, in the plating solution under the conditions of: temperature 25 °C; and no stir and stir by a stirer. Thickness of the alloy layer is 6-7µm. Current density, amount of silver (WT%) in the alloy layer, current efficiency and external appearance of the alloy layer ( ○ is white and no glossy; and o ○ is white and If glossy) are shown in TABLE 16. In spite of adjusting the pH and the stir, an external appearance of the plating solution did not change. Embodiment 21 The alloy plating solution of the Embodiment 8 is air-stirred, but an external appearance of the plating solution did not change. Pure copper substrates are electroplated, based on Hull cell test, with direct current, in the plating solution of the Embodiment 8 under the conditions of: air-stir; and temperature 25 °C. Current density, amount of silver (WT%) in the alloy layer and external appearance of the alloy layer (○ is white and no glossy; and o ○ is white and half glossy) are shown in TABLE 17. Current Density (A/dm 2 ) 0.5 1 2 4 6 8 10 Amount of Silver (%) 34.5 12.1 5.5 2.6 2.3 1.7 1.6 External Appearance ○ ○ ○ ○ ○ ○ ○ Embodiment 22 Tin-silver alloy layer (amount of silver: 3.3 WT%), whose thickness is about 80 µm, is formed in the plating solution of the Embodiment 8, and a sectional face of the alloy layer is analyzed by an electron probe X-ray micro analyzer. Composition of the alloy layer is uniformly formed in the direction of thickness. Embodiment 23 Tin-silver alloy layer including 3.5 WT% of silver is formed on a pure copper substrate, in the plating solution of the Embodiment 8, on the basis of the Hull cell test, under the conditions of: temperature 25 °C ; stir (air-stir and stir by the stirer), then adhesivility of the alloy layer is tested by a bending test of JIS-H-8504 so that no separation and no expansion are observed. Embodiment 24 Melting point of a tin-silver alloy ingot (made by Senju Kinzoku Corporation), which is made by casing and which included 3.5 WT% of silver, and that of the tin-silver alloy layers (including 2-5 WT% of silver) formed in the plating solution of the Embodiment 8 are observed by a thermal analyzer (DSC). The melting point (temperature of beginning to melt) of all the tin-silver alloy layers are 221°C, which is equal to the melting point of the casted tin-silver alloy ingot (including 3.5 WT% of silver). Embodiment 25 A pure copper substrate, whose size is 5 ×30×0.5 mm, is plated in the plating solution of the Embodiment 8 under the conditions, described later. Solderability test of the copper substrate plated with tin-sliver alloy is executed, by a solder checker (type: SAT-2000 made by Rhesca Corporation), under the conditions of: tin-silver solder (including 3.5 WT% of silver); temperature 250 °C; 30 %-WW rosin or no rinse type flux. As the result of the test, the plated layer has good wettability without dewetting of soft solder. The plating conditions are: Cathode Current Density 1.5-2.5 A/dm 2 ; Temperature of Solution 20 or 50°C; Stir no stir or stir by the stirer; and Thickness of The Layer about 6 µm. Embodiment 26 Pure copper substrates, whose size is 5×30×0.5 mm, are wholly plated, namely plated 30 mm from the end, in the plating solution of the Embodiment 8. The solderability test of the copper substrates plated with tin-silver alloy are executed in the air, by the solder checker (type: SAT-2000 made by Rhesca Corporation), under the conditions of: tin-silver codeposition solder (including 3.5 WT% of silver); temperature 250 °C; 30 %-WW rosin; and soaking depth 4 mm. Relationship between the thickness of the tin-silver alloy layer, which includes 3.0 WT% of silver, and wetting time is shown in TABLE 18; relationship between the composition of the tin-silver alloy layer (thickness: about 6 µm) and the wetting time is shown in TABLE 19. After the solderability test, the soaked part of the substrate has a good glossy external appearance without dewetting of soft solder. Thickness (µm) 0.5 1 2.5 5 10 Wetting Time (sec.) 1.74 1.76 1.72 1.69 1.67 Amount of Silver(WT%) 2.8 3.1 5.0 7.3 18.4 Wetting Time (sec.) 1.68 1.69 1.88 1.92 1.95 Embodiment 27 A photo-sensitive resin film ( a resist film layer), whose thickness is about 25 µm, is formed on a pure copper substrate, then 50 rows and lines of holes, namely 2500 holes, each of which has diameter of 100µm, and which are longitudinally and latitudinally arranged with the space of 100 µm, are bored in the resist film layer by a manner of photo-lithograph, so that the copper surfaces are exposed as inner bottom faces of the holes. About 25 µm thick tin-silver alloy layers are formed on the exposed copper surfaces under the conditions of: using the tin-silver alloy plating solution of the Embodiment 8; the current density 1.5 A/dm 2 ; no stir; and temperature 25°C. After forming the alloy layers, the resist film layer is removed, then the plated parts (the tin-silver alloy parts) are observed by an electron microscope; the alloy layers are correctly formed along inner shapes of the holes. Composition of the alloy layers are analyzed by an electron probe X-ray micro analyzer; the alloy layers are tin-silver layers including 2.8 WT% of silver, and the thickness of the alloy layers are almost equal. The pH of the plating solution of the Embodiment 8 is 8.9, but the resist film layer is not removed therein. Note that, non-photo-sensitive resist film layer may be employed. In this case, the resist film layer may be formed into desired patterns by laser, e.g., excimer laser. A semiconductor chip is plated with the tin-silver alloy layer as well, then the alloy layers are melted in hydrogen atmosphere, so that they are formed into hemispheres. The hemispherical alloy can be employed as electric terminals (bumps) for flip chip connection of the semiconductor chip. A plastic substrate (e.g., BGA (Ball Grid Alley)), on which copper printed circuits are formed, is selected as a work to be plated, and the resist film layer is formed to cover the plastic substrate except some parts of the patterns, at which electric terminals are formed, then the exposed parts are plated with the tin-silver alloy layers to form the electric terminals (bumps). Note that, bump forming area of patterns, e.g., gold plated patterns, which have been formed on a surface of polyimide film, may be plated, as well, to form the bumps. Embodiment 28 A photo-sensitive resin film ( a resist film layer), whose thickness is about 25 µm, is formed on a pure copper substrate, then 50 rows and lines of holes, namely 2500 holes, each of which has diameter of 100µm, and which are longitudinally and latitudinally arranged with the space of 100 µm, are bored in the resist film layer by a manner of photo-lithograph, so that the copper surfaces are exposed as inner bottom faces of the holes. Nickel layers are formed on the exposed copper surfaces about 5 µm thick. Then tin-silver alloy layers are formed on nickel layer under the conditions of: using the tin-silver alloy plating solution of the Embodiment 8; the current density 2.0 A/dm 2 ; no stir; temperature 25°C; and supplied quantity of electricity 50 C/cm 2 . After forming the alloy layers, the resist film layer is removed, then the plated alloy parts are observed by an electron microscope; the alloy layers are thicker than the resist film layer, and they are formed like mushrooms (diameter of the parts projected from the surface of the resist film layer are greater than that of the holes). The mushroom-shaped alloy layers are melted in hydrogen atmosphere, so that they are formed into hemispheres having the diameter of 100 µm and the height of 70µm. The hemispherical alloy is analyzed by the electron probe X-ray micro analyzer; tin and silver are uniformly distributed in the hemispherical alloy, and amount of the silver is 3.0 WT%. Note that, non-photo-sensitive resist film layer may be employed. In this case, the resist film layer may be formed into desired patterns by laser, e.g., excimer laser. A semiconductor chip is plated with the tin-silver alloy layer as well, then the alloy layers are melted in hydrogen atmosphere, so that they are formed into hemispheres. The hemispherical alloy can be employed as electric terminals (bumps) for flip chip connection of the semiconductor chip. A plastic substrate (e.g., BGA (Ball Grid Alley)), on which copper printed circuits are formed, is selected as a work to be plated, and the resist film layer is formed to cover the plastic substrate except some parts of the patterns, at which electric terminals are formed, then the exposed parts are plated with nickel and further plated with the tin-silver alloy to form the electric terminals (bumps). Note that, bump forming area of patterns, e.g., gold plated patterns, which have been formed on a surface of polyimide film, may be plated, as well, to form the bumps. EFFECTS OF THE INVENTION In the aqueous solution for forming metallic complexies , many kinds of metals can be stabilized complex ions. Namely, the combination of the two compounds, which never cause bad influence mutually, and which have effective complexing function to many kinds of metals, is novel and quite advantageous. In the tin-silver alloy plating solution of the present invention, the tin-silver alloy plating layers, which have optional composition, can be formed with high current efficiency and without using harmful cyanides, so that the plating solution is more advantageous than conventional cyanic tin-silver plating solution. The plating solution of the present invention can resist to the air-stir; the plating solution is very stable; the external shape, the adhesivility and the solder-wettability of the tin-silver alloy layer are satisfactory; and the alloy of the present invention, which includes no harmful lead, is more advantageous than the conventional tin-lead solder alloy. Bumps or electric terminals can be properly formed on semiconductor chips or printed circuits on substrates by the tin-silver alloy plating solution of the present invention.

Claims: Tin-silver alloy plating solution, comprising: a tin compound; a silver compound; and a complexing agent including a pyrophosphoric compound and an iodic compound. The tin-silver alloy plating solution according to claim 1, wherein said pyrophosphoric compound includes pyrophosphate and/or pyrophosphoric acid. The tin-silver alloy plating solution according to claim 1 or 2, wherein said iodic compound includes iodide, iodite and/or iodine. The tin-silver alloy plating solution according to claim 1, 2 or 3 wherein said tin compound includes a tin compound of inorganic acid or a tin compound of organic acid, and wherein said silver compound includes a silver compound of inorganic acid or a silver compound of organic acid. The tin-silver alloy plating solution according to claim 4, wherein said tin compound is at least one kind of tin compound of inorganic acid or tin compound of organic acid, which is selected from following tin compounds: tin chloride; tin sulfate; tin pyrophosphate; tin iodide; stannic acid; potassium stannate; tin acetate; tin methanesulfonate; tin alkanolsulfonate; and tin phenolsulfonate. The tin-silver alloy plating solution according to claim 4, wherein said silver compound is at least one kind of silver compound of inorganic acid or silver compound of organic acid, which is selected from following silver compounds: silver iodide; silver chloride; silver nitrate; silver sulfate; silver pyrophosphate; silver iodate; silver acetate; silver methanesulfonate; silver alkanolsulfonate; and silver phenolsulfonate. The tin-silver alloy plating solution according to any one of the preceding claims wherein prescribed amount of said pyrophosphoric compound and said iodic compound are included so as to exist tin and silver, as complex ions, in said plating solution. A method of electrolytic plating, characterized in that plating solution comprising: a tin compound; a silver compound; and a complexing agent including a pyrophosphoric compound and an iodic compound, whereby a work is plated with tin-silver alloy. A method of plating, comprising the steps of: forming a resin layer on a surface of a work; forming the resin layer into a prescribed pattern as a plating mask; executing electrolytic plating on the surface of the work in plating solution comprising: a tin compound; a silver compound; and a complexing agent including a pyrophosphoric compound and an iodic compound, whereby the work is plated with tin-silver alloy. The method of plating according to claim 9, wherein the resin layer is a layer of photosensitive resin, and the photosensitive resin layer is formed into the prescribed pattern by a manner of photo-lithograph. The method of plating according to claim 9 or 10, wherein the work is a semiconductor chip, and wherein the connecting terminals are plated with the tin-silver alloy. The method of plating according to claim 9 or 10, wherein the work is a substrate including printed circuits and connecting terminals, which are connected with the printed circuits, are plated with the tin-silver alloy. The method of plating according to any one of claims 8 to 12 further comprising the step of executing heat treatment to a plating part of the work. The method of plating according to any one of claims 8 to 13, wherein said pyrophosphoric compound includes pyrophosphate and/or pyrophosphoric acid. The method of plating according to any one of claims 8 to 14 wherein said iodic compound includes iodide, iodite and/or iodine. The method of plating according to any one of claims 8 to 15, wherein said tin compound includes a tin compound of inorganic acid or a tin compound of organic acid, and wherein said silver compound includes a silver compound of inorganic acid or a silver compound of organic acid. The method of plating according to claim 16, wherein said tin compound is at least one kind of tin compound of inorganic acid or tin compound of organic acid, which is selected from following tin compounds: tin chloride; tin sulfate; tin pyrophosphate; tin iodide; stannic acid; potassium stannate; tin acetate; tin methanesulfonate; tin alkanolsulfonate; and tin phenolsulfonate. The method of plating according to claim 16, wherein said silver compound is at least one kind of silver compound of inorganic acid or silver compound of organic acid, which is selected from following silver compounds: silver iodide; silver chloride; silver nitrate; silver sulfate; silver pyrophosphate; silver iodate; silver acetate; silver methanesulfonate; silver alkanolsulfonate; and silver phenolsulfonate.

Inventors: ARAI SUSUMU, HIGASHI MITSUTOSHI, WATANABE TOHRU, ARAI, SUSUMU, HIGASHI, MITSUTOSHI, WATANABE, TOHRU, Arai, Susumu, c/o Nagano Prefecture
Applicants: NAGANO PREFECTURE, SHINKO ELECTRIC IND CO, NAGANOKEN, SHINKO ELECTRIC INDUSTRIES CO. LTD.
Application year: 1997
Main classification: C25D3
IPC codes: C25D3