JPH0140119B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a plating method for imparting solderability and wire bonding properties to stainless steel by directly applying extremely thin plating of pure gold to stainless steel. (Conventional technology) Stainless steel has been gaining popularity in recent years because it has excellent rigidity, springiness, and heat resistance, and is cheaper than copper alloys and nickel alloys.
It is attracting attention as a material for electronic components such as IC lead frames. However, when using stainless steel for electronic components, it is essential that it has excellent solderability and wire bonding properties.
Since the surface of stainless steel is covered with a strong passivation film, direct soldering or wire bonding cannot be performed on it. For this reason, various methods have been attempted to impart solderability and wire bondability to stainless steel. For example, methods for imparting solderability include (a) a method in which stainless steel is treated with hydrofluoric acid alone or with a mixed acid of hydrofluoric acid and hydrochloric acid or nitric acid, and the passivation film on the surface is removed, followed by soldering immediately; (b) A method in which stainless steel is pretreated with a mixed acid such as hydrofluoric acid, hydrochloric acid, or sulfuric acid, then nickel or copper plating is applied, and solder is immersed on top of that, (c) Pretreatment with acid. A method for applying thick nickel plating has been developed. (Problem to be solved by the invention) However, method (a) can only barely perform soldering, and the acid remains and corrodes the soldered area and its surroundings, so it has not been put to practical use to date. It has not been. In addition, in method (b), when solder is plated by immersion plating, it must pass through molten solder at a temperature of about 230 to 250 degrees Celsius, which causes thermal distortion in stainless steel and prevents the use of electronic components that require precision. This method was not suitable for application to stainless steel plating. Furthermore, in method (c), the surface of the nickel turns into nickel oxide due to changes over time, resulting in extremely poor solderability, and the method requires pickling before soldering, making it difficult to adopt in practice. It was hot. On the other hand, as a method for imparting wire bonding properties, these methods (a) to (c) are not possible. Conventionally, the surface of stainless steel is plated with Ni undercoat, and then a thick layer of Au or Ag is plated on top of that. (1 μm or more) has been developed. This method also imparts solderability, but it results in two-step soldering.
In addition, since a large amount of precious metal is used, the cost is high, and there are problems in applying it to plating electronic parts, where cost reduction is difficult. (Means for Solving the Problems) Therefore, the inventors of the present invention have conducted repeated research on a method of imparting solderability and wire bondability to stainless steel at low cost. I found something good. That is, according to this method, plating is performed in one stage, and the amount of pure gold deposited is extremely small, and high-speed plating is possible, resulting in low costs. In addition, since the plating metal is pure gold, it has excellent corrosion resistance and
It has excellent heat resistance, chemical resistance, and resistance to changes over time, and can provide stable solderability and bonding properties. . Table 1 shows the aging resistance and heat resistance of stainless steel plated with various metals. Since the surface is covered with oxides, the solderability rapidly decreases even at room temperature, and the heat resistance is also poor.

[Table] Ru,
However, the plating thickness targeted by the present invention is 0.01 to 0.1μ.
It was found that when conventional methods were applied as a pretreatment when applying ultra-thin pure gold plating of 500 mm, the joint strength of soldering and bonding was weak, which caused a practical problem. For example, in Special Publication No. 58-57520,
A method is shown in which gold plating is applied directly to stainless steel by pretreating stainless steel by immersion treatment and cathodic electrolysis treatment in a bath mainly composed of inorganic acids and organic acids, and then applying gold plating. However, when using this method to apply extremely thin plating as described above, a fine passivation film remains after the pretreatment, and water drainage occurs between the pretreatment and the plating bath. Since the passivation film is re-formed and gold plating is not applied to those parts, the bonding strength is weak. Therefore, as a result of research on a pretreatment method to improve bonding strength, it was found that if a pyrrolidone derivative and acetylene glycol were added to the treatment solution in the second pretreatment step in this Japanese Patent Application Laid-Open No. 58-61293, the above-mentioned problems could be solved. He discovered that the problem could be solved. This method uses (a) hydrochloric acid (35% solution)15 as an inorganic acid.
~30% by volume, sulfuric acid (85% solution) 5-15% by volume, nitric acid (68% solution) 4-6% by volume, and as organic acids citric acid powder 5-15% by weight, acetic acid (90% solution)
Contains 0.5-1.5% by volume, plus 0.1-0.3% by weight of nonionic or amphoteric surfactants, and 0.05% corrosion inhibitors.
A chemical polishing process in which stainless steel is immersed in an acidic activation bath containing ~0.15% by weight, and (b) phosphoric acid (85% solution) 5-15% by volume and sulfuric acid (85%) as inorganic acids. Solution) 5-15% by volume, and as organic acids citric acid powder 5-15% by weight, acetic acid (90%
Solution) Contains 0.5-1.5% by volume, including 0.1-0.3% by weight of nonionic or amphoteric surfactant, 2-20% by volume of pyrrolidone derivative, and acetylene glycol.
A method comprising an electrolytic activation step of immersing chemically polished stainless steel in an electrolytic bath containing 0.5 to 7% by weight and 0.05 to 0.15% by weight of a corrosion inhibitor and cathodic electrolysis to activate the surface. According to this method, the passivation film on stainless steel is completely removed, leaving an activated surface with exposed chromium atoms, making it a suitable substrate for pure gold plating. For this reason, when pure gold plating is applied after this pretreatment, the exposed chromium atoms are selectively plated with gold, enveloping the chromium atoms, and preventing the re-formation of a passivation film due to the reaction between chromium and iron. It is. The reason why a nonionic or amphoteric surfactant is added to each bath in the above pretreatment method is to prevent water splashing when washing with water after each step, which prevents the surface from being activated until the next step. Preferred examples include polyethylene glycol alkyl ether and polyethylene glycol fatty acid ester. In addition, a corrosion inhibitor is added to prevent excessive pickling with _inorganic acids and to remove only the passivation film. It may be either a film type (bicarbonate type, phosphoric acid type, etc.) or a passive type. Furthermore, the reason why pyrrolidone derivatives and acetylene glycol are blended into the electrolytic bath is to reliably remove the passivation film dissolved by inorganic and organic acids in the case of pyrrolidone derivatives, and to prevent skin roughness and water drainage in the case of acetylene glycol. It's for a reason. In other words, when a pyrrolidone derivative is added, chromium atoms are exposed throughout the material, and gold is selectively plated there, preventing the re-formation of the passivation film due to the iron-chromium reaction after plating. be. In addition, the addition of acetylene glycol can prevent the roughening caused by strong electrolytic activation treatment despite the addition of corrosion inhibitors, eliminating the recesses that are difficult to plate in the case of extremely thin plating, and the passivation in the recesses. It can prevent film re-formation, and
Since water always adheres to the stainless steel while it is being transferred from pretreatment to the plating bath to protect the surface, it is also possible to prevent the passivation film from forming again due to water drainage. In each of the above baths, if the concentration of the inorganic acid is higher than the upper limit, the surface of the stainless steel will be over-pickled despite the addition of a corrosion inhibitor, and if it is lower than the lower limit, activation will be insufficient, which is not preferable. Further, even if the concentration of the organic acid is higher than the upper limit, the surface of the stainless steel cannot be activated, but if it is lower than the lower limit, activation becomes insufficient. Furthermore, surfactants, corrosion inhibitors, pyrrolidone derivatives, and acetylene glycol are all not very effective even if the concentration is higher than the upper limit, so it is uneconomical to increase the concentration, and the effect is insufficient when lower than the lower limit. . In the present invention, pure gold plating is performed after the above-mentioned pretreatment, and during this pure gold plating, the gold purity of the plating layer is preferably 99.70% or more, which is the purity of pure gold specified by JIS. Plate to 99.90-99.99%. The plating thickness is calculated by dividing the deposited amount per unit area by the specific gravity of the plated pure gold, from 0.01 to 0.01.
Make it extremely thin at 0.1 μm. The plating is performed by electroplating, but the bath composition, plating conditions, etc. may be any known ones. According to the present invention, stainless steel can be plated regardless of the type of steel, such as austenitic, ferritic, or martensitic, and the plated material is imparted with solderability and wire bondability. (Example) Various stainless steels were pretreated in the following steps (a) to (c) and then plated with pure gold in the step (d). (a) Alkaline degreasing process 70-80℃ in a commercially available stainless steel tank
After primary degreasing by sequentially passing stainless steel through the tank,
DC electrolytic degreasing was carried out in an alkaline bath heated to 0.degree. C. by applying a voltage of 6 volts using the stainless steel plate as an anode and the stainless steel after primary degreasing as a cathode. (b) Chemical polishing process The degreased stainless steel is then treated with 20% by weight of hydrochloric acid (35% solution), 10% by weight of sulfuric acid (85% solution), 10% by weight of citric acid powder, 1% by volume of acetic acid (90% solution) and A mixed acid consisting of 5% by volume of nitric acid (68% solution), 0.2% by weight of a nonionic or amino acid amphoteric surfactant such as polyethylene glycol alkyl ether or polyethylene glycol fatty acid ester, and an amine corrosion inhibitor (for example, Lion Armor's Armovibu) 28) 0.1
% by weight bath for 20 seconds to remove surface oxides and impurities. (c) Electrolytic activation process Phosphoric acid (85% solution) 10% by volume, sulfuric acid (85% solution) 10% by volume, citric acid powder 5% by weight, acetic acid (90% solution) 1% by volume, N-methyl-2-pyrrolidone 5% by volume, 2% by weight of 2-butyne-1,4-diol, 0.2% by weight of a nonionic or amphoteric surfactant as above and 0.1% by weight of a corrosion inhibitor.
A (-) current was applied to the stainless steel and a (+) current was applied to the titanium platinum plated plate. was activated. (d) Pure gold plating process Pure gold plating bath with a metal content of 10g/bath temperature 60℃
℃, current density 1A/ ^Dm2 , and for 1 second.
Plating was carried out for a time corresponding to the plating thickness of Examples 1 to 6 based on an electrodeposition amount of 80 Å. Table 2 shows stainless steel plated with pure gold through the above process.

[Table] Thickness.
Next, the following tests were conducted to confirm the plating adhesion, solderability, and wire bondability of pure gold-plated stainless steel. (1) Plating adhesion (a) Cross-cut test The plated products of Examples 1 to 4 and 6 were cut with a cutter to make a cross-cut with a width of 1 mm, reaching the steel substrate, and then heated at 160°C for 10 minutes. After heating, adhesive tape was applied to the grid area and peeled off, but no peeling of the plating layer was observed from any of the plated products. (b) Bending test The plated products of Examples 1 to 6 were repeatedly bent 180 degrees to cause them to break, but the plated layer on the broken surface did not peel off. (2) Solderability (a) Method using a solder test machine The plated products of Examples 1 to 6 were set in a test machine and the wetting phenomenon of the solder was measured electrically. There were few phenomena and the wettability was very good, superior to phosphor bronze. (b) Test using a solder bath Temperature at which solder with a mixing ratio of 6 tin and 4 lead is melted
The plated products of Examples 1 to 6 were immersed in a solder bath at 230° C. for 3 seconds and 5 seconds to be soldered, and all exhibited "solder adhesiveness" of 95% or more and had good solderability. (c) Test using an electric soldering iron Using a commercially available electric soldering iron and resin-cored solder wire (6 tin, 4 lead), soldering was carried out between the plated products of Examples 1 to 6 and between different plated products. When I tried to pull it apart, I found that the solder adhered to both stainless steel surfaces. (3) Wire bonding properties Gold wires with a diameter of 25 μm were bonded to the plated products of Examples 1 to 4 and 6 using a high-speed wire bonding machine, and the adhesion was good, with an adhesive strength of 7.
It showed a tensile strength of g. (Effects) As explained above, according to the present invention, stainless steel can be efficiently plated with ultra-thin pure gold with excellent adhesion at low cost. Furthermore, since the plating is pure gold plating, it is not affected by changes over time or heat, and stable solderability and wire bonding properties are always maintained.

Claims (1)

[Claims] 1 (a) Hydrochloric acid (35% solution) 15-30% by volume, sulfuric acid (85% solution) 5-15% by volume, nitric acid (68% solution)
4-6% by volume, citric acid powder 5-15% by weight, acetic acid (90% solution) 0.5-1.5% by volume, nonionic or amphoteric surfactant 0.1-0.3% by weight, corrosion inhibitor
A chemical polishing process in which stainless steel is immersed in an acidic activation bath containing 0.05 to 0.15% by weight, and (b) 5 to 15% by volume of phosphoric acid (85% solution) and sulfuric acid (85%).
solution) 5-15% by volume, citric acid powder 5-15% by weight, acetic acid (90% solution) 0.5-1.5% by volume, nonionic or amphoteric surfactant 0.1-0.3% by volume, pyrrolidone derivative 2-20% by volume, (c) An electrolytic activation step in which the stainless steel after chemical polishing is immersed in an electrolytic bath containing 0.5 to 7% by weight of acetylene glycol and 0.05 to 0.15% by weight of a corrosion inhibitor and subjected to cathodic electrolysis to activate the surface; A method for directly plating pure gold on stainless steel, comprising the steps of: applying an ultra-thin pure gold plating with a thickness of 0.01 to 0.1 μm to stainless steel after surface activation in a pure gold plating bath;