JPS61110792A - Method for plating directly pure gold to stainless steel - Google Patents

Abstract

PURPOSE:To perform a ultrathin pure gold plating whose adhesive strength is excellent, by performing the gold plating after passing through a chemical polishing process and an electrolytic activating process of specified conditions. CONSTITUTION:Stainless steel is immersed into an acidic activating bath which has compounded 15-30 hydrochloric acid (35% solution) by volume %, 5-15 sulphuric acid (80% solution), 4-6 nitric acid (68% solution), 5-15wt% citric acid powder, 0.5-1.5 acetic acid (90% solution, 0.1-0.3wt% non-ion or amphoteric surface active agent, and 0.05-0.15wt% corrosion inhibitor, and chemical polishing is performed. Subsequently, it is immersed into an electrolytic bath which has compounded 5-15 by volume % phosphoric acid (85% solution), 5-15 sulphuric acid (85% solution), 5-15wt% citric acid powder, 0.5-1.5 acetic acid (90% solution), 0.1-0.3wt% non-ion or amphoteric surface active agent, 2-20 pyrolidone derivative, 0.5-7wt% acetylene glycol, and 0.05-0.15wt% corrosion inhibitor, and the surface is activated by a cathode electrolysis. Subsequently, ultrathin pure gold plating is performed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a plating method for imparting solderability and wire bondability to stainless steel by directly applying extremely thin plating of pure gold to stainless steel.

(Prior Art) Stainless steel has been attracting attention in recent years as a material for electronic parts such as ICl7-board frames because it has excellent rigidity, springiness, heat resistance, etc., and is cheaper than copper alloys and nickel alloys. However, when using stainless steel for electronic components, it is essential that it has excellent solderability and wire bonding properties, but since the surface of stainless steel is covered with a strong passivation film, it cannot be directly soldered. For this reason, various methods have been tried to impart solderability and wire bondability to stainless steel.1 For example, methods for imparting solderability include (a) A method in which the passivation film on the surface is removed by treatment with a simple acid or a mixed acid of hydrofluoric acid and hydrochloric acid or nitric acid, followed by soldering immediately; (b) Stainless steel is treated with a mixed acid of hydrofluoric acid, hydrochloric acid, sulfuric acid, etc. The following methods have been developed: (c) pretreatment with acid, followed by nickel or copper plating, and immersion solder plating thereon, and (c) pretreatment with acid followed by thick nickel plating.

(Problem to be solved by the invention) However, the method (,) can barely perform soldering, and the acid remains and corrodes the soldering area and its surroundings, so it has not been put into practical use to date. In method (b), when solder is plated by dip plating, it must pass through molten solder at a temperature of about 230 to 250°C, which causes thermal distortion to the stainless steel.
This method was not suitable for plating stainless steel for electronic parts, which requires precision. Furthermore (e)
In this method, the surface of the nickel turns into nickel oxide due to aging, resulting in extremely poor solderability, and requires pickling before soldering, making it difficult to adopt in practice.

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 coated with a Ni base plating, and then a thick plating of ^U or 8g is applied to the surface of the stainless steel. (1μ- or more) has been developed. This method also imparts solderability, but since it involves two-stage plating and uses a large amount of precious metals, it is expensive and poses a problem when applied to plating electronic components, where cost reduction is difficult. Ta.

(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.

Furthermore, since the plating metal is pure gold, it has excellent corrosion resistance, heat resistance, chemical resistance, and change over time, and can provide stable solderability and bonding properties.

Table 1 shows the 1tJ1 time variation and heat resistance when stainless steel is plated with various metals, but when gold is plated with nickel or cobalt alloys, the nickel and cobalt are oxidized. , the surface is covered with oxides, so the solderability decreases rapidly even at room temperature.
Heat resistance is also poor.

(Note 1) The plating thickness of ^U is 200A.

(Note 2) O is good, Δ is slightly inferior, and X is considerably inferior. However, when applying pure gold plating, it was found that applying the conventional method as a pretreatment would weaken the joint strength of solder and bonding, causing a practical problem. did. As a result of further research on pretreatment methods to improve bonding strength, they developed a method of performing immersion treatment and cathodic electrolysis treatment in a bath containing *a acid and W1eIl as main components.

This method is as follows: (a) Hydrochloric acid (35% solution [)1] as an inorganic acid
5-30% by weight, side 185% solution) 5-15% by volume, nitric acid (68% solution) 4-6% by volume, and as an organic acid citric acid powder 5-15% by weight, acetic acid (90% solution) 0.
5 to 1.5% by volume, plus 0.1 to 0.3% by weight of a nonionic or amphoteric surfactant, and 0.0% of a corrosion inhibitor.
A chemical polishing process in which stainless steel is immersed in an acidic activation bath containing 5 to 0.15% by weight and chemically polished, and (b) 5 to 15% by volume of phosphoric acid (85% solution) as a lfi-free acid.
, 5-15% by volume of sulfuric acid (85% solution), 5-15% by weight of citric acid powder as organic acids, 0.5-1.5% by volume of vinegar (90% solution), and non-ionic Or amphoteric surfactant 0.1-0.311t%, pyrrolidone derivative 2-20% by volume, acetylene glycol 0.5-0.
A method consisting of an electrolytic activation step in which chemically polished stainless steel is immersed in an electrolytic bath containing 7% by weight and 0.05 to 0.15% by weight of a corrosion inhibitor, cathodic electrolyzed, and surface activated. According to this method, the passivation film of stainless steel is completely removed, leaving an activated surface with exposed chromium atoms, which becomes a substrate suitable 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. be.

The reason why nonionic or amphoteric surfactants are commonly added in the above pretreatment method is that water splashing occurs when washing with water after each step, making it impossible to maintain the surface activated state until the next step. Preferred examples include polyethylene glycol alkyl ether and polyethylene glycol fatty acid ester. Also, it is j! that contains corrosion inhibitors! ! This is to prevent excessive pickling with IF acid and remove only the passivation film. As a corrosion inhibitor, adsorption type (having Ni12 group, Sll group, OH group, etc.), film type (severe A type, phosphoric acid, etc.)
or passive type, ′! - Furthermore, the reason why pyrrolidone derivatives and acetylene glycol were blended into the electrolytic bath was to reliably remove the passivation group dissolved by inorganic and organic acids in the case of pyrrolidone derivatives, and to prevent rough skin in the case of acetylene glycol. This is to prevent water from running out.

In each of the above baths, if the concentration of p&organic 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 undesirable. The concentration of organic acid is higher than the upper limit (even if it is higher, it will not activate the stainless steel surface, but if it is lower than the lower limit, the activation will be insufficient. In addition, surfactants, corrosion inhibitors, Both pyrrolidone derivatives and acetylene glycol do not produce much effect even when the concentration is higher than the upper limit, so increasing the concentration is uneconomical, and lowering the concentration below the lower limit results in insufficient effects.

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 99.70%, which is the purity of pure gold specified by JIS.
Plating is preferably carried out to achieve a ratio of 99.90 to 99.99%. *The plating thickness is calculated by dividing the plating thickness by the coating amount per unit area by the specific gravity of the plated pure gold, so that it is extremely thin at 0.01 to 0.1μ. 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, hepterythic, or martensitic, and plated materials have solderability and wire bondability.

(Example) Various stainless steels were pretreated in the following steps (1) to (C), and then pure gold plating was performed in the step (d).

(a) Alkaline degreasing process Commercially available stainless steel! ! After primary degreasing by heating the stainless steel plate to 70 to 80°C in a bath and passing the stainless steel one after another through the tank, the stainless steel plate is heated to 40 to 60°C in an alkaline bath. DC electrolytic degreasing was performed by applying a voltage of 6 volts using the degreased stainless steel as a cathode.

(b) Chemical polishing process followed by degreased stainless steel with salt fi (35% solution 81)
20% by volume, 10% by volume of sulfuric acid (85% solution), 10% by weight of citric acid powder, 1% by volume of acetic acid (90% solution) and 5% by volume of nitric acid (68% solution), polyethylene glycol alkyl ether, In a bath to which 0.2% by weight of a nonionic or amino acid amphoteric surfactant such as polyethylene glycol fatty acid ester and 0.1% by weight of an amine corrosion inhibitor (for example, Armovibou 28 manufactured by Lion 7-Ma) were added. It was passed for 20 seconds to remove surface oxides and impurities.

(c) Electrolytic activation step Phosphoric acid (85% i8) 10%, sulfuric acid (85% solution)
lQ$ amount%, citric acid powder 5% by weight, vinegar 1! ! (90%
solution) 1% by volume, N-methyl-2-pyrrolidone 5% by volume,
A bath containing 2% by weight of 2-butyne-1,4-phor, 0.2% by weight of a nonionic or amphoteric surfactant similar to the above and 0.1% by weight of a corrosion inhibitor was heated to 60°C. hand,
Apply (-) current to stainless steel and (-) to titanium-platinum plated plate.
+) An electric current was passed through the bath, set at 4 volts, to activate the stainless steel surface.

(d) Pure gold plating process Metal content 10? In a pure gold plating bath, the bath temperature was set to 60°C and the current density was set to IA/DI 6'', and plating was carried out for a time corresponding to the plating thickness of Examples 6 to 6, with an electrodeposition amount of about 8 OA per second.

Table 2 shows stainless steel plated with pure gold through the above steps.

Next, the following tests were conducted to confirm the plating adhesion, solderability, and V-wire bonding properties of pure gold-plated stainless steel.

(1) Plating adhesion (a) Base grain test Examples 1 to 4 After making a base grain with a width of 1 m to reach the steel substrate using a cutter to check the plating of the upper plate 6, heat it at 160°C for 10 minutes. After heating, an adhesive tape was attached to the base grain portion 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 plating layer on the broken surface did not peel off.

(2) Method using a solderability test machine The plated products of Examples 1 to 6 were set in a test machine and the solder wetting phenomenon was electrically measured. It has very good wettability,
Superior to phosphor bronze.

(b) Test using a solder bath The plated products of Examples 1 to 6 were immersed and soldered for 3 seconds and 5 seconds in a solder bath containing melted solder with a mixing ratio of 6 tin and 4 lead at a temperature of 230°C. Also exhibited a "solder adhesiveness" of 95% or more, and the solderability was good.

(c) Test using an electric soldering iron After soldering between the plated products of Examples 1 to 6 and between different plated products using a commercially available electric soldering iron and resin-cored solder wire (6 tin, 4 lead). When I pulled it apart, I found that the solder adhered to both stainless steel surfaces.

(3) Wire bonding properties Examples 1 to 4 A gold wire with a diameter of 25 μ- was bonded to a light item with a high speed wire bonding machine.
Adhesion was good, and the adhesive strength showed a tensile strength of 7 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, it is not affected by changes over time or heat, and stable solderability and wire bondability are always maintained.

Claims (1)

[Claims] (a) Hydrochloric acid (35% solution) 15-30% by volume, sulfuric acid (8% by volume)
5% 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 0. 05~0.
A chemical polishing process in which stainless steel is chemically polished by immersing it in an acidic activation bath containing 15% by weight of phosphoric acid (85% solution) and 5 to 15% by volume of phosphoric acid (85% solution) and sulfuric acid (85% by volume).
% 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 weight, pyrrolidone derivative 2-20% by volume, acetylene glycol 0.5
an electrolytic activation process in which the stainless steel after chemical polishing is immersed in an electrolytic bath containing ~7% by weight and 0.05~0.15% by weight of a corrosion inhibitor, and cathodic electrolysis is performed to activate the surface; ) A method for directly plating pure gold on stainless steel, comprising the steps of: applying ultra-thin pure gold plating to stainless steel after surface activation in a pure gold plating bath;