Case: Failure analysis of gold-plated layer peeling off in a research institute

Because of its inertness, gold has almost no effect on various acids and alkalis. It has strong corrosion resistance, low contact resistance, excellent electrical conductivity, and is easy to weld. The field has important application value as an electronic reliability guarantee.

Many electronic products with high safety requirements in the aviation and aerospace fields (such as printed boards, connectors and connectors, etc.) are gold-plated.

Gold-plated parts are mostly copper matrix. In order to improve the functionality of gold, prevent the interdiffusion of copper-gold and later the penetration of copper on the outer surface of the gold plating layer, causing discoloration and forming a high resistance film there, gold plating on copper parts often requires a layer in between. Nickel barrier layer.

The problem often arises from the treatment before plating and the control of the plating solution.

This article tracked a gold-plated layer shedding accident, investigated various steps and analyzed the cause, and solved the problem in a timely and effective manner.

01 Fault description

The author’s research institute has been using acidic wear-resistant gold plating process. The resulting coating has a uniform, bright and smooth surface, low porosity, high hardness (120~190HV), wear resistance, good weldability, and gold-plated printed board taps The number of mating and unplugging can reach 1000 times.

The brief process flow of copper and copper alloy gold plating is: hanging, degreasing, washing, activation, washing, nickel plating, washing and gold plating. At that time, when a kind of yellow actinium part was plated with gold according to the process, an accident of peeling and falling off of the gold plating occurred. Observed with a magnifying glass, it was found that only the gold layer fell off, while the nickel layer was still completely attached to the surface of the substrate.

02 Reason Analysis

According to the peeling phenomenon of the gold layer, the main reason is that the bonding force between the nickel layer and the gold layer is not good. Combined with the maintenance and experience of each process step and process parameters on site, there are four main possible reasons:

(1) The nickel plating bath contains impurities such as oil or organic matter, or the parts are touched by hand after nickel plating, causing the surface of the nickel layer to be contaminated.

(2) The nickel layer is passivated after being stopped in the air for too long.

(3) The process parameters are inappropriate. If the bath temperature is too high, the current density is too high, and the main salt concentration is too high, the reaction rate will be too fast, and the resulting coating will be loose, rough, and easy to fall off. However, during the operation, all the factors mentioned meet the requirements and are therefore excluded.

(4) After nickel plating, the cleaning is not clean, and the parts are charged with Ni2+ when they enter the tank. If the pH of the gold plating solution rises, the Ni2+ is hydrolyzed to form nickel hydroxide and other basic salts attached to the surface of the plating parts. The bonding force of the coating is not strong.

03 Trial

For the above reasons, the following process tests were carried out.

1. Replace the nickel plating bath

After cleaning the plating tank, prepare a new tank solution according to the process specification.

After processing according to the original process, refer to the heating method in HB5052-1977 “Gold Coating Quality Inspection” to test the bonding force, and the coating is still peeling. Therefore, the elimination of nickel plating tip liquid is the cause of peeling and peeling of the plating layer.

2, increase the activation process

An activation step is added to the original process flow, that is, the process flow is “hanging, degreasing, washing, activation, washing, nickel plating, washing, activation, washing, gold plating”. The parts are activated immediately after washing with nickel plating. After the processing is completed, the adhesion is tested, and the coating is still peeling.

3. Adjust the pH of the gold plating solution and increase water washing

Use citrate and citric acid to adjust the pH of the gold plating bath to 4.2~4.5. At the same time, add a washing step before the parts are put into the gold plating bath, that is, according to the process flow: “Water washing-nickel plating-water washing-activation-water washing-water washing-gold plating” for processing. After testing the bonding force, the coating is intact and there is no peeling or falling off.

Through experiments, it can be seen that there are two main reasons for this quality scene accident:

(1) The nickel layer is passivated;

(2) After nickel plating, Ni2+ was brought into the gold plating solution with high pH.

Therefore, the process parameters and flow are improved, and the pH of the gold plating solution is adjusted to 4.2~4.5, and the flow is as shown above. After processing, the bonding force is tested, and the coating has no peeling or falling off, and the bonding force is good.

04 Summary

The main cause of this gold plating peeling quality accident is that the nickel layer stays in the air for too long and is passivated, and the parts are not washed clean after nickel plating, and the Ni2+ water brought into the gold plating solution The hydrolyzed product adheres to the nickel layer, resulting in poor bonding between the nickel layer and the gold layer. The corresponding countermeasures are as follows:

(1) Adjust the pH of the gold plating solution to 4.2~4.5 and strictly control it.

(2) Add an activation and cleaning process after nickel plating.

(3) Increase detection methods. Each batch of parts is tested for bonding force according to the heating method of HB5052-1977.

Through the improvement, the bonding force of the gold plating has been greatly improved, and the product qualification rate has reached 100%. There has been no such quality accident so far.