Introduction: Pitting refers to small pits formed on the surface of the coating during electroplating for various reasons. Pitting is distinctly different from coating roughness, burrs, and orange peel.
Pitting on copper deposits is one of the main faults encountered in acid copper plating. The causes of pitting on the deposit are analyzed as follows.
1 Causes of pitting
1. Improper pretreatment
1) Residual polishing compound: The polishing compound remaining on polished parts is not completely removed during degreasing, causing a greater accumulation of colloid-type impurities in the nickel strike solution, which leads to pitting in the nickel strike layer and pitting in the copper deposit.
In the subsequent acid bright copper plating process, the pits become enlarged and look very obvious.
2) The degreasing solution has been used for a long time, and the parts are removed from the tank at a relatively high temperature, causing oil droplets adhering to the part surfaces to dry out. The dried small oil droplets are non-conductive, and the affected areas have no coating or plate more slowly than other areas, appearing as pitting.
3) The activating solution is contaminated by Cu2+ and Fe2+.
The base metal iron or zinc alloy undergoes a displacement reaction with Cu2+ in the activating solution, producing a loose displacement copper layer. When plating is performed on this loose displacement copper layer, pitting is prone to occur.
For zinc alloy parts, Fe2+ can also be displaced, similarly causing pitting. The activating solution for iron parts and that for zinc alloy parts cannot be used interchangeably and should be set up in separate tanks.
2. Poor quality of the strike layer
When the cyanide copper strike layer or the nickel strike layer is too thin or has excessive porosity, displacement layers will form in the pores in the acid copper plating solution. These loose displacement layers cause pitting in the acid bright copper layer.
Therefore, the cyanide copper strike solution should not be too dilute, the free NaCN should not be too high, the current density should not be too high, and the striking time should not be too short. The solution should also be properly warmed.
When using a low-concentration nickel plating solution for striking, the solution should not be too dilute, the pH should not be too low, and a medium current density is preferable. If the current density is too low, hydrogen evolution is more severe and easily leaves many pinholes; the striking time should be somewhat longer. In short, the strike layer must not be too thin, and the porosity must not be too high.
3. Problems in the acid copper plating solution
1) Excessive leveling agent M. When M is excessive, pitting appears on the copper deposit.
2) Excessive brightener SP. When M is excessive, it causes roughness and burrs on the deposit, which is generally solved by adding brightener SP. SP has a negative leveling effect; when excessive, it makes recesses even deeper and turns small pits into large pits. When the SP content is too low, pitting and dendritic striations may also occur.
3) Insufficient sodium lauryl sulfate, causing excessive interfacial tension.
4) Excessive chloride (Cl−) content.
5) Too little polyethylene glycol.
6) The copper plating solution may contain silver impurities.
7) Excessive accumulation of brightener decomposition products.
8) Compatibility issues among additives. If two additives produce side effects such as pitting, roughness, or haze when used together, they are incompatible.
Janus Green B is a phenazine dye that exists as cations in solution, while sodium octane sulfonate and sulfonated nonionic surfactants exist as anions in solution; the two form macromolecular precipitates.
Fatty amine polyoxyethylene ether (AEO) also exists as cations in solution. When it meets anionic surfactants such as sodium lauryl sulfate, macromolecular precipitates likewise form.
These water-insoluble macromolecular precipitates adsorb on the coating surface, exhibit poor conductivity, and co-deposit with copper. In severe cases, they appear as large pits.
9) Poor quality of copper anodes.
When the phosphorus content on local anode surfaces is too low, Cu+ in the plating solution tends to increase, causing pitting and roughness in the copper deposit.
2 Two troubleshooting cases
Case 1: In a large automatic line in a factory, with 8000 L of acid copper plating solution, the copper coating on zinc alloy parts always showed pitting and a hazy appearance.
After inspection and analysis, the commonly used coating combination was adopted, namely two cyanide copper strikes followed by pyrophosphate copper plating for thickening; the strike process was not a problem.
Second, the activating solution was checked and did not appear blue, indicating that no Cu2+ was present.
The line also plated iron parts. Inspection revealed that the activating solution for iron parts and that for zinc alloy parts were not in the same tank, so there was no Fe2+ contamination.
For the factory's zinc alloy parts, degreasing was carried out with heated concentrated H2SO4, which fully met the requirements for ordinary production lines, and there was no residual polishing compound.
In addition, iron parts had no pitting, indicating that the acid copper plating solution was problem-free, with no brightener imbalance or excessive accumulation of organic matter.
Then where exactly was the problem? Finally, it was found that the cyanide copper strike tank was very small; parts took only 1.5 min from entering to leaving the tank, and the total time for two cyanide copper strikes was less than 4 min. Combined with the low solution concentration and low current efficiency, it was estimated that the strike layer thickness was insufficient, and a slight displacement copper layer appeared during pyrophosphate copper plating, leaving a hidden hazard.
The factory added brightener twice a day. When the zinc alloy parts were plated, the brightener had just been added, so the brightener concentration was somewhat excessive; therefore, the bright copper layer had pitting and slight haze. After the zinc alloy parts were plated, the brightener was consumed to a normal level before plating steel parts, so the bright copper layer on steel parts was normal.
Troubleshooting: Improve the method of adding brightener. Under the condition of unchanged total brightener dosage, change from adding twice a day to adding four times a day, once every 2 h, so that the brightener concentration remains relatively stable and avoids sudden highs and lows.
Case 2: In a factory, a 1300 L tank on a conventional manual rack plating line showed a fault combining roughness, pitting, and haze after acid copper plating, with the brightening time at 15–20 min.
Inspection showed that the copper plating solution concentration was not low, the anode area was not small, and no anode passivation was found. The sodium lauryl sulfate content was normal.
Later it was discovered that the emulsifier AEO had been added to the plating solution, and the sodium lauryl sulfate and AEO formed insoluble macromolecular precipitates.
After the copper plating solution was treated with activated carbon adsorption and filtration, and brightener was replenished, the plating returned to normal. Therefore, if sodium lauryl sulfate has been added to a copper plating solution, AEO must not be added.