Letter 5001

Pitting of acidic copper plating 

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I found pitting on the plated copper layer (acid copper plating). I hope i can have a smooth copper surface, not the surface full of pitting. 60 ppm of chloride ion is in the plating bath.

I have tried to do the plating at both 1.9 ASD and 4 ASD current density However pitting on the plated Cu surface is still observed. I use photoresist as plating resist to plate the desired pattern. I would like to know how to minimize the pitting. Thanks.

Kelvin YU
- Hong Kong

Kelvin, there are quite a few items you left out. Are you using an oil-free source of air agitation? Are you utilizing phos-copper anodes (phos .02-.08 per cent) and they must have anode bags. Oil will cause pitting. Should be filtering constantly. Photo resist breakdown causes pitting.

I like the chloride to be 75-90 ppm. You should be utilizing a proprietary additive, manufacturer makes chloride recommendation. The bath temperature should be 70-90 degrees F. 90-120 degrees must have a proprietary brightener. If the acid copper bath has turned dark green, heavy organic contamination. The anode area should be 1:1 cathode/anode ratio for pattern plating. Hope this helps.

Bill Hemp
chemical process supplier - Grand Rapids,MI

Pitting of acid Cu plating is almost always a surface contamination problem, and you solve it in your preplate cleaning. A common cause is incomplete development of the photoresist, with pitting along the resist sidewalls. "Tacky" rollers used for dust removal can also cause pitting.

James Totter, CEF - Tallahassee, FL

Kelvin

In our experience James Totters remarks are on target. Often it can be of the greatest importance how you are pre-plate cleaning and what you are using. You must start with proper photo resist exposure and follow that with complete & clean development. Side wall attack usually is due to using the wrong pre-plate cleaning chemistry for the resist being used. Side wall attack will be seen at this point. You never stated your process, ie., are you putting your dry film over treated electroless, or are you striking the boards before applying the resist? You also did not say if you were using a microetch after the pre-plate cleaning chemistry and if so how much electroless or strike copper you are removing. Also if would be helpful to know if your pits show up on the pattern plated copper, or after they have been SnPb plated. If the pits are on the copper traces are they on the outer edges as little half-moon cutouts, or what we term "mouse- bites".

James Taylor
- Orange, California

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