Letter 36008

Tin plate peeling  

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Copper strips plated with 100-300 micro inches of pyro-copper then pure tin plated parts pass tape-bend-blast test but when a teflon coat is applied the tin lifts off heated to 350degrees for teflon,the copper is black when tin is removed with teflon but if you strip the tin copper is nice looking.

Daniel Casey
plating shop - EVERETT, MASS, U.S.A.

I waited 24 hours to respond, hoping that some more qualified metallurgist would sound off. I am a little out of my area here but in absence of any other advice here is my 2 cents.

At the temperature you are curing the Teflon the copper and tin and organic plating additives are fusing and cooking together to form some sort of brittle and black intermetallic that is falling apart when you peel.

Try a nickel or paladium difusion barrier between the copper and the tin, but be careful as tin will also difuse into nickel.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

There is not enough information here to really answer the problem. However, it could be that the tin layer is too thin and porous. You do not say what type of tin you are putting down or whether it is flow melted etc etc. Unless it is pore free, it will allow gases to seep through onto the copper and react there, especially if the metal is heated to 350C. Secondly, tin has a melting point of 232C, so heating it this high will cause the diffusion of tin into the copper go very quickly. This will form an intermetallic compound of Cu-Sn, which could also account for the blackening. This compound will be quite brittle. I suggest you try this: take your tin plated copper and put it through the PTFE process, but DO NOT put any PTFE down. Then heat it to 350C for the same time and see how the adhesion and bend tests stand up. If the same problem occurs, it will be the processing; if it doesn't, the problem lies with the PTFE. Perhaps more information will be beneficial to solving your problem.

Trevor Crichton R&D practical scientist - UK

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