Letter 24114

Discoloration of immersion tin plating  

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I am running an immersion tin plating line currently. Tin is plated onto Copper surface. Recently we encountered discoloration issue with various severity. The tin plating solution is MSA based.
The Copper surface is plating with tin just after an aggressive etching process.

Anybody has similar experience to mine?

What could have gone wrong in the tin plating line besides the possibility that the Copper surface is not even or clean?

Thanks.

Peter
consumer electronics mfgr. - Singapore

In my process, we already done discoloration analysis in tin plating line. We have some type of discoloration. One yellow discoloration come from plating chemical composition. Maybe additive too high. We contact chemical maker to change plating bath composition, especially additive composition. Other yellowish discoloration come from back end operation. Plating chemical contain corrosive acid. If rinse/drag out not effective, acid will be trapped in plating pores. So must check rinse condition. Other type discoloration is black/brown. Maybe it come from bath composition, temperature of plating, agitation which related to current density. It also can be from drying system.

Alidah
- Indonesia

Most likely you are not completely removing the tin plating solution because it is very difficult to remove. You cannot merely rinse it off. Take the freshly plated work back thru an alkali cleaner and then rinse again.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como

Have a 3 stage rinse after Plating Sn from MSA bath followed by Hot-10% TSP (Trisodium Phosphate [link is to product info at Amazon]) dip, rinse and finally rinse with Hot DI water. This will help remove staining caused by tin plating solution.

Good Luck...

Karthik
- Norwood, Massachusetts

Great information on the Tin Discoloration.

I have similar issues with discoloration (Yellow tint & black stains) on parts that Tin is plated on Copper surface.

However, the discoloration always appears 2,3,4,5 months after the production of the part. For this reason, I was wondering if anyone knows of a Highly Accelerated System Test (HAST) that I can run after the product is made to predict if the parts are going to fail later.

I would like to use the test as a LOT acceptance Test.

I would really appreciate any suggestions you might have.

Thanks,

Gus Mavrogiannis
- Orland Hills, Illinois

January 13, 2008 Well I am back with lots more experience now after posting my first question above. the discoloration in our case is generally caused by different tin grain structures deposited on the Copper surface. Various factors, such as plating temp, additive content, bath age, and of course the stress level in the Copper, are all proven to be significant factors in one way or the other depending on different situations. One thing for sure is that immersion plating, including tin, is one of the most difficult processes in all circuit board/flex manufacturing. happy troubleshooting. Peter - Singapore December 3, 2008 I think the copper base and plating tin over it will not work,because of copper oxidizes and migrates copper on the surface and makes the tin surface discolor.It will fail solderability and it doesn't pass 85/85 test.If you want to make it work you must plate nickel over copper,it will stop copper from migration to surface.It will solve a lot of headache.If you want your process non-magnetic than you cannot use nickel, but you can use palladium to protect your copper and than plate tin over the palladium.The process on the palladium is not easy,if you know the technic you can make it work. Good Luck, Duang Bouphasavanh - Myrtle Beach, South Carolina