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Immersion silver tarnish issue after wave reflow in RoHS environment

November 13, 2008

I am running a lead-free build thru a solder wave process by using Alpha Metal water-soluble flux 3355 in spray method. The board surface finishing is immersion silver without the PCB mask on top of pcb(expose to air). I found all the silver surface became yellowish or brown color & some area around the solder joint even turn black after the wave process. It still the same after the washing process. In conclusion, all the unsolderable pad was tarnished terribly. May I know is immersion silver plating are not recommended for the PCB with unsolderable pad? Will the water-soluble flux "attack" the immersion silver pad & cause the tarnish?

Jih Sheng Tan
product engineer - Johor Bahru, Johor, Malaysia

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December 11, 2008

Does your immersion silver have an anti-tarnish post treatment before the reflow? Does your flux contain Sulphur?

In my experience, the Immersion silver will tarnish easily when react with the sulphur. My previous company do have an anti tarnish solution which pro-long the life of the immersion silver before tarnishing (around 3 months).

Immersion silver for PCB act only as a protection layer for the Cu circuit. If you are not using the pad for any contact purposes, then it should be not a problem even with the tarnishing. However, there is no literature or study to prove that. If you do have, please share it here.

Robin C.Y. Chua
Freelance - Singapore

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