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Pcb immersion tin thickness

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Pcb immersion tin thickness | 7 August, 2006

Hi everyone,

I am starting to use the lead free process and I had a few wetting problems (solder did not reach top side) at the wave soldering process once the pcb had passed our smt line (one heat pass). From what I understand, I have had a problem with the thickness of the tin layer on the pcb. I did not have a sufficient tin layer 14 micron. What would be the recommended thickness of immersion tin for a pcb for at least three heat passes ???

Thanks for taking time to answer!!!!!

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Pcb immersion tin thickness | 9 August, 2006

See "IPC-4554 Specification for Immersion Tin Plating for Printed Circuit Boards" 3.2.1 Immersion Tin Thickness.

The most common reason for solderability issues with the white tin surface coating during multiple thermal excursions is a thin white tin deposit. Florida Cirtech recommends 0.65 microns, that has proven to be sufficient for at least one year shelf life and up to four passes in assembly. In almost every case I've seen, when the first pass or two solders fine and then problems arise, it is an insufficient thickness of white tin that was on the board. [S Wentz Florida Cirtech ]

Additional points: * Heat cycles deplete the pure tin layer. Each solder cycle reduces the tin thickness by ~0.1 micron (4 uin). * A highly activated flux will begin to corrode the imm tin coating after 10 minutes. Corroded tin has poor solderability.

Finally, white tin thickness has substantial variability, depending on the process used by the licensee [your fabricator]. So, that is where you need to start, with the process of the licensor.

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