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Flat Solderable Tin as a PWB solderable surface finish

Carol Stirling


Flat Solderable Tin as a PWB solderable surface finish | 16 November, 2001

Hello Everyone,

Our board supplier changed our board surface finish on a prototype to flat solderable tin (FST) because of the PWB thickness (.125"). The pads seem to be contaminated, particularly on second side pass through reflow. The solder appears to draw into the center of the pads. Has anyone any experience with this? I was wondering if our screen cleaner was affecting the process. It seems to leave a greasy film on the screen, but the instructions don't require a water rinse. Thanks in advance...

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Flat Solderable Tin as a PWB solderable surface finish | 19 November, 2001

Now, that is one thickass board!!!

The rant first: If our �board supplier changed our board surface finish on� anything without our approval, the board supplier would be an �X�. Life has enough surprises.

And of all the flat finishes available, how on this green earth did they choose immersion tin? Lordy!!!

We have one immersion tin [imtin] board, but have never seen the �dewettingish� problem with the greasy smutz [colloquially, not technically speaking of the smutz], that you describe. If we did, we quickly would check our printing process to be sure nothing has come unscrewed. Then we would, as you are, proceed to try and figure-out what�s going on with the board, starting with beating the living stuffing out of our fab.

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, which 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.

Imtin is a bath that works best if used often, and is not allowed to sit idle for long periods .... whereas OSP can tolerate long periods of non-use more easily. The two white tin suppliers [ Omikron / Florida Cirtech & Dexter (although something makes me want to say Dexter was acquired recently by someone like Entek)] require their licensees to maintain and record conformance to strict process controls.

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Keith Stone


Flat Solderable Tin as a PWB solderable surface finish | 22 November, 2001

I have heard of this but do not have direct experience. Immersion tin is susceptible to moisture absorption particularly after washing off solder paste. So it sounds like your boards are picking up the solvent off the screen. Water rinse or degrease the screen after cleaning would be advisable to see if that cures it.


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