Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

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Solder Wetting

Kris Wiederhold


Solder Wetting | 14 December, 1999

We recently reduced our standard aperture reduction to aid in the reduction of solder on gold defects. Since this reduction in aperture size, we have been experiencing exposed copper at the end of the component pads. The heel and toe joints are perfectly formed but as the solder tapers to the pad surface it turns a dull grey color and then disappears to expose the copper underneath. The components are primarily TSOPs and the solder paste is Kester RMA. The stencil is 5 mil printing up to 6 mil in height, brick placement is consistant and accurate. The oven profile matches the Kester data sheet with the exception of time above liquidous, which is 65 - 70 seconds, peak temperature is ~220C.

Thanks in advance!!


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Curtis T


Re: Solder Wetting | 14 December, 1999

What is your board finish it sounds like an osp. That type of thing is common with osp finishes.

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Re: Solder Wetting | 14 December, 1999

Hello Kris If you do have and OSP coating I'd say Curtis has nailed it. But you seem to have references to gold. Does this board have a standard nickle/gold finish. If so what sort of "solder on gold" problems were you having? Reading between the lines I'm guessing that you now have two problems. The one you started with and the one you added by reducing the aperture. I think you started with what's commonly called black pad disease. This is where the underlying nickle is corroded and does not present a solderable surface once the gold is taken up into solution with the solder. The nickle often looks dark and can be mistaken for copper. This is generally an excess phosphorous situation and a plating problem with the fab vendor. If it's indeed copper, where did the nickle go? Search the archives for more info. When you reduced the aperture you reduced the amount of solder and flux available. Because there was less flux there was less wetting of the already bad pad and the smaller amount of solder drew back to the more accommodating component lead. Because the solder brick melted over the gold flash it dissolved as it should. The grey appearance is typical of a gold percentage of over 3% and probably occurred because the of the gold covering the non-wetting area concentrating when the surface tension pulled the solder back. A reliability problem would not be unlikely. To test, try some new fabs, possibly from a different vendor or with the co-operation of your present vendor. Solderability problems with Ni/Au are hardly unheard of. If your lead pitch allows it try some HASL coated boards with your old stencil. Good luck. John Thorup

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Curtis T.


Re: Solder Wetting | 14 December, 1999

John, I have never seen black pad covering a majority of the board, that would be a scarry thought. I think he is talking about building modules (sims dims) and getting soldering spatering on the gold edge connectors. We went the rounds with that some time ago, and the paste volumes did not do the trick. It was the paste type and oven profile. Talk with your paste rep and he/she will be a good help. We have done extensive work with Kester and Alpha on this very problem and they were very helpfull. Curtis

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