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Tin-lead Reflowed at Lead-free TempsBoard Talk
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TranscriptPhil Jim The question, "We recently reflowed a batch of tin-lead soldered circuit board assemblies with a lead-free profile. This was a mistake. The lead-free profile temperatures were significantly higher than specified for these tin-lead assemblies. The assemblies appear fine, but we're concerned about reliability. Would you scrap these board assemblies? What additional testing can we do to confirm the long-term reliability of these assemblies?" Phil We're assuming that the joints look okay from visual inspection and there is no conspicuous blistering of boards, or parts, or strange chemicals oozing around. So starting at that point, visually, at least, things look good. The concerns with the metal, Jim would be? Jim The higher temps might have exceeded your specifications, and then you would be concerned about inner metallics forming on your joints, unless you have a nickel-gold board, in which case, that would not be a problem. The real concern that we have are components and the printed circuit board. For components, you should be able to go back to your spec sheets, and find out what the peak temperature rating is, and what the moisture sensitivity rating was. If you had a part that was only rated as a tin-lead part, at a level three, and it was exposed for six days, so you're near the seven day limit for tin-lead, then you may very well have exceeded the moisture sensitivity limit by going to the higher temperature.
The only way to test that is a scanning electron microscopy. That is really the only testing that I know, short of accelerated life-testing them, and testing them to failure, and see how they correlate. That's a really long, expensive, and rigorous process. So should you throw them away? It depends on the reliability requirements, but I think you can get most of the way by looking at the material specs on your components, and your circuit board. What was the T-Sub-G of the laminate used? And then, as Phil said, visual inspection, looking for blistering on the boards, bad solder joints, warping, cracking of your components due to moisture sensitivity. But regarding testing, there's not a lot of non-destructive testing you can do. Phil Jim Phil Jim |
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