SMT007 Magazine

SMT007-Oct2020

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78 SMT007 MAGAZINE I OCTOBER 2020 There are two terms in the electronics indus- try that I feel should be more widely used. The first is an acronym for "violates minimum elec- trical clearance," which is obviously, and sim- ply, VMEC. It has always confused me why this isn't already heavily used. We use TLAs for everything (TLA is an acronym for three- letter acronyms, by the way). The second term I use on a regular basis, and the focus of this month's column, is "con- tamination relocation." I use this term mostly when I test a PCBA that has gone through some sort of localized cleaning process after a man- ual or selective soldering operation. In most of the cases I have seen, when a no-clean flux is used, localized cleaning is a totally useless practice intended only to improve a cosmetic issue. Cosmetic issues are usually just that and don't have any impact at all on functionality or reliability. I'd rather have an ugly, reliable board than a sparkling clean field failure. What's Lurking in the Shadows? In some cases, there are legitimate reasons to perform localized cleaning. The number one reason is when you are using a water-sol- uble flux for the soldering process. As with any water-soluble flux, the activators are never rendered near benign through a thermal excur- sion and will always be hygroscopic, as well as corrosive. That means you don't have to worry only about electrical leakage failures. The residues can cause corrosion without any bias differential or available atmospheric mois- ture. That's one reason I really like working on PCBA failures if they were built with a water- soluble flux. If you see flux residues, that is more than likely your root cause of failure. It's a short day, and I can go fishing in no time. Another valid reason to perform localized cleaning is when you are planning to use con- formal coating on the PCBA. Flux residues can cause adhesion issues in some cases, and if you use parylene, a full cleaning is required. You Quest for Reliability by Eric Camden, FORESITE INC.

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