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80 SMT007 MAGAZINE I OCTOBER 2020 may also need to clean the residues to access test points if you're not using a probable flux. Sensitive RF circuits also can be affected by flux residues and need to be removed to ensure proper operation. I think that pretty much exhausts the good reasons to perform localized cleaning. It's not a long list. Please note that cosmetic appearance did not make the list. What is lurking in the shadow? It is most likely active flux residues spread around by the localized cleaning process. And much like a teenager standing on your porch, trying to score some candy from you by stuffing a few pens in their shirt pocket and saying they're a nerd, you just don't want that. And ouch, by the way, that's just rude. This month, I want to highlight localized cleaning that is performed correctly and incorrectly, as well as the impact on cleanliness and reliability. I often go into a production facility for a pro- cess review and see operators using an acid brush of questionable cleanliness that's been dipped in IPA, scrubbing away after a manual soldering process and then inspecting the area of concern for residues. If none are found, they simply pass the boards down the line for subse- quent processing. What I never see is that the same operator looking in the adjacent areas for residues. Those areas are the shadows I men- tioned. In almost every case, you can't inspect monolayers for residue under components with low standoff height for some time. That same low standoff height makes them very difficult to clean under the best of circum- stances. When you add IPA or other solvents to the flux residue, all that does is make it sol- uble. What it does not do is magically make detrimental flux residues disappear like many seem to think it does. Don't believe me? Let's look at some ion chromatography (IC) data from a field failure. Touch-up soldering was done to an SMT component, and according to the work instruc- tion, this process includes a quick cleaning with IPA. The boards are already built with no- clean flux, but any touch-up requires cleaning. Figure 1 shows the SMT part that was cleaned, as well as the capacitor and PTH pins that were tested. The IC data in Table 1 shows low levels of ionics at the SMT location, but much higher Figure 1: Area after localized cleaning. Table 1: IC data results.

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