| | | We recently made the switch to no-clean and the wurface mount is turning out beautifully. However, the wave solder process continues to put up a fight; we are getting a white residue on the surface of the PCB. The residue is not localized to just the solder joints, but is similar to water spots on a car, it is on the PCB surface. We have played with the preheat temperatures and flux volume. But the flux volume cannot change too much as we are not getting very many solder problems (i.e. bridging, opens, etc.) Any ideas how to get rid of this residue. I realize some residue is expected, but the literature says the flux leaves none. Any ideas? We are using alcohol-based flux, for now. | | Back when, before DF and LPI solder masks, cure was a big issue. With DF type SM's it was difficult to determine cure. Then, as now, SM's | | are epoxy types and it is possible to determine | | cure using methelyne chloride (a drop at a time | | for one minute), in accordance with IPC-TM-650. If cured, the mask will show no degredation and | | will effectively resist nearly all other chemical | | assaults. | | A longer shot, but it might be helpful to make | | this determination if all else fails. | | Earl Moon | Earl: Must you run the methelyne chloride (a drop at a time for one minute), in accordance with IPC-TM-650 on an unprocessed board that you suspect is not cured properly? Or can you test a soldered board that you're sick about? ... In other words, does processing cure the uncured solder mask? Dave F
Dave, It would be a crap shoot. One of the reasons epoxy type solder masks are so desirable, over DF types, is that boards, for the most part are epoxy. Therefore, during the curing process, there occurs a degree of molecular x-linking between the mask and board surface - just as in the lamination and re-lamination process. Any additional attempts to achieve the desired effect would be unplanned and likely uncontrolled. Having said this, the mask may become "dry" but not achieve all specified requirements and still render undesirable effects as the problem discussed. Again, it's a supplier process control issue that must be addressed to prevent defect later. Sincerely, Earl Moon
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