|
|
|
|
||||||||
QFN Rework: No-Clean or Water Soluble Flux?
Board Talk
|
||||||||
TranscriptPhil And welcome to Board Talk. This is Phil Zarrow and Jim Hall of ITM Consulting here to discuss with you SMT process, methodologies and materials. And I do believe we have exactly that kind of a question today. Jim This comes from M.D. No clean or water soluble for QFN components. We're replacing 32 lead QFN and 14 lead DFN components. We can use either a no clean or a water soluble process. Which do you suggest? Is there a formula or a standard that explains when to use a no-clean or water soluble process with these component types? Well, I don't know of any formula or standard, it's certainly done both ways. Ideally, it would be wonderful to just use the no clean and assume the residues are safe and not have to go to a cleaning process. Phil In many cases and in many applications that is the case. There are risks involved, particularly when we talk about QFNs. Jim A paper published by our favorite cleaning gurus, Terry Munson from Foresight showed that they removed QFNs that had been processed with no clean and found active residue under the paste. Terry theorized in his paper that the component and all of the paste under it had in fact seen the proper time/temperature profile during the re-flow process should have deactivated the paste. But, because of the configuration of the QFN, with a large amount of solder paste in the center for the thermal pad, what was actually happening was the flux residue around the perimeter of the package was hardening sooner than the center, preventing the evaporation or the escape of the evaporated materials from the no-clean paste. Remember, no cleans are deactivated by three mechanisms: chemical reaction of the components, evaporation, and encapsulation. So even if we have the proper cycle, if you suppress that evaporation and trap those materials, you have the potential of having some corrosive materials. So my instinct is to use a water soluble process. You need to make sure that you use a saponifier, because in most cases the high surface tension of pure water alone will not penetrate under the QFNs to clean out all of the residues. And with a water soluble you must remove the residue. Phil So probably the best of all courses for QFN component rework is to use no-clean flux, but clean the no-clean flux using an engineered aqueous saponifier. As our other favorite cleaning guru, mild mannered Mike Bixenman from Kyzen, will tell you, you must be certain that you match the solvent - in this case the engineered aqueous, to the soil, and the residue of the no clean. Jim I absolutely agree with that, but it's always been my feeling, if I know I'm going to clean, why not use the water soluble which is designed to be clean, and not a no clean which is designed not to be cleaned? Now it's done all the time, but it seems to me if you've got this flexibility and you're going to end up with a cleaning process with an engineered chemistry and some sort of saponifier, that it always seemed more logical to me to go with a water soluble that's designed to be clean. But check it out for yourself. Make sure you run the tests on your parts and your components with your solder paste. Phil I suspect we'll get a bunch of mail on this one, Jim. Jim But whatever you do, don't solder like my brother. Phil And don't solder like my brother. |
||||||||
Comments
|
||||||||
|
|
|