Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Jacqueline Coia

#16366

white residue after wash | 3 April, 1998

At this moment in time we are using water soluble solder paste on our PCBs these are cleaned using water only, agitated with ultrasonics. After drying a white/grey residue is left around solder joints. Reflow profile is fine and a reliable wash procedure is followed. Advice would be appreciated.

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Scott Cook

#16369

Re: white residue after wash | 3 April, 1998

| At this moment in time we are using water soluble solder paste on our PCBs these are cleaned using water only, agitated with ultrasonics. After drying a white/grey residue is left around solder joints. Reflow profile is fine and a reliable wash procedure is followed. Advice would be appreciated. Hi there. Although you fail to mention it, I will assume you are using an off the shelf paste, with about 90% metals. Are you using compatible chemistry at the wave, after reflow? Are you cleaning the assemblies after reflow, prior to wave? 1st I'd absolutely verify the following potential: Do you also utilize any other chemistry in your facility? Like no-clean? If perchance a no-clean was introduced to your printer through a mistake, then run through the aqueous wash after reflow......voila; white residue. Try this test: spray water soluble flux onto a sample of the boards with residue, brush the joints vigorously with an acid brush. Run through the wave preheat (without the wave itself turned on) to activate the OA flux, then wash again. See if the residue disappears. Next scenario to investigate is improperly cured solder mask on the bare PCB. I have experienced the problem you describe with a mask which requires a UV bump to cure the mask. The amount of UV energy (joules) the PCB house used to cure the mask did not reach the mask mfg'ers recommendation. This problem, however, usually manifests itself as a white, milky, opaque looking finish UNDER THE MASK itself. However, outgassing of uncured masks can cause very strange symptoms. Need more ideas? e-mail me at scottc@t-com.com or call at 850-580-0282 Scott Cook Mgr, Comm. Business Dev. Talla-com Industries

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Steve Gregory

#16368

Re: white residue after wash | 3 April, 1998

Hi Jacqueline! The dreaded white residue huh? YUCK! This seems to be a ongoing issue...probably because there are many different things that can possibly cause this. There's a real good article about the different causes of this problem at Kester's WEB site...in fact, I'll just paste the URL below so you can cruise it yourself. The article gives ALL possible causes of the residues, and there are quite a few. But the few times that it's happened to me (and it's been water soluble paste too), was heat related. Once because the board was liquidous too long, and the other time was because the wash tank temp and dryer temp at the exit end had been cranked-up way too high...ya' might wanna check that too. -Steve Gregory-

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Graham Naisbitt

#16367

Re: white residue after wash | 5 April, 1998

Jaqueline There are so many variables in the electronic assembly manufacturing process involving more than 12 different chemistries, that it is impossible to easily identify and determine the nature and reason of such residues. GEC Hirst Research in the UK, had identified more than 16 different white powder residues! What are they? How are they caused?....Consider the following, and I believe I may be shot at... The "rosin" particles that may be present in certain types of flux, are usually based upon resins that are derived from trees. As such they are seasonal and their quality varies from year to year, season to season. When these particulates are exposed to a more efficient solvent, as was the case when DuPont first introduced an alcohol/azeotrope of 113 (Freon); then the "cleansing" action was greatly improved. However, in so doing, the alcohol (having a higher KB factor or solvency capacity)partially dissolved such particles that were trapped in the "sponge" like surface of the board. Take a look at your board under a scanning electron microscope and you will see what I mean. So... the particle was rendered a different light refractive condition which showed up as white and proved that your 113 "Freon" type solvent alone, was a lousy cleaning solvent and that the alcohol was actually doing a better cleaning job....but there were occasional "white residues". Modern process chemistries may be similarly affected or may exhibit OTHER forms of white residue... the essential message being: DO Not clean No-Clean! There is no such thing as no residue flux - there is fluxless soldering, but that is another technology. Thus, if you leave residues, they WILL react unpredicatbly maybe, with different types of cleaning chemistry - alcohol, methanol, glycol based??? Who knows for sure? HOWEVER, a simple suggestion, check with your flux/paste supplier, and get them or others, to supply a Water SOLUABLE (not washable) alternative and you should find your problem has gone away. I hope I did not bore you... Regards Graham Naisbitt | At this moment in time we are using water soluble solder paste on our PCBs these are cleaned using water only, agitated with ultrasonics. After drying a white/grey residue is left around solder joints. Reflow profile is fine and a reliable wash procedure is followed. Advice would be appreciated.

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