I will summarize some points re your problem which hopefully will make it
easier for others to add their thoughts-
1. The residue- while there are many causes to white, tan, yellow, gray
residues; the most likely cause in your case would be the insoluble activator
residues from your no clean flux. As Brian points out, these residues are meant to
be left behind, and their presence is not supposed to be harmful. Most likely
they are a dibasic acid, generally adipic acid, although succinnic, tartaric
and malic acids have also been used. Check in a Chemical Handbook and you will
find that adipic acid is not really very soluble in anything, which gives some
insight on why it remains on the completed assembly.
2. Quick identification of the (flux) residue-- if it is a dibasic acid, dip
a Q-tip in 0.1 N NaOH, touch it to the residue, rinse with water, rinse with
IPA, and blow dry with dry, oil-free compressed air. Best to do this on a scrap
board that is run through the soldering process so it can be discarded
afterwards. If the spot is now clean, contaminant is most likely dibasic acid.
3. Quick identification of the (solder mask) residue- if it is the white
powder fumed silica used to thicken liquid solder masks, you will often see it
appear as Brian has noted. If the solder mask is undercured, any cleaning step
may strip off part of the green solder mask, exposing the white thickener powder
(typical trade name is 'Cab-O-Sil"). Heat the white area with a heat gun. If
the residue is the Cab-O-Sil, the heat softened solder mask will allow the
Cab-O-Sil to sink back into the green solder mask. Residue disappears, problem is
insufficient UV cure during solder mask processing. [Quick, cheap tests are
great !]
4. A-610 residue allowance- Most folks using 610 follow the rules that
visible residues are not acceptable. Arguing with customers that the residue is x or
y or z normally tends to be an uphill battle, especially when the customer
asks you to accept the liability that the residue will be x every time... You
will sleep much easier if you can find a way to eliminate the residue, even if a
cleaning step is required.
5. Many US companies offer cleaning agents that will quickly remove no clean
flux residues. Certain of these companies even offer contract cleaning
services, where they maintain a cleaning centre staffed by experts with all different
types and sizes of cleaning equipment and appropriate cleaning agents, plus
testing equipment. They will accept the problem PWAs, clean them, test them and
return them to you via express shipping. In the long run, this is much more
economical than maintaining cleaning experts and processes in house.
Let me know if clarification or elaboration of any of these points is needed.
Bill Kenyon
Global Centre Consulting
3336 Birmingham Drive
Fort Collins, CO 80526 USA
Tel: 970.207.9586 Cell: 302.377.4272
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