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Date: | Tue, 07 May 96 13:16:49 PST |
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I have my doubts re your analysis of the residue. Lead oxide wouldn't
be white but would vary from yellow-red to brown color (depending on
what chemical form the oxide took). We've had a variety of "white
residues" and they have generally been one of four types:
a. Polymerized flux residue (due to either inadequate cleaning leaving
flux which reacts with water cleaner and/or excessive heat in
soldering) (this one is ~90% of the time)
b. "Etching" or chemical attack of solder (typically related to use of
hot/high resistivity DI water (7-12 Megohms); surface affect only no
structural impact (5-10%)
c. Bromide salts (reaction between flux and bromide in fire retardant
used in Gl/Ep) (1-3%)
d. Discolored solder mask (typically etching/chemical attach of
undercured mask) (1-3%)
What we do to differentiate is a) look at location; (c) and (d) are
typically on laminate, not on joints themselves). We have found that using
the "like disolves like" idea; a mildly acivated flux (e.g. Kester 197) on
the residue followed by mild heating (100-150F) to increase activity of
acid in activator then "normal" cleaning can remove most of the residue.
Note that we had a LOT of trouble with white residue on initially using
semi-aqueous cleaning. We found reducing preheat helped, reducing conveyor
speed/dwell time helped a lot, reducing solids content in flux helped some;
going to a more thermally stable (e.g. tall oil) type flux (e.g. Alpha 615
or Kester 186) eliminated the problem. We also found some cleaners more
effective than others in removing the residues (e.g. Kyzen FCR).
If you have any questions, call.
Jim Maguire
Senior Principal Engineer
Boeing Defense & Space Group
PO Box 3999 M/S 3W-97
Seattle, WA 98124-2499
Phone 206-657-9063
FAX 206-657-8903
Email: [log in to unmask]
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Subject: White residue/lead oxide
Author: [log in to unmask] at esdigate
Date: 5/7/96 10:04 AM
We have a white residue problem at another division that has been analyzed
using FTIR and SEM EDAX that has shown it to be lead oxide. The white residue
is located on pads/solder fillets. The following process is used:
a. Yokota flow solder (preheat 220F topside, 495F solder pot (less than 3
seconds), conveyor speed 3.5 ft/min)
b. Kester 197 foam fluxer (rma)
c. Cleaner is Bioact EC7R in a Ney spray under immersion or ECD batch
diswasher. Temperature of EC7R 90-95F, Rinse waters 130F.
The white residue is observed after cleaning and is insoluble in alcohol
water/EC7R.
Any input on things to look for would be greatly appreciated. We are running
tests at other divisions with different flux chemistries/board design/process
parameters.
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