SOunds like a residue removal problem and the yellowy white colour indicates
rosin.
You can verify if it is rosin by warming it to say 80-90C - use hot air or a hot
plate .
If it is rosin then it will melt and become transparent and therefore
practically invisible. 
It will still be there of course, just not so easy to see. 
If it resin then check your cleaning process, this sort of thing was very common
with solvent cleaning so may be you are on HCFC/alcohol and need to improve your
process control. 
If you are using alcohol it may not be possible to eliminate this problem. 
Alternatively/additionally look at a different paste.
All RMA pastes are based on Rosin but  solder pastes have other things in the
mix as well and some products will clean off more completely/easily  than
others.
If it doesn't melt then you have an unidentified residue which is most likely
not relatively benign resin. 
This being the case I would suggest assigning a higher priority to action. 
Hope this helps
Mike Fenner
BSP
---------------------------------------------------------------------------------
------------------------------

Ed Knowles wrote

We are noticing a film on our SMT solder joints.  It doesn't seem to 
affect ionic cleanliness or conformal coating adherence, but can only 
be removed mechanically with a brush or Q-tip.  It has a slightly 
yellowish color in most cases and appears powdery.  Is this 
contamination from the PWB pads or part leads floating to the top, or 
a compound derived from the solder paste (63/37 RMA)?  Any insight 
into what this contamination is would be appreciated.
Thanks.

Ed Knowles
Lockheed Martin Astronautics

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