Larry: There's a variety of possible causes. Take a close look at the
bottom side of the board under magnification. Is the white appearance on  both
the soldermask and the open laminate areas? If it is, the problem is probably
related to your solder/clean process. If it is only on the soldermask it could
be under cured mask. I'm assuming you are seeing NO RESIDUE on the board top
side.
A.) Solder/Clean posible causes:
1. Overheating or oxidation of the RMA flux; try running a board at twice your
normal flow solder conveyor speed.See if the problem goes away.
2. Formation of Pb or Sn halides from the activator in the flux reacting
with the metal.
3. Polymerization of the rosin in the presnts of moisture in your flux or
cleaner chemistry. Change them out, see if the problem goes away.

B.) Uncured soldermask:(soldermask resin is being dissolved exposing the filler)
1. Check for cure and complete crosslinking. Rub a cotton swab dipped in
methylene chloride across the soldermask, use moderate pressure for 5
sec. If it picks up the color of the  mask it is uncured;(a light or
faint color is acceptable). You should also see the same effect on the board top
side.
2. If possible try baking an unloaded board at 300F for 90 minutes. Do
the same swab test,it should be clean.If it looks the same as the
unbaked test swab i.e. color on the swab, the mask is not poperly
catalyzed/reacted and will never cure. Not Good.

There are many causes for white residues as you will probably see in many
responses. However for your particular application these are some of the most
common.

                                                Regards
                                                        Michael Barmuta
                                                        Staff Engineer
                                                        Fluke Electronics
                                                        Everett Wa.
                                                        425-356-6076

On Fri, 19 Dec 1997 06:58:00 -0800 Larry Davis wrote:

> From: Larry Davis <[log in to unmask]>
> Date: Fri, 19 Dec 1997 06:58:00 -0800
> Subject: [TN] ASSY:White residue after cleaning
> To: [log in to unmask]
>
> Has anyone come across a white residue on the circuit side of board
> assemblies after cleaning?  We are using Axarel 32 in a semi-aqueous
> cleaner to remove an RMA flux after the wave solder operation.
> Thanks for your help!
> Larry Davis
>
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