I agree it probably is under cured resist, but as Brian says it may not be.
If you were using a no clean and had this the explanation could be unremoved
resin which is melting back to its normal translucent state, the white would
be analogous to the water mark left by a coffee (tea for me) cup on a
polished table. Probably not too bad a risk. The downside with unremoved
water soluble flux residues is much higher. You need to be more certain. So
you have done a simple test and have an indicative, but not absolute result.
So you need to do some more work.
If it is under cured resist then boards from a different batch may not show
it, so all else being the same it's your board supplier's problem.
You could also try finishing off the cure by baking some unpopulated boards
for say a couple of hours at 100C. If it goes away on the baked boards, but
not on unbaked boards then it's your PCB suppliers problem. Meanwhile you
have a keep you going fix till the problem is rectified.
Another thought is - has PCB supplier changed the resist in some way, if it
is now shinier than before this may be highlighting an existing problem.
So you could also contact your PCB supplier outline the problem, ignore the
never had that before etc, ask them to collaborate with you to identify the
problem by elimination all possible causes.
Anyway their contribution in the elimination process would be to supply a
batch which is definitely cured and do another side by side test. No doubt
you already doing that though.
Best Wishes
Mike
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: Thursday, May 23, 2013 7:38 AM
To: [log in to unmask]
Subject: Re: [TN] White Residue Disappears with heat gun, what does that
mean?
There are a number of potential causes. A common one is the exposure of
solder mask fillers because the cleaning removes the surface molecules
of the solder mask. This is largely cosmetic but it may mean the mask is
insufficiently cured. As you state (or I did in the earlier exchange!),
heat will allow the filler to sink in.
Another common cause is that a resin (synthetic or rosin) or carboxylic
activator from the flux has spread and hydrolysed. This indicates a
potential incompatibility of the flux and cleaning chemistries. This was
common in the days of cleaning DIN 8511 F-SW32 type fluxes in CFC-113
azeotropes, as well as aqueous cleaning of many modern fluxes. The
residues, which are not necessarily ionic, may form a hydrogen bond to
low MW components of the solder mask whose surface may be incompletely
polymerised.
There are five pages devoted to these phenomena described in more detail
in my book from p. 157 ff.
You should determine the cause and take measures to eliminate it, rather
than work in the dark. This means a series of systematic diagnostic
tests and can be a painful process because you may have combinations of
up to about half-a-dozen contributory factors, some of which may require
expensive analyses. Empirical trial-and-error is usually unsatisfactory.
Hope this helps.
Brian
On 23.05.2013 02:37, Steve Gregory wrote:
> Hi all,
>
>
>
> I know we've talked about this before, but I seem to forget the fine
details lately. (blame it on me getting old or something),
>
>
>
> Anyways, we've got a different (newer)cleaner and we've just gotten it
going over the last few days. The cleaner is close-looped which is different
than the way the old cleaner operated. Had a little foaming issue that was
solved by putting in another carbon bed. But today I washed a board after it
had come off the wave, and the bottom surface of the board had what I call
"Zebra Stripes" of what looked like white residue along the bottom of the
board. They are pretty much shadows of the conveyer mesh chain that the
board was sitting on through the cleaner:
>
>
>
> http://stevezeva.homestead.com/ZebraStripes.jpg
>
>
>
> I ran the board through the Omegameter we have and of course it passed at
4.3 ?g NaCl/sq. in.. I took the board out of the Omegameter tank and the
stripes were gone, which didn't puzzle me too much. Boards always seem to
come sparkly clean out of an Omegameter tank.
>
>
>
> I ran the board through the cleaner again and the white stripes
re-appeared. Our cleaner is a Austin America Microjet, our water is at 13
megohm, I'm running 135 F. in my wash at a belt speed of 2 fpm. My dry
section is 100 F. We're using a Indium 1095 water soluble flux in a foam
fluxer in our wave.
>
>
>
> I remembered something from the TechNet a long time ago about heating the
board with a heat gun to see if the white residue disappears. I searched the
archives and found this from Bill Kenyon:
>
>
>
> "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."
>
> So I tried that, and the stripes disappeared. I cleared almost all of the
stripes with the heat gun and re-ran the board through the cleaner and the
stripes pretty much stayed gone...I can barely see a shadow of them. The
information that Bill Kenyon shared with us was from quite some time ago,
but that still applies, correct?
>
>
>
> Steve Gregory
>
>
>
>
> ______________________________________________________________________
> This email has been scanned by the Symantec Email Security.cloud service.
> For more information please contact helpdesk at x2960 or [log in to unmask]
> ______________________________________________________________________
>
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask]
______________________________________________________________________
|
