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May 2013

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Subject:
From:
Steve Gregory <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steve Gregory <[log in to unmask]>
Date:
Thu, 23 May 2013 10:13:47 -0600
Content-Type:
text/plain
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text/plain (197 lines)
Hi Mike,

Unfortunately a lot of what we build here is consigned stuff...customer
supplied boards and parts. So often times I won't even know who built
the fabs.

One of the questions I have is; by using the heat gun am I actually
curing the mask? Why doesn't that happen during reflow or wave solder?

Brian,

Guess what? I found a used copy of your book on Amazon that I'm buying.
It should arrive in 2-3 weeks. It will be one that I'm going to
study....


Steve

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Mike Fenner
Sent: Thursday, May 23, 2013 3:34 AM
To: [log in to unmask]
Subject: Re: [TN] White Residue Disappears with heat gun, what does that
mean?

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
>
>
>
>
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