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September 2000

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From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 7 Sep 2000 15:17:05 +0300
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Steve

The most common cause of dewetting is still the use of abrasive cleaning. This is
inevitable if the abrasive is insoluble and almost so if it isn't. I have seen it so
many times with woven and non-woven abrasive brushes of all types, pumice slurries
(hand, rotary brush or hydraulic), abrasive papers incl W&D etc. The crystal formation
of the copper does not help. This may be exacerbated by epoxy contamination (bleed
through) which is much more common than is generally thought, at a sub-microscopic
level. Believe it or not, the EDCF used for rigid lamninates is actually a sieve with
holes in the range 0.01 to 5 micrometres. During pressing, residual solvent in the
prepreg has little place to go except through the copper, as a vapour. It takes some
epoxy with it. This helps peel strength, but it plays havoc with solderability. Do an
edge-dip test with "as received" laminate which has been lightly persulfate
microetched, and you will almost surely see some dewetting. Brush it, and the dewetting
will be a lot worse a) because the bled-through epoxy will be smeared and b) abrasive
particles will be implanted into the copper, some of them even swaged over with copper
if the brushing is even slightly vigourous. If subsequent copper plating is epitaxial,
even that can be epoxy contaminated at temperatures exceeding Tg. However, brushing is
still the worst culprit (I gave a paper on this at InterNepcon in Brighton, England, in
1969, believe it or not).

Another common source is a plating bath with excessive organics co-deposited. At the
time I was director of a PCB fab co., I had one BIG customer who insisted on
mirror-finish copper tracks. We set up a special bath for this one guy and he got what
he asked for, but he had hell's own job getting decent solder joints. At the time, we
offered a solderability guarantee, but these PCBs were excepted, purely because the
brighteners in the copper bath were causing very severe dewetting. The same may also
apply to electroless deposits, as well as electroplated.

Hope this helps

Brian

"Stephen R. Gregory" wrote:

> Hi All!!
>
> Got some boards in at receiving that are exhibiting dewetting on the pads.
> You can see what I mean at:
>
> http://www.driveway.com/share?sid=e25a88c4.8e904&name=Pictures
>
> I'm rejecting them of course, but what would be the most likely cause of
> something like that? I'm pretty sure it's some form of contamination, but
> what?
>
> Thanks!
>
> -Steve Gregory-
>
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