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May 2005

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Thu, 26 May 2005 11:36:11 +0300
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For me, the wetting balance has one great weakness: yes, you can get a 
comparative measure of how well something will wet, but it's next to 
useless to get a reliable indication of dewetting, maybe a very slight 
drop after the rise. Meniscus testing, likewise. The good old edge-dip 
test with pure 25% WW rosin flux, immersion into skimmed Sn63Pb at 230°C 
at 1 cm/s for 5 cm, no dwell time, withdrawal at the same speed, giving 
0 sec at the top and 10 sec at the bottom demonstrates the wetting time 
and the dewetting time with a band of good wetting in between. Fine for 
PCBs and OK for wires, useless for SMDs, of course. We used this test 
for many years when we were making PCBs with guaranteed solderability in 
the 1960s, when the little-understood dewetting on print'n'etch, aka 
punch'n'crunch, was a terrible pest. Dewetting is still badly understood 
by many. I presented a paper to InterNepcon '69 in Brighton on this, 
with a photo of our DIY edge dip tester, explaining the causes of 
dewetting, yes 36 years ago!!! Our internal specs stated wetting at 
<0.75 sec and zero visible dewetting at <9.5 sec. Those boards 
conforming to this wave soldered beautifully. I still have a sample of a 
test board that wave soldered without any bridging with 4 tracks between 
adjacent DIL pads on a 0.1" pitch.

Luigi Cantagallo wrote:
> Hello Technetters,
> 
> I have questions about wetting balance.
> We intend to use a wetting balance not to accept/reject supplied SMD's (Our
> SMD's are 1 to 5 years old) but to minimize the risk of solderability
> defects in production (Low volume, SnPb technology).
> So we don't apply J-STD-002D criterium but we try to find them to
> corroborate wetting balance and production results.
> On some tests (Wetting balance calibrated and in order, same type of flux,
> same alloy) on same component lots, we have not a perfect correspondence
> between wetting balance and visual inspections results in production (Vapor
> phase soldering). One of the case is "Good at the solderability test/Defect
> in production" and this one is the most risky.
> Somebody have experience with that kind of problem?
> What actions have you made ?
> 
> Thanks for answers.
> 
> Best regards,
> 
> CANTAGALLO Luigi
> 
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