Luigi

You didn't say what flux you were using. I assume it's the flux called out
in the spec.

My experience at AT&T (and resulting contributions to the IPC wetting
balance test) are that if you use ANY flux but water white rosin (WWR) the
results you get in production will NOT equal the results of the wetting
test. The test will always look better.

I and my co-workers published this as a reliability study at the SMI
conference in San Jose, CA in 1995.

I have heard the arguments that wetting balance is inherently too noisy to
provide good repeatable data from site to site if one uses WWR. Yup! A
little activation in the flux definitely improves the repeatability: It
makes everything good. And when that happens, from an assembly standpoint,
you're the loser.

I suggest a simpler alternative: use the "dip and look" test with WWR and
the wetting balance to control immersion depth and speed. But only accept
the parts if the area wetted exceeds the area dipped. Why? Simple! In
production you want the solder to wet "up" the part/wet "over" the lead.
When you put the part in the solder with WWR and the solder climbs up the
lead you know it's good! If all the solder does is wet the immersed area . .
. well, on my desk I have some carbon fiber bundles that exhibit good
"wetting" per the current test. They're well covered with SnPb over the area
immersed. But I defy anyone to actually make good connections in production
with leads like that. (Years ago John Devore, if I remember correctly, would
show people a solder surface that was well wetted. Then he'd hand it to
them. It was a wood toothpick with solder adhered to part of the wood. Sure
looked it'd solder per the spec!)

So, per my opinion, WWR and demonstrated active wetting of the parts are the
only way to go in solderability testing.

Greg Munie



-----Original Message-----
From: Luigi Cantagallo [mailto:[log in to unmask]]
Sent: Wednesday, May 25, 2005 2:00 AM
To: [log in to unmask]
Subject: [TN] Wetting balance


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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