Luigi,
There are those who might say "there goes Munie again preaching hiearchy". How could anyone recommend to use a wetting balance as a method to control the depth and immersion speed for a dip-&-look solderability evaluation? Well I must be one of those hieratics that agree with him. One of my first jobs in the Western Electric Company in 1971 was to computerize an old G.E.C. Meniscograph using a DEC PDP-8 computer to automatically collect wetting data to take out the subjectiveness of the dip-&-look procedure. Well after 34 years I'm still relying on use WWR and the dip-&-look procedure for solderability testing. The only thing that I've changed in 34 years is that I now use "Active Wetting" as my criteria for good solderability. I too remember John's solder coated toothpick. Also, I used to have samples of axial leaded components that had "black" colored leads. When I dipped the entire lead in molten solder solder stuck to the leads. When removed from the sodler pot 100% of the lead was coated with solder. According to the dip-&-look criteria they pass solderability. Our problem was that when they were through-hole insereted and wave soldered the solder joints looked "bad" and we couldn't get electrical connections. We went back to the dip-&-look test and found that if we dipped only one-half of the lead length in solder that although solder would sstick to the lead there was a slight negative meniscus to the lead rather than a positive menisus. That is when we adopted the "Positive Menisus" or "Active Wetting" as our criteria for good solderability.
Regards,
George (The Hieratic)
George M. Wenger
Reliability / FMA Engineer
Base Station and Subsystems Group
Andrew Corporation, 40 Technology Drive, Warren, NJ 07059 (908) 546-4531 [log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Munie, Gregory
Sent: Wednesday, May 25, 2005 9:18 AM
To: [log in to unmask]
Subject: Re: [TN] Wetting balance
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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