In the old days, HASL was not as robust of a process as it is today. PWB
vendors often had to put the boards through the HASL process several
times, each time building up the IMF layer. This left a copper-enriched
pad for the user to solder too, and led to many copper dewetting issues.
As a result, the 5 second dwell was put into the wetting test procedure
in order to try and detect dewetting and copper migration problems ahead
of time.
My experience with wetting testers is exactly that of others who have
responded; you can get whatever results you want, and none of them have
any correlation as to how well the pwb or components will actually
solder. Solder will wet to a copper pad that displays heavy copper
oxides, a green pad. Does it mean you have a good IMF? No, of course
not. Good wetting and good intermetallic formation are two entirely
different things.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Richard Kraszewski
Sent: Wednesday, May 25, 2005 3:36 PM
To: [log in to unmask]
Subject: Re: [TN] Wetting balance
Few IOI's on this subject, that in my mind brings to light some
important aspects:
- One has to remember that the J-Std-002 dip&look test stipulates ~ 5
second dwell time in the molten solder bath. You normally don't see
times that long in Wave or Selective soldering applications.
- Problem with both tests is the neither really take preheat into proper
consideration. This why we've found it difficult to correlate WB
results with the actual performance of Pb-free fluxes we've been
evaluating as of late. Different products have varying heat stability.
WB doesn't adequately address this.
- Has anyone noticed that neither of the presented WB methods (Test E &
F) of J-stnd-002 actually deal with thru hole components? The former is
for leaded and latter is for lead-less, but early in the same spec. a
distinction appears to be made between leaded & through whole (see
figures 4-2 & 4-3). Do you think this meant that the WB wasn't thought
to even be worthy of a test "without established accept or reject
criteria" for thru-hole components?
- To my way of thinking, the strength of a WB test is unfortunately
after the fact of discovering a soldering issue. If given set A versus
set B, you can normally determine/confirm which had the solder issue on
the line . Most engineers like to quantify findings and the WB gives
you ample opportunity to do statistical analysis... for whatever it's
worth. Most of the time with ~<10 runs of each set you can convince
yourself that you have enough statistical significance to present a
plausible story.
Rich K KEDS
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wenger, George M.
Sent: Wednesday, May 25, 2005 2:37 PM
To: [log in to unmask]
Subject: Re: [TN] Wetting balance
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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