Ramon,
Although our Failure Mode Analysis Lab is small we have FMA
responsibility for our entire corporation. We don't have time or the
resources to put together a sophisticated automatically digitized
solderability test system. When we get a request to test solderability
it typically comes from a manufacturing process engineer who already
knows he has a solderability issue and he wants to know why and what to
do about it. Rather than run a solderability test to find out what we
already know, we examine the surface finish thickness and composition
using X-Ray Fluorescence (XRF). This usually lets us know why we are
having a solderability issue. If our internal customer or a component
vendor wants proof of poor solderability we apply J-STD-002 Test "A" but
I use a "Wenger" modification to the 4.2.1.4.2 Accept/Reject Criterion:
Wenger's Accept/Reject Criterion
Leads shall exhibit a continuous solder coating free from defects for a
minimum of 95% of the area immersed and the component must exhibit
"Active Wetting" (i.e., the area of the test sample with fresh solder
adhesion must be greater than the area that was immersed in the solder
bath).
The "Active Wetting" wording was added to the suggested criteria for
solderability evaluations for the wetting balance Test "E" but wasn't
added to the dip-&-look Test "A". Greg Munie and I gave up trying to
get it added to Test "A". Neither of use wants to write our own
solderability specification. It makes a lot more sense to use industry
specifications whenever possible. During our employment with Bell
Laboratories we contributed to many internal specifications which were
very simple. They referred to industry specifications and listed only
exceptions. The "Active Wetting" criteria is one of those exceptions.
Sorry for the long winded explanation. I did put together this
afternoon a five slide presentation showing how I do solderability
tested when required. I'll forward it to you in a separate email since
it won't come through in a TN email. I'll also send an email to Steve
Zeva to see if he's willing to put it up on his web site in case others
are interested.
Regards,
George
George M. Wenger, Andrew Corporation
Reliability / FMA Engineer
Base Station & Subsystems Group
40 Technology Drive, Warren, NJ 07059
(908) 546-4531 [log in to unmask]
-----Original Message-----
From: Dehoyos, Ramon [mailto:[log in to unmask]]
Sent: Thursday, May 26, 2005 8:37 AM
To: Wenger, George M.
Subject: RE: [TN] Wetting balance
Great job George. So how do you inspect the solderability? Do
you use a camera whose image is automatically digitized and fed to a
computer and compared? Tell us the details, please.
Best regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Wenger, George M.
Sent: Wednesday, May 25, 2005 3:37 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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