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May 2003

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From:
"Wenger, George M." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 8 May 2003 11:48:22 -0500
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John,

I don't have all of the gauge R&R measurements in front of me but I'm sure you can get the data from IPC or the JSTD-002 committee.  As for what influences the variation, the answer is easy; everything that could cause a weight difference.  Wetting balance gauge R&R measurements are usually well controlled and use "ideal" samples to avoid sample to sample variations so you can quantify the tester/machine to tester/machine variations.  So you start out with solid copper coupons of given size.  Well there is some sample to sample weight variation due to the variation is sample preparation.  Calibration  of the wetting balance weight measuring scale is also easy because you can weigh a paper clip on a precision balance and send the paper clip to each of the testers and have them weigh the paper clip on their wetting balance.  The tester to tester variation comes in because you need to flux the samples and make sure the density of flux at each tester location is correct.  That is measurable by a simple density measurement but testers poor their measured flux into a sample beaker and dip the part to be tested to a given length.  The length in most cases is "eyeballed" not measured.  There are wetting balances that do flux samples and know how deep the samples were immersed into the flux.  But even if you know how much flux is on the sample you need to control the hang time and distance so that there is the same amount of flux on the sample from tester to tester when the test begins.  Flux evaporation from tester to tester is a variable, immersion depth control from tester to tester is a variable, solder temperature, immersion speed, etc. are all variables that make it almost impossible to get accurate weight measurements from tester to tester.

The main reason I like the "Active wetting" requirement is it doesn't require the variables to be "accurately" controlled.  My solder temperature can be 235C +/-10C, my flux immersion depth needs to be greater than my immersion depth, my immersion depth needs to be 50% +/- 25% of the wettable length of my sample, etc.  All I want the wetting balance for is to immerse my sample to the approximate position on the sample where I want to know if it is Solderable and I'll watch the solder meniscus.  If the meniscus is positive I have "active wetting" if the meniscus is negative I don't have "active wetting' and would have a high probability of having soldering problems in production.
   

Regards,
George

George M. Wenger (908)-546-4531 
Reliability Engineer
RF Power Amplifier Group
Andrew Corporation,  40 Technology Drive, Warren, NJ 07059
[log in to unmask]



-----Original Message-----
From: John S. Grosso [mailto:[log in to unmask]]
Sent: Thursday, May 08, 2003 11:34 AM
To: TechNet E-Mail Forum.; Wenger, George M.
Subject: RE: [TN] Wetting Balance Testing


George,

I was interested in more commentary about your comment:

"One of the major reasons accept/reject criteria limits have not yet been
established is because every time you run a wetting balance evaluation you
get slightly different weight measurements."

What influences the variation? Can't this quantified / anticipated through
normal gauge R & R?

Regards,

John S. Grosso
Waytec Electronics Corporation
Vice President of Quality & Engineering
(434) 237-6391 x130
[log in to unmask]



-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Wenger, George M.
Sent: Thursday, May 08, 2003 11:27 AM
To: [log in to unmask]
Subject: Re: [TN] Wetting Balance Testing


Neil,

I agree with Greg Munie.  I've done extensive wetting balance evaluations
with at least six different wetting balances since 1971.  Four of the six
wetting balances were commercially available units and two were experimental
laboratory units devolved by Dr. Kon Lin.  As Dave Hillman indicated that
although the wetting balance test methodology in JSTD-002B does include some
suggested criteria which can be used, industry acceptable wetting balance
accept;/reject criteria limits has not yet been established.  The primary
problem with wetting balance testing is that the wetting balance measures
weight change (like a fish scale) due to forces acting on the sample.  One
of the major reasons accept/reject criteria limits have not yet been
established is because every time you run a wetting balance evaluation you
get slightly different weight measurements.  Dave's correct that it is too
early in the JSTD-002 committee studies to know if a valid limit for
industry wide application can be achieved but if we haven't been able to
achieve a a valid limit in the past 33 years I'd guess a valid limit isn't
just around the corner.

In soldering one looks for and expects "good wetting" (i.e., good spreading
of molten solder on surfaces along with adequate metallurgical bonding of
the solder to the surfaces).  As Greg indicated in his #2 point, the wetting
balance is a good tool to control solder temperature and immersion depth and
speed.  Using the wetting balance for a Dip-&-Look solderability evaluation
takes out the variability of the dipping process.  Although Greg's "active
wetting" in his #3 point doesn't give a quantitative measure of the wetting
force it provides an excellent accept/reject criteria.

Regards,
George

George M. Wenger (908)-546-4531
Reliability Engineer
RF Power Amplifier Group
Andrew Corporation,  40 Technology Drive, Warren, NJ 07059
[log in to unmask]

-----Original Message-----
From: Munie, Gregory [mailto:[log in to unmask]]
Sent: Thursday, May 08, 2003 10:11 AM
To: [log in to unmask]
Subject: Re: [TN] Wetting Balance Testing


Neil

I've done a lot of wetting balance but I have to add a few  comments on the
"dip and look." (I agree with all Bev's other comments and suggestions.)

The dip and look as it's traditionally done does deserve to die. But if done
correctly it can tell you almost as much as wetting balance.

And I define correctly as:

1) Use ONLY water white rosin flux. NEVER use any activators in the flux.

2) Control your immersion (wetting balance is good for this :-)

3) Coverage criteria means nothing if you don't see active wetting, i.e.
solder rises ABOVE the surface of the bath to wick UP the part lead.

# 3 is absolutely critical: in a real world soldering situation that is
exactly what you want to happen for either wave or reflow soldering. It's
what happens to the section of the lead that's NOT in direct contact with
the solder that tells you whether the part is solderable.

Greg Munie










Original Message-----
From: Bev Christian [mailto:[log in to unmask]]
Sent: Thursday, May 08, 2003 4:25 AM
To: [log in to unmask]
Subject: Re: [TN] Wetting Balance Testing



Neil,

You also need to consider what alloy you are using.  Dr. Lee of Indium has
shown that 62/36/2 Sn/Pb/Ag gives significantly different results.  And then
there is the work Chris Hunt et al of the British NPL have done on lead free
solders.  And then are you using a solder globule or solder bath?  Don't
forget temperature, atmosphere (most likely air, unless like Chris you can
afford a dry box) and preheat (or is that what you mean by hang time?).  If
you are using a globule block we have also found that how centered your
component lead is over the solder globule affects your results as well.

I would say it is not so much the length of the lead (unless you are talking
about something obscenely long - no comments please), but rather what the
lead is connected to - like an internal heat sink.  And, yes, we have had
ONE problem with the heat sinking ability of a component affecting the
results.  I gave a short presentation to the J-STD-002 committee at APEX.  I
will send you a copy.  It will not show up for other TechNetters,
unfortunately.  In essence, we found that wrapping the wetting balance clip
in polyimide tape allowed the component in question to pass solderability
testing!

All said, we have had excellent results using a wetting balance and much
prefer it to the dinosaur "dip and look" test, which I would like to see DIE
for SMT components.  Now if we can just agree on what pass/fail criteria
will be....

regards,
Bev Christian
Research in Motion

-----Original Message-----
From: Neil Flatter [mailto:[log in to unmask]]
Sent: May 8, 2003 1:17 AM
To: [log in to unmask]
Subject: [TN] Wetting Balance Testing


For those of you doing solderability testing, what sorts of parameters
should we be concerned about for reliable results?  Our tester already
allows inputs for lead perimeter, cross sectional area, and hang time.  My
company's testing also specifies immersion angle, depth, and speed as well
as the type of flux.  By controlling these factors, can I expect repeatable
results?

It has been suggested that we also need to limit the length of the leads for
our testing as wall as installing a thermal break between the lead and the
hanger.  Both have been blamed for failing test results.  Has anyone
experienced changes in test results from these variables?

Neil Flatter
TRW-Automotive
Process Quality

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