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May 2005

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Subject:
From:
Bev Christian <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Bev Christian <[log in to unmask]>
Date:
Tue, 31 May 2005 15:39:47 -0400
Content-Type:
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text/plain (291 lines)
Richard,
It is as feasible as a dip and look test which is no more inherently
statistically significant than the wetting balance, since you are not
usually doing 100 parts to my 10.

The beauty of a wetting balance test is that it is on the conservative
side.  A dip and look test is pretty optimistic - you have got one
really, really big thermal heat source compared to a globule block on a
wetting balance.

A real time DPMO would make the most sense, if you really have it, if it
is really watched and you haven't made $$$ worth of product before you
realize you have a mistake.  Of course even further upstream, pick good
suppliers and audit them.

We are not using the wetting balance as a form of blind incoming
inspection, as I agree with you components are better than they used to
be. However, we use it to:
1) qualify new components
2) check components from distributors (And where did you say you were
storing those again - under your bed you say - in your unair-conditioned
Malaysian apartment?!)
3) check part lots from suppliers where we have had problems recently
4) confirm line problems
5) solderability research

Regards,
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard
Sent: May 31, 2005 3:18 PM
To: [log in to unmask]
Subject: Re: [TN] Wetting balance

Bev/Ingemar,
Is the wetting balance test still feasible, based on your comments that
the wetting balance test results will not be statistically significant
for small DPMO? In other words, can running a daily test on a small
batch of components detect enough lots of non-solderable components and
prevent them from reaching the production floor often enough to justify
the cost of performing the testing? Are there more "escapes" of bad lots
to the production floor than there are "detections"?
I would assume that it is only useful in high-volume production
scenarios where a bad lot of components with poor solderability could
create a real nightmare in terms of rework cost, scrap, etc.
Does a real-time solder process DPMO rate detection make more sense? I
understand that in some cases a certain amount of part preparation has
already taken place by the time the solder defects are discovered,
especially with leaded through-hole parts (cut and form ops).
With current improvements in component solderability, I personally
seldom see a lot of solder defects caused by component solderability
issues anymore. This is why I question whether performing WB testing is
still a value-added activity.
Just asking.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
Sent: Tuesday, May 31, 2005 8:15 AM
To: [log in to unmask]
Subject: Re: [TN] Wetting balance

Ingemar,
I am somewhat surprised by your abandonment of your wetting balance.
Although I am "married" to it, here it is usually run by a continuing
rotation of co-op students, changing every four months.  Are you sure
the variation is not in the components rather than the operators?

If there is one downfall of a wetting balance, or for that matter ANY
solderability test, short of building actual product, it is THEY WILL
NEVER BE STATISTICALLY SIGNIFICANT FOR SMALL DPMO.  Do the math, the
number of parts you would have to test is horrendous.  So I am not
surprised by Luigi talking about pass wetting balance/fail on line.  It
is NOT a function of the relatively conservative wetting balance test,
but the statistics of where you took the parts out of the reel.  We use
the wetting balance to get a snapshot.  Sometimes we are lucky.  And if
the whole reel is indeed crummy then we have the evidence to bludgeon
the supplier.  

And you are 100% right about needing a person who knows about soldering
and the book you mention is THE BEST.
Regards,
Bev
RIM

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ingemar Hernefjord
(KC/EMW)
Sent: May 31, 2005 3:38 AM
To: [log in to unmask]
Subject: Re: [TN] Wetting balance

Bon giorno,

My response is late, been in Rome for some days, 35 Centigrades and
polluted city air, could study the wetting of sweat very well. If it
were not for the watching police, I had jumped into Fontana di Trevi.

Very interesting to see the answers on solder wetting. We have a
Multicore WB, but it's abandoned, been placed in a corner and never
asked for. The reason is that we could not find a person that got enough
in love and married the machine and who promised to be a good husband
for decades. With different operators we got too much variations of
parameters. 

If inspecting just a few items per day, I think Bev's recipe is working,
but if you have hundreds and hundreds of samples to test, I ask myself
if not WB is necessary to use. At least if you have a WB enthusiast and
someone who can use statistics. 

We have found that better than a machine or any dip pots or whatever, is
a person who KNOWS about soldering. Even if samples are WB tested from a
lot of 100,000 chip caps, and that sample looks nice, we know that there
can still be soldering trouble. When you get soldering problems in the
line, the root cause must be found in hours. A person who KNOWS about
soldering use to have that ability. Usually, those guys are greyhaired
gentlemen with shit under the nails despite a PhD grade, and with no
take-overs. Two of the guys at our company left big holes when they
withdraw. There are a few left, don't see how we will manage the time
they too leave. Knowing a lot about soldering is not on the hit
list.....

For those who are interested in KNOWING about soldering and wetting, I
recommend this book:

"The Mechanisms of Solder Alloy Wetting and Spreading" written by
Frederick G. Yost, F.Michael Hosking and Darrel R. Frear. Sandia Lab
guys. ISBN-0-442-01752-9 Van Nostrand Reinhold, 115 Fifth Avenue, New
York, NY 10003. 
300 pages about wetting and only wetting!

Ingemar Hernefjord
Ericsson Microwave Systems















































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