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January 2004

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Subject:
From:
Doug Pauls <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 9 Jan 2004 16:17:47 -0600
Content-Type:
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text/plain (130 lines)
What da heck, its only a few minutes left in the day.

Ioan,
As usual for me, short question, long response.

First, the best thing you can do is attend the IPC meetings upcoming in
February.  All the best minds to pick on this topic will be there.

1.    Your questions of qualify the process, begs the return question of
qualify the process to what standard or specification?  MIL-STD-2000?
J-STD-001?  An internal standard?  A customer levied standard?  Such
standards may tell you what tests to run and in what sample amounts.
2.    If you are not qualifying to an external standard, and are doing an
engineering evaluation, how much data do YOU or your internal decision
makers need to see to feel confident that the change is reliable?  Are your
internal decision makers well grounded in the area of materials and
processes (that would be a first)?
3.    Can you choose the Saber board?  Sure.  But, ask your self if the
technologies represented on that board are representative of your process.
Will you be doing thermal cycling or vibration testing?  Only two patterns
(one BGA and one QFP) are wired for continuity testing.  Other boards have
more patterns.  Debbie Goodwin at Practical Components can talk you through
some other options.  If you do choose the Saber, or any other board, are
the materials representative of what you plan on using.  If they are using
a difunctional FR4 solder mask, will it withstand the higher reflow
temperatures.  Whatever board you get, get the Gerbers and have your board
house make up the board with your intended material set.
4.    Ask yourself if the following materials can take the 20-30C higher
reflow temperatures of the lead free alternatives (bismuth is for sissies):
laminate, solder mask, component adhesives, thermal greases, marking inks.
Does your flux work at 20-30C higher than current?  If so, and you use
cleaning, will your cleaning chemicals handle a possibly tougher cleaning
chore?  If conformal coating, will the surface of your mask change causing
more problems with adhesion?
5.    The number of test vehicles depends on the number of tests you plan
on running.  How is reliability defined in your company?  Jim Maguire, now
with Intel, once pointed out that a sample size of 3 boards equated to an
error rate of 60%.  Consider 10 a minimum for a reasonable error rate.
6.    OK, if it were me in your shoes (hopefully size 10 or my feet would
hurt), I would use the Sabre board or a near equivalent as the test
vehicle, made by one of my approved board shops, for the material screening
tests.  Vibration and thermal cycle would give indications of the
reliability of the solder joint.  Ion chromatography should be used to
benchmark the cleanliness of the board.  Dielectric strength or some other
accelerated electrical test should be used to determine if the dielectric
properties have been degraded by the higher temperatures.
7.    After your materials screening work is done, and you have a candidate
material and process, select one or more pieces of your hardware, build
them and then life test them.  Probably 3 sets:  accelerated humidity with
bias, thermal cycling to failure, and vibration to failure.

Expensive.  Oh yes. Time consuming.  Start now, we only have 4-5 years.
But, no one can do this work for you for your product.

My 2 cents worth.

Doug Pauls
Rockwell Collins



                      "Tempea, Ioan"
                      <itempea@POSITRON        To:       [log in to unmask]
                      .QC.CA>                  cc:
                      Sent by: TechNet         Subject:  [TN] Qualification of a lead-free process
                      <[log in to unmask]>


                      01/09/2004 02:45
                      PM
                      Please respond to
                      "TechNet E-Mail
                      Forum."; Please
                      respond to
                      "Tempea, Ioan"






Hi Netters and Free folks,

I have a good one about lead-fee. It is common knowledge that lead-free
materials and a process have to be qualified. But what would this mean?

OK, I chose the SMTA Saber board as a test vehicle. I approached a few
paste
suppliers and I will probably use the products of 2 or 3 of them to stuff a
certain number of boards. I will get support and tweak the reflow and wave
recipes, let's consider this done. But, what next?

First of all, what quantity of boards would be a good test batch for each
different paste?

Then, I must confess that I am not a reliability guy, so what tests should
I
perform? Cleanliness (is it really necessary?), pull tests, vibration? What
accelerated joint reliability tests to do? Simply go through IPC 9701 and
pick something that matches the application?

Any clue would be largely appreciated.

Thanks,
Ioan

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