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October 1998

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Subject:
From:
Evan D Jones <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 2 Oct 1998 16:51:43 +1000
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> Steve,
>  We do a percentage of lot inspection for ISO.  If you are considering
> becoming certified then you will need to do process control which will
> require a need for inspection.  BGA's can only be inspected by x-ray so
how
> else would you do it?
>
>  Linda

Hi Linda!

    How would I do it you ask? Easy, we got this guy workin' for us named
Clark Kent and he...what?....not buyin' that are ya?...didn't think ya
would...hehehe.

Seriously, my logic is that when you're pumping out production boards, you
should have your processes dialed-in.

Production scenario: You've got your stencil tweeked, paste is just the
stuff
you want, and you know you can print 15 times and then program a cleaning
cycle to keep the image crisp. You also know that each print takes 3-grams
of
paste, so you'll program a dispense cycle in every 30 prints to keep a nice
roll of paste in front of the squeegee. You know your placement programs
are
dead on because they came from cad data, you also verified that on your
first
article. Reflow profile was created, plotted, and given a fine adjust for
the
first article, which was followed with the xray inspection. Now you're
gonna
run some revenue.

You've got your people doing periodic, random, process checks and logging
them
after solderpaste print, placement, and reflow, and they're doing a great
job
making sure that happens. So you tell me, when should I fire up the ol'
x-ray
machine again? When would I r-e-a-l-l-y need to if what I described above
was
happening? See my point? Things normally don't all of a sudden go wacko for
three boards and fall back in line again. If you have your process set and
being monitored, I have a hard time coming up with a good reason to justify
x-
ray for anything other than initial process set-up and verification. If you
find something during a production run, in my mind, it's too late. What
would
you do? Stop production and go back and x-ray all the boards that were
built
from the last time a board was
x-rayed?

Don't get me wrong, I'm not saying what you're doing is a waste, but these
are
things that I've been thinking about trying to really justify the expense
of a
x-ray. Will it really be earning it's keep while every other machine is
producing something? Food for thought huh?

-Steve Gregory-

Steve,
      I would have to agree somewhat with you. We have built a couple of
hundred thousand, or more, cards with BGA's (both ceramic and plastic). We
don't use x-ray inspection for two reasons, expense of the x-ray machine,
and question marks over it's ability to reliably detect contact opens.
 We verify our processes by cross-sectioning, on the first few cards. In
production the two sets of parameters that seem to affect repeatability are
paste application and reflow profile. We use a cybercentry in line to
verify that paste has been deposited correctly, and a continuously
monitored reflow oven to control reflow profile. Once optimised these two
mechanisms allow us to maintain paste and reflow conditions within
acceptable limits.

Regards Evan

Evan Jones
Manufacturing/Test Engineer
Bluegum Technology Pty Ltd
PO Box 609, Wangaratta, Australia, 3676
Ph: 61 3 5720 2539     Fax: 61 3 5720 2412
Internet:  [log in to unmask]

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