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October 2001

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Subject:
From:
Hinners Hans M Civ WRALC/LUGE <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 25 Oct 2001 11:44:00 -0400
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Good Morning All!

I think I've got a case of folks comparing apples to bananas.

I'm trying to predict % defects due to inner layer shorts and compare across
part numbers.  Folks are looking at one part number and saying, "12 layer
board with 7% inner layer shorts (of the ones tested) and looking at another
part number - 24 layer board has 18% inner layer shorts.  You are screwing
up more on the big $$$ board - fix it!"  Shouldn't inner layer shorts be a
linear or geometric relationship?

For example, take a 10 layer board, it has 5 signal layers (50-50 signal &
plane).  Each signal layer will have a certain surface area where traces are
packed close enough together that a sliver of metal could cause a short.  At
electrical test I fail a certain percentage of boards due to shorts - say
10%.  Now take a 20 layer board and say it doubles the surface area that
could have a short.  The failures due to inner layer shorts should be double
the 10 layer board or 20%.  If the boards were manufactured at the same time
the process should, on average, contribute the same number of shorts/area to
each.

Inner layer short defects come from multiple sources that produces a finely
boned fish - metallic contamination, poor IL etching, poor IL
imaging/development, missed in IL AOI, equipment, material, method, people -
ad nausem. . . And for now I'm ignoring test escapes.  The inner layer short
generation fluctuates over time but has some average value per area.  So I
propose a simple equation:

Inner Layer Defects = (average # inner layer shorts/surface area) (total
critical surface area)

Looking at the artwork should get me the critical surface area, the inner
layer defects we know from Electrical Test so a graph of defects vs. surface
area should get me the shorts/area for the total process, no?  Then I can
say when the process has changed versus merely a higher tech board being
processed.

Anybody hit this wall before?  Is there a flaw in my logic?

Hans

PS.  If memory serves, I remember Bev mentioning Technetters' lack of gender
identification skills at that "Stump the Chumps" session we had a few years
ago.  And I can relate - every once in a while people see the middle name
(Michel) and think "must be a groovy chick".

Integrity First  -  Service Before Self  -  Excellence in All We Do
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Hans M. Hinners
Electronics Engineer
Warner Robins - Air Logistics Center (WR-ALC/LUGE)
Special Operations Forces System Program Office (SOF - SPO)
Gunship Team
226 Cochran Street
Robins AFB GA 31098-1622

mailto:[log in to unmask]

Com: (478) 926 - 5224
Fax:   (478) 926 - 4911
DSN Prefix: 468

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