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July 1997

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Subject:
From:
Greg Bartlett <[log in to unmask]>
Reply To:
Greg Bartlett <[log in to unmask]>
Date:
01 Aug 97 09:32:03 +0000
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I agree that copper foil tensile strength and elongation values as
derived from methods in IPC TM 650 should not be used to estimate reliability. 
They should serve only as a process control tool, and even then I wouldn't
place too much confidence in them.  Tensile tests on such fragile foils
(which aren't very representative of PTH plating, as has been pointed out)
are extremely tricky to do and have many error sources that may be not be
entirely obvious.  Jaw type, tightening, foil alignment, foil creases, and
edge imperfections are some of the factors which can affect strength
values.  In addition, the inability to attach an extensometer to the foil
means that you're going to have to rely on the crosshead displacement readout,
which can differ markedly from actual elongation.   (Note:  this is NOT
to say that the folks who have written about their test experiences didn't
do a proper job.  I used to perform these tests too many years ago and can
appreciate the difficulty with them.  My point is that these tests were
derived from those used on "bulk" tensile samples which are much easier to
work with and have fewer error sources.)

Too often we confuse "numbers" that a machine spots out with actual
"data".  Reliance on such numbers can be dangerous, particularly if you don't
have a feeling for statistical significance, error sources, data scatter,
test relevance, etc.  I've heard of board shops who have tensile tests
performed routinely on a few copper strips.  They get back numbers of 30+% in
some cases and (in my opinion) get a false sense of security.  ("We don't
need to plate as much copper in the barrels because we have a 'Super
Copper'....")

Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
[log in to unmask]
-----
Hintpwb1 wrote:
>Ted 
>
>You will note that in IPC-6012 that the copper purity, tensile strenght,
>elongation has been moved to the Special Requirements-(when specified)
>section in Table 4-3.  If you specify, it is hoped that you put in you
own
>numbers.  The 6% was based on pyro copper and the 36 KSI was based on
acid
>copper.  Neither probably have any relevency to good/bad boards.  The
pyro
>should have a KSI of about 55 KSI and the acid copper should have a a
15%
>elongation as a miimum.  The task group moved the requirement to the
special
>requirements because it is a internal process control requirement and
should
>not be used as a conformance requirement.  Additionally, since ithe test
>requires a surface plating of 2 mils, it is probably not very
representative
>of the hole plating at 1 mil.  I encourage users to apply their own
numbers
>if they want to specify it, and I encourage suppliers to set up there
own
>standards to be used as a an internal process control on their copper
bath as
>we do with Hull Cell, CVS, grain size, Knoop hardness, TOC, copper
content,
>acid content etc.  This test belongs in the process control area and not
a
>conformance requirement.
>
>Phil Hinton   
>
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