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July 2003

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Subject:
From:
Robert Tarzwell <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 11 Jul 2003 08:03:56 -0500
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The best test method I have found for a multiple temp  cycle test is to
design a small coupon the goes on the thief of your board, we made it 10-20
holes in series and would use each drill bit a couple of times. the coupon
was cut out and  using a stanford 4 wire resistance measuring unit the
coupon was placed in a cycle oven and the resistance measured. allowing time
to stabilze between each temp rise.If the resistance increases by as little
as 5 % after or during a cycle you have a crack starting. this saves cross
sectioning and the fact that a cross section only looks at two very small
spots aroung the hole. A far as what you can do is a big question, usualy
its copper ductility or not enough copper in the hole, but as mentioned
thicker boards with stange designs can be a problem. my research has shown
you can all ways add more copper to the hole ( 2 to 3 oz) and it always gets
you through the tests. In fact the tests I have done would convinse me if I
were ever to start up a PCB shop again, the standard board would start with
1/4 oz copper and I would plate 2 oz . it gives you better fine line control
, less polution from etching ( less etch) and better reliabilty. Just to
confuse the issue.
robert Tarzwell    megadawn.com

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