Paul,

I don't think you message was too long either ... it was well worth the
read.

Richard,

While I'd like a time/temp rating on a PCB, practically I think we will
have to
settle for an empirical go/no-go type of coupon.

The problem is that the laminates, prepregs, and coppers can combine
into too many different "things"; different thicknesses, different area,
different
expansion even along the same board. We have boards that call for the
same material and those boards are extremely different in what they will
see when we assembly them - one for instance is all digital while the
other is loaded with electro-mechanical devices that just suck up heat ...

I agree that delamination will be a big problem to deal with - we are
seeing
the same things that you describe. We also have seen a material make
through assembly one time, but not the second build ... if we establish
that
a material is "capable" of a time/temperature then that would seem to
imply that the material will continue to be capable at that
time/temperature
however, the reality is that isn't necessarily so. Which would then seem
to mean that every PCB would need to be tested (batch by batch) for it's
time/temp rating (and we have not as yet established a test to do that),
which would then mean that the time/temp rating is a variable by board
not by material. Whereas I think for components, every component of
that "type" has the same rating so long as the same compounds are
used... I don' think that is going to work for PCBs...

Valerie






"Stadem, Richard" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
12/13/2005 08:01 AM
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Subject
Re: [TN] Qualification of the Pb-free capable manufacturers






Paul,
This is not too long of a message at all. This is an excellent message.
I worked in the industry as a consulting engineer prior to coming to
work at GD. Many of my clients are currently reporting that PWB
delamination is a major problem for them as they go to higher temps to
accomodate Pb-free BGAs and flipchips. Repeatedly, the pwb materials are
rated for Pb-free processing, and the Tg and Td ratings are purportedly
good enough to withstand the higher temps, but the boards delaminate,
even after baking to remove moisture. They are delaminating at
temperatures as low as 230 C, and that is the chamber temperature, not
the board. I brought this up in several postings to the Technet forum
last month. There has been much discussion about it.

There needs to be a time/temperature rating on pwbs for resistance to
delamination, just like there is a moisture classification on
components. The other properties of the materials are meaningless if
they pop in the reflow oven or during hot-gas rework.








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