I'm on a roll today ...
we have also seen laminates with cohesive failure ... sure, the prepreg
and or the
prepreg to copper bond are the weaker links, but you could in fact see
failure in
the core even when fully cured.
My issue is with the laminators, the resin suppliers, the glass weavers,
the glass
makers, and oh, lets throw in the copper folks too.... that whole supply
chain is going
to have to work together like they never have had to before or we will not
get LF
capable laminate "systems" (which include all the individual pieces).
The finish on the woven fiberglass has turned out to be a very important
component
of a reliable laminate; it is a key to the resin to glass adhesion ... and
there are other
areas for more investigation which would ultimately increase the
reliability of the product.
All of those players are "holding their cards close to their chests" and
even if they
don't want to spill their recipes to the OEMS, they had better be forming
working
groups to optimize their end product which is the entire "system".
The PCB fabricators aren't straying from the guidance for full cure of the
multilayer
board. They have so much value-added labor and process consumables in that
final
board that they do not want to risk making boards that delaminate. They
will nail
down a lamination recipe for a particular multilayer based on the laminate
system,
the thickness, and so forth and once they nail it down they will lock it
up. The press
will use the same lamination recipe every single time that same PCB goes
into
the press.
I have already had a similar discussion with a Laminator who wanted to
know before
hand if our application (meaning PCB) was going to be LF processed. My
reply is
that "I should not have to tell you what I am doing with the laminate if
it falls within
your stated laminate's capabilities. You have a laminate which is heavily
advertised
as Lead Free Compatible or Capable and if I intend to use it in a
lead-free assembly
profile I SHOULD NOT have to let you know first!" Well, the same thing
applies to
the fabricator: we are probably going to add some kind of symbol and
terminology
to new designs, but we are already in trouble with some components only
coming
in with SAC balls when we have no desire or other need to go LF right now.
IF we
order a board and we call for a laminate which is one of the newer
laminates that
touts itself as LF capable - the supplier better assume that the board has
a high
chance of being LF reflow profile assembled - and quote it accordingly if
in fact
there is a price difference.
Valerie
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Tuesday, December 13, 2005 10:11 AM
To: [log in to unmask]
Subject: Re: [TN] Qualification of the Pb-free capable manufacturers
Hi Susan,
The degree of cure is certainly a concern. I am pretty certain that the
laminates are fully cured--to what degree the prepreg layers are depends
on a lot of variables, but they are hardly ever cured to the same degree
as the laminates.
So my issue is not so much with the laminators, but with the resin
suppliers [sometimes of course, they are the same]. The resin/prepreg
suppliers need to give valid data for the materials they supplied, when
they are fully cured.
The PWB fab houses need to make sure that their PWBs are in the
laminating presses long enough and at sufficient temperature to achieve
the necessary levels of cure--what it will take will vary with resin,
glass-cloths style, lay-up, etc.
As to your question: "when will a fabricator know that the board they
make is to be assembled in a LF process?"-- the answer is that his
customer needs to tell him. For LF-PWBs, the customer had better spec'ed
the PWBs properly in the procurement RFQ, in terms materials, plating
thickness, drill hits, etc.
Unless this is done it will be quoted for SnPb-soldering capability,
which for higher end PWBs is inadequate for LF-soldering.
Werner
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