Hi All
Werner is correct on all counts.
We have known about this for many years and have compensated for the
foil thickness issue by using foils weights as they typically come in on
PWBs. How do we know this? We know by the HUGH accumulation of
mechanical data via microsections from our lab.
In addition, our internal research regarding PTH interconnect
reliability strongly indicated that the flexural fatigue ductility of
the foil (as well as plated PTH copper "films") stood out as the one
true parameter that explained low cycle fatigue failures in foils and
PTHs. The fatigue ductility values obtained from our research are used
in our FEA models for PTH reliability simulation. With this data, the
"modeled" results more closely match real and tested data.....never
exact but very close.
Regards
Gerry
-----Original Message-----
From: Engelmaier [SMTP:[log in to unmask]]
Sent: Thursday, March 26, 1998 10:50 AM
To: [log in to unmask]
Subject: Re: [TN] Copper foil thickness
Hi Hillel,
The problem with copper foil that you mention is nothing new. The cost
of
producing copper foil is in the copper weight provided to the customer
as well
as the electrical energy required for the plating; with some additional
cost
for the other plating chemicals consumed as well. A 5 to 10% thinner
(lighter)
foil translates directly into a 5 to 10% higher production capacity as
well as
an increased profit margin. In most cases, copper foil is, however,
bought by
weight. Thus, the direct customers of foil manufacturers, the
laminators, not
only get what they pay for, but get more foil in terms of area received,
increasing their profit margin as well. The real users of foils, the
customers
of the laminators, however, get a thinner foil on the laminates they
buy.
However, not all foil manufacturers are the same, and measuring actual
foil
thickness is not a totally straightforward process (that is why weight
is
specified) (see IPC-TR-484, Results of IPC Copper Foil Ductility Round
Robin
Study).
But thickness of plated copper foil is only a relatively minor problem.
When
ANSI/IPC-MF-150F, issued in October 1991, was produced, I was the
chairman of
the copper foil committee. I and the few other users of copper foil-not
counting the laminators, who are part of the problem-tried very hard to
have a
specification that would give the users some protection. Our efforts
were
unsuccessful, since we were unable to get sufficient user participation
in
the committee to balance the overwhelming presence of the copper foil
manufacturers on this committee. I resigned the chairmanship in protest
prior
to the completion of ANSI/IPC-MF-150F; one can play Don Quixote only so
long.
The foil thickness (weight) issue is, however, only a small part of the
issue-and at least it is consistent for a given foil manufacturer. Thus,
the
user can adjust the current carrying trace cross-section by designing
wider
traces. For a real problem issue, take a look at the Elongation values
for all
CU-E (plated) foils. The foil manufacturers labeled Fatigue Ductility,
which
is a much better measure than Elongation for the ductility of foils for
which
elongation is a notoriously bad measure, "NA" (Not Applicable). How can
a real
ductility measure of an ASTM test method specifically developed for
foils be
"Not Applicable"? The minimum Elongation values listed are for the most
used
foils in the 2 to 3% range. Most of the foil produced has significantly
higher
elongation values.
The foil product getting close to the specification minimums are in the
2 to
3-sigma tail of the production. Thus, it does not happen very often and
may
not be used in a product that runs into trouble at those values. Low
elongation (ductility) values like that are produced when the plating
chemistry goes off-balance. Not only will the ductility be lower, but
also the
tensile strength and perhaps also the density, the etch rate will be
higher as
will be the electrical resistivity.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]
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