Material Witness
by: Chet Guiles
February 2010
Slash Sheet Chaos:
Is What You See, What You Get?
With the demise some years ago of MIL-S-13949
during
the
“commercialization”
of
military
procurement, when then Secretary of Defense William
Perry instituted moving to “best commercial practice,”
our industry lost something that a few of us were
concerned about then, and that maybe still should be
cause for some concern. I think most OEMs were
disturbed to discover that while IPC 4101 was taken
more or less intact from MIL-S-13949, there is now no
enforcement or validation of certifications to IPC-4101
slash sheets, and changing materials based purely on
slash sheets can be hazardous when considering a
new supplier or unproven product. Moreover, periodic
audits by an independent auditor (DESC) gave a sense
of quality system stability, and the annual required
testing and data submission on each material
approved to the QPL for MIL-S-13949 provided an
ongoing assurance of product performance.
My old friend Dick Grannells (of Hamilton Standard
in Connecticut) and myself had a number of long
discussions about the “Perryization” of laminate and
prepreg specifications, and we both would have
preferred to have retained the MIL-S-13949/DESC
audited approach. Perhaps that’s a bit of “good old
days” nostalgia on the part of an “Alz-Timer,” but in the
real globalized world of the 21st century, I wonder if
the risk that you won’t get what you think you ordered
is increasing.
It is possible to take a lot of liberty with interpreting
slash sheets, sometimes ignoring the header definition
on chemistry or filler. For example, it has been known
for suppliers to certify the same polyimide product to
IPC-4101 /40, /41, and /42, convenient when a
customer orders solely by slash sheet designator and
a supplier has only one product, but potentially
dangerous since /42 specifications are mutually
exclusive of /40 & /41 by definition since the properties
of, for instance, blended polyimides are not the same
as pure polyimides. How certain can you be when
you change suppliers based on price that the product
you are getting is the same kind of polyimide you
have been using, and not some sort of blended,
filled or otherwise modified variant with compromised
properties?
Blended materials do not necessarily copolymerize to
form a homogeneous final product. Work done some
years ago suggests that during thermal cycling, it is
possible for discrete domains of the two blend
components to develop cracks, almost like “grain
boundary” failure in metals, which can weaken the
physical and deteriorate the electrical properties of the
composite. The presence of fillers, used to reduce
cost or to “extend” the resin system, can also affect or
enhance properties. These differences make it critical
to understand not only the industry specifications, but
the details behind the materials being qualified. If your
www.arlon-med.com
supplier is not willing or able to tell you how their
product differs from the “standard” polyimide you are
using, it may very well not be.
Luckily, it is possible to evaluate some of these things
by doing some basic analysis. In fact it wouldn’t be a
bad idea to use the Ronald Reagan US/Soviet
mutual disarmament principle: “Trust, but Verify.” IR
Spectrophotometry, for instance, can detect the
“fingerprint” of epoxy reactive groups in polyimide
prepreg.
Dynamic Mechanical Analysis can
determine whether the fully cured resin system is
mechanically significantly different from a control.
Thermo-Mechanical Analysis (TMA) may be able to
detect dual Tg’s in a cured two component product.
That doesn’t even begin to touch on the confusion
about what is “FR-4”! “Families” of FR-4 products can
be cross-certified to a variety of slash sheets, and
ordering “FR-4” without getting very specific could be
an interesting exercise in chaos. In the end, a slash
sheet and material specification is a baseline tool that
we need to have, but the principle of “Caveat Emptor”
(“Let the Buyer Beware”) is especially important today
with the plethora of materials all being certified
to a somewhat confusing panoply of pro-forma
“specifications” that never seem to quite specify
enough. With over 50 IPC slash sheets, and the
complications arising from what is “Lead-Free” and
what is “Green” further multiplying the number of
possible slash sheets by which “FR-4” products can
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be designated, designers, manufacturers and material
suppliers all need to be talking the same language
when materials are specified and purchased. All this of
course begs the question, “Do we really need all these
FR-4 variants?”
Do your homework! It has been said that there is
always someone willing to sell a product that is
cheaper, and those who buy only on price are that
man’s legitimate prey. Don’t be a victim of slash sheet
confusion. Ask for and expect your suppliers to
provide clean and real differentiation between their
products. Make sure when you buy a lower cost
“Alternative” that it is really the same thing you’ve been
using. It is possible for legitimate competitors to have
different cost/price structures for truly equivalent
products – just make sure that’s what you’re looking
at. Don’t find out after a few thousand thermal cycles
that /40, /41 and /42 (with and without filler!) really are
different products and perform differently in
service. If there are good reasons for differentiated
designations, make sure you are not being deceived
by the data sheet!
Does anybody have suggestions about how to avoid
these issues that you want to share with the industry?
Please send me a note with your thoughts.
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