Hi Valerie,
x,y-TE is below Tg, so it is low. Almost all FR4s with woven glass are
somewhere between 10 and 14; and that number would also change in practice
when one moves from a raw laminate core to a cured multilayer board.
EW: Well, those numbers do not make sense--where do 10 to 14 come from. I
have never seen values this low-they typically are 16+/-2 ppm/C. When I
had one of these materials measured, the values turned out to be
CTE(x,<Tg)=21.4 and CTE(y,<Tg)=16.0. Can you imagine the incorrect Design
for Reliability for the SMT solder joints based on CTE's of 11 and 13?
Valerie: This answer will have to go on a little bit: to get equivalent
results you would need to be measuring the same thickness material, with
the same glass style, with the same percentage resin. The TM 2.4.41 states
that the test specimen shall be between 0.05 and 0.30 inches !! Further,
it may be as received material or it may be laminated from b-stage. For
the test, the upper temperature is "the temperature of interest", and I
think we would both agree that it matters what that temperature is. Also,
the population is three specimens from the same piece of material for each
direction....
Poly 370Hr is 14,16; IS 408 is 13,13; IS410 is 11,13; MEM R1755 is 10,14;
Nelco 4000-11 is 12,14 .... The value of 16 +/-2 ppm/C is what we use for
multilayers which combine epoxy, fiberglass and copper into a composite
CTE; copper being usually quoted as 17 ppm/C.
I agree that using 11 or 13 or something as input into a formula to
determine Design for Reliability with certain device packages would be a
problem. Which is why nobody pays much attention to it anyway and uses 16
or 17 (anyone I have talked to about this, at any rate).
EW: Somewhere here, there is a disconnect. Either we do not properly specify
how testing is to be done... or what? Actually, for CTE(x,y), the T to which
you test does not matter much, since it is the glass cloths that dominates
everywhere. In fact, because of the drastic decrease in the modulus, I have seen
the CTE above Tg go down in the x/y.
What I had tested was actually laminated PCB coupons. How there could be a
legitimate difference from 11-13 from the data sheet and 16.0 and 21.4 actually
measured in x and y puzzles the heck out of me.
Werner
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