I have a question for the forum:

We have an engineer who will be using an MCM on his PCB, and he has said
that the CTE is 11-12 ppm/C (I have no other information as yet on this
MCM). He believes that because of this he needs to use a material with a
closer x,y CTE. There is a material where the specs give the x,y CTE as up
to 14, which is the material we would typically use; and there is an
alternate material where the specs say the max CTE is 13. Of course the
alternate material costs more.

First, is there any practical information on matching MCM CTE? Would a 1
ppm per deg C make a difference?

Second, I was under the impression that to match CTE we would only have to
change the top layer, where the joint between the device and the board is
formed. He believes that we would need to change all the cores and
prepregs in the stackup. This will be somewhere over 24 layers (26, 28 ,
30?? don't know just yet) so the cost delta for the material will be
magnified if the entire stackup is converted.

The alternative material which is proposed is not Thermount; but I did go
out to Dupont's web pages and read all their papers on CTE matching. I
sent those to the Engineer but it wasn't enough to convince him. Does
someone have any alternative links to papers detailing real life projects
for CTE matching and what they did and what was achieved, and so forth?

Is there a board fabber on distribution with a lot of background/customer
demand for this who would care to comment on my two questions?

Thank you all,

Valerie

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