Jeff,

I agree with most all you say. Just can't buy the thought of not considering
who and who cannot work with what materials. To ignore this is violating one
 of many important rules of DFM/CE. The very high speed requirements, as in
Jim Marsico's pending designs, certainly demand high performance, exotic
materials. Not having a highly qualified supplier puts these requirements in
great jeopardy. Of course, this should not be the case for Jim - should it Jim?

I certainly agree with your technical assessment of material performance
properties. Some other physical properties simply add to the extent of the
compromise.

I started working with Arlon's CLTE material, I belive in 1997, before
Rogers had its own treaters and presses to make this type prepreg and core
material (though they own the patents for it), respectively, and had mixed
results. It must be said, that in the begining, there were but two or three
shops capable of successfully processing CLTE. It must also be said that
material was hard to come by and deliveries were slow.

Now, Rogers is making its prepreg and core as well, so deliveries and
material consistency is much better though the glass styles still (typically
106, or there abouts, glass styles) can cause issues especially when thicker
layer structures are needed. However, most high speed dielectric thicknesses
are in the 4 to 6 mil range so this is not such a big problem. Also, there
are fab shops more capable of managing their processes properly and ensuring
some initial "defects" I found do not occur. Initial defects, and currently
potential types, consisted of very low foil and innerlaminar bond strengths
as well as some a little wierd looking like "mush" on the surface layers.

In agreeing with most all you say, part of that means compromise always is a
factor in any design. The compromises here are, as always, design rules,
materials selections based on design need, fabrication process capabilities,
and assembly process capabilities.

What ultimately must come out of all this is quality, and reliability - over
a long performance period, meeting clearly specified contract requirements.
Other factors must enter the equation as well and, of course, are
availability, delivery, and cost.

DFM/CE, when properly practiced, ensures all this is taken into
consideration. Many other issues must be considered at the design level as
well. Plated holes, blind vias, buried vias, micro vias and the impact they
have on the critical, high speed equation and, again, reliabilty together
with specified performance. All this, and so much more, is part of the
designer's realm while working concurrently with everyone else on the DFM/CE
team as board fab and assembly process and manufacturing engineers
notwithstanding component engineers plus many others.

There's so much more for another discussion, but you say it well,

Earl

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