It is both for assembly and performance when considering some past
applications and studies. The EMPF used to be part, or all, of the Navy labs
in Indy.

I and others were involved with building some of the first digital engine
controls for Pratt & Whitney evolutionary engine technologh. Some planes and
pilots were lost when first launched with the product.

I can't remember everything, as I'm sure many of you can, but the FA
findings came down to, basically, one thing. Polyimide MLB's had a problem
because they were being cured/burned/cooked in the press too long trying to
reach their maximum Tg of 270 C.

It also was found that the initial foil and innerlaminar bond strength were
reduced siginificantly over short periods at that Tg. Foil bond strength, as
an example, was reduced from about 7 pounds to less than 1. Shocking enough
unto itself but consider all the other 80's type historical facts concerning
polyimide wiring insulation and its problems associated with fly by wire
technology early on the the F-16 prgram where, again, several planes and
pilots were lost.

For me, though not a normal personal trait, moderation is the key to success
in taking any design to successful verification and validation. Lowering the
final cured Tg of polyimide, to about 250 C, enhances both its foil and
innerlaminar bond strengths. The reason, to put it simply, is reduced
oxidation of all materials in the MLB structure.

In my most recent endeavors, as well as many others, a BT/epoxy blend,
providing a "good" all around Tg is the material of choice. This is
evidenced by my work, with many others, concerning very high layer count,
very complex MLB's in excess of 50 layers with all manner of blind, buried,
and micro vias.

Still, as having before said, there is nothing like polyimide in an open
press like some of the old PSI version. Again, to specify high Tg materials
for high Tg's sake, without taking into critical consiserations and plating
thickness and ductility, is ludicrous.

Earl Moon

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