Technet A question for bare board gurus. Is there a difference in
reliability between high Tg and regular Tg FR-4 ( IPC-4101/24 vs
IPC-4101/21) in a vehicle type application? The end use application is less
than Tg of /21 material, so not in the glass transition range for either
material.

Typically - is the CTE different between these two? (I expect that depends
on the manufacturer to some degree). If there isn't a high temperature
operating requirement, why select the high temp FR-4?

Why indeed? Using high Tg materials primarily is for one purpose. That is to
extend the board operating temperature range even though the z axis CTE
still is in the range of 50 ppm/C for whatever material. Running TMA or DSC
confirms this as higher Tg materials, as polyimide or BT, turn to dust after
severe delamination - after a prolonged period compared with shorter times
for "normal" epoxy resin systems. Expasion rates are the same. High Tg stuff
is for higher reliability (long term) applications exposed to higher
temperatures for longer times and to fly once into whatever inferno.

However, cure requirements can contradict this. Polyimide, taken to full
cure at an advertised 270 Tg, can have other problems. This is why it should
not be taken to full cure but should be moderated at, say, 250 C in a
modified press cycle. Laminate and foil bond strengths have been proven to
diminish rapidly with the higher Tg requirement.

Everything, again and again, is a compromise. Determine initial quality
requirements, as thermal stress and shock, and long term requirements as
prolonged temperature excursions. In other words, polyimide can be a good
all around choice, but why go the the extra expense when a modified epoxy or
BT will do? And on it goes.

MoonMan

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