We are working on a laminate package product in which we wish to embed Alloy 42 or Kovar layers. We also need to make PTH contact to these layers. Due to some design constraints, we must be able to make to contacts using electroless, direct metalization, or some other comparable process. That is we cannot directly electroplate because some features of the buried Fe/Ni alloy are electrically isolated prior to metalization.

These PTHs also need to be somewhat high aspect ration, at least 8:1 and desirably more than 12:1. Fortunately, the hole sizes are fairly large though, i.e. 0.015" to 0.030". We have though seen some difficulty in drilling in that there seems to be excessive "nail heading" and sometimes the bits seem to "wander" excessively from the top to the bottom of these boards which are relatively thick also, i.e. 0.140" to 0.250".

I would very much appreciate any perspective and/or advice on such PWB structures.

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