Hi Glenn,

We have explored the Hole fill problem here at TI for a number of years with
the constrained core boards that we build here. We tried to hole fill boards
with solder prior to the thermal plane bonding and always, I mean always, had
problems with the solder erupting out of the holes. We finally went with a
polymer hole fill ( either thermally conductive or not ). The polymer is cured
prior to the thermal plane bonding. We also tried to hole fill with soldermask,
but always had problems with incomplete fill with all the masks on the market.
We also found, through thermal cycling, that an incomplete fill impacts long
term reliability

Wish You luck,

Les Connally,

[log in to unmask]

>  From: [log in to unmask], on 8/1/97 10:45 AM:
>  Thanks everybody for your input.  Let me throw out some more information and
>  see what you think.
>
>  The current process is direct die attach to a heatsink through a cutout in
>  the
>  PCB.  We want to save some process steps without losing the thermal
>  dissipation.  So, we looked at thermal vias.  But the problem we ran into
>  was
>  amount of metal to pull the heat away and the requirement to attach the
>  heatsink first.  By attaching the heatsink first and applying conductive
>  epoxy
>  for die mount, the entrapped air blows holes in the epoxy and displaces the
>  die.  We didn't want to tent the die side with soldermask since we also need
>  electrical contact.  That's when we looked at some sort of fill method.
>
>  Anyway, more info to think about, but you all have provided enough info to
>  keep me going for a while.  Thanks again.
>
>  Glenn Pelkey
>  Quality/Reliability Engineer
>  Maxtek Components Corp.
>
>