Ahhhh, Monday, Monday...can't trust that day...'member that song? (OOPS, I'm
dating myself!) Oh well, I'm a ol' fart and I know it...hehehe.

Just spent the better part of the afternoon and well, early evening with the
internal short problem I asked about a week ago. This was a meeting between
the fab vendor, our customer, and us.

Sent one of the suspect assemblies to Trace labs for a few microsections and
they showed fracturing within the laminate at just about every large diameter
plated through hole of the terminal blocks that are installed on the
assembly. Plating chemicals were trapped within the fractures in the
laminate, and upon power-up the assembly shorted within just a few hours
after voltage was applied.

A number of key points were brought-up that were deemed contributors to this
problem, and I wanted to share them with ya'll so maybe you won't wind-up in
the same creek that we're in now:

The laminate that was used is a high Tg GIL polyimid, very brittle and
difficult to drill and/or machine. The fab vendor is going to use a GIJ on a
re-run of these boards which was stated is easier to drill.

The angle of the cutting flutes at the drill bit tip changes from smaller
diameters to larger diameters. This is standard with the drill bits used in
the industry. On smaller holes I believe they're 65-degrees, but on the
larger diameters I believe the angle that was stated is 145-degrees. Which
contributes to the crushing effect that drilling larger holes experience,
especially in hard, brittle resin systems.

The holes in question had the un-used pads removed in the artwork of the
inner layers. If they were left intact, they may have provided some
additional support in the barrels during drilling to prevent the fracturing,
as the fractures appeared to have originated from where the pads were removed.

The test coupon did not have a full representation of all the hole diameters
that were drilled into the fab...only the smaller diameters where there
wasn't an issue, mainly because of the different drill bit design of smaller
diameter bits. So the issue didn't show itself in the microsections done on
the coupons by the fab house.

Hey Earl! Maybe you're right...I might become a PC Fab guy after all! Boy
though, this sure is a tough way to learn about this stuff!

-Steve Gregory-

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