Steve,

Some more board and polyimide stuff (from the old days):

1) Critical press cycle thermal rise time between 190 and 280 F. must be
about 3 degrees per second.

2) Other type polyimide resin systems now available may provide easier press
cycles and different Tg's so maximum avoided, etc.

3) Drill flute angles ok as you stated but critical feeds and speeds
required to match, or holes punched or burned through board.

4) If holes punched (feeds too fast and speeds too slow), gouging and other
rough conditions contributing to poor
plating as folds, nodules, entrapped chemical residues, voiding, etc.  Also,
don't forget drill spindle runout (unlikely with modern high speed air
bearing spindles and good PM) could be culprit.

5) If burned (speeds too fast, feeds too slow), excess resin smear effected
requiring more etch back time
effecting more exposure of glass fiber bundles in hole walls requiring
ammonium bi-fluoride removal (always required with etch back). If not,
bundles become conduits for moisture exit as they protrude into hole plating
also reducing it and constituting voiding, etc.

6) Ensure relamination oxide treatment red (couprous), not black (cupric) as
the latter will not effect sufficient inner laminar bond and "pink ring"
occurs more readily also allowing delamination near all internal pads (this
stuff is "mush" and is vehicle for chemical entrapment)

7) Trace and other MIL qualification type labs very valuable as they have
seen it all - if "old guard" personnel handed down information based on old
MIL Specifications (55110, etc.).

8) Ensure board shop capable as many newer shops jumping into
"advanced/exotic" material fray for marketing fluff.

All I can remember now and sorry I didn't provide it last night but time ran
out.

Go get 'em "board man",

MoonMan


----- Original Message -----
From: Stephen R. Gregory <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, September 20, 1999 5:46 PM
Subject: [TN] Internal Fab Shorts...a follow-up...


> 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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