Steve,

Your posting impressed me and I'm really an old fart when it comes,
especially to polyimide. Great work and information. Also, nice to have a
lab handy and quality conformance test circuitry with totally representative
coupons for quality verification, or not.

One serious additional note. No matter the polyimide, ensure the supplier
does not go full tilt boogie with cure. Don't seek the highest Tg possible
with this stuff. Go for less as about 250 C and be sure the shop verifies
this using DSC or TMA - usually done at the material supplier site.

Full cure renders polyimide near its useful life, for lack of better terms.
It becomes extremely hard and brittle. With additional thermal excursions,
as in assembly and during operational performance, the bond strength (inner
laminar and pad) drops off rapidly.

Nice job,

Earl


----- 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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