Hi Dean and everybody,

Sounds like you have a problem (well duh, Hans).

What's your minimum allowed dielectric thickness?

Checked with our Design Engineers and we can't go below 3.5 mils according
to the Mil Spec (55110F).

Our fix for was to replace one sheet of 1080 with one sheet of 2113 and
optimize the press cycle.

150 F and 50% RH doesn't sound too extreme.  What's the base laminate FR-4?
T sub g?

Let me know if you want to talk to our gurus.

Best of Luck!

Hans

~~~~~~~~~~~~~~~~~~~
Hans M. Hinners
Materials (Process & Manufacturing) Engineer
Warner Robins Air Logistics Center
Avionics Production Division
Manufacturing Branch
380 Second Street, Suite 104
Building: 640, Mail Stop: LYPME
Robins AFB, GA 31098-1638
Voice: (912) 926 - 1970 Fax: (912) 926 - 7164
mailto:[log in to unmask]
http://www.robins.af.mil



-----Original Message-----
From: May William D (Dean) CNIN [mailto:[log in to unmask]]
Sent: Monday, May 15, 2000 9:22 AM
To: [log in to unmask]
Subject: [TN] (FAB) Shorts Through Thin Epoxy Dielectric at 150V


Interlayer shorts developed in modules subjected to elevated humidity,
temperature, voltage testing.  The shorts go through a 2 - 2.5 mils thick
epoxy glass dielectric (2 plies 1080, expectations were that the dielectric
would be closer to 4 mils thick).  Test conditions were 150V (DC), 150*F,
50%RH.  Time to failure varied from 130 to 1011 hours.  Current was limited
to 1mA.  Failure was defined as (leakage) current above 0.5mA.  The shorts
go nearly straight through the epoxy (not tree-like dendrites and do not
pass-through glass bundles), usually from a pad on one internal layer to the
solid plane on the adjacent internal layer.  Voids were observed in the
dielectric layer, but none were noted at the failure sites.  SEM analysis
did not identify any contamination.

Questions
1.  Are these "real" dielectric breakdown failures or have we induced some
odd (irrelevant) failure mode?

2.  Search of TechNet archives uncovered a 1998 (R Hersey) post stating that
dielectric strength should be derated to 50 V/mil for long-term reliability
of "thin" dielectrics.  Our results seem to support this post.  Is there
other data available to support (or refute) the 50 V/mil derating?

Thanks,
Wm. Dean May
NSWC Crane
Bldg. 38, Code 8086
300 Highway 361
Crane, IN 47522
812-854-3073

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