All:

A few weeks ago I sent out a question to the community - Why do I have
low resistance isolation between isolated circuits.  The general answer
that came back was  - I have CAF.  Fast forward a couple weeks and I
have more information - I don't think I have a CAF issue - I may have
invented something new.

Quick description: 14 layer, polyimide material.  Single lamination
cycle (no blind/buried vias), .093" thick.  We have identified the
shorts to be created between layers 7/8.  I have gone as far as milling
down through layers 1 - 7 and evaluated the entire board.  There are no
anomalies until we get to the dielectric layer between 7/8.  There are
cracks/delaminations between holes in the BGA socket area but nowhere
else on the board.

Pictures of what we are seeing are below.

The first horizontal section shows cracks/delamination - the resistance
measured 150 ohms (or so), That coupon was sent out for analysis.  We
received back some SEM photos with an explanation that the copper plated
from one hole to the other.  One hole acted as a cathode, the other an
anode and the moisture in the board was the carrier. We in essence
created a mini plating cell.  In these pictures you'll see copper
plating actually plating around the fiberglass bundles.  The voltage on
this device is 3.3 volts.  The shorts showed up in hours of operation
(maybe up to 20- 40 hours).  The boards were not baked prior to
assembly.  The boards do get temperature cycled (-53 - +54 C) but this
short will appear with just 0 or 1 cycle.  (I have 4 boards with no
thermal cycles other than assembly).

Any suggestions on what I actually have?

Thanks,
Pete




Horizontal Cross section
 <<Picture (Device Independent Bitmap)>> 

You can see three areas with delamination - all three areas have reduced
resistance.

We had this section evaluated with SEM/EDS which revealed:

 <<Picture (Device Independent Bitmap)>> 

Higher magnification of the same location:
 <<Picture (Device Independent Bitmap)>> 

The light areas are identified as copper.

 <<Picture (Device Independent Bitmap)>> 
New sample - horzonatal view of copper connecting holes.

Pete Menuez 
Supplier Quality Engineering Manager 
L-3 Communications Cincinnati Electronics 
7500 Innovation Way 
Mason, Ohio 45040 
[log in to unmask] 
513-573-6401 Voice 
513-573-6767 Fax