We've had several boards burn like this after some hours of operation.  The best guess of the cause was 'resin starvation'.  In particular at via locations where unconnected pads had been removed.  Removal of unused internal pads is one of those 'religious' issues, some people always do it, some never do.  I've followed a few online debates over the issue, and after the last one, did some of my own searching and kept the following notes that seem to apply:


http://www.protonique.com/plcom/files/desig.htm

"It is well-known that internal power planes must be connected through thermal 
breaks. It is also good practice to use annular rings (or squares), rather 
than solid discs for the artwork, where no connection is to be made to the 
power plane. This ensures there is a pad on all layers. This ensures a 
constant build-up at all plated-through holes and vias. Particularly where 
there are many layers, the lack of internal layers can physically distort the 
board at the most critical point. Delamination or conductive anodic filaments 
may result from this weakness. On the other hand, it is advisable to leave a 
1 mm gap between the power planes and the edge of the board for a number of 
reasons. This gap should never exceed 2.5 mm, for risk of resin starvation 
and delamination."

Note that this issue is exacerbated by thick copper and thin prepreg.  We also had a couple of boards burn at the corners, where there were no components or vias, but where there were power planes, which seems to support the resin starvation idea.


Gary Crowell
Micron Technology
















> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Douglas O. Pauls
> Sent: Tuesday, April 27, 2004 1:09 PM
> To: [log in to unmask]
> Subject: Re: [TN] Interesting Occurance...
> 
> 
> Steve,
> Was the board lying flat in the chamber during burn in 
> testing?  Was it
> possible that a drop of humidity could have formed on the 
> chamber ceiling
> or walls and gotten blown onto the energized part?  Smoking 
> Hole 2 looks
> like either a nasty dendrite or a crack in the component 
> body.  I have seen
> such failures in a previous life using low voltage high 
> current testing.  I
> began using current limiting resistors shortly thereafter.  
> Nice welding
> demonstration though.
> 
> Doug Pauls
> 
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