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Subject: Filleting of via pads
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WE currently use vias with pads of .024" to .028" and holes of .014" to
.018".  
The spec on the holes is +0"/-closure.  There is no filleting allowed
presently, 
but we to permit 90 degrees of breakout(not at the conductor junction).

My questions are as follows:

1)	What are the risks of the present specification?
2.)	What are the risks and benefits of filleting the via pads?  Can any 	
electrical problems arise from this?  If so, what?
3.)	If you recommend filleting the via pads, do you do it in house or do you

	have your supplier add the fillets?  What do you recommend for existing 
	designs where there is a cost to revise the artwork?

Tim Fowler
BENTLY NEVADA CORPORATION
======== Fwd by: Tony King / N ========
Hi Tim,

I am not sure of your definition for fillets, but I believe this is "tear
dropped" pads. I would recommend the addition of tear drops to pad to
improve electrical integrity when the pad to hole is too tight to guarantee
annular ring. There is no other way to prevent breakout at the egress. We
add the tear drops in-house using a new CAM system by Valor called Genesis
2000.  The Genesis uses intelligence in application of the tear drop to
insure against possible shorts.  A tear drop in Genesis has several
components which allows the possibility to apply a tear drop to one side of
the attached trace while not the other side if an interference would occur,
basically half a tear drop or fillet. All this and in an automated mode !

Your specification is not bad, though  tight designs could have problems. It
is a good specification to allow the +0/-hole size tolerance to allow the
manufacturer to choose the minimum hole size best suited for his process.
Breakout will help eliminate the problem of too little pad, but an ever
increasing problem is the reduction of spacing in new designs. If product
spacing is designed to 0.004"-0.005", shorting could occur.  An important
factor to consider when designing product to a breakout specification is the
plated through hole to copper distance. Most board manufacturers prefer to
release a minimum 0.007" annular ring for tangency (pad is 0.014" larger
than drilled hole) at a minimum spacing of 0.004". A good value to use for
minimum plated hole to copper would be 0.010". If your spacing pad to line
is 0.004" there is a potential with your design of 0.014" hole into a 0.024"
pad to reduce spacing to 0.002" or less.


Tony King
Elexsys International
Nashua N.H.
603-886-0066