Hi Phil,
Y.C. Koo gave you a good answer; however it is somewhat incomplete. Indeed, 
Sn gets depleted from the solder leaving behind a weaker Pb-rich layer; 
however, the depletion of Sn results from the formation of Sn-Ag 
intermetallic compounds, which solder at least have been implicated in solder 
joint failures similar to 'Au-embrittlement', but worse. Thus, even small 
tensile forces on the via, the thermal expansion of the PCB is a likely 
source, may break the connection. Heat transfer is almost eqyally as bad with 
small physical separations and larger voids——and your original purpose of 
heat transfer is defeated.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com 

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