Hi Pete, 
You got some good answers to your question--but they are not differentiated 
enough.
First, from purely a processing point of view, you would like to have the 
least number of pads. For run-of-the-mill PCBs I would go with processing.
Second, from a reliability perspective, you need to differentiate between 
vias [small diameter PTVs] and holes with larger diameters like component lead 
holes (PTHs). 
PTHs have a higher propensity of inner-layer separation, less for barrel 
cracks. Innerlayer separation occur predominantly at layer 2, 3, n-2, and n-1—the 
reason for this is the combined stresses from land rotation near the PCB 
surfaces and barrel compression [the later shows up in analysis as resin 
recession]. Barrel compression is the result of the 'hydro'-static pressures from the 
expanding resin surrounding the barrel [think of a submerged submarine]. Thicker 
plated barrels, smaller-diameter barrels and barrels with closely spaced pads 
will resist this barrel compression better.
PTVs show more barrel cracks. Barrel cracks occur almost invariably at the 
B-stage layers [higher resin content than laminates gives higher localized CTE-z 
above Tg] near the PCB center [location of highest barrel stresses]. For 
really thick PCBs [>150 mils] there is a high-stress center section of 
near-uniform barrel stresses.
Thicker plated barrels, good quality drilling [smooth walls without fiber 
rip-outs] and good quality ED copper [ductility, plating uniformity] will resist 
barrel fractures better. The number of pads and their spacing has little 
influence beyond simply replacing some of the high-CTE resin with low-CTE Cu.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com


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