Roger, the via or PTH cracking is due to differential expansion of plated
copper and FR-4 material.     CTE of copper -- 1.6 to 1.7 x 10*-5 per deg C
                  FR-4           --  >= 40x10*-5 per deg C above its glass
transition (110 degC)
The expansion for FR-4 goes exponential above it's TG. The increase in length
being directly propoetional to temperature, copper is going to be stretched
by a factor of 2.2 X.
The stress is therefore great and will exceed its tensile value, thereby
cracking it. At 270 deg C, expansion is almost twice that at 125 deg C.

When thermal cycle (-65 to +125 C) is used fatigue becomes dominant factor
for via cracking and will take few cycles before failure takes place. Also
the thicker the board, greater the chance of cracking in the via.

Pratap Singh
RAMP Labs
(512) 255-6820