junkeymonkey :
also worry about the solder
I guess it comes down to how cheap they go will they use high quality silver or cheap as china can provide lead or tin ?? are the parts going to get so hot to melt / stress crack the solder ?
Most solder alloys do not even reach their eutectic phase until 180C, so the chip would fail long before the solder joints across the stack might melt. Silver and tin are much stiffer than lead and lead is not as susceptible to tin pest so were it not for RoHS compliance, you would actually want them to use leaded solder. Since the HBM stacks are relatively low power and tightly coupled thermally, it should be fairly easy to match thermal expansion coefficients and temperature across the stack to mostly eliminate mechanical failure. The heat output from the HBM stacks will also be fairly constant, so the HBM stacks will not need to put up with the sort of temperature gradients present across the GPU die.
I bet thermal management accounts for a large chunk of the reasons why AMD decided to go with a silicon interposer instead of putting the RAM directly under the GPU.
Look at it this way: before HBM, the path from GPU to GDDR dies was: GPU die -> micro-BGA balls to GPU substrate -> BGA balls to PCB -> BGA balls to GDDR package substrate -> uBGA balls to GDDR die. With HBM, you have the same path, except the PCB gets replaced by the silicon interposer and there is a logic chip added to the HBM stack. The total potential points of failure remains mostly unchanged. They have only been relocated.