When the 360 was in its preproduction phase Microsoft used Lead based solder. It was what the industry at that time was using. Microsoft didn't notice a problem regarding heat in these units simply because lead based solder is much kinder to heat cycles.

EU at this time put a deadline to the end of use for lead based solder, so the move to leadfree solder was done right before moving to mass manufacturing. The poor cooling solution on the 360 compared to its heat output didn't do the system justice. The PCB would flex as it would not be heated evenly.

After Die shrinks and manufacturing revisions, more mature lead free solder also used. The Jasper units were pretty much tanks, and the cooling solution was oversized as they were the same as before with the much hotter and power demanding lager die first gen chips.

Video Cards from the era of the launch 360 are notoriously unreliable for the same reasons. Heat cycles were a death wish, luckily for videocards they normally had a cool down period after gaming idling at a desktop. While the 360 would commonly go from 100% load term load to off.