Big deal, right?
Alas, not everything expands and contracts at the same rate. Old-style tin-lead solder has 
=17, new-style lead-free solder is around 22, aluminum is 23, glass fibers are 16-50, epoxy ranges from 15 to 100, and plastics are all over the map. Circuit boards contain all of those materials, firmly bonded together, and all undergoing the same temperature cycles.
The power transistor in Figure 1 has four mechanical connections: its aluminum heatsink and three copper leads. It's firmly mounted to the heatsink with a steel screw through a plastic bushing, with a thermally conductive plastic sheet separating it from the heatsink. The center of the screw is 21 mm above the solder joints on the other side of the circuit board.
If the transistor normally operates at 70 C°, about 40 C° above room temperature, its copper leads and tab will expand by 21×40×17×10-6=0.014 mm. The aluminum heatsink, however, expands by 21×40×23×10-6=0.019 mm, a difference of 0.005 mm. That lengthening occurs every time the power turns on, followed by relaxation when the power goes off.
Below the transistor, the 2-mm thick circuit board is mostly epoxy with some glass fibers. It may heat up by 30 degrees C° and expand 0.006 mm, nearly as much as the difference between the transistor's two metals, with the net effect that the distance between the mounting screws and the solder joints varies by about 0.01 mm.
That doesn't sound like much, but it turns out that solder isn't particularly ductile. After years of repeated thermal cycles, the solder can crack around a copper lead, electrically isolating that lead from the circuit board. Of course, the connection becomes intermittent before it fails completely, so whacking the thing upside the head helps for a while, until at the end, "it just stops working" forever.
I talked to my buddy Eks, an ingenious fellow closing in on his 100th patent, and he mentioned he's seeing this problem in a disturbing number of fairly recent electronic gadgets. He drew the sketch in Figure 2 to show where the cracks occur. Figure 3 shows the nice-looking solder fillet (pronounced "fil-IT" in the metalworking trades) around each lead of a dual diode, which Eks says are typical of the failed joints he's seen. You generally cannot identify the cracked joints by eye, even under a microscope.
Eks says the only certain repair involves resoldering all the joints on a circuit board to fix the single failure, a tedious, labor-intensive process that he's willing to perform on his pet gadgets, but is obviously out of the question for most boards and most folks.
This is not something you can cure with software, although if the specs call for frequent on-off cycles for a high-power gadget, you might ask the hardware folks if they've really considered the effect of thermal stress.
Eks and I agree that most gadgets will outlive their warranty long before their solder joints crack. However, in deeply embedded and long-lived applications, this is precisely the sort of failure that drives repair techs over the edge: Reliable gear that slowly goes crazy, then fails completely, with no obvious cause.
Bottom line: It doesn't matter unless you're worried about the power bill.
