PCB Rework / PCB failure

[JivenJim]

Hey Designers,

My QA guy asked me how many times they could Reheat a PCB (FR4, 6 layer, .3 mm small vias, 1.6mm thick) I couldn't remember a spec but the the number 5 poped up from memory. Anyone got any guidlines to how many times you can reheat/rework before causing a significant number of board level failures to thermal expansion and thus via failure? Or is this a question for the Black Arts master?

Thanks,

Jim

[robert Tarzwell]

that is a good question that most experianced designers or board fabracators dont know.
also the answer depends on a couple of factors, how thick is the board and how thick is the copper plating in the holes.
normal 1 oz .059 board can stand about 3-4 solder cycles above that the vias will start to crack and problems will follow.
in the pcb shop we test to 3 cycles normal and 5 for a tough test but we add one or two cycles in curring the solder mask and one more for solder hot air leveling after the test.

if the board is .093 you may only get 2-3 cycles .
however you can ask the board manufacture to increase the copper plating thickness , which significantly increases the number of solder cycles.

robert tarzwell pcb books at megadawn.com