Hi Technos,

we are faced here with a very delicate problem: the customer suspects a workmanship issue on our side, we are suspecting a design or component problem.

Here it goes: 50% of the cards do not function due to the same problem, something wrong with the same PBGA. We tweaked the thermal recipe, we are at about 220C peak; no improvement. We played with lower temperatures for fear not to thermally stress the part; no improvement.

Now I get into the weird zone:
*       if the BGA is heated, it instantly starts working, but also starts working if the BGA is frozen
*       it also works if we freeze the opposite side of the PCB
*       We use the flying probe to test for open joints, everything seems to be OK, but the card does not work
*       we did all kinds of re-reflow of the BGA on the bad boards, in the oven, but also on the rework station, even at extreme temperatures that send the top of the part at 280C. Same pattern, heat/freeze, it works, ambient temperature - fail. So we didn't even manage to kill the part for good.
*       we change the component, bingo, it works in 90% of the cases

I am sure the soldering is OK and the joints are properly formed, but don't have a solid proof, for some people BGA soldering is still the cause of all problems.

My question is: if it's not the soldering, what else can it be? Can an electronic design be borderline so that it does not work with the first time reflowed BGA, but works right after replacing the part with parts from the same tray? Can something be in the design of the PCB, in the manufacturing of the PCB?

And a last one, what is the test that can confirm the absence of open joints under a BGA? Can the balls be cross-sectioned parallell with the PCB, like half-ball, so an eventual open be redily spotted?

Thanks,
Ioan

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