Ioan,
You don't say what your flux is or what your cleaning solution is. A
possibility is flux residue, flux/cleaning agent residue, or residual
cleaning agent. When you heat the part, you rob the circuit of moisure and
the problem goes away. If you freeze it, the water is taken out of the
equation.
Doug Pauls
"Tempea, Ioan"
<itempea@POSITRON
.QC.CA> To
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09/23/2005 02:39 [TN] Soldering or design problem?
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"Tempea, Ioan"
<itempea@POSITRON
.QC.CA>
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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