Preliminary thanks to all that responded yet!
A few answers:
no pop corning, wires look OK under X-Ray. My X-Ray is 2D, can't see opens. Board finish is ENIG, no buried vias, 8 layers. Board does not warp, it is a 9"x4", 0.63" PCB, that passes in the reflow oven in the right orientation, length parallel with the oven axis. BGA is 487 pin, 1mm pitch, the "easiest" on the PCB. Right besides it I have a BGA about the same size, but with a round heat sink, and like 3" away I have a metal top BGA, larger size; none of these 2 give me a problem.
Now, is it lead-free or not, this is what we are investigating right now. I would say no, judging by the look of the balls.
Werner,
there is something here I miss: freezing. Shouldn't freezing increase an existing gap, both sides of a crack actually contract, increasing the relative distance?
Doug,
your answer is how wonderful the eclecticism of this group is. I would have never thought of this kind of effects of the entrapped water. Unfortunately it is on no-clean and there are no dendrites in the critical area.
Thanks,
Ioan
> -----Original Message-----
> From: TechNet [SMTP:[log in to unmask]] On Behalf Of Bev Christian
> Sent: Friday, September 23, 2005 4:07 PM
> To: [log in to unmask]
> Subject: Re: [TN] Soldering or design problem?
>
> Ioan,
> Any chance the parts are MSL parts that were not properly stored and
> handled and they have pop corned, resulting in broken bond wires that
> are really, really close and when a temp change happens things touch.
> The other suggestions have been good too.
> Bev
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Brinkman, Mark
> Sent: September 23, 2005 3:59 PM
> To: [log in to unmask]
> Subject: Re: [TN] Soldering or design problem?
>
> At this point I think I would try to isolate the problem by debugging
> the failing board. The designer should be able to find what is not
> working. He should know the sequence of events that should take place
> once power is applied to get the design up and running. He's going to
> have get dirty. Find the event that is not working. What would cause
> this event not to happen? Find it and see if it's the same on the other
> failed boards.
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
> Sent: Friday, September 23, 2005 2:40 PM
> To: [log in to unmask]
> Subject: [TN] Soldering or design problem?
>
> 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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