Ioan,
I agree we need more info to help direct you further such as specifics about the BGA and the board structure beneath it. Werner is right also about pressing down on the part to see it that makes it connect as well.
How long is the component held above 208 would my first question?
Also, you did not say if your reflow at 220 C was measured in the BGA solder connection or elsewhere on the board. Make sure that you don't have a thermal design issue in the board that is keeping the BGA from coming all the way up to temp
I have had similar problems with BGAs at .5 mm pitch which contained Lead Free solderballs without our knowledge. We had to start reflowing them at 230 in order to get them all to connect. Make sure you have Sn63Pb37 solderballs
Xray is the best means of determining the ball to pad soldered connection but you have to have a good eye to see it. It helps if the designers can tell you what ball is open.
The reason why the part works if replaced is that the replacement cycle is typically hotter than the reflow cycle and the pads have been nicely tinned and conditions are ideal for making the solder connection.
I would watch your part very carefully during the rework heating cycle to see if it drops (ball collapse) at all when copious amounts of flux and adequate reflow heat is applied. Xray the BGA before and after the rework heat application. You will see a marked difference if the part is dropping a lot.
Warpage of the pwb could also be interacting with the open connection so watch the entire assembly when it is heated and cooled.
I have to admit I miss the challenge (or headache) of solving problems such as this one on a daily basis.
I had one component that I never did resolve as to why it's solderballs would not collapse properly every time. It only had 4, 6, or 12 solderballs attached directly to a piece of ceramic. The solderballs were Sn63Pb37 but usually one out three components would not drop equally all the way around and were constantly being rejected by Quality. We were able to get them to drop if we applied extra liquid flux to them prior to reflow, but we should not have had to do this in the first place. I wanted to change the pad size but never got to finish the project. It was a .5 mm pitch part as well and the paste print sometimes was a little starved on some of the pads.
Hope this helps...
Phil
"Wenger, George M." <[log in to unmask]> wrote:
Ioan,
There isn't enough information to know what the root cause is. If replacing the PBGA makes a board work only 90% of the time do you then start blaming the PBGA's. What if the intermittent isn't a solder joint or a bad PBGA? What if it is a bad PBGA?
What is the surface finish on the board?
How thick is the board and how many layers?
Does the board have blind vias?
-----Original Message-----
From: TechNet on behalf of Tempea, Ioan
Sent: Fri 9/23/2005 3:39 PM
To: [log in to unmask]
Cc:
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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