Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

BGA rework



BGA rework | 14 May, 2002

Hello All, I've got a little problem here. I have a 456 ball PBGA 1mm pitch, .5mm balls, with .6mm pads on the board. given that the pad size is way to big (S/B .45mm or so)I need any and all advice on how to rework these things without shorting or opens, I currently have about a 25-50% yield. Most of the shorts are at the corners. And just to throw another wrench into this the board has some pretty significant copper (2oz.) on one side of the BGA with not much on the other. I have tried to "shim" two sides with Kapton tape. This got rid of the shorts but now I get opens most likely due to blocking the air flow from two sides. I have a small rework station with a bottom side IR and convection nozzles for heat.



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BGA rework | 14 May, 2002

Maybe the issue is not a proper rework method. Your shorts on the corners are caused by too fast of a profile causing the corners of the BGA to get too hot, too fast, which made them curl, like tatter chips.

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BGA rework | 15 May, 2002

Good point, I am preheating for 90 sec to a temp of 170 C. (PCBA) then reflowing for 40 sec. to bring it to 215 C. I will try a softer hammer. This has been working on other assemblies with the same package however.

Do you know if a water soluble paste flux can stay active for a long period of time? I contacted my rep for this stuff and I got "try it and see" (I'll be switching reps soon).

Thanks Dave, always a pleasure

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BGA rework | 22 May, 2002

2 suggestions - Get a good scope - at least a 4X, but more like a 6X, 10X is best. You have a choice on used equipment for a substantial savings. A Leica Stereo 4 zoom is it. You can look for BGA scopes that have a 90` angle for looking at the balls under your CSP/BGA/Flip Chip pkg. These are like 9X or 10X so they really bring out detail. Just a regular scope will do if you tilt the board. If you are reworking by eyeballing it, then youre like me. Then after reflow, put it under the scope, check reflow, if more heat is needed - keep it under the scope and watch for the solder to flow. I get very good results this way. I am using a tacky no clean flux to hold IC in place, brand name Kester, in a 30CC syringe for applying. It works great, it must be stored refrigerated at 36-40`F, the results are good enough to not warrant a BGA placer/ rework station at $30K.

Regards, Phil Johnson Sr Eng Tech

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BGA rework | 27 May, 2002

I use the exact same paste flux that we use in our solder paste for SMT manufacturing with great success. I have also had great success using the Air Vac DRS Hot Gas units with the same size BGA's. If to much heat is applied to fast to the BGA be it a plastic or metal device, the package will expand faster on the top side due to the temperature delta between top side and bottom side. This causes the package corners to bend downward. On very large devices say over 44mm BGA's the opposite can happen where the corners bend upward. I have found that if the delta across the BGA package is not even you will get this warpage. You can increase your heat rates to help acheive more even heating through the package and reduce the warpage. Try a multi-step reflow profile with say four steps, preheat, soak, ramp, reflow, each step applying heat at an increasing rate. It has worked with verygood results for me, also try to increase your board temp by say five degrees, I like to stay around 90 to 100 on my preheat stage, and by the time I get to reflow the top side temp is around 110 to 120C for the last fifty to sixty seconds.

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