| | | We are experiencing shorting on a BGA. The part has a 1mm pitch with a 196 I/O count. The problem seems to pop up without warning & disappear in the same manner. | | | Our pad design is .020" diameter pads with .025" solder resist diameter. The center pads on the PCB are all tied together as a ground plane, therefore, the pads are solder mask defined and are .025" diameter. The shorts mostly occur on the center pads on this ground plane. | | | One thing that is strange, is that there is another 144 I/O BGA very close to this one on the PCB, with the same pitch and pad with center ground plane designs, but we do not experience any shorting problem. | | | We are printing with a .020" diameter stencil aperature. | | | Any advice or input would be appreciated. I would be interested to hear what other people are doing for pad and aperature designs for similar components. | | Hi Kelly, | | A couple of questions first. what type of BGA are these components? CBGA, PBGA, TBGA, etc. What is your aperture diameter on your stencil and what stencil thickness are you using? I think I can be of more help once I know the answers to those questions. | | Regards, | | Justin | Since the problem seems to come and go, Consider the operators as the source of the variability. (aside from baking to prevent moisture induced delimitation) | A few things to check | 1) Placement, Some operators will tweak a part if not perfectly placed on pad. This is a bad thing. | If yes slap the operator, if no continue to step 2 | 2) Handling, paste and/or part becoming disturbed after the printing process. | If yes slap the operator, if no continue to step 3 | 3) Printing, Repeatable? Are you measuring the Paste volume? Automatic screen printer? Are all shifts and/or operators setting up the screen printer the same? | If yes train the operators to follow the same procedures, if no continue to step 4 | 4) Are the boards warping in the reflow oven? | If yes, slap yourself and fix the oven, if no Continue to step 5 | 5) If you are still having problems you should do some imperical modeling to take advantage of the inherent continuity of quantitative variables. | Good luck | Mike Post Script Oh yea one more thing to check 1) If that Imperical modeling BS does not work, check the placement machine to make sure the part is not being smashed into the paste, Bad placement of board supports could cause the board to rest at a higher level. I agree with Justin more info is needed. The frequency of the problem would be nice. Is it 10% or 1% of the parts are shorting? How often does it show up, what equipment are you using? Questions questions questions Mike
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