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Surface Finish of Substrate BGA Pad Effect on Flip Chip Ball Grid Array Package Solder Joint Reliability
Keywords: flip chip, surface finish, solder joint reliability
Because flip chips can shorter possible leads, achieve high electrical interconnect speed, high density, low profile and allow higher frequency switching with better noise control. Xilinx is using flip chip technology for next generation high I/O device. Assurance of board level reliability is necessary and required for employing any new packages into products. Recently many works have been devoted to board level solder joint reliability of ball grid array packages. In this paper, the effect of BGA pad surface finish on large flip chip package solder joint reliability is studied. The test vehicle is 42.5x42.5mm body size flip chip BGA package with 23x20mm die size. During this study, Cu/SOP, Cu/Au, ENIG and electrolytic Ni/Au surface finish is tested. The flip chip package with different surface finish is mounted on the FR4 test board with each board carrying 8 test vehicles. The package under investigation was assembled using daisy-chained substrate following standard assembly process. Prior to mounting packages onto the test board, they are baked at 125 oC for 24 hours to remove any potential moisture inside. After surface mount was completed, all test vehicles were put into T/C chamber for cyclic testing. The test temperature ranges from 0 oC to 100 oC with a frequency of two cycles per hour. 5 minutes of transition time and 10 minutes of dwell time composed the cycle at each extreme temperature. All of the packages have been daisy chain connected. In this way, all solder balls can be monitored simultaneously. The resistance of each daisy chain unit is monitored through test board edge connector that is wired to terminal module connected to a switching board. The resistance is then detected and recorded periodically and in a real-time manner via data acquisition system. Failure criterion is defined as resistance reaches 500 ohms. The results reveal that electrolytic Ni/Au surface finish has the best solder joint reliability. Electrolytic Ni/Au surface finish units also passed 3500 cycles without any failure Surface Finish Cycles Completed # Tested # Failed 1st. Failure (Cycles) Characteristic Life (Cycles) Cu/Au 4372 36 28 2879 4027 Cu/SOP 3678 36 35 2666 3369 ENIG 4925 36 32 3403 4273 Electrolytic 5332 36 33 3664 4551
Leilei Zhang , Staff Packaging Engineer
San Jose, CA

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