This paper discusses the design optimization of Bail Grid Array(BGA) packages with the help of FEM analysis, placing a special emphasis on solder ball reliability and package warpage after board assembly. The solder ball interconnection betweenBGA package and printed circuit board(PCB) is susceptible to shear stress during thermal storage due to the mismatch in coefficient of thermal expansion(CTE) between them. Compared to other BGAs, Tape Ball Grid Array(TBGA) has lower thermal resistance and lower impedance as well as the greater capability of accommodating fairly high counts of I/Os. Therefore, an extensive study was given in regard to the optimization of TBGA structure. In TBGA, a metal plate called stiffener is installed to improve theability of keeping the package flat. An adhesive material to attach the TAB tape and the stiffener plays a role of stress buffer which is caused from the thermal mismatch between the package and PCB. Two crucial issues are associated with the designoptimization of the package. One is the improvement of solder joint reliability and the other is the prevention of package warpage.In this study, elastic-creep analyses of solder bumps and elastic analyses of package coplanarity were performed. The largest creep strain among all solder bumps was calculated using both a global package model and a local model. Then thermal fatigue life was estimated from the Coffin-Manson's equation which was obtained from a low cycle fatigue test. A number of analyses were carried out varying thickness and the CTE of the stiffener. Thickness of the stiffener tuned out to be the most influential factoron package warpage. Also we found, compared to the effect from thickness, the CTE of the stiffener has much greater effect on solder bump stress. In conclusion, a significant progress was achieved in TBGA design optimization and we believe the methodology in this study can also be applied to the designing of any other BGA package.
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