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In a Plastic Ball Grid Array (PBGA) package, the glass transition temperature of 170-215 deg C for the PCB substrate puts an upper
limit to the wirebonding temperature. The low thermal conductivity of the PCB substrate and the need for a thicker material to prevent package warpage make fine pad pitch wirebonding even more difficult to perform. To compensate for the thermal limitations of the package, high frequency thermosonic bonding was proposed. This study was able to successfully demonstrate the use of a 100-kHz thermosonic gold ball bonder for fine pitch bonding. Of the four major bonding parameters investigated, bond force appeared to be the most important control factor for ball size and ball shear force optimization. On the other hand, both bond power and bond force were important for optimizing ball shear stress. The study predicted a minimum ball size of Xs=2.6 mils and Ys = 2.7 mils, which has a ball shear force of 29.5 g and a ball shear stress of 5.24 g/mil^2. At the predicted maximum ball shear force of 41.9 g, Xs = 3.2 mils and Ys = 3.3 mils, with the ball shear stress equal to 5.03 g/mil^2. At the predicted maximum ball shear stress of 5.6 g/mil^2, Xs=3.0 mils and Ys=3.0 mils, with ball shear force equal to 40.2g. The results of the study showed that trade-offs are necessary to cater to a given application.
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