Effect of Au ball bond geometry on bond strength and process parameters, and assessing reliability on Al bond pad using integrated stress sensors

The effect of bond geometry on bond strength and reliability is studied for 25 μm Au wire bonds on standard CMOS Al pads. An accelerated method for parameter optimization is used to produce ball bonds with optimized shear strength (SS) and target values for ball diameter of 56.5 μm, bonded ball height (BH) of 12 μm, 16 μm, and 20 μm for low, medium, and high BH, respectively. Low BH bonds achieve 126.1 MPa strength and endure up to 41.9% US before deformation begins, whereas high BH bonds achieve only 109.8 MPa strength and withstand a maximum of 39.8% US. Low BH bonds achieve higher SS possibly due to the higher level of US that can be used. Bond reliability is assessed non-destructively in high temperature storage (HTS) tests using piezoresistive stress sensors. The bonds are aged at 200 °C for ~500 h. From features in the stress sensor signals, a characteristic time (t_c) is derived during aging using an improved method. The t_c value indicates the time it takes for the bonds to degrade severely due to intermetallics and crack formation. The t_c values for low, medium, and high BH bonds were found to be 199.9±8.7 h, 225.9±9.5 h, and 231.9±6.8 h respectively. The result suggests ball bonds with higher BH values are more reliable than the bonds with lower BH for a constant ball bond diameter. This result is corroborated by the SS before and after aging being the same for the high BH bonds whereas SS dropped by 3% for the low BH bonds. The non-destructive method was demonstrated to have an estimated time resolution better than 10 h which allows for precise comparisons of the reliability of various processes.