Rationalization of gold ball bond shear strengths

Bond shear testing is becoming an indispensable tool for wirebonder machine set-up and bonding process monitoring. The lower acceptable limits, however, are defined based on historical data. This paper establishes the theoretical and statistical correlation between bond size, wire grain size, strain hardening and ultimate tensile strengths of the gold wire in relation to the Mode 1 (through the gold ball) shear strengths. Results of the study showed that the ball shear strength is strongly correlated with the bond aspect ratio which is the ratio of the ball diameter to the bond height. This parameter is easily obtainable from an automated vision inspection system and could provide initial information on the quality of the bond. The study also showed that the gold ball undergoes strain or work hardening during bonding. Bond shear strengths predicted from this work hardening conforms well with the actual data