Effect of Capillary Trace on Dynamic Loop Profile Evolution in Thermosonic Wire Bonding

Thermosonic wire bonding remains the most commonly used interconnection technology in microelectronic packaging, and looping is an important aspect in modern wire bonders. To identify the loop formation mechanism, the effect of capillary trace on the standard wire looping process was studied. Dynamic looping processes with different capillary trace parameters and reverse motions of 4, 8, and 16 mil were recorded by a high-speed camera. The capillary trace and wire profile evolution were obtained from the looping videos using a digital image processing program, and the relationship between capillary trace and loop profiles was analyzed. A finite-element model was established to study the strain distribution on wire during looping. Experimental and simulation results show that the wire profile of the standard loop is mainly affected by capillary position and is not sensitive to capillary velocity. The upward capillary trace mainly affects the loop configuration, including the number, position, and deformation of kinks and the loop length. The downward capillary trace affects the stress states, loop height, kink deformation, and loop profiles. A kink is the wire with the largest local curvature, and it is a plastically deformed wire segment with little elastic core. The kink has two functions: 1) shaping the loop and 2) isolating the pulling force on the first bond and neck caused by capillary movement. This paper can be of great help in loop profile optimization in the industry and in academic research of loop dynamics.