Influence of gold pick up on the hardness of copper free air ball

The effect of substrate material pick up by the Cu wire tail on the hardness of the subsequently formed free air ball (FAB) is investigated with scanning electron microscopy (SEM) and micro-hardness test. The Cu wire bonds are made on Au metallization. Wire residue is not found on the imprint the tail bond leaves on the metallization, but fracture of the substrate metallization is evident. SEM images of the Cu wire tail end clearly show Au residue (pick up). The amount of Au pick up is estimated higher than 0.03% of the volume of a subsequently formed 50 μm diameter FAB, exceeding typical impurity and dopant concentrations (0.01%). Lowering the impact force process parameter is found to strongly increase the amount of substrate material pick up. Cu free air balls (50 μm in diameter) formed with an estimated Au pick up volume of 50 μm3 are found to be between 1.2 HV and 4.3 HV softer than those without pick up. However, the hardness varies significantly more than that of FABs without pick up.