Gold to gold thermosonic flip-chip bonding

The aim of the project is to establish a thermosonic flip-chip assembly process to replace the anisotropic conductive adhesive films (ACFs) assembly method. The thermosonic bonding technology has the following advantages over the existing ACFs mounting method: (i) metallurgical joining is more reliable than conductive particles and adhesive joining, (ii) process cycle time can be reduced from several minutes to less than 10 seconds, and (iii) lower manufacturing cost per unit. In this paper, the process development of gold bumped flip-chip bonded on substrate with different gold finishing pads is described. Thermosonic bonding test was conducted using test chips with different I/O counts and bump geometry. The assembled dice and boards were subjected to die shear test to determine the bond strength. SEM was used to determine the bonding interface. Thermal cycling and thermal shock tests were performed on the optimized assembly process. It is observed that the die shear force can range between 32.16 and 52.20 gram per bump for gold bumps bonded on thick-film and thin-film substrates respectively. Cross-sections were carried out to inspect the degree of deformation on the gold bumps after thermosonic bonding. Coplanarity issue is addressed and parallelism adjustment is crucial for good bonding.