Investigation on the high frequency thermosonic flip chip bonding under low temperature

The feasibility of using high frequency (109 kHz) thermosonic bonding for a flip chip package under 60°C bonding temperature is studied, and the effects of bond power and bond force on die shear strength are elucidated. It is noticed that the die shear strength increased with the bond power initially, and reached a maximum level at 2 W and attained a plateau level eventually. Furthermore, it appears that the increase in bond power substantially reduced the package standoff height and strengthened the die shear strength. Essentially, the package standoff height is dependent on the bump deformation during the thermosonic bonding, in which the increase in bond power imparted significant amounts of bump deformation. By analogy, the increase in bump deformation due to the elevated bond force also enhanced the die shear strength. Comparing the performance between 63 kHz and 109 kHz bonding, it is interesting to note that satisfactory die shear strength was only achieved in 63 kHz bonding at higher bond power level, while the 109 kHz bonding demonstrated the distinctive characteristic in establishing the joints even at lower power level under 60°C. From the SEM micrography, it is observed that the deformation of gold bumps under low frequency bonding was more severe. The bonding mechanisms due to the power and vibrating frequency are also discussed schematically.