Comparison of bonding defects for longitudinal and transverse thermosonic flip-chip

Thermosonic flip-chip technique can be divided into transverse and longitudinal approach depending on the direction of ultrasonic vibration of the bonding tool (collet). The collet vibrates in-plane and out-of-plane with the silicon chip for transverse and longitudinal bonding, respectively. In this paper, two common defects: chip tilting and silicon cratering were compared for the two modes of thermosonic flip-chip bonding. Finite element analysis (FEM) using ANSYS was used to study the effect of rigidity of transducer and the stress induced on the silicon layer during bonding. It was found from both simulation and experiments that the deformation of the transverse transducer under a loading contribute to the chip tilting defect. For longitudinal bonding, the collet can maintain a perfect planarity even under high loading and hence eliminate the tilting problem. Actual vibration amplitudes of both types of transducers were measured by laser vibrometers at different ultrasonic power and loadings. The measurement data were fed into the FEM, together with the compressive loading, to study the stress generated at the silicon layer. Using the maximum principal stress criterion, the stress level induced by longitudinal bonding was higher than that of the transverse mode. A maximum stress of 1.2GPa occurs at a bond force of 200gf per bump and an ultrasonic power at 2W for the longitudinal mode and, this is 20% higher than the transverse mode. Silicon cratering was also observed from experiment on the longitudinal bonding but not on the transverse mode. It matches with our FEM prediction that longitudinal bonding induces higher stress level at the silicon layer and hence it is more susceptible to silicon cratering.