Thermosonic fine-pitch flipchip bonding of silicon chips on screen printed paper and PET substrates

Abstract

In light of the necessity to introduce new techniques for hybrid integration of semiconductor dies on temperature and pressure-sensitive substrates, the feasibility of employing thermosonic flipchip bonding on screen printed paper and PET substrates was the objective of this study. Gold-bumped silicon test chips with internal daisy chain structures were bonded to the corresponding screen printed silver pads on PET and paper substrates by using both thermocompression and thermosonic techniques. The contact resistance of the interconnecting bumps and the die-shear strength of the resulting assembly were evaluated and the optimized bonding conditions on paper and PET substrates were identified. It was proposed that by the introduction of ultrasonic energy, diffusion bonding between the silver flakes and the gold bump were facilitated, whereas in the thermocompression bonding the mechanical interlocking was the main bonding mechanism. As a result, thermosonic flipchip bonding could be conducted in a much shorter time (milliseconds) and at lower bonding pressure (5 N) and temperature (100 °C). It was also found that for screen printed flexible substrates, thermosonic flipchip bonding at lower ultrasonic energy levels is preferential, while the possible bonding-induced damages to the pads and substrates can be avoided. Conclusively, It was proposed that thermosonic flip chip bonding can be regarded as a promising solution for fine-pitch system on paper (SOP) and system on PET (SOPE) technologies.