Thermosonic flip-chip bonding system with a self-planarization feature using polymer

Thermosonic flip-chip bonding is a wire bonding technology modified for flip-chip assembly. Compared with the soldering technology, it is simpler, faster and more cost-effective. Unfortunately, the yield of thermosonic bonding is low and unreliable because it is difficult to control the ultrasonic energy transmission. A small planarity angle between the bonding tool and stage can result in a nonuniform ultrasonic energy distribution. A self-planarization concept was proposed to solve this problem. A layer of polymer was placed between the bonding tool and the chip to smooth the nonplanar contact. Experimental measurements and finite element modeling were used to study the effect of the polymer layer. Results showed that the polymer layer could assure a uniform ultrasonic energy distribution; however, it also reduced the energy transmission efficiency. A case study for optimization was conducted based on finite element modeling. For a 1000-I/O flip chip assembly with a 250 /spl mu/m pitch using a bonder with a 0.01/spl deg/ planarity angle, polymer thickness of 350 /spl mu/m and a Young's modulus of 2 GPa were selected.