Flip chip bonding using ink-jet printing technology

In this paper, a bump-forming method for flip chip bonding using ink-jet printing technology is proposed. A flip chip bonded transmission line using ink-jet printed silver bumps consisting of conductive ink containing silver nanoparticles was fabricated to verify the electrical characteristics after the flip chip bonding process. The transmission line was designed according to microstrip theory for a frequency range of 300 kHz to 3 GHz, with the size of fabricated line being 35 (width) × 8.64 (length) × 0.5 (thickness) mm³. The electrical characteristics of a reference microstrip and the flip chip bonded line were compared with FEM simulation and measurement results. Direct current (DC) resistances of both the reference line and the flip chip bonded line were measured to be 3.1 Ω and 3.2 Ω, respectively. The discrepancy between measured insertion loss and the simulation result was only 0.04 dB at 3 GHz, and the return loss was greater than 15 dB in the measurement frequency range. As a result of the analysis, it was confirmed that the DC resistance of ink-jet printed bumps account for 0.56% of the total DC resistance, and the ink-jet printed bumps hardly affected the radio frequency (RF) characteristics of the RF transmission line at low frequencies. The results demonstrate that ink-jet printed silver bumps can be used for a simple and low-cost flip chip bonding process. We expect that the proposed method can be applied to the packaging of various electronics such as flexible, wearable devices and RF applications.