Inkjet printed metallic micropillars for bare die flip-chip bonding

Abstract

Inkjet printed metal micropillars have been developed to help meet the demands for novel and highly adaptable microelectronics fabrication processes. The digitally printed silver pillar arrays in this study have been utilized in place of wafer-level solder bump processes or chip-level wire-bonded stud bumps. These three-dimensional silver pillars were printed with a drop-on-demand piezoelectric inkjet printer utilizing silver nanoparticle ink. The inkjet printed micropillars were found to have 22 μm diameters and a height equivalent to approximately 3 μm per droplet. In our study, we chose pillars for further use as stud bumps with 8, 10, 12 and 14 droplets, with heights of approximately 20.9 μm, 25.9 μm, 33.3 μm and 35.9 μm respectively. After printing on the bare dies the bumps were subsequently used to increase the contact reliability of flip-chip bonded samples. It was found that the bumped chips dramatically improved the reliability of the I/O connection as compared to unbumped samples. In fact nearly 88% of the bumped pads had a resistance less than 2.5 Ω/bump (no noticeable variation between bump heights) as compared to 17% for the unbumped bare dies. This study clearly demonstrates the fabrication of inkjet printed silver micropillars for use in uniform stud bump arrays. Furthermore, the feasibility of incorporating inkjet printed silver stud bumps for use in flip-chip fabrication methods was demonstrated.