Inkjet printed metallic micropillars for bare die flip-chip bonding

Inkjet printed metal micropillars have been developed to help meet the demands for novel and highlyadaptable microelectronics fabrication processes. The digitally printed silver pillar arrays in this studyhave been utilized in place of wafer-level solder bump processes or chip-level wire-bonded studbumps. These three-dimensional silver pillars were printed with a drop-on-demand piezoelectricinkjet printer utilizing silver nanoparticle ink. The inkjet printed micropillars were found to have22 μmdiameters and a height equivalent to approximately 3 μmper droplet. In our study, we chosepillars for further use as stud bumps with 8, 10, 12 and 14 droplets, with heights of approximately20.9 μm, 25.9 μm, 33.3 μmand 35.9 μmrespectively. After printing on the bare dies the bumps weresubsequently used to increase the contact reliability of flip-chip bonded samples. It was found that thebumped chips dramatically improved the reliability of the I/Oconnection as compared to unbumpedsamples. In fact nearly 88% of the bumped pads had a resistance less than 2.5Ω/bump (no noticeablevariation between bump heights) as compared to 17% for the unbumped bare dies. This study clearlydemonstrates 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-chipfabrication methods was demonstrated.