Fabrication and Reliability Assessment of Cu Pillar Microbumps With Printed Polymer Cores

Cu pillar microbumps with polymer cores have been demonstrated to effectively reduce thermomechanical stress and improve joint reliability. Fabricating polymer cores by a printing approach was proposed to overcome the limitations in conventional fabrication process. Cylindrical polymer cores with diameter of 20 $μm$ and height of 30 $μm$ were successfully printed. Surface metallization was subsequently applied on the printed polymer cores and Cu pillar microbumps with printed polymer cores with diameter of 35 $μm$ and height of 35 $μm$ were eventually achieved. To study the reliability performance of the interconnect joints made of Cu pillar microbumps with printed polymer cores, flip-chip bonding technology was successfully introduced and the interconnect joints between a designed bismaleimide triazine (BT) substrate and a silicon chip were formed. The interconnect joints made of conventional Cu pillars with identical dimensions were prepared for comparison. The reliability performance of the joints was investigated under temperature cycling condition and drop condition, respectively. Printed polymer cores increased the characteristic life by 32% in a temperature cycling test (0–100 $°C$⁠), while the drop test showed that printed polymer cores increased the characteristic life by four times due to the extra compliance provided by the printed polymer cores. It can be concluded that Cu pillar microbumps with printed polymer cores can effectively reduce stress and improve joint reliability.