Laser-Assisted Micro-Solder Bumping for Copper and Nickel-Gold Pad Finish

Flip-chip bonding is a key packaging technology to achieve the smallest form factor possible. Using copper as a direct under-bump metal and performing bonding under little force and at a low temperature eliminates the processing step for the deposition of a suitable wetting metal and offers an economical solution for electronic chip packaging. In this paper, various samples with copper and nickel-gold surface finishes are used to apply an in-house solder bumping, flip-chip bonding and reflow process to exhibit the bump-bond feasibility. Native oxides are reduced using process gases, and copper surface protection and solder wetting are achieved using copper formate. Lead-free 40 mu m solder balls were bumped on 80 mu m copper pads and 120 mu m copper pillars to demonstrate a full intermetallic Cu-Cu bond as a base study for stacking applications. Using a low-force bonding technique, various chips with different dimensions were bonded at 0.5-16 MPa, followed by a reflow step at a maximum temperature of 270 degrees C. Then, 30 mu m solder balls are utilized to bump the samples with NiAu and Cu bond pads at 50 mu m pitch. A mean shear strength of 44 MPa was obtained for the 30 mu m Cu samples. To the best of our knowledge, 30 mu m solder bumping directly on the copper pads by producing copper formate is a novel research contribution.

Automated summary

Flip-chip bonding is a critical packaging technology that allows for the smallest possible form factor. This paper demonstrates the feasibility of using copper and nickel-gold surface finishes in a solder bumping, flip-chip bonding, and reflow process. By reducing native oxides and using copper formate, the copper surface is protected and solder wetting is achieved. The study also shows that bumping solder directly on copper pads using copper formate is a novel research contribution.