Fine pitch and high density Sn bump fabrication

Three-dimensional packaging technology, which requires fine pitch and high density of solder bumps, has been developed recently for system-in-package applications. There are several methods being used for solder bumping process for now. As the sphere pitch decreases to below 100 mum, electrodepositing has an advantage over robotic ball placement and screen printing in the cost per ball, according to the study done by ROHM HAAS electronic materials. The current environmentally conscious manufacturing moves toward Pb-free schemes for electronic devices and components. Sn- Ag and Sn-Cu alloy system is an acknowledged candidate among Pb-free solders. However, it is difficult to fabricate alloy-plated bumps. In this paper, the fabrication of lead-free bumps made of pure-tin, as a basic study, is described. Area-array tin solder bumps each of size 60 mum diameter on an 180 mum pitch with very tight height variation were obtained. In addition, tin bumps can also be applied in a new chip-to-chip interconnection method, in which bonding between the chips is achieved by deformation-injection of tin stud bump on a chip into the through via hole in the other chip. SEM was used to observe the microstructure of tin bumps. At last, the reflow was carried out in a glycerol bath in order to get a high quality of the solder bumps.