Fine pitch solder-less bonding using ultrasonic technique

Abstract

Industry is adapting micro-bumps in the device structures in order to having module with multiple functions and capabilities within smaller area. Micro-bumps is coated with Tin (Sn) cap to facilitates solder interconnects formation between the chip and substrate. Electrochemical migration failure is a known issue related to flux residue on the solder joints after the thermal compression of the chip with solder cap micro-bumps on substrate. Electromigration is another issue related to shrinking interconnects. It is related to atomic displacement in a conductor line due to an applied current. In this study, the micro bumps are directly bonded to the substrate without solder cap and thus there is no electro migration failure concern. The chip used in this study is of size 7mm × 7mm × 0.05mm and consists of peripheral micro-solder bumps at 40μm pitch with no solder cap. Ultra-sonic process was adopted to form the direct metal to metal joint between the chip and substrate. Ultrasonic process offered several advantages such as lower bonding temperature and shorter bonding duration over thermal compression process. However, the US process demand bumps with good co-planity of less than 0.6μm and good surface finishing. The copper bumps were coated either with TiAu, ENEPIG, and ENEP to prevent oxidation occurring during the bonding process. Detail DOE experiment was conducted to evaluate the bonding quality. Shear test and x-section analysis revealed that chips coated with either TiAu or ENEPIG could form a bond on silicon substrate coated with TiAu with optimized US parameters. The developed US bonding process successfully demonstrated on C2C application.