Abstract
High melting point (HMP) lead-free solder, hybrid sinter and transient liquidus phase sinter (TLPS) are the emerging lead-free alternatives for the potential replacement of high-lead solder. Lead-free solder is perfectly compatible with existing high-lead soldering processes for clip bond packages. The benefit of hybrid sinter is that it has much higher thermal and electrical conductivity than lead-free or high-lead solder. In this study, ten materials (including lead-free solders, hybrid sinter paste and TLPS) were first evaluated via die shear test. With the initial material screening, two lead-free solders (solder 1 and 2), two hybrid Ag sinter pastes (sinter i and ii) and one TLPS proceeded to internal sample assembly. For the lead-free solders, a process optimization with the aid of vacuum reflow was made to reduce void rate. Due to the slow and unbalanced inter-diffusion of Ag-Cu sintering than Ag-Ag sintering, optimizations to enhance the hybrid Ag sintering include Ag finishing for the die metallization and Ag plating for the clip and bond area of the leadframe. In 0-hour package electrical test, solder 1 and sinter i passed and were sent for reliability testing while solder 2, sinter ii and TLPS failed due to intermetallic compound (IMC) cracking, material bleeding and die cracking, respectively. In the reliability testing, a basic scheme of thermal cycling (TC) 1000 cycles, intermittent operating life (IOL) 750 hrs and highly accelerated temperature and humidity stress test (HAST) 96 hrs was defined for the early feasibility study. 1 of 75 sinter i units failed by TC 1000 cycles due to separation between silver sinter structure and die bottom metallization. Solder 1 passed the basic scheme without defects, and next the material workability and clip bond strength need to be improved to the equivalent level of high-lead solders.
Wetting characterization of Flip chip’s Lead-Free Solder Interconnect Using Surface Evolver.
