Flip Chip LED Assembly By Solder Stamping/Pin-Transfer

摘要

Flip-Chip and Chip Scale Package (CSP) Light Emitting Diodes (LEDs) are being increasingly adopted for applications in TV backlight and mobile flash. Lately they are also being used for automotive interior, street lighting and even and general lighting applications. The advantages of very small form factor, easier optics, improved thermal dissipation and no wire-bond result in unrivaled high lumen density at lower cost. Eutectic gold tin (AuSn 80/20) is the die attach material of choice for flip-chip LEDS. Lately, there has been a significant effort to make these devices compatible with SMT. However, SMT assembly of these small packages is challenging. Package float and tilt can result in sub-par assembly yields. Flip-chip LEDs are tricky because of their rectangular interconnect pads with small gaps (which are getting even smaller). Finally, performance issues like luminous flux degradation due to flux residue and current leakage during reverse bias remain. In this study, pin transfer (also called stamping) process was adapted to assemble flip-chip CSP LEDs with fine pitch solder paste. Solder reservoir height and die attach conditions were varied to optimize solder spread, voiding and die shear for commercial flip-chip CSPs. Also preliminary results on the effect of cleaning of LEDs (after assembly) on light output and color are also presented. This study is relevant for LED packaging and LED module assembly makers who use flip chip for automotive, backlight and general lighting applications.