Electronics Assembly Knowledge, Vision & Wisdom
Flip Chip LED Solder Assembly
Flip Chip LED Solder Assembly
A pin transfer process was adapted to assemble flip-chip CSP LEDs with fine pitch solder paste to assemble lateral and vertical LEDs on flat substrates.
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Authored By:
Gyan Dutt, Srinath Himanshu, Nicholas Herrick, Amit Patel and Ranjit Pandher, Ph.D.
Alpha Assembly Solutions
South Plainfield, NJ, USA

Summary
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.

In this study, a pin transfer (also called stamping) process was adapted to assemble flip-chip CSP LEDs with fine pitch solder paste. Pin transfer or stamping is a popular method to assemble small lateral and vertical LEDs on flat substrates. However flip-chip LEDs are tricky because of their rectangular interconnect pads with small gaps (that are increasingly getting smaller).

In this presentation we will present the findings of this flipchip assembly process development by the pin transfer process. 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 moduleassembly makers who use flip chip for automotive, backlight and general lighting applications.

Conclusions
A pin transfer / stamping process was successfully adapted for high throughput assembly of flip-chip LEDs. The paste volume was optimized to achieve high die-shear, low voiding and minimal spread-out for highest light extraction. Solder paste stability over 8 hours of the stamping / pin transfer process was also demonstrated.

Preliminary functional performance testing of the assembled UV LEDs suggest that cleaning after assembly can have significant positive impact on the radiant flux output.

Initially Published in the SMTA Proceedings

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