Electrical characteristics of fine pitch flip chip solder joints fabricated using low temperature solders

The electrical characteristics of low temperature solder joints were investigated for an LCD application. The contact resistances of ultra-small eutectic 97In-3Ag (m.p.: 143°C) and eutectic 58Bi-42Sn (m.p.: 138°C) solder joints were measured and the microstructures of solder joints were characterized after flip chip joining, underfill, and reliability testing. The octagonal shaped UBMs of 50 μm and 80 μm pitches were fabricated through a photolithographic process and wet chemical etching. Low temperature solders were evaporated on UBMs, and solder bumps were formed by the lift-off and reflow process. After flip chip bonding, the resistance of the solder joint was measured from a daisy chain test structure using a 4-point technique. The real contact resistances of eutectic In-Ag and eutectic Bi-Sn solder joints were calculated as 5 mΩ and 8 mΩ, respectively, after subtracting conductor line resistances. The contact resistance of the 50 μm pitch In-Ag solder joint was similar to that of the 80 μm pitch. The 50 μm pitch Bi-Sn solder joint had higher contact resistance than that of the 80 μm pitch. The contact resistance of the In-Ag solder joint did not increase after thermal cycling test (-55°C to 125°C/1000 cycles). The contact resistance of the Bi-Sn solder joint increased up to the four times as large as the initial value after 611 cycles.