Pb/In solder bump formation for a flip-chip bonding technique at high speed optical communication devices

The increasing speed of advanced chip technologies has greatly challenged the interconnection methods and processes in order to achieve enhanced capability. We have successfully fabricated the solder bump and it's reflowing process for flip-chip bonding interconnection technique instead of conventional wire bonding for high speed devices. The lead(Pb: 350 degrees C) and the Indium(In: 157 degrees C) were used for solder bump and deposited by using thermal evaporation. The thickness of the deposited metal for solder bump was in the range of 5 similar to 6 mu m thickness. Specially, to increase the accuracy and the reliability of the flip-chip bonding Technique, 3 layer thick photoresist about 30 mu m was used to control the deposition area for solder bump. It was also used for the lift-off process of excess deposited metal for solder bump. The height of solder bump through the reflowing process was controlled in the range of 10 similar to 40 mu m according to the deposited area and-shape. Also, the deposited area and shape was one of the most important parameters for solder bump fabrication. In addition, it was found that an oxidized surface layer effects on the increased melting temperature of deposited metal for solder bump. In this process, the reflowing temperature of PB/In (60:40 wt%) solder bumps was 230+/-5 degrees C.