Kenji Shimokawa

Micro-ball bump for flip chip interconnections

Micro-ball bump technology has been developed for flip chip (FC) interconnections. This technology is based on (1) a production method of fine metal balls (micro-balls) and (2) a gang-bonding method for forming bumps (micro-ball bumps) on chip electrodes. Solder balls of 60-150 mm and gold balls of 35-100 mm in diameter were prepared with extremely uniform diameters and high sphericity. After holding these micro-balls on through-holes of an arrangement plate by a vacuum suction method, the micro-balls were transferred onto the electrodes of the chips in order to form the micro-ball bumps. An excess ball eliminating system and a ball bouncing system were developed for arranging the ball successfully on the plate. The cycle time of the originally developed mounter was 20 seconds for a chip with 300 bumps. Both bumping on a single chip and on multiple chips in a wafer were possible. The micro-solder bumps were formed onto the electrodes covered with under bump metals (UBMs). The micro-solder-balls of 150 mm in diameter were transferred onto the flux printed electrodes of a chip with 220 mm pitch and 45/spl times/45 area array. The micro-solder bumps were uniform in composition, volume, and height because of the use of the micro-solder-balls with precisely controlled diameter and composition. Using the micro-gold-balls of 35 mm in diameter, the bumps with 50 mm pitch were formed on Al pads by means of thermocompression bonding. The proposed micro-ball bump technology could be applied to bumping not only for FC interconnections, but also for TABs.

Micro-ball wafer bumping for flip chip interconnection

A new wafer bumping method using micro-balls was developed that can be used for high-density LSI assembly, specifically for Flip Chip interconnection. Micro solder balls with the diameter ranging from 60 /spl mu/m to 200 /spl mu/m were first formed with a high level of accuracy and sphericity. These balls were transferred and bonded to the whole electrode-pads of an 8-inch wafer in one stroke using a fully automated micro ball mounter, which was newly developed. The balls were held on fluxed pads and melted in a reflow furnace. The fluxing was performed using unique stamp system. The productivity and the yield were evaluated under the following conditions. The number of chips on an 8 inch wafer was 616, Pad pitch was 250 /spl mu/m, Pad number of a chip was 625 (25/spl times/25 area array), and the total number of balls on a wafer was 385,000. The yield of forming bumps was confirmed to be higher than 99.995% without repairing and the cycle time of micro ball bumping was ca. 5 min. For an 8 inch wafer. The bump height variation, the bump shear strength and the bond reliability were evaluated in comparison with other methods.