Eiji Hashino

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.

Lead-Free Solder Micro-Ball Bumps for the Next Generation of Flip Chip Interconnection: Micro-Ball Materials, Bump Formation Process and Reliability

Micro-ball wafer bumping (MBB) technology, which has capability of fine pitch bumping such as 150-mum pad pitch and the advantages of adjusting the optimum material combinations of joints, was brought into practice. High productivity and yield were achieved by employing an inspection and the repair process with special equipment, and a void-less reflow process was established. Package-level reliability tests were performed for chips with Ti/NiV/Cu-UBM and Sn-Ag-Cu solder bumps using MBB technology with excellent results. An evaluation on reliability was also conducted with Sn-1.2Ag-0.5Cu-Ni solder and pure tin solder bumps. As a result of multiple reflow cycle tests for Ti/NiV/Cu-UBM and various lead-free solders, the shape of IMC and the spalling of IMC were influenced by the total amount of Cu and Ni in the solder and the UBM composition. Ti/NiV/Cu-UBM can be applied to various solders by changing the thickness of the surface Cu layer according to the composition of the solder.

An Applicatin of Micro-Ball Wafer Bumping to Double Ball Bump for Flip Chip Interconnection

Microball wafer bumping method was applied to forming double ball bumps, which increased the bump height to improve the reliability of the flip chip interconnection. The micro solder balls of 100mum in diameter were transferred and connected to the whole electrode-pads covered with UBMs (under bump metals) of an 8 inch wafer in one stroke using a fully automated micro ball mounter, which was originally developed. The balls were held on fluxed pads and melted in a reflow furnace. After cleaning the flux residue, the wafer bumped with microballs was then encapsulated with epoxy resin containing silica fillers by using Apic Yamadas wafer level molding system. The molding resin was spread to the whole wafer by compressing with a heated flat plate, where the top of the solder bumps were covered with an elastic parting film. The supplied resin volume was previously adjusted to the desired molding thickness. The exposed top of the ball bumps was cleaned and then second microball bumping was processed on the top of the first bumps. The first and second balls were connected by reflowing to form the double ball bumps. The height variation and shear strength of double ball bumps were evaluated. To compare the reliability for the different type of bumps the TCTs and FEM analysis were performed for the chips connected with PCBs