Koichi Fujimaru

High throughput thermal compression NCF bonding

High through put thermal compression NCF bonding was studied and the new process consisting of dividing pre and main bonding, and the multi die gang main bonding has been developed. The dividing could change the process from serial to parallel and enabled to use the constant heated bonder head, which eliminated the time consuming head cooling process of the conventional serial thermal compression bonding. The die of 7.3 × 7.3 × 0.1 mm size with bumps of 38 × 38 μm2 square Cu pillar covered by Sn-Ag cap, which had the pitches of 80 μm at peripheral and 300 μm at corer area, and the organic laminated substrate with Cu/OSP trace were used as the test vehicle in this study. Firstly, the dividing of pre and main bo ndi ng process in the case of si ngle die was investigated. The prebonding was the die placement to the NCF on the substrate, which was carried out at 150°C for 0.5 second. The substrate was kept at 80°C during the process. After the pre bonding the test vehicle was removed out from the equipment and cooled down to room temperature. And then it was mounted back to the equipment again and main bonding was carried out at 240°C for 20 seconds. The same substrate temperature as the pre bonding process was kept. Solder joint formation and NCF curing was made at the process. The assembled test vehicle was evaluated. The cross sectional observation results showed that the bump solder wetted the Cu trace on the substrate and no void was detected in the NCF by C-SAM observation. Secondly, the multi die main gang bonding was studied. The equipment was newly designed and built. 15 dies were pre bonded on the substrate with the same condition as that of the single die experiment. After the pre bonding was finished, the substrate was moved to the main gang bonder. During the transportation the substrate was cooled down to room temperature. The 15 dies were bonded at one time at 240°C for 10 seconds. The substrate was heated at 240°C during the process. The evaluation of the assembled dies revealed that the solder wettability of the joints and void detection in the NCF was almost the same as those of the single die pre and main divided bonding. This main bonding process time corresponded to 2700 UPH.

Low temperature touch down and suppressing filler trapping bonding process with a wafer level pre-applied underfilling film adhesive

Flip chip bonding process of the chip touch down at 40°C and suppressing the material trapping at the joint area with the wafer level NCF (Non conductive film), which is pre applied underfilling film adhesive, has been investigated. The test vehicle wafer has 25 μm diameter and 50 μm height bumps which are 10 μm height Cu pillar and 40 μm height Sn-Ag solder cap. The bump pitch was 200 μm. The 55 μm thickness 50 wt% filler loaded NCF was laminated on the wafer and then the surface was planarized with the bump solder layer exposing by the bit cutting technique. Such prepared chip was bonded as the top chip to the bottom chip which has the 25 μm diameter pad of 3 μm Cu bottom, 2 μm Ni middle and 0.1 μm Au top. To insert the sticking step in the bonding process, which melts and flows down the NCF underneath the top chip to the bottom chip partially, the chips were held well the aligned position during the successive processes. The gang bonding possibility was also proved with the four chips together bonding. PCT (pressure cooker test, 121°C and 100%Rh for 168 hours) was performed to the gang bonded samples. By shortening the joint formation step time form 25 to 5 seconds Cu diffusion into the solder bulk area was suppressed and the durable joint to the PCT was formed. It was confirmed by the cross sectional observations.

Wafer and/or chip bonding adhesives for 3D package

For 3D package application the demanded feature of chip bonding adhesive was discussed and the material was developed, which was wafer level process compatible NCF (Non conductive sheet). It should be high flowability for lamination on a bumped wafer surface without void, diced with wafer without deformation or sticking dust, transparent for alignment mark detection and low coefficient of thermal expansion (CTE) for the package thermal cycle durability. The nano particle dispersed and highly loaded NCF has been developed to satisfy the demanded characteristics. The CTE was 37°C/ppm and 1% weight loss temperature is higher than 350°C. The photosensitive NCF has been also developed. Bump top adhesive can be removed completely by photolithography before bonding process. It can wipe off the anxiety of adhesive residue between bump and pad. This material is also good for lamination and has high heat resistance of 1% weight loss temperature higher than 300°C.