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.