Filler trap and solder extrusion in 3D IC thermo-compression bonded microbumps

As the Moore´s law is drawing to an end, there is a growing consensus that 3D stacking of Si integrated circuit chips is necessary to continue the current technological trend. In our work, a test 3D IC structure is successfully achieved and the effect of filler trap on microbump of solder joints will be discussed. In our test samples, the two Si chips are connected through thermo-compression bonding of 20 μm diameter microbumps. After bonding the two chips together, some underfill materials are found to stay on the interface of joining in the microbumps; the residual underfill in the microbump is defined as “filler trap”. Together with the formation of filler trap, a ring-type solder extrusion also forms on the circumference of the microbump. Although the filler trap is an electrical insulator, it has been found that the solder extrusion is a good electricity conductor. But the filler trap in solder can cause a detour of electric current path in solder. The detoured current potentially increases current crowding in the joint which might be accelerating electromigration. To investigate this potential issue, specially processed test vehicle which intentionally has higher filler traps and solder protrusion was fabricated for easier detection. The synchrotron radiation x-ray tomography results show that the solder extrusion in microbumps have a random shape, which may cause a random distribution of early failure in electromigration.