The purpose of this paper is to model and understand waferlevel underfill (WLUF) flow behavior during thermocompression in order to predict the risk of residual polymer entrapment between top and bottom interconnects, which drastically reduces electrical yield.In the first part of this paper, the fluidic equation describing remaining polymer entrapment thickness has been established. It depends on layout such as interconnect diameter, thermocompression parameters such as force and time as well as WLUF initial thickness and viscosity. Secondly, comparison with experimental results has been made. This work highlighted that residual polymer thickness cannot only be predicted by WLUF flow ability phenomenon. Procedure installation by Fourier transform infrared spectroscopy analysis has allowed verifying that WLUF cross-linkage did not happen during thermocompression. However, it has been demonstrated that plastic deformations of the solder joint could occur and dramatically increased residual underfill thicknesses in solder joint during thermocompression.
This paper deals with the performances of Wafer-Level UnderFill in ultra-fine 50 microns pitch interconnections. Firstly, dry-film WLUF assembly feasibility has been demonstrated by checking lamination and planarity in the pillar areas. Well-formed Pb-free joints have been obtained after thermocompression with limited void or WLUF entrapment. Secondly, to have a better understanding of this innovative material performances during thermal and moisture tests, the electrical behavior of Copper pillars chains surrounded by WLUF has been compared with those surrounded by classic Capillary Underfill used in the semiconductor industry. We have successfully demonstrated that chains surrounded by WLUF are stable during JEDEC level 4 preconditioning and moisture storage. During thermal cycling, behavior variation of chains surrounded by WLUF has been observed, depending on interconnects quantity. However, electrical performances of the longest 100 pillars chain respond to industrial specifications and are similar for both underfill. After thermal and moisture tests, a good adhesion of WLUF stacks has been verified by shear tests. Those results have demonstrated the pertinence of this innovative WLUF material with 50µm pitch interconnections.
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