Underfill Dispensing for 3D Die Stacking with Through Silicon Vias

In this paper, a 3D stacked-die package is developed for the miniaturization and integration of electronic devices. The developed package has a stacked flip-chip-on-chip structure and eight flip chips are arranged in four vertical layers using four silicon chip carriers with through silicon vias (TSVs). In each layer, two flip chips are mounted on the silicon chip carrier with 100 um solder bumps, and multiple TSVs are fabricated in each silicon chip carrier for underfill dispensing purpose. The 3D module with four stacked layers is sequentially assembled by the standard surface mount reflow process and finally mounted to a substrate. In the underfill process, conventional I-pass underfill is used to fill up the gaps of the bottom two layers as it has relatively fast spreading speed. For the top two chip carriers, underfill is dispensed through TSVs to fill the gaps. Unlike the conventional underfill process, the encapsulant in this case would not flow in the gaps by the capillary effect unless the dispensed materials can obtain enough kinetic energy to overcome the surface tension at the end of TSVs, and thus, smooth sidewall, proper dispensing settings and optimized TSV pattern are needed. After underfill, detailed inspections are performed to verify the quality of encapsulation. The results show that the combined I-pass/TSV underfill process gives void-free encapsulation and perfect fillets for the stacked 3D package.