Through silicon underfill dispensing for 3D die/interposer stacking

Abstract

This study describes a through-silicon-underfill dispensing that the encapsulant is dispensed through through-silicon-vias (TSVs). The TSVs function as entrances for encapsulant dispensing or paths for fluid flow. Typically, the inflow for TSV dispensing may be flow with a constant speed or free droplets. A model was developed to investigate the filling time and the pressure distribution for the quasi-steady, radial and laminar flow between parallel plates. Compared with free droplets, a constant inflow has shorter filling time at the expense of increasing the fluid pressure. 3D stacking with the same-size interposers forms several planar sidewalls. Encapsulant may flow out from the edges of the sidewalls and form an edge flood failure if the edge flow of an underfill can overcome the surface force. An optimized pattern of TSVs was designed for the underfill of a 3D package to identify the trade-off between the filling efficiency and the lower risk of the edge flood. In each interposer, the TSVs are classified into two groups: the central group is dispensed by a constant inflow whereas the outer group is dispensed by free droplets; the inflow of free droplets eliminates the risk of an edge flood. A four-stack 3D package with the optimized TSV pattern was developed for validation. Edge dispensing was used to fill the gaps at the bottom levels as much as possible. Subsequently, the remaining gaps were encapsulated by TSV dispensing (with both a constant inflow and an inflow of free droplets). Inspections by scanning acoustic microscopy and crosssectioning verified that the combined underfill could result in a void-free encapsulation and suitable fillets.