Exploring Process Interaction of No-Flow Underfill and Thermo-Compression Bonding in D2D Stacking

Abstract

ABSTRACT Conceptualization of the No-Flow (NUF) or any pre-applied underfill in encapsulating bump joints in flip chip and 3D IC; defined the direction of industry in manufacturability of scaling 3D IC. It replaces capillary underfill (CUF) which is process time and real-state consuming. Not to mention the overflow for thin die stack platform. One single event in a very short process called the Thermo-Compression Bonding (TCB), where you apply heat and pressure force in joining 2 die in D2D configuration, will make or break an effective stacking and good bump joint. Interaction of the underfill material behavior, the bond profile and the tool used is the purpose of this paper. First of all, No-Flow should be applied or be dispenseable. Material viscosity has to be in process-able range in which volume control can be a challenge. Filler has its purpose of lowering CTE but may pose issues in filler entrapment and viscosity. The isothermal viscosity at lower temperature ranges help to define dispense process. Dispensing process should prevent No-Flow underfill from starting the gelation or curing in which jet dispense process uses nozzle heat. Jet dispensing is very much preferable in terms of volume control accuracy especially for a sub-milligram requirements of low profiled 3D IC stacks. Fluxing is a major component in the NUF to aid the bump joining process but the effect of volatility of the material might affect voiding. Ramp rate and dwell time of force and heat that influence bump connection and material cure will impact how good the bump formation will be. This paper will also take a look on the cure rate and dynamic change in viscosity with a corresponding bond profile coinciding with the melting of bump.