Novel method for NCF flow simulation in HBM thermal compression bonding process to optimize the NCF shape

Abstract

A typical stack bonding process of HBM core dies is 1) lamination of nonconductive film (NCF) over the bumps of core dies, 2) thermal compression bonding (TC bonding) of core dies, and 3) molding EMC around stacked dies. The main advantage of TC bonding is being able to control joint void by pre-filling the bump area with NCF lamination prior to reflow. TC bonding, however, has a fillet at the die joint gap edge and unfilled gap risk at the die corner need to be controlled, in turn, the flow in TC Bonding process. We put much of our effort to improve the accuracy of NCF flow simulation to understand the mechanism for fillet shape formation and to help the development of material and bonding process.To simulate the NCF flow in TC Bonding for stacking dies, we must acquire the viscosity profile during process in temperature and time scale, process pressure and initial shape of the laminated NCF surface. However, TC Bonding equipment has high temperature and rate of temperature rise comparing with the conventional rheometer for the high viscosity such as NCF, rheometer data is not adequate for our simulation and to describe the NCF surface evolution at the die joint gap edge in simulation, we must consider two phase flow and NCF Zone which is very high aspect ratio comparing the thickness with size. Using commercial code, we need to use the volume of fraction (VOF) model with billions mesh but it needs several month to solve the NCF fillet shape under single condition.In our study, we set up the methodology for NCF viscosity profile during the die stacking process from the pressure and monitoring the joint gap height, and to define the surface shape of the laminated NCF, we trying to find out the relationship between the NCF height and the bump density of the laminated face. After then, we establish the in-house simulation code based on Hele-Shaw flow formulation and we can diminish the calculation time dramatically to several minutes. Finally we verify and find out the fillet shape over the conditions of the process pressure and NCF viscosity and suggest a bump layout design for the optimized fillet shape.