Vapor pressure prediction in reflow for stacked-chip packages by convection-diffusion model.

摘要

Moisture plays a critical role in the reliability of electronic devices, especially in the desorption process at reflow temperatures (around 270° C) where severe damages may occur due to high-pressure vapor accrued from condensed moisture. Such pressure-driven vapor flow, however, could not be described by conventional Fick's Law. Furthermore, using conventional Fick's Law for multi-materials always encounters interface discontinuity issues. Therefore, this paper adopts a Convection-Diffusion Model that is able to describe complex desorption behavior in a multi-material media without the discontinuity issue. Both pressure gradient-driven (convection) and concentration gradient-driven (diffusion) moisture transports are considered in the model. To achieve this, absorbed moisture is partitioned into vapor phase and liquid phase (condensed water), with the vapor flux governed by Darcy's Law and the water flux by Fick's Law. Henry's Law is also implemented so that the Fickian term is converted to pressure, resulting in a unified vapor pressure model. Numerical validations are also performed to show that the Convection-Diffusion Model (CD Model) can be reduced to traditional Fickian Model and Convection-Only Model. The model is applied to analyze a stacked-chip package by two numerical cases: desorption under two typical reflow temperature profiles. The differences of the two reflow profiles are heavily dependent on the material properties, especially diffusivity. A high diffusivity will result in a diffusion dominant case, and a small diffusivity will result in a convection dominant case. A sensitivity analysis is done to show the most important parameters are as follows: diffusivity, permeability, and porosity. This analysis shows the importance of using accurate material properties. The overall benefit of the CD Model is that it accurately predicts desorption of moisture out of the material through a wide range of temperatures. This paper details the CD Model applied to multi-material configurations, which show accurate pressure, concentration desorption and concentration contours.