Vapor pressure development in a FR4-CU composite structure during solder reflow

Abstract

This article presents a study on the development of vapor pressure a FR4-Cu composite structure when heated to a solder reflow temperature of 215 °C. Abaqus® finite element software was used to develop a representative two-dimensional model of the composite structure and to simulate moisture absorption and desorption processes. Simulation of transient moisture absorption was performed to predict moisture concentration distribution in the structure after being preconditioned in 85°C/85 % RH environment for 15 days. Simulation of transient moisture desorption was carried out at the solder reflow temperature to predict the redistribution of the moisture. Results of the moisture desorption simulation were used to compute magnitude of the vapor pressure in the structure. It was found that the moisture redistributes itself during the solder reflow process. Moisture concentration in the vicinity of the FR4-Cu interface, below the longer copper trace increases during the solder reflow. The vapor pressure in nearly 70 % of the FR4 material and close to the FR4-Cu interface, below the longer copper trace is almost equal to the saturation pressure of vapor at 215 °C. Distribution of the vapor pressure is very similar to the new distribution of moisture concentration resulting from moisture desorption process.