Electrochemical migration failure on FR-4 PCB by hygro-thermo-vapor pressure coupled analysis

Abstract

INTRODUCTIONCorrosion reliability recently becomes an issue due to the trend of miniaturization of electronic products. The phenomenon of electronic corrosion is ionic migration which is also known as electrochemical migration (ECM) by surplus moisture remained inside the package. High temperature may also deteriorate the ECM failure. The hydrated metal ions (positively charged) will migrate towards the cathode and form a dendrite. In this paper, electrochemical migration failure caused by hygro-thermal swelling and residual moisture has been carefully investigated for FR-4 PCB board plated with Cu. An analytical moisture diffusion solution is proposed to determine the moisture distribution and consequent hygroscopic induced strain as well as stress. By applying Fickian diffusion law, the "thermal wetness" analogous technique is used to solve moisture absorption and desorption models. The analytical expression for total expansion strain due to hygro-thermo-vapor pressure coupled effect is implemented using finite element software ANSYS. Finite element predictions reveal the significance of contribution of hygroswelling induced effective stress/strain. Hygroscopic properties such as moisture diffusivity and coefficient of moisture expansion are characterized by an integrated TMA/TGA scheme. Solubility and vapor pressure effect are included to study popcorn failure during reflow process.