A novel study of mold compound effect towards TCoB and process integration for power leadless package

Abstract

Solder joint crack in thermal cycling on board (TCoB) for surface mount devices (SMD) is becoming more stringent in semiconductor market. Current TCoB literature mainly focuses on Finite Element Analysis simulation with ANSYS software to identify the most sensitive parameters affecting TCoB performance and estimating the solder fatigue life. By a given Driver Mosfet Power QFN, this paper is focusing on actual TCoB by modifying mold compound's coefficient of thermal expansion (CTE) and storage modulus by altering the filler loading percentage and adding in additive either stress relief additive (SRA) or silicone. These modifications reduce mold compound viscosity, increase CTE1, mold shrinkage potentially increases challenges in wire sweep and map molding panel warpage. Six different mold compounds are assembled and soldered on a PCB board with TCoB condition of -40°C to 125°C. The judging criteria is concentrating on solder joint crack. Package level reliability MSL3@260°C, 500 cycle TC(-65/150°C) and 96h PCT are evaluated. Material analysis of Thermal Mechanical Analysis (TMA) and Dynamic Mechanical Analysis (DMA) are employed to analyze the coefficient of thermal expansion (CTE), glass transition temperature (Tg) and storage modulus of six different mold compounds. Filler loading reduction increase liquid to solid particles ratio, therefore wire sweep and process mapping result for all mold compounds are comparable. Increment of resin induce criticalness of panel warpage as mold shrinkage increases, result shows that filler content ≤87wt% increases panel warpage > 0.7mm which increase the difficulty in package singulation. Experimental TCoB reviews that corner leads are subjected to the highest thermal mechanical stress and becomes the initiation point of solder joint crack. Result further validates storage modulus is primary factor of mold compound instead of CTE1. Higher solder joint thickness ≥34μm is sufficient to prevent solder joint crack and meet TCoB requirement.