Impact Of Viscoelastic Material Characterization Of PCB's On Thermomechanical Properties Of QFN Thick Package Under Thermal Cycling

Abstract

The QFN component package is a quad flat pack (QFP) with “no-leads”, where the electrical contact to the printed circuit board (PCB) is made through soldering of the lands underneath the package body rather than the traditional leads formed along the perimeter. The popularity of this device package style is primarily due to the superior electrical and thermal performance demonstrated. It is one of the most cutting-edge technologies emerged in the market, exhibiting high performance with unparalleled cost effectiveness. But using thick PCBs (>3mm) is detrimental to the package reliability. The motivation of this work is to understand and mitigate the failures associated with QFN packages on thick boards.ANSYS workbench is used in Finite Element (FE) modelling to benchmark with the already existing literature to propose a best-known-method (BKM) for modelling. The analysis includes solving a “Global model” (relatively coarse mesh) with the quarter symmetry QFN model under Accelerated Thermal Cycling (ATC). The viscoelastic orthotropic material properties of the PCB are determined using Thermomechanical Analysis (TMA) and Dynamic Mechanical Analysis (DMA). The properties are determined in various sub-steps. Initially, the whole board level material characterization is determined which is linear elastic in nature. Sub-step involves the layer removal material characterization of the QFN thick board which helps in determining the complex time and temperature dependent deformation due to mismatch in the Co-efficient of Thermal Expansion (CTE) of each layer. An exhaustive experimental and FE analysis is performed to validate experiment and simulation results.