A New Efficient Thermomechanical Reliability Model for Lead-Free Solder Joints

Abstract

The thermomechanical reliability of solder joints remains a challenge for companies whose electronic equipment is used in harsh thermal conditions throughout their lifecycle, such as in the aeronautics, space, military or medical applications. With the RoHS legislation and REACh directive preventing lead from being used in electronic equipment, 96.5Sn-3.0Ag-0.5Cu (SAC305) composition has become one of the most widely used lead-free solder alloys. Various models are available to evaluate the reliability of solder joints under temperature cycles, ranging from simple analytical models to complex Finite Element Analysis (FEA). While analytical models are easy-to-use, they may lack the precision needed for accurate lifetime assessments. Conversely, FEA, though more precise, requires expertise, is time-consuming, and may not be cost-efficient.

This study aims to present a new analytical model that accounts for the shape of solder joints, with a special emphasis on the determination of the mechanical properties of the Printed Circuit Boards (PCBs) and components. A simplified elastic-plastic constitutive relationship coupled with a viscoplastic model is used to describe the behavior of SAC305 solder joints under temperature cycling. The reliability model presented in this study is calibrated and validated using over 100 durability data points from various leadless packages, PCB stack-ups, and thermal cycling conditions. It represents an efficient and easy-to-use tool for electronic designers to quickly and accurately evaluate the thermomechanical reliability of their electronic assemblies.