Efficient Thermomechanical Reliability Model for Lead-Free Solder Joints

J-B. Libot, P. Milesi
Hooke Electronics

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.

In a world where electronic system failures can result in catastrophic accidents and erode customer trust, performing reliability calculations is crucial to ensure that equipment functions reliably throughout its lifecycle. The analytical SAC305 reliability model developed in this study is designed to be practically useful for designers and reliability engineers, offering quick and accurate results without the need for Finite Element Modeling (FEM), which requires specialized skills and can be time-consuming.

The SAC305 reliability model aims to retain the simplicity of use characteristic of analytical models while enhancing certain aspects, such as the effects of array configurations in BGAs and solder joint geometry. It has been calibrated using 83 durability data points and validated with an additional 23. This model is particularly useful for common SMT components (such as BGAs, chip-like components, and CLCCs). However, when dealing with new or complex components, FEM is often necessary. In such cases, combining FEM with experimental work can be a valuable approach for capturing key effects that may eventually be incorporated into an analytical model.

Finally, the PCB material properties model discussed in this paper, while essential for assessing solder joint durability, is also practically useful for thermal analysis. The model allows for the evaluation of global density, thermal conductivity, and specific heat, which are crucial for thermal engineers conducting both steady-state and transient thermal analyses.