Enhanced Analytical Fatigue Model for Sac305 Solder Joints Subjected to Temperature Cycles Considering Geometrical Parameters

Ensuring the thermomechanical reliability of leadfree solder joints remains a significant challenge for companies whose electronic equipment operates in harsh thermal environments throughout their life cycle, such as in aeronautics, space, military, and medical applications. Due to the restrictions imposed by the RoHS legislation and the REACh directive on the use of lead in electronic devices, the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$96.5 \text{Sn}-3.0 \text{Ag}-0.5 \text{Cu}$</tex> (SAC305) alloy has emerged as one of the most commonly used lead-free solder compositions. Several analytical models exist to assess the reliability of solder joints under temperature cycling conditions. However, due to their intrinsic simplicity, they often lack the precision needed for accurate quantitative estimations of solder joint durability. This paper presents an analytical fatigue model for SAC305 solder joints in Wafer Level Packages (WLP) subjected to temperature cycles. Unlike most existing analytical models, this work focuses on the influence of geometrical parameters, including solder joint shape and matrix array, on the thermomechanical durability of 400 I/O and 900 I/O WLP. Two Printed Circuit Board (PCB) copper pad configurations were considered: unbalanced pads, where the PCB pad diameter is larger than the component's pads, and balanced pads, where both pad diameters approximately match. These configurations resulted in different solder ball shapes. Using experimental durability data and Finite Element Analysis (FEA), an analytical fatigue model accounting for solder joint shape and matrix array effects was developed and validated with independent data.