The importance of Printed Circuit Board (PCB) material characterization in solder joint reliability

Electronic assemblies exposed to severe thermal environments can fail due to thermomechanical fatigue of solder joints. During temperature cycling, solder joints experience shear strains caused by the Coefficient of Thermal Expansion (CTE) mismatch between the component and the PCB:

Solder joints subjected to shear loading during temperature cycles due to CTE mismatch between the PCB and the component

Accurately assessing the CTE of both the PCB and the component is crucial for evaluating the reliability of solder joints. PCBs are complex structures composed of alternating layers of copper, prepregs, and laminates, which themselves consist of epoxy resin and glass fabrics:

Complex internal structure of different PCB stack-ups

In many research studies on solder joint reliability, it is common to assume a temperature-independent CTE for the PCB without accounting for the complexity of PCB stack-ups. However, when comparing different PCBs with varying stack-ups, this simplification can lead to significant errors in fatigue evaluation:

Inaccurate fatigue model due to poor PCB material properties evaluation

The abnormal shift between the two curves for each PCB is due to the inaccurate evaluation of their material properties. By combining experimental characterization with modeling, it is possible to accurately evaluate the CTE of any complex PCB stack-up as a function of temperature:

Accurate material characterization of the different PCB stack-ups

The temperature-dependent CTE accounts for the complexity of each PCB stack-up and enables an accurate estimation of solder joint reliability by determining a robust fatigue curve with well-aligned data points:

Accurate fatigue model thanks to precise PCB material characterization

The key takeaway is that for fatigue analysis, input data such as the material properties of PCBs, are crucial. Without accurate material properties, reliability analysis can lead to significant errors.

Hooke Electronics has developed a micromechanical model that integrates strength of materials and laminate theories to accurately assess the physical properties of PCBs and ensure precise reliability estimation of solder joints: https://www.hooke-electronics.com/category/all-products