How does coefficient of thermal expansion mismatch affect solder joints during thermal cycles?

• A. It reduces joint length but not stress.
• B. It has no effect on the joint.
• C. It reduces the risk of cracking.
• D. It induces thermo-mechanical stresses at joints, risking cracks or joint failure.

Answer: (D)

CTE mismatch between solder and its surrounding materials means they don’t expand or contract uniformly as temperature changes. In a solder joint, the solder, pads, and substrates each have a different tendency to grow with heat. When things heat up, parts with higher CTE try to expand more and push on the joint; when they cool, they contract and pull, creating shear and tensile stresses at the interfaces. Repeated thermal cycling magnifies these stresses, leading to crack initiation and growth in the solder or at the interfaces, and eventual joint failure. This is why mismatch increases the risk of cracking during cycling—there’s no stress-free way for the joint to accommodate the differing expansions.