Expansion Mismatch
Stress levels in a thermal cycle result from the unequal expansion or contraction of bonded materials during temperature changes. In electronic assemblies, thermomechanical strain occurs because the silicon chip, the solder joints, and the circuit board have different coefficients of thermal expansion. This mismatch creates internal stresses that can eventually lead to the fatigue failure of the interconnects.
Stress Relaxation
Solder is a relatively soft metal that can deform plastically to relieve some of the applied force over time. This process is known as creep and it is highly dependent on both temperature and the duration of the thermal cycle. While it reduces the peak stress, the repeated plastic deformation causes microscopic damage to the metal structure.
Fatigue Life
The number of cycles an assembly can survive is determined by the magnitude of the strain and the properties of the solder. Cracks usually start at the corners of a package where the displacement is greatest and propagate through the joint. Finite element analysis is used during the design phase to predict these strain levels and optimize the layout.
Material Response
A typical printed circuit board expands significantly more than the ceramic or silicon components attached to it. When the assembly is heated during operation, the board pulls on the solder joints, stretching them. When it cools, the board shrinks and pushes the joints in the opposite direction.
Cumulative damage is measured through accelerated aging tests in a thermal chamber.