Failure Mode
Structural failure in microelectronics occurs when materials are subjected to simultaneous temperature fluctuations and mechanical stress. Thermomechanical fatigue represents the damage and cracking that results from the mismatch in coefficients of thermal expansion between adjacent bonded materials during thermal cycling. Unlike isothermal mechanical fatigue, this degradation mechanism involves changing temperatures that continuously alter the material properties and stress levels.
It is the dominant failure mode for solder joints and vias in high-reliability electronic assemblies.
Stress Profile
Inphase and out-of-phase thermal cycling produce different damage rates depending on whether the peak stress occurs at the highest or lowest temperature. In the study of thermomechanical fatigue, these stress profiles must be carefully simulated to understand the creep and plastic deformation behavior of the solder. The interaction of thermal and mechanical cycles accelerates grain structure coarsening and microvoid coalescence.
This localized damage leads to crack initiation and eventually complete electrical open circuits.
Material Selection
Selecting materials with closely matched expansion coefficients is the primary method for reducing this damage. Designers often use composite substrates or specialized underfills to minimize the displacement at the joint interfaces.
Testing Validation
Accelerated thermal chambers and mechanical test fixtures are used to evaluate the resistance of new designs to this fatigue. Continuous monitoring of the electrical loop resistance detects the onset of crack propagation before complete failure occurs.
