Meaning
The non-reversible physical deformation of an assembly as it returns to its original temperature after being heated demonstrates the presence of residual stress within the joint or material. Measuring thermal expansion hysteresis helps engineers identify the risk of mechanical instability or electrical failure in multi-material structures like printed circuit boards and metal enclosures. This behavior is characterized by a difference in the dimension of the part at the same temperature during the heating and cooling cycles.
The hysteresis is bounded by the elastic limit of the materials involved.
Stress Accumulation
Plastic deformation occurs when the thermal stresses exceed the yield strength of the material or the adhesive joint holding the assembly together. This leaves the part with a permanent set once it returns to room temperature. The mismatch in expansion coefficients between the metal and plastic components is the main cause of this stress.
Reliability Impact
Repeated thermal cycles can cause the residual stresses to accumulate, leading to solder joint fatigue, board warpage, or delamination of layers. In wireless devices, this deformation can shift the position of antennas or RF shield cans, detuning the radio or causing intermittent contact failures. Engineers use thermal cycling tests to evaluate the severity of the hysteresis and ensure that the assembly remains functional.
This testing is conducted over the expected operating temperature range of the product.
Material Selection
Selecting materials with closely matched expansion coefficients or using flexible adhesives can reduce the hysteresis. This prevents the build-up of destructive internal forces during operation. It ensures the mechanical stability of the product over its lifetime.
