In mechanics and thermodynamics, thermal stress is mechanical stress created by any change in temperature of a material. These stresses can lead to fracturing or plastic deformation depending on the other variables of heating, which include material types and constraints. Temperature gradients, thermal expansion or contraction and thermal shocks are things that can lead to thermal stress. This type of stress is highly dependent on the thermal expansion coefficient which varies from material to material. In general, the greater the temperature change, the higher the level of stress that can occur. Thermal shock can result from a rapid change in temperature, resulting in cracking or shattering.
The IPC-TM-650, Method 2.6.27, Thermal Stress is intended to establish a relative ability of printed boards, or representative coupons, to survive the thermal excursions associated with assembly and rework in a tin/lead or lead-free application using a convection oven, or alternate equipment with the capability to match the reflow profile of a convection oven. The test embraces relative robustness of the copper interconnection and dielectric materials subjected to the strain and resulting stress associated with a standardized thermal profile. The purpose is to establish an objective measurement of relative robustness ranking or comparing variables, or establishing minimum reliability requirements for copper interconnections and dielectric material in a printed board. The purpose of the test method is to provide the procedure for conditioning and reflowing of the test specimen prior to evaluation for compliance to the applicable performance specification, i.e., IPC-6012, IPC-6013, IPC-6018, etc.
In PCB manufacturing field, the thermal stress Test is an IPC standard test for PCB thermal reliability. In a test, a PCB specimen is fluxed then floated on the top a molten solder pot at 288°C (T288) for 10 seconds with 3 cycles.