Thermomechanical Analysis (TMA) is a thermal analysis method in which linear dimensional change is measured as a function of temperature. TMA is mostly used to determine the coefficient of thermal expansion (CTE) before and after Tg of PCB base materials. This test can also signal important transitions in the material structure (glass transition temperature, time to delamination) and the presence of internal molded-in stress.
One valuable application of thermal analysis is for the characterization of electronic materials and components, including printed circuit boards (PCB) and encapsulants. In PCB industry, linear dimensional change is measured as a function of temperature.
A technique in which a deformation of the sample under non-oscillating stress is monitored against time or temperature while the temperature of the sample, in a specified atmosphere, is programmed. The stress may be compression, tension, flexure or torsion.
This is definition of TG by ICTAC.
In FR-4, the CTE increases once the board’s temperature rises above the Tg temperature. It is always best to use a high-Tg CCL if your board will run at a high temperature as you want to stay below the Tg temperature. As everything in your board expands, stress accumulates and causes microscopic cracks to propagate in the conductors. After repeated cycling between high and low temperatures, these cracks can merge, which leads to fracture. More ductile materials (i.e., solder with added indium) can experience more cycles before failure. This relationship between heating and thermal expansion is central in thermomechanical analysis.
The thermomechanical analysis process for electronics proceeds in the following order:
- Steady-state temperature calculation: Calculate the steady-state temperature while a board is in operation. This should include the operation of all cooling fans and components.
- Convert temperature rise to volume expansion: Once the temperature in each region of a board is calculated, it can be converted to a volume change using a CTE value.
- Calculate strain on various structures due to expansion: The strain experienced by different structures in the board is equal to the volume expansion. For a via, which gets mechanically stressed under expansion, the via experiences some additional mechanical strain.