Dynamic Mechanical Analysis or DMA measures the storage and loss modulus and Tan Delta of a material. Storage or elastic modulus is a material’s ability to store energy applied to it for future use or to spring back when deformed. Loss modulus is a material’s ability to dissipate energy that is applied to it. For example, when silly putty is stretched the energy that goes into it is lost and it cannot return to its original shape. Tan delta is the ratio of the storage to loss modulus.
These measurements are calculated from a sample’s response to a change in position when a known force is applied to it. The most common clamps are dual cantilever, single cantilever and thin film. It is important to remember that because the force is applied to the sample differently with each clamp, the results cannot be compared. Unless otherwise noted, the results reported by Isola's Analytical Services Lab use the dual cantilever clamp.
The temperature of the sample can be held isothermally or changed at a fixed ramp rate. Sample temperature is measured indirectly by a thermocouple. Careful calibration and a slow ramp rate of 3°C per minute ensure the temperature of the sample is the same as the reported temperature within plus or minus 2°C.
The glass transition is very complex and causes changes in many measurable characteristics over a temperature range; a significant decrease in storage modulus, a peak in loss modulus and a peak in tan delta. Several methods have been defined to assign a single temperature. It is important that temperatures be assigned by the same method if they are to be compared. Unless otherwise noted, the glass transition temperature reported by the Isola ASL is the temperature of the peak in the tan delta curve.
Laminates and printed wiring boards are composites and many factors affect their modulus value, therefore, modulus values are not reported. An incomplete list includes overall resin content, glass type, stack up of laminate and prepreg, grain direction and presence of copper. Changes in modulus, indicating the glass transition region are not affected by these factors, because they only occur within the resin component.
Two characteristics of printed wiring boards will affect the glass transition temperature. Samples with large copper content heat slowly and the temperature error increases with the amount of copper. Results for these samples will be noted as “clad." Plated thru holes also cause a temperature error and affect the sample’s ability to respond to the instrument force. The glass transition temperature cannot be measured with confidence when they are present.