All materials expand when heated. The coefficient of thermal expansion or CTE is the measure of the expansion of the material per degree in temperature change when compared to the materials original size. Thermomechanical Analysis is used to measure the initial height of the sample, the finite changes which occur as the temperature is increased and calculate the CTE.
The coefficient of thermal expansion of laminates can be measured in three directions, x, y and z. Because the expansion of a laminate in the x and y direction is restricted by the weave of the glass it is independent of the resin grade. Much historical data has been collected on the various glass styles, therefore evaluation of X and Y direction CTEs are rarely done by the Isola Analytical Services Lab.
The coefficient of thermal expansion measured in the Z direction is the composite result of the expansion of all of the components of the sample. The reported coefficient of thermal expansion of an unclad laminate will reflect the combination of the resin grade, the ratio of the resin to the glass and the glass style. These factors are also reflected in the coefficient of thermal expansion of printing wiring boards as well as the specific prepreg and laminate stack up and the amount of copper present.
Plated through holes and blind vias will restrict the expansion of the sample in the Z direction. Samples for Coefficient of Thermal Expansion of Printed Wiring Boards must be taken from areas of the board which do not have these features.
Samples thinner than 20 thousandths of an inch are not evaluated for Z axis CTE. Materials this thin are not rigid enough to undergo sample preparation and remain flat enough to maintain their original thickness. Because the coefficient thermal expansion is directly related to the original size, this distortion introduces a significant error in the result.