Coefficient of Thermal Expansion Measurement Case Study
The following example illustrates a typical material coefficient of thermal expansion test using the Analysis Tech CTE Tester. The material tested is a yellow silicone material of the type commonly used in electronic packaging. This material was subjected to a total temperature span of 70°C in two discrete temperature 'bands' (25°C to 60°C & 60°C to 95°C). Steady-state thermal equilibrium was attained at each temperature 'step' (25°C, 60°C, and 90°C), with the temperature and linear displacement measured and recorded between each temperature plateau.
The software generated a plot of Displacement vs Temperature showing the change in total material length at each temperature, and generated a plot of Displacement vs Time showing that the material had sufficient time to reach steady-state thermal equilibrium at each temperature.
The overall coefficient of thermal expansion (CTE) value for this sample, from 25°C to 95°C was 184.9 x 10-6/°C (μ-inch/°C).
CTE-360 Example Test Data
[CTE-360 Test Summary #1]
Sample ID= Yellow Silicone
Sample Length= 2.015 inches
Raw Data File= 177-15-23-x1_CTE.dat
CTE: 1=25°C to 95°C: 184.9 x 10-6/ °C +/- 3.2%
CTE: 2=25°C to 60°C: 184.0 x 10-6/ °C +/- 3.5%
CTE: 3=60°C to 95°C: 185.8 x 10-6/ °C +/- 3.5%
Date/Time= 03-21-2016/19:41:17 S/N:1160220 WinCTE V1.0.0
The Importance of Thermal Cycling
The initial temperature point and final temperature point should be the same to ensure that the sample returns to the original dimension. If not, issues such as stress-relaxation, dimensional creep, or phase-change may be occurring. If the sample fails to substantially return to the original length, annealing the sample to relieve internal stresses (oven-soaking at or above the maximum CTE temperature) for an hour or more can be beneficial.
When testing a new material, general best-practice is to perform a few temperature cycles to accurately assess inelastic/plastic behavior associated with stress relaxation and dimensional creep. These issues generally become more severe for materials closer to their melting or softening temperature. Cyclic testing is shown in the following plots.
