Determination of Non-Uniform Microscopic Distributions of Thermal Strains in Glass-Fiber Cloth/Epoxy Laminates Using DIC

In electronic components, various materials with different thermal expansion coefficients are joined. For reliability evaluation, it is important to understand the thermal stresses and strains caused by such thermal expansion mismatches. In this investigation, the microscopic thermal strain distribution during the heating of glass-fiber cloth/epoxy laminates was measured via the digital image correlation method using a micro-speckle pattern under the microscope. The micro-speckle patterns were painted on a base coat to track the deformation of the epoxy resin during thermal expansion. The trackability was validated by a silicon single-crystal standard reference material. The results demonstrate that the non-uniform thermal strain distribution reflects the periodic structure of the glass-fiber cloth material at temperatures ranging from 30 °C to 140 °C, i.e., in the region of the laminate surface where the interspaces among the glass-fiber bundles are wide, the thermal strain during heating is large, but the thermal strain on the glass-fiber bundle is small. The thermal strain in the thickness direction is large in the resin part and small on the glass-fiber bundle.