Effect of rolling deformation on the thermal expansion properties of ultra-thin Kovar alloy foils

Lightweight, flexible, and highly integrated electronic packaging demands the micron-scale ultra-thin (<50 µm) Kovar alloy foils to meet the stringent dimension requirements. In this study, we fabricated the micron-scale ultra-thin Kovar alloy foils by rolling deformation and systematically investigated the effects of rolling deformation on them. It was found that the deformation mechanism of ultra-thin Kovar alloy foils diverges significantly from that of their thicker sub-millimeter (>100 µm) counterparts. Severe rolling deformation induces a structural phase transformation from the face-centered cubic (FCC) austenite to the body-centered cubic (BCC) martensite. Moreover, the local lattices of the FCC austenite are substantially distorted and deviate from the cubic symmetry by severe rolling. These microstructural changes increase the coefficient of thermal expansion (CTE) and introduce in-plane CTE anisotropy of the ultra-thin Kovar alloy foils, which has not been observed in the corresponding sub-millimeter (>100 µm) foils. Additionally, it was found that the saturation magnetization of Kovar alloy foils increases with decreasing foil thickness, which is due to the increased volume fraction of BCC martensite caused by severe rolling. These findings provide important insights for tailoring thermal and magnetic properties of the ultra-thin Kovar alloy foils, which are beneficial to the advanced electronic packaging applications.