Effects of physical properties of components on reactive nanolayer joining

We studied the effects of the physical properties of components on a reactive joining process that uses freestanding nanostructured AlNi multilayer foils as local heat sources to melt AuSn solder layers and thereby bond the components. Stainless-steel reactive joints were compared with Al reactive joints. The strengths of both the stainless-steel and the Al joints increase as the foil thickness and thus the total heat of reaction increases until the foil thickness reaches a critical value. For foils thicker than the critical value, the shear strengths are constant at approximately 48 and 32 MPa for the stainless-steel joints and Al joints, respectively. The critical foil thickness for stainless-steel joining is 40 μm, compared with 80 μm for the joining of Al. Numerical studies of heat transfer during reactive joining and the experimental results suggest that the duration of melting of the AuSn solder is shorter when Al specimens are joined. Thus, a thicker foil is required to enable a sufficient duration (0.5 ms) of melting of the AuSn solder and full wetting of the metallic samples in order to form a strong joint. In general, when components with higher thermal conductivity, higher heat capacity, and higher density are joined, the duration of melting of the solder or braze layer is shorter and therefore a thicker foil is required to ensure the formation of a strong joint.