Topological Optimization of Microstructure of Adhesives under Thermal and Mechanical Loads

The paper presents a mathematical model and method for solving a wide class of problems in topological optimization of an adhesive joint to obtain an optimal microstructure and gradient properties in order to reduce the level of stresses arising from both thermal and mechanical loads therein.Adhesive joints have advantages over alternative bonding methods. The paper shows that the introduction of graduating properties in thickness or along the adhesive layer is the most promising strategy to optimize the adhesive. The approach is to modify the material properties or the geometry of the adhesive, varying along the joint.In all the papers known to authors, the shape of the elements to be joined, or the shape and location of the adhesive layer, were subject to optimization. The topological optimization methods to determine the optimal distribution / change of the gradient properties of the adhesive layer itself were not used.In the paper, the stresses arising in the solder joints are analyzed; it is shown that due to the small solder thickness, shear stresses are basic in it. The shear stresses are concentrated near the ends of the solder, and have the lowest values in the middle. The objective of the optimization problem is to reduce the peak values of the shear and peeling stresses in the solder layer. The topological optimization of the solder microstructure is to find the best distribution of a given amount of solder in the region in order to reach minimum peak values of stresses. The advantage of using topological optimization is that the microstructure of the solder should not be known a priori, and, thus, any designs can be optimized without first studying the effect of the original geometric parameters on the strength of the joint.The algorithm is implemented using the finite element method and the method of movable asymptotes. A number of examples are considered in order to obtain the solder microstructure to be optimal for reducing the peak values of shear stresses and delamination in a three-layer package.The results show that optimal microstructures significantly reduce peak stresses compared to a uniform layer. The obtained results reveal the potential of the developed algorithm and show that it can find practical use.