In situ analysis of the stress development during fabrication processes of micro-assemblies

Fabrication processes, like die attachment or plastic encapsulation, lead to thermal stresses in micro-assemblies due to CTE mismatches of the involved materials. In this paper, stress- and strain-sensitive test chips were used to measure in situ absolute stresses caused by die attach and encapsulation processes. Furthermore, deformations of assemblies over temperature were observed by white light interferometry and electronic speckle pattern interferometry. Applying these methods, e.g. the effect of entrapped air in thin adhesive layers of die attach assemblies on the absolute stresses in the silicon chip surface could be observed during cure. The resulting stress offset accounted up to 68% of the thermal stresses at RT in the assembly.