Combined simulation and optical measurement technique for investigation of system effects on components solder fatigue

Among others, physics of failure related concepts are being developed to address the thermo-mechanical reliability challenges in automotive electronics. Limitations in particular applied for finite element (FE-) analyses are models of limited size, which rarely address the system character of failure. Also in testing a system view on fully mounted electronic control units (ECU) and loaded by environmental and active loading cannot be taken performing end-of-life tests for time limitations. Accelerated testing is done instead, however, mostly on board level. To overcome some of these limitations, a combined measuring-simulation technique is being developed, which is described in the paper. System level view on boards mounted in automotive ECUs is taken by a newly developed high-precision optical deformation measuring system. The multi-sensor measuring method combines a chromatic sensor for topography and warping analysis with an optical sensor for in-plane deformation and strain field analysis. By this combination, a high resolution can be reached for all three components of displacement vectors. Additionally, software tools allow the determination of derived quantities like strains, local curvatures and local warpage radius. The latter can be taken as input for FE-simulations. It is shown that some components, in particular QFNs, are sensitive to thermally induced cyclic warpage even if the corresponding bending deflections are in the micrometers range. Worst case in-plane stretching and cyclic warpage of a board mounted in an ECU have been measured. By corresponding simulations on QFN solder fatigue, mounted on a special test board, the critical fatigue life can be determined dependent on the interaction to the case, which differs by several hundred percent from a free-standing assembly