Reliability analysis of solder joints under different thermal and thermo-mechanical loading conditions: Case study of automotive ECUs

Electronic control units (ECUs) in automotive industry are exposed to extreme harsh electrical and physical working conditions. Moreover, due to the continuous innovation in automotive industry, complexity and miniaturization of these systems are increasing. Beside the environmental factors, there are other new requirements facing the electronic components reliability in automotive industry accompanied by `electromobility'. These new requirements represented in charging have increased the operating time of the involved ECUs. In addition to that, Reliability as a definition is the ability of the device to perform its designed function in terms of years of useful lifetime. Furthermore, internal heat dissipation of electronic devices is increased which is caused by increased power densities of electronic devices needed for electromobility requirements. One of the governing factors determining the reliability of electronic control units are first level interconnect technologies; solder joints between electronic components and the printed circuit board. Accelerated testing experiments are used to obtain reliability information by visualizing the possible failure mechanisms during the useful lifetime relatively quick compared to the actual useful lifetime. Systems are subjected to more severe stress levels than those specified in the normal operating conditions. Interfacial structure of solder joints is a main factor determining solder joint reliability. The concerns are not only void formation between the solder and copper substrate, however, formation and growth of inter-metallic compounds (IMCs) is a concern as well. These compounds are brittle materials in nature which could crack and fracture when subjected to mechanical or thermo-mechanical loading leading to possible electrical or overheating failures. A design of experiment has been prepared using industrial electronic modules out of production line with an aim to understand the governing factors determining the reliability of electronic control units under environmental loading conditions. The modules have been stressed in active mode using different designed stress conditions. Failure analysis follows the aging process including functional testing, metallography and microscopy investigations. Evolution of the interfacial structure as well as the mechanical reliability of the solder joints under the different stressing conditions is discussed. Based on the observations obtained from this study, a deep understanding of the aging mechanisms under different environmental stress conditions has been reached. It has been concluded that the solder joint failure mechanisms are more dominant than failure mechanisms of other components. IMCs growth, voiding and cracking in the solder joints under thermal and thermo-mechanical loading is observed which could lead to malfunctioning due to electrical failures or overheating.