Browsing by Author "Mattila, Toni, Docent, Aalto University, Department of Electrical Engineering and Automation, Finland"

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Effects of accelerated lifetime test parameters and failure mechanisms on the reliability of electronic assemblies
(2014) Hokka, Jussi
School of Electrical Engineering | Doctoral dissertation (article-based)
This dissertation presents the results of accelerated reliability assessment methods employed on lead-free component board assemblies. Temperature cycling and mechanical shock tests are commonly used to assess the reliability of portable electronic products. Higher acceleration factors can be achieved by exposing the devices under test to higher loadings than those experienced in operation conditions or producing the loadings more frequently. Recently there has been an increasing interest towards the optimization of test parameters in order to minimize the time required for testing. However, if the effects of acceleration procedures, especially on solder interconnection microstructures, are not well-understood, misleading conclusions can be made that can lead to poor product reliability having disastrous consequences in the worst case. The results of this work demonstrate that the highly accelerated test conditions can lead to excessive lifetime acceleration and misleading failure mechanisms. It is shown that relaxation of the residual stresses has a significant effect on the shock impact lifetime of component boards while the failure mechanism(s) do not change with the increased impact repetition frequency. Relaxation of the residual stresses in load bearing materials takes place during the time between the impacts. The extent to which they can operate affects the way how the stresses/strains in the solder interconnections develop during further impacts. Similarly, lifetimes and failure mechanisms of component boards under thermomechanical cyclic conditions are shown to be dependent on the accelerated test parameters. In the highly accelerated tests, the microstructural evolution (recrystallization) controls the propagation of cracks, while in the real-use conditions, significantly less microstructural evolution takes place and the rate of crack propagation through the solder is notably lower. Re-assessment of the standardised test parameters and lifetime prediction models is therefore necessary in order to achieve better correlation between test conditions and real-use conditions. This work discusses different ways of achieving this target.