Numerical simulation approach for consideration of ageing effects in PCB substrates by modifying viscoelastic materials properties

During operating time of electronic systems, the used materials in such devices are potentially subjected to ageing effects, which might limit the lifetime. Therefore, knowledge about the used materials and the way the materials are affected by ageing effects is of key importance to develop reliable products.

In this study, a simulation approach is discussed that is able to consider ageing effects caused by oxidation at elevated temperature of a printed circuit board material, typically used for high frequency applications. The material was characterized for its thermomechanical properties with state-of-the-art techniques for different ageing durations. Ageing was accelerated by storing the samples in an oven at 175 °C for up to 1000 h.

Within the simulation workflow, the thermomechanical properties of the different aged states are defined by modifying the pristine viscoelastic properties. Four exponential functions are derived modifying the initial modulus, the characteristic time constants, the shift function and the coefficient of thermal expansion, all in dependency of ageing time.

To demonstrate the approach, the soldered interconnection lifetime of a theoretical chip-size-package on a printed circuit board is studied. State-of-the-art lifetime predictions of such interconnections only include thermomechanical ageing effects, for example by creep effects of the solder. By additionally considering the ageing of the printed circuit board, thermal ageing is combined with thermomechanical ageing.

Results in the soldered interconnection are compared between either considering additional ageing effects of the printed circuit board or neglecting this behavior. Thus it is shown that thermal ageing plays a significant role in the development of accumulated creep strain which becomes increasingly important with increasing expected lifetime.