Effect of thermal and vibrational combined ageing on QFN terminal pads solder reliability. (November 2020) - Arabi, F.
Abstract: Automotive environment generates vibration and temperature fluctuation, which reduces the life of electronic boards. The heavy trucks passing at relatively high speed on uneven road pavement is considered as the main reason of the traffic vibration. In fact, interaction between wheels and road surface causes a dynamic excitation which generates waves propagating in the soil affecting nearby structures. In addition, Thermal Cycling Test reveals the strict impact of environment conditions and slow cycles of temperature caused during operation. In this paper, vibration tests are performed continuously throughout thermal cycling to investigate the combined effect of temperature and vibration on the SAC305 solder of Quad Flat No-lead (QFN) terminals pads bonded on Printed Circuit Board (PCB). Power Spectral Density (PSD), which can be linked to the strain rate, is calculated from the acceleration, during combined ageing. Experimental results imply that the number of combined cycles affects the PCB responses: the value of the first natural frequencies and the corresponding PSDs. Fifty thermal cycles of combined thermal cycling and fatigue random vibration tests have been carried out on 100 QFN terminal pads solder. Statistical evaluation of the State of Health of the solder showed that cracking has started in 68% of them. An in-depth analysis was conducted to understand the reasons behind this degradation. On the other hand, an accurate numerical model for simultaneous combined thermal cycling and vibration simulation is developed and validated using experiments. These simulations make it possible to assess the displacement evolution due to combined loadings. Highlights: Thermal and Vibrational Combined Ageing methodology Effects of simultaneous thermal cycling and random vibration on the state of health of QFN Terminal Pads Solder Difference between fully exposed terminal pads and Wettable Flanks terminal pads (WF) Development of Multiphysics model able to quantify the stress accumulation during cycling Distribution of simulated equivalent stresses of terminal pads, terminal and thermal pads solders' using FE simulation