Reliability of thermally stressed rigid–flex printed circuit boards for High Density Interconnect applications

• Rigid flex PCBs are designed for High Density Interconnect applications.
• Thermal cycling (−55 °C, +125 °C) of PCBs up to 6500 cycles is conducted.
• For the rigid flex design, failure occurs in the copper plating of buried holes.
• Finite Element Analysis is used to model the thermo-mechanical response of PCBs.
• Fatigue life increases with thick copper plating, large hole diameter and thin PCB.

The thermal fatigue of vias in rigid–flex printed circuit boards (PCB) is considered in the paper. Dedicated printed circuit boards have been designed with different geometrical configurations (plating thickness, drilled hole diameter and PCB thickness). The PCB is made of hundreds of vias or holes which are wired by copper path to create a daisy chain. The PCB is subject to cyclic thermal loading (−55 °C, +125 °C). Electrical connectivity is recorded during tests. Cross sectioning is performed finally to characterize the loss of electrical connectivity. Fracture of plated copper, due to the thermal expansion mismatch between constituents, is shown to be responsible for the failure of the PCB. In addition to environmental tests, finite element model is developed to analyze the deformation of PCBs during thermal cycling. Areas of strain concentration determined by Finite Element Analysis (FEA) are consistent with locations where cracks were observed in experiments. In addition, the numerical estimation of the plastic strain increment per cycle enables the prediction of the fatigue life. The results confirm that for rigid flex boards, the fatigue life of vias increases with higher plating thickness, larger drilled hole size and lower PCB thickness. Numerical results are shown to be in good agreement with experiments.