Adhesion of printed circuit boards with bending and the effect of reflow cycles

During manufacturing and use printed circuit boards (PCB) are subjected to different mechanical and thermal loads. These loads can cause the PCBs to develop a delamination between the insulating layers of pre-preg and conducting copper which can lead to failure of the entire electronic device. Therefore, it is critical to understand the delamination process and to know the adhesion strengths of the interfaces in a PCB to improve the device reliability. To evaluate the copper/pre-preg interface properties in PCBs a combination of experiments and modeling is used. The experimental characterization of interfacial adhesion strength was measured with a 4 point bending (4PB) technique. To find a context with application, the adhesion strength was determined as a function of reflow cycles. Finite element (FE) modeling was utilized to determine the optimum layer structure and the stiffness for the test specimens. In a second step, the FE model was used to study the influence of plastic deformation of the copper foils and the residual stresses developing during the reflow process on the adhesion strength. It could be shown that the calculated adhesion strength changed with the number of reflow cycles. The measured adhesion strengths were influenced by plastic deformation of the specimen and by residual stresses within the specimen.