Towards a new methodology to characterize the fracture energies of the woven composite/copper interface in mode I and mode II: Application to printed circuit boards

The continuous trend towards miniaturization in electronic devices has stimulated the development of a new generation of printed circuit boards (PCBs) with embedded components. Throughout their lifespan, PCBs are subjected to thermal loads generated by heat from active components or the surrounding environment. In particular, mismatches in thermal expansion coefficients between materials are a leading cause of thermal stresses, often resulting in layer delamination, either between insulating substrates or at the copper-substrate interface. Traditionally, the peel test has been the dominant method for evaluating interfacial energy within PCBs, offering an estimate of the interface energy based on the IPC standard. During peeling, the copper layer often undergoes significant plastic deformation, complicating the precise determination of the fracture energy. Thus, achieving an accurate assessment of the mechanical response at the interface remains a challenging task. To overcome these limitations, we have designed a new specimen and adapted the Double Cantilever Beam (DCB) and End Notched Flexure (ENF) tests to the PCB context where layer thickness is significantly constrained (with copper layers ranging from 17 to 70 μ m ). Prior to experimentation, simulations demonstrate that, unlike the peel test, the DCB and ENF configurations exhibit minimal plastic dissipation. One of the main outcomes of the work is that a precise description of the plastic behavior of copper is not necessary to determine accurate estimations of the critical strain energy release rates in mode I and mode II. Furthermore, a notable advantage of these methods is their ability to maintain a controlled fracture mode, whereas the peel test inherently involves a mixed-mode (I and II) fracture process. The synergy between finite element analysis and experimental testing provides critical insights about the framework of application of the methods. • Copper/woven composite interface strength is evaluated in pure modes I and II. • A specific sample containing the copper/composite interface is proposed. • DCB and ENF tests enable to evaluate fracture energies in pure modes I and II. • DCB and ENF numerical models with CZM validate the identified fracture energies. • Plastic dissipation in copper during delamination is negligible unlike peel test.