Pull-off test in the assessment of adhesion at printed wiring board metallisation/epoxy interface

摘要

It is shown that the value of adhesion strength at the metallisation/polymer interface can be determined directly from a printed wiring board (PWB) by a pull-off method. However, careful optimisation of the test geometry is required. In particular, the mechanical properties of the substrate are important. The use of flexible substrate in the test results in severe underestimation of the adhesion strength. There was a considerable stress peak at the edge of the test area, as shown by a finite element method (FEM) calculation with the flexible construction. This unevenly distributed stress decreases the force needed to detach the coating. In addition, if the test area is not defined to match the stud of the test equipment, the stress peak appears at the adhesive/coating interface rather than at the coating/metallisation interface. The experimental data and the examination of the fracture surfaces by scanning electron microscope (SEM) were consistent with the FEM results. In addition, the effect of environmental stresses on the adhesion was investigated. The environmental tests consisted of exposures to (1) thermal shock, (2) elevated temperature and relative humidity and (3) corrosive gases NO_2, SO_2, H_2S and Cl_2. The employment of embedded capacitors during the exposures revealed the diffusion of water into the epoxy, which weakened the adhesion. The corrosive gases induced irreversible deterioration of the dielectric material. An adhesion promoter enhanced the durability of the coating/Cu interface in the environmental tests. X-ray photoelectron spectroscopy was used to analyse the chemical states of the pre-treated copper surfaces.