Modeling Slip Velocity and Pressure Gradients across Smooth Copper Foils
Modelling slip velocity and pressure gradients across smooth copper foils prevents resin starvation and locks dielectric thickness tolerances during lamination.
Post-Cure Residual Stress
Internal Tension
Mechanical energy remains trapped within a polymer matrix after the completion of the chemical hardening and cooling process. The development of post-cure residual stress occurs when the resin shrinks during polymerization or when different materials in the assembly contract at different rates. This phenomenon can lead to the warping of the board or the cracking of sensitive components.
Designers must account for these forces when choosing the thickness of the dielectric and the distribution of the copper.
Dimensional Stability
Thermal expansion mismatches between the glass reinforcement and the epoxy resin are the primary source of these forces. In high-density interconnects, post-cure residual stress can cause the micro-vias to pull away from their target pads. This tension persists even when the board is at room temperature.
Warpage Control
Annealing cycles help to relax the molecular structure and reduce the magnitude of the internal forces. By holding the board at a temperature just below the glass transition for an extended period, the post-cure residual stress is slowly dissipated. This step is particularly effective for thick multilayer boards that would otherwise bow during the assembly process.
Monitoring the flatness of the substrate before and after the baking cycle provides a measure of the effectiveness of the stress relief.