Internal Stress
Latent mechanical deformation within a composite circuit board structure originates from locked-in stresses generated during high-temperature manufacturing. This physical condition, known as residual strain, occurs because of mismatched thermal contraction rates between the epoxy resin and the glass fabric. It remains inactive within the material until subsequently released by mechanical processes or thermal changes.
Cure Dynamics
Lamination temperatures exceeding the glass transition point of the resin can solidify the matrix while the copper and glass are in an expanded state. As the laminate cools, the differential shrinkage between these materials creates a complex network of internal forces. If a large copper plane is subsequently etched away from one side of the board, the unbalanced residual strain will cause the thin laminate to twist or bow.
Fabricators use symmetric stack-up designs to ensure that these internal forces remain balanced across the neutral axis of the board.
Warpage Prevention
Thermal assembly processes, such as lead-free reflow, can trigger the release of these locked-in forces, resulting in dynamic coplanarity failures. Components with tight-pitch solder spheres may fail to make contact if the board undergoes severe board warpage during the heating cycle. Post-lamination baking represents an effective method to relieve these internal stresses before final assembly.
