Verification of Thermal Rework Degradation Limits for Multi-Layer Board Assemblies
Verification of thermal rework degradation limits relies on coupon continuous resistance tracking and microsectioning to prevent latent inner-layer via cracking.
Barrel Plating Cratering
Morphological Failure
Voids appearing in the resin adjacent to a copper-plated hole define a condition of mechanical separation. This physical event, known as barrel plating cratering, occurs when the thermal expansion of the board material exceeds the tensile strength of the resin system during soldering. It identifies a breakdown in the bond between the dielectric and the copper wall.
The condition is limited to the interface area surrounding the plated through hole.
Stress Mechanism
Repeated thermal excursions during the assembly process exert considerable pressure on the copper barrel. Large differences in the coefficient of thermal expansion between the z-axis of the laminate and the copper plating generate these forces. Because the epoxy resin becomes softer above the glass transition temperature, the metal wall pulls away from the supporting glass fibers.
Such movement creates micro-cracks. When the resin fails to recover its original shape, the permanent gap interrupts heat transfer and weakens the mechanical support of the via.
Verification Method
Microsectioning provides the evidence needed to confirm the presence of these fractures. Technicians polish the sample to a central plane to view the barrel wall under high magnification. An absence of resin contact indicates a failure to maintain structural integrity.
This check ensures the board meets IPC-A-600 standards for internal layer separation.