Selective Soldering Preheat Windows for Multi Layer High Thermal Mass Assemblies
Selective preheat windows for heavy multi-layer boards require top-side convection to achieve 110-130°C without exceeding flux thermal activation limits.
Z-Axis Delamination
Layer Separation
Internal failure of a multi layer printed circuit board happens when the adhesive bond between the copper and the resin breaks down. During thermal stress, z-axis delamination appears as a physical gap or crack that runs parallel to the board surface. This defect often originates at the interface of the glass reinforcement and the epoxy matrix.
It destroys the structural integrity of the assembly and can lead to the fracture of copper vias. The separation usually begins in areas with high resin content or near large internal copper planes.
Stress Origin
Differences in the coefficient of thermal expansion drive the separation of the layers. Because the resin expands much more in the vertical direction than the copper or the glass fibers, z-axis delamination occurs when the internal tension exceeds the bond strength. This is particularly common during lead free soldering where the peak temperatures are higher.
Repeated thermal cycling further widens the cracks.
Inspection Criteria
Visual identification of the defect is often difficult because the separation occurs inside the board. Technicians use micro sectioning or acoustic microscopy to confirm the presence of z-axis delamination after a failed reliability test. If a gap is found, the entire batch is usually quarantined.
This failure is a major reason for rejecting boards intended for harsh environments.