Quantifying Inner Layer Interconnect Strain during Solder Reflow
Matching dielectric CTE above Tg to copper ductility prevents inner layer interconnect fracture during high temperature reflow thermal profile cycles.
Blind Microvia Strain
Material Stress
Mechanical displacement occurs when thermal expansion coefficients diverge between the copper wall of a via and the surrounding dielectric resin. Blind microvia strain represents the physical tension loaded onto the base of a laser-drilled hole during reflow or subsequent thermal cycling. Excessive force leads to barrel cracking or pad cratering at the interface between the via structure and the internal capture land.
This failure mode initiates when the vertical expansion of the laminate exceeds the elastic limit of the copper plating.
Structural Impact
High aspect ratio structures experience heightened force concentration at the bottom corner of the blind hole due to the restricted movement of the copper column. Uneven heating across the printed circuit board assembly accelerates fatigue in these copper connections. Microscopic fractures propagate from the stress point into the pad area after repeated exposure to operational heat.
Thermal excursions beyond the glass transition temperature of the substrate significantly increase the magnitude of this internal load.
Detection Methods
Cross-sectional analysis provides the primary verification for integrity within the internal via structure. Micro-ohm resistance monitoring during accelerated life testing detects intermittent open circuits caused by structural deformation. Thermal shock testing forces the material to exhibit latent defects that remain hidden under static conditions.
Automated optical inspection fails to identify these internal separations because the surface pads often remain intact while the internal connection compromises electrical continuity. Reliable performance depends on matching the mechanical properties of the plating chemistry to the expansion profile of the base material.