Microvia Target Pad Interfacial Separation under Reflow Thermal Stress
Microvia target pad separation stems from z-axis thermal expansion strain exceeding electroless copper interfacial bond strength during assembly reflow.
Target Pad Copper Foil
Landing Surface
Internal copper layers designated as the end point for laser drilled microvias provide the necessary electrical and mechanical connection for vertical interconnects. The target pad copper foil must be thick enough to withstand the energy of the laser beam without being completely pierced. It acts as a stop for the drilling process, ensuring that the via depth is consistent across the entire panel.
This layer is usually a part of the inner layer core material before the buildup process begins.
Laser Interaction
Absorption of infrared or ultraviolet energy by the metallic surface prevents the beam from penetrating into the underlying dielectric. When the laser hits the target pad copper foil, the heat must be dissipated quickly to avoid delamination at the interface. High quality foil with a controlled grain structure ensures a clean ablation of the resin above it.
If the copper is too thin, the laser may burn through, creating a hole that cannot be plated reliably.
Thickness Requirement
Specifications for the base metal often demand a minimum of half ounce or one ounce copper to ensure a stable landing. Proper target pad copper foil selection prevents the formation of knee defects during the subsequent plating cycle.