Microvia Integrity
The complete closure of blind, laser-drilled microvias with epoxy resin during the lamination process or through a subsequent paste printing process prevents air entrapment in the multi-layer structure. Microvia resin fill must be managed to ensure that no voids remain inside the via cavity before the outer layer copper is plated. This process is used to build high-density interconnect substrates where vias are stacked or placed within component solder pads.
It defines the flatness of the surface for subsequent assembly.
Filling Mechanism
Capillary action and vacuum pressure during lamination drive the liquefied prepreg resin into the microscopic via holes, or a dedicated squeegee system applies conductive or non-conductive fill pastes. If the resin viscosity is too high or the vacuum is insufficient, the resin cannot fully penetrate to the bottom of the microvia, leaving a hollow pocket. This trapped air expands when the board passes through the reflow oven, creating high-pressure stresses that can crack the copper plating or cause laminate delamination.
To ensure a reliable joint, the filled via must be cured and then planarized to a flush finish with the surrounding laminate surface.
Plating Connection
Once the microvia is filled and planarized, a capping layer of copper is electroplated over the top of the via. This copper cap provides a solderable surface for mounting ball grid array components. This arrangement allows for increased trace routing density.
