The latest increase in wiring density has been achieved in Surface Laminar Circuit cards and boards through the use of photosensitive dielectrics and
microvias. The Surface Laminar Circuit process starts with an FR4 printed wiring board without plated through holes (PTH). A liquid or film photosensitive dielectric is applied over the surface, and micro
vias connected to the layer directly below are formed by exposure and developing of the dielectric material. Through holes are then drilled, followed by a plating and etch process or pattern plate process
to form the circuitry.
The Surface Laminar Circuit microvia process can provide large numbers of small (4 mil) blind vias, consuming far less real estate than with normal PTHs. With blind vias, real estate is consumed only
on the Surface Laminar Circuit layer side, whereas a plated through hole consumes real estate on all layers —- even though the desired interconnection may be only from the top layer to the layer
below. The blind vias of Surface Laminar Circuit are smaller than mechanical blind vias or PTHs. This allows ultra high density wiring using 2 mil lines.
