Accuracy enhancement of blind via depth-controlled drilling

Plated-thru-hole (PTH) vias extend from an external layer to the opposite external layer of a multilayer printed wiring board (PWB). Whereas, blind vias begin at an external layer - do not propagate to the opposite external layer - and end at an internal layer, hence the term "blind" via. While blind vias are more complex to fabricate the advantages are that layers beyond the blind via interconnect do not need to have a clearance area, as with a PTH via. Blind vias can also be stacked in the same collinear axis. This optimizes the use of available real estate for inner layer pads, planes, and traces. Furthermore, this aids in reducing the PWB layer count, This is critical in today's high density interconnection (HDI) designs. The drilling process to create blind vias contains additive and subtractive accuracy tolerance errors causing drill depths to be either too shallow or too deep. If too shallow, there will either be no conductive path to the target layer, or insufficiently exposed target layer copper, compromising the integrity and reliability of copper plating adhesion. If too deep, an electrical short to successive layers may be created. In addition to the inherent accuracy bandwidth of the electro-mechanical drilling system, thermally, induced errors of the mechanical subsystem introduce additional z-depth error. A new procedure will be introduced that allows accurate and repeatable drilling of blind vias at an optimum depth, regardless of the number of vias drilled.