 Buried microvias are required to be filled, while blind microvias on the external layers usually do not have any fill requirements.A stacked microvia is usually filled with electroplated copper to make electrical interconnections between multiple HDI layers and provide structural support for the outer level(s) of the microvia or for a component mounted on the outermost copper pad.The reliability of HDI structure is one of the major constraints for its successful widespread implementation in the PCB industry.