An important HDI PCB design practice is the via-in-pad technology of placing eyelets in transition holes within the contact pad to miniaturize the size of the PCB by reducing the space required for routing traces.
It is common for solder to seep onto the back layer of the PCB. To prevent this, the via-in-pad requires the holes to be filled and sealed. This is accomplished by filling through-holes or blind holes with conductive or non-conductive material. Micro-holes can also be filled and plated.
The holes are typically filled with conductive epoxy. The use of conductive materials increases the bandwidth of the PCB. Non-conductive materials block the flow of solder through the channels, but do not affect the DC electrical performance.
Characteristics of Ultra HDI (UHDI) PCBs: conductor width and spacing are less than 50 µm, dielectric thickness is less than 50 µm, and microwire diameter is less than 75 µm.
The holes are typically filled with conductive epoxy. The use of conductive materials increases the bandwidth of the PCB. Non-conductive materials block the flow of solder through the channels, but do not affect the DC electrical performance.
Characteristics of Ultra HDI (UHDI) PCBs: conductor width and spacing are less than 50 µm, dielectric thickness is less than 50 µm, and microwire diameter is less than 75 µm.