Laser Drill Micro Vias

HDI and Micro-Vias

As printed Circuit Boards continue to decrease in size and increase in complexity, your PCB fabricator has to keep up. As a designer, you know old processes are revised and new ones are added, including those needed for High Density Interconnect (HDI), is your PCB fabricator ready?
HDI is one of RUSH PCB Inc specialties. HDI requires more than fine lines in the foil pattern, it requires Sequential Build-Up (SBU), and laser drilled Micro-Vias.

Sequential Build-Up

Multilayer technology using SBU is the addition of one layer of dielectric and foil pattern at time, and allowing for processing HDI Micro-Vias before adding another layer.
These layers are added to the substrate in pairs, one layer on each side of the substrate, to balance the stresses to the substrate. This is 1, 2, 3, etc. layers on each side of the substrate (N), or 1+N+1, 2+N+2, 3+N+3, etc.

Laser Drilled Micro-Vias

The whole idea behind laser drilled Micro-Vias is to reduce the space consumed by through the board vias. The Micro-Vias, having been drilled by laser, are smaller in diameter than through the board vias, and because they’re smaller, more space can be devoted to circuit traces.

A Micro-Via links only one layer of foil to the next; layers above and below these are available for foil lines, further increasing overall circuit density.

As each layer of SBU is added, a laser drills a hole from the outer layer of foil, through the dielectric that was just laid down, to the next inner layer of foil. This hole is then plated and filled. If there is an additional layer of SBU, the Micro-Via becomes buried.

Not all fabricators fill their Micro-Vias. RushPCB fills the Micro-Vias with solid metal for stronger connections and to obtain better thermal management, increasing the overall board reliability.

High density I/O components sometimes require Via-In-Pads (VIP) just to connect traces to all solder pads. When VIPs are used, filling the Micro-Via helps keep the pad smooth, allowing for a better reflow of solder.

Filling also allows the Micro-Vias to be stacked on top of each other to connect three or more layers of foil, further increasing density.

Contact RUSH PCB for more information about HDI technology at

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