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Connect the Dots: Designing for the Reality of UHDI PCBs—Drilling

Ultra high density interconnect (UHDI) PCBs are changing the game in designing for the reality of manufacturing. With both consumer and industrial electronic devices becoming more advanced, the demand for UHDI PCBs will grow. That means we’re all likely to be designing more UHDI boards. UHDI advanced miniaturization technology challenges designers with regard to both board thickness and footprint. Designers will face more variables in every aspect of design creation. This is certainly the case with drilling.

Drilling is already one of the more complex and tricky steps in the manufacturing process. Designers must consider everything from board thickness and via size to material type to craft their designs and avoid problems during drilling that can drive up costs and increase the risk of board performance issues. With UHDI designs there is a lot for the designer to consider.

A Quick Refresher on Sound Drilling Practice
Drilling follows lamination in the multilayer PCB board manufacturing process. Once the lamination process is complete, we have a book—a stack of panels manufactured into a multilayer PCB. Panel preparation takes place before drilling can begin. After cleaning, we remove the flash around the edges to create a perfect rectangle that can be handled safely.

To ensure drilling accuracy, we use a test coupon. The coupon contains drill targets for each layer, and the movement of the layer from nominal can be calculated using an X-ray on each layer, hitting all connections and missing clearances.

However, it’s more than simply programming the drill. If you've read my articles over the years, you know that I consider much of PCB design and manufacturing to be both art and science. Drilling is no exception.

The following are examples of how attention to detail, sound communication, and good old-fashioned know-how ensure that quality panels leave the drill department:

• Good design notes help the drill team balance constraints during manufacturing
• Designers collaborating closely with the CAM team on components like hole sizes, vias, and through-hole components eliminate uncertainty about instructions in the notes.
• A good CAM department accounts for anything that goes in the hole, adjusts the hole size to account for this, and ensures the drilling process finishes with the proper tolerances
• Cohesion between the CAM and drill teams is critical. The book’s inner layers skew and stretch once mashed together, and the drill team will need more than the right bit size from CAM. They’ll need help predicting layer behavior.

It all begins with the designer, the design, and the design notes. For a full rundown of drilling best practices, check out this recent episode of PCB007’s On the Line With… podcast.

Design for Drilling More Complex UHDI Boards
There’s a lot for the designer to consider. Even so-called simple decisions can become complicated. Conventional wisdom might suggest that laser drilling is necessary for UHDI PCBs, but not always. The space you save by miniaturizing the circuitry in UHDI can allow for more room for through-hole vias and less need for blind or buried vias. Using UHDI can also reduce layer count and PCB thickness, making through-vias more successful. Using through-vias instead of blind and/or buried vias can save PCB costs and shorten lead times. However, the high layer counts and UHDI’s fine features require more advanced via structures. These could include laser-drilled microvias and buried vias, etc., which are not a one-size-fits-all proposition.

Drilling creates heat, melting the epoxy materials inside the hole, and can create defects in the via walls, making the drilling process sensitive to material composition. There are two main types of lasers used in PCB drilling: the more precise ultraviolet (UV) laser, which is best for drilling through copper, and the carbon-dioxide laser (CO2), which is more suitable for dielectric material.

Laser drilling creates less heat than mechanical drilling, and UV laser drilling generates less heat than CO2, so materials requirements can have a significant impact on how your manufacturer’s drill team approaches the process. It isn’t hard to imagine a design requiring three unique drilling methods that could slow manufacturing and drive up costs, so designers must keep materials considerations in mind as they lay out their boards.

UHDI needs precision drilling to create tiny microvias for these compact designs. Designers should account for aspect ratios, via structures, and connectivity requirements to ensure the finished board’s reliability and signal integrity. 

Specific design considerations include:

• To ensure reliable plating, keep the ratio of the via's depth to its diameter (aspect ratio) very low. Drill depth should not exceed hole diameter, meaning your aspect ratio should not go above 1:1. For advanced UHDI designs, the ratio may be as low as 0.8:1 or 0.75:1.
• Watch your blind and buried vias. Blind vias connect an outer layer to an inner layer, while buried vias connect two or more inner layers. If you are working on a tight budget and want to leverage mechanical drilling where possible, you can drill buried vias mechanically, depending on the aspect ratio, while blind vias are laser-drilled.
• Depending on the environment your board will need to operate in, staggered vias—those offset from one another—are considered more reliable than vias placed directly atop each other (stacked) because they relieve stress on the board and improve its manufacturability.
• If you have extremely high connectivity requirements and need to use stacked vias, designers can use every-layer interconnect (ELIC) structures—a type of stacked microvia that connects every adjacent layer. The trade-off here is higher cost and decreased manufacturability. 

Drilling is a critical part of the manufacturing process. With UHDI projects in particular, getting quality, manufacturable boards starts with a design optimized for drilling. To learn more, please download my book The Printed Circuit Board Designer’s Guide to... Designing for Reality.

This column originally appeared in the October 2025 issue of Design007 Magazine.