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48 DESIGN007 MAGAZINE I OCTOBER 2025 The coupon contains drill targets for each layer, and the movement of the layer from nominal can be cal- culated using an X-ray on each layer, hitting all con- nections 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 man- ufacturing 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 bal- ance 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 any- thing 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 I-007e'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 drill- ing 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 thick- ness, 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 materi- als 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 ultra- violet (UV) laser, which is best for drilling through copper, and the carbon-dioxide laser (CO 2 ), which is more suitable for dielectric material. Laser drilling creates less heat than mechanical drilling, and UV laser drilling generates less heat than CO 2 , so materials requirements can have a significant impact on how your manufacturer's drill team approaches the process. It isn't hard to imag- ine a design requiring three unique drilling meth- ods that could slow manufacturing and drive up costs, so designers must keep materials consider- ations 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 fin- ished 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, depend- ing on the aspect ratio, while blind vias are laser-drilled. C O N N ECT T H E D OTS