In PCB manufacturing, the demand for increasingly complex and miniaturized designs continually pushes the boundaries of traditional fabrication methods, including depth routing.

Depth routing is indispensable in several key PCB applications:

• Rigid-flex PCBs: These hybrid boards combine rigid sections with flexible interconnections, requiring precise depth routing for removing the “cup” on top and/or bottom of the flex part.
• PCBs with cavities: Cavities are strategically created within PCB layers to integrate or embed components directly into the board.
• Copper coin PCBs for heat sinks: In high-power applications, copper coins are often embedded within PCBs to serve as highly efficient localized heat sinks. Depth routing is used to create precise pockets for these coins, ensuring a perfect fit.

Success in these applications demands not only on robust machinery but also sophisticated control functions. PCB manufacturers rely on advanced machine features and process methodologies to meet their precise depth routing goals. Here, I’ll explore some crucial functions that empower manufacturers to master complex depth routing challenges.

Depth Routing With a Second Measuring System
A common scenario in depth routing involves creating a constant depth of cut, even when the PCB panel itself is not perfectly flat—a frequent occurrence in manufacturing. In such cases, relying solely on a pre-programmed Z-axis depth from a fixed reference point can lead to inconsistent results.

To overcome this, machines use a second measuring system, typically involving a pressure foot equipped with a specialized insert or brush that precisely touches the surface of the PCB. The machine calculates the depth from the exact moment of contact and maintains it consistently throughout the routing path. This dynamic adjustment ensures that the routed depth is accurate relative to the potentially uneven panel surface. A typical and critical application of this technology is depth routing for cup elimination in rigid-flex PCB manufacturing.

This method is particularly advantageous for applications requiring extremely tight tolerances and direct reference to a conductive layer. It eliminates the minor mechanical variations that can occur with pressure foot systems, offering superior repeatability when the target is a copper feature.

This iterative micro-movement ensures that the machine is continuously “feeling” the copper surface. The result is a highly precise depth routing with minimal penetration into the copper, effectively “polishing” the surface by removing any remaining dielectric residue or micro-burrs, leading to a clean copper finish ready for subsequent processes.

Advanced Mapping Possibilities
For applications demanding an equal thickness of the remaining web (the precise distance between the end of the depth routing and the bottom surface of the panel), simple constant depth routing is often insufficient because of inherent variations in panel thickness. In such intricate cases, it’s imperative to create a “map” of the back-up panel (the bottom surface) before the routing process begins.

This mapping involves high-resolution scanning or measurement of the bottom surface topography. The resulting data creates a digital “map” that precisely informs the routing machine about the local thickness variations. The machine's control system then adjusts the Z-axis depth for each routed point according to this map. This ensures that even if the original panel thickness varies, the remaining web thickness is consistently uniform.

Vacuum Table With the Adaptor
For the most demanding depth routing applications, particularly those involving extremely thin or small panels, or panels with multiple cavities, maintaining absolute panel flatness and stability during machining is critical. A vacuum table or a specialized vacuum adaptor is imperative. By firmly holding the panel in place, the vacuum eliminates vibrations, prevents panel movement, and ensures optimal flatness across the working area. This rigid, consistent hold allows the machine's tools to perform accurate and repeatable depth routing processes, leading to superior quality results with the most delicate and challenging PCB designs.

Touch Probe
Building on the stability provided by a vacuum table or adaptor, the depth routing with touch probe control offers the highest level of accuracy for cavity creation. A specialized touch probe is mounted on the Z-axis, providing an independent and highly precise means of verifying and controlling the actual depth during the routing process.

There are several ways to use the touch probe technology:

1. Single Measuring + Routing:
   • Measure the panel surface: The touch probe precisely measures a single point on the panel surface to establish a definitive reference
   • Rout to defined depth based on panel surface: The routing path executes to the programmed depth, referencing the measured surface point
2. Multi-Measuring + Routing:
   • Measure the panel surface with a touch probe: The touch probe scans multiple points across the panel's surface
   • Different measuring points on panel surface/mapping process: This creates a more detailed topographical map of the needed area
   • Rout to defined depth based on mean value of mapped data: The routing path is dynamically adjusted based on an average or interpolated value from the mapped data points, compensating for localized surface irregularities across larger areas
3. Measuring after Routing:
   • Measure the panel surface: Take an initial surface reference
   • Measure a lower, already processed level and check the depth: After an initial routing pass, the touch probe measures the depth of the routed feature to verify precision. This could be a critical quality control step.
4. Adaptive Cavity Routing: This represents the pinnacle of precision, employing a closed-loop feedback system:
   • Measure surface by touch probe: Establishes the starting reference
   • Routing: Performs an initial routing pass
   • Measure depth with a with a touch probe: The probe measures the depth achieved after the pass
   • Final routing if necessary: If the measured depth deviates from the target, the machine performs a final, adaptive routing pass to achieve the specified depth. This process ensures unparalleled accuracy.

Laser Cases: When Mechanical Routing Reaches Its Limits
While mechanical depth routing offers immense capabilities, there are instances where its limitations require laser technology for skiving processes. These cases often arise when dealing with materials that are difficult to machine mechanically, when the required feature size and precision exceed mechanical capabilities, or when thermal impact needs to be minimized. Lasers, particularly high-end ones such as PICO-green, are perfect for tasks like removing thin coatings from copper pads or accessing contact pads with minimal damage.

A PICO-green laser can ablate microns with minimal impact on the next layer. This non-contact, highly precise, and localized energy delivery ensures minimal thermal impact on the surrounding material, preserving the integrity of sensitive copper pads and fine-line structures. Laser skiving provides superior control and cleanliness compared to mechanical methods for specific applications.

Summary
The ability to perform highly accurate depth routing is fundamental for producing today's complex PCB designs, including rigid-flex boards, PCBs with embedded components, and those requiring integrated heat sinks. Several machine features play a crucial role in ensuring depth routing accuracy. These include the previously discussed functions and capabilities, as well as clamped spindles that enhance tool stability, linear drives that enable precise and rapid movement, and adjustable valves that allow for accurate pressure control of the pressure foot, among other factors.

The strategic combination and correct use of these machine functions (or mix of functions), together with proper routing tools, suitable backup materials, and the expert work of a skilled process engineer, make it possible to produce the most intricate and demanding PCB designs. These advanced capabilities are essential for meeting the complex requirements of modern electronics, ensuring superior performance, reliability, and miniaturization.

This column originally appeared in the August 2025 issue of PCB007 Magazine.