Manufacturers require accurate documentation from the design team to translate the manufacturing files into the expected board dimensions, performance, and other critical characteristics. These files correspond to individual steps in the manufacturing process, like the XY-data for pick-and-place assembly, Gerber/ODB++ for etch-resist photoimaging, and the PCB drill chart for drill hit location. The placement of drill hits is critical to ensure vertical connections between same-net pads while the table associated with the drill chart communicates vital information about the stackup and lamination process in an easily digestible standard format.
| What’s in a PCB Drill Chart? | |
|---|---|
| Drill Legend | Drill Map |
|
The xy-location of the individual drill hits as indicated by the corresponding symbol. Operators can see the total number of hits and the local density of drill hits at a glance. |
PCB Drill Chart Elements
The drill chart showcases the different hole sizes and characteristics across the board for manufacturing purposes. It primarily offers a quick reference for the hole locations; for legibility, it’s important to use simple legend characters that are visually distinct and easily discernible. Like all PCB artwork, a technician viewing a printed image cannot zoom in to see the difference between two nearly identical and overlapping drill symbols; legibility is essential, even if file viewing only ever occurs electronically. Ensure the placement of drill chart objects in the ECAD tool on the appropriate layer for output.
The drill legend contains all of the information related to the drilling cycle and generally includes the following categories for each unique hole template:
- The symbol ties the legend and drill map together. Use simple shapes (circles, squares, triangles) and easily distinguishable uppercase letters for a single legend to promote readability. Similar or complicated symbols make the drill map difficult to read.
- Hole counts indicate the number of unique drill bit hits across the board.
- Hole size expresses the final diameter after plating. Plated through-hole drilling oversizes the drill bit to account for plating, while non-plated through-hole drilling is to the exact specification listed. In other words, designers must only indicate the finished hole size to the manufacturer.
- Tolerance indicates the acceptable deviation from the nominal value due to drill wander, drill bit wear (for mechanical drilling), substrate thermal expansion during processing, and plating. Tolerances will depend on the board’s IPC Classification (for example, Class 2 boards use +/- 3 mils for PTH +/- 2 mils for NPTH for hole sizes less than 250 mils). Tighter tolerances incur higher processing costs and may be unnecessarily strict for the board application; consult with a manufacturer to optimize per-board costs on larger production lots.
- Hole type indicates a round (standard) hole or slot/cutout. Slots are generally more challenging to fabricate and can become a significant cost adder; for plated slots, a larger hole can sometimes suffice if board real estate allows it.
- Drill layer pairs indicate the terminal layers of the hole, e.g., a through-hole would have the top and bottom layers. For stackups with microvias, each laminate-and-drill cycle will require separate drill files.
- Some drill charts may differentiate between hole types, i.e., via or standard through-holes. While a via is a through-hole, it doesn’t offer mechanical support/stability for through-hole package assemblies; it only carries signals.
Via and Microvia Drilling
Typically, designers save the drill chart generation as part of the final project packaging and documentation, but it reflects critical stackup decisions decided earlier in the board’s development. As mentioned, microvias require a separate treatment during the stackup design and laminate-drill cycle. While microvias are cost adders relative to regular through-hole vias, they are often necessary for high-density interconnect applications where board real estate is at a premium due to their significantly smaller hole diameter.
Designers may wonder, “Why can’t I shrink a through-hole via diameter down to microvia hole diameters and save on the fabrication?” During thermal expansion inherent to PCB manufacturing processes (like soldering), the copper plating in the through holes experiences a significantly different expansion rate than the surrounding substrate in the z-axis, causing a stress-strain matrix to develop and intensify. The longer a via barrel is relative to its diameter, the less mechanical integrity it has and the more likely barrel cracking, pad lift, or other signal discontinuity defect modes occur. For reliability, manufacturers typically cap the maximum aspect ratio at 10:1 depth to diameter. Due to their unique depth-drilled construction, microvias can bypass this restriction.
Your Contract Manufacturer Drills Down into the Details of PCB Quality
A PCB drill chart communicates the essential and comprehensive information necessary for post-lamination drilling and helps operators meet the project’s design intent. Designers want to ensure the drill chart and files are easy to read with simple, distinguishable drill symbols that indicate hole location by template. At VSE, we’re a team of engineers committed to building electronics for our customers, including a thorough review of project documentation. We’ve been realizing life-changing and life-saving devices for over forty years with our valued manufacturing partners.
