One of the hallmarks of a great detective is perceiving the importance of something that appears to be completely unobtrusive on the surface. A simple footprint unnoticed by others may be the clue that solves the mystery. Similarly, some of the holes in a printed circuit board may seem unimportant, but without these “tooling holes,” the board couldn’t be used as intended or even manufactured correctly.
Taken by themselves, the tooling holes in a circuit board would seem commonplace as they are regular-looking thru-holes drilled in the board. But a closer look at their purpose will reveal their importance. Some are used for attaching hardware on a circuit board, while others are designed to help with the board’s fabrication and assembly. We will examine these different holes in greater detail as we discuss PCB tooling holes and the design guidelines that accompany them.
PCB Tooling Holes: Why They are There, and What They Do
Circuit boards will typically have holes of many sizes drilled in them, used for different functions. Usually, the majority of the holes are for vias that interconnect electrical signals between layers. There will also be holes that component leads will solder into and holes used for mounting different hardware types. Tooling holes used for attaching hardware, also known as “mounting holes,” can be associated with component footprint patterns or isolated by themselves.
In addition to having leads or pins that solder to the circuit board, several components have mounting hardware for added physical support. Connectors often bolt to the board to protect their solder connections from the stress of being plugged and unplugged from their cable. Other components that are heavy or serve as interfaces may also be bolted down—including switches, fans, batteries, and speakers. These holes may also need to connect the mounting hardware to electrical ground or conduct heat through the board to be dispersed on interior plane layers. Lastly, to mount a circuit board into its system, mounting hardware or brackets are often attached to the board using isolated holes.
PCB tooling holes can be put into the board to assist with the different phases of manufacturing. These holes guide the board through its assembly processes, such as solder reflow, wave solder, and automated optical inspection (AOI). Tooling holes are also used during the testing process if the board needs additional test fixture support. However, most of the tooling holes for production are added to the PCB contract manufacturer’s panel based on their requirements instead of the board.
Creation of the Different Holes on Your PCB Design
If tooling holes are needed in the board for any manufacturing processes, the PCB contract manufacturer may request they be added to the design. Typically the CM will either use holes that are already on the board or add their own to the fabrication files. Creating the holes used for mounting hardware is a different matter, and they must be made with the correct design parameters to function as expected:
- Location: The number of holes used for tooling and where they are located is dependent on what the circuit board and its components require. Component datasheets typically specify the location of these holes for the designer to include in their PCB footprints.
- Preferred size: This again is dependent on the requirements of the components. Usually, these parts will use 4-40 or 6-32 screws, and the holes must be drilled to match the mounting hardware. If no system mounting holes have been included in the design, PCB CMs will recommend adding some holes with a generic size of 0.100 or 0.125 inches to give their clients mounting options for the future.
- Plating: This is also dependent on the hardware’s needs that will be attached to the hole. For hardware that needs to be connected electrically to the board, the hole must be plated and connected to the appropriate net. Otherwise, a non-plated hole will be used.
- Plane connections: A plated hole connected to either a power or ground plane must not be connected to any other planes to prevent shorting power and ground nets together. While CAD tools have net checking built into them to prevent this, incorrectly designed mounting holes may end up fooling design rule checks.
- Spacing: Holes that house mounting hardware typically require a larger keep-out zone around them. There needs to be plenty of room for the mounting hardware to firmly contact the board without damaging component leads or metal traces and planes. Additional clearance is also crucial around plated holes, which are always drilled oversized so that the added plating material will finish them at their desired hole size.
While tooling and mounting holes may seem neutral to PCB layout, the truth is that they need to be designed with the same attention to detail as any other part of the board. There are also some other elements of the design that are important for manufacturing.
Preparing Your Board with the Design Elements Necessary for PCB Manufacturing
In addition to the different PCB tooling holes required for mounting hardware and production aids, there are other PCB elements that your design will need for manufacturing. These include solder paste pads for surface mount component pins, solder mask to protect the board and silkscreen markings for component reference designators, and corporate identification.
Another critical design element that you should include are fiducials, which are small pads used for circuit board alignment during automated assembly. Fiducial pads are usually placed in the corners of the board and at the center of large SMT devices with high pin-counts.
It can be challenging for designers to second-guess all of the requirements necessary for successful circuit board fabrication and assembly. The best thing you can do is work together with your PCB CM to ensure that your design has all of the necessary elements needed for error-free manufacturing.
At VSE, we will review the PCB layout and confirm whether or not it has the minimum features it needs. Our engineers will also work with you on any design areas that we see as potentially problematic and recommend changes that will improve your design’s manufacturing, test, and reliability.