Importance of Tool Holes in PCBs
Most printed circuit boards from Rush PCB have holes in their periphery. These are Tool Holes, Locating Holes, or Mounting Holes, necessary for fixing the board on different machines during assembly. The tool holes allow the board to achieve the expected functioning for proper manufacturing.
Although the tool holes present on printed circuit boards are very ordinary, they are regular through holes that we drill on the circuit board during its fabrication. However, it is necessary to understand their importance and functionality. Some tool holes allow the board to connect to different tools, while others help in aligning the board during assembly and testing.
PCB Tool Holes and Their Functions
When we design and fabricate printed circuit boards, we drill many holes of different sizes on the board, suitable for different purposes. Most holes are via holes that route electrical signals between tracks on different layers, while others are necessary to hold the leads of through hole components for soldering. Usually, the designer associates the holes for mounting through-hole components with their specific package pattern, and marks them separately.
Some components, especially heavy ones, need additional mounting holes to give them increased physical support. An example of this is the connector, which requires bolting to the board to prevent its solder connections from the stress of plugging and unplugging. Other examples of such components are switches, batteries, fans, and speakers.
Some holes may also have the additional function of connecting the component to electrical ground or of conducting heat through the board to an outside layer.
Other mounting holes are necessary that assist the operators to install the PCB assembly into the system. The operator may install different hardware such as brackets on to the mounting holes.
PCB tool holes, on the other hand, are present on the board to facilitate mounting the board on different machines during the manufacturing and assembly stages. With the help of tool holes, the operator guides the PCB through its many assembly processes, such as drilling, stencil printing, component placement in a pick-and-place machine, automated optical inspection, and during testing.
PCB tool holes help to keep the board aligned and still during a specific operation. For instance, the board must not move laterally when drilling to achieve proper drilling accuracy. Similarly, the operator locks the board in place with the tool holes when using a stencil to deposit solder paste on the board. Tool holes also restrict lateral movement of the board when the operator is placing components using the pick-and-place machine.
PCB Tool Holes and their Design Criteria
If the manufacturing process requires PCB tool holes, the designer must add them when designing the board. In many cases, the manufacturer uses existing holes in the design or adds the required holes in the design documents.
We need the tool holes meant for mounting the PCB on different tools with proper design parameters, so they can perform their desired functions. Tool holes must meet the following design criteria:
Number and Location: Usually, there are three tool holes in a PCB. One at the bottom left-hand corner, one at the right-hand end of the longer edge of the board along the X-axis, and the other at the top left hand corner along the shorter edge of the board along the y-axis. In a panel, the location of the tool holes are usually on the outermost rails or tooling strips. The perimeter of the tooling holes must cover the center of gravity of the board for stability. Large boards may have an additional tool hole in the top right corner.
Preferred Size: The tool hole at the bottom left-hand corner of the PCB is typically round, with diameters of 1 mm, 1.5 mm, and 2 mm being the most commonly accepted size. The other two holes are also of similar diameter, but slightly elongated along the X- and Y-axis to account for differences in different tools. On a PCB, all tooling holes are of the same size for uniformity.
It is important to not confuse tool holes in a PCB with fiducial markers. The designer creates fiducial markers on the PCB as pads that provide a reference for optical pick-and -place machines.
Plating: It is critical that tool holes in a PCB do not have any plating. A non-plated hole has a higher accuracy compared to that of a plated one. For the same reason, the solder mask opening should be larger than the tool hole diameter.
Clearance: There must be adequate clearance between the edge of the tool hole and the nearest trace or component on the board.
Tool Holes on Small Boards
As PCB sizes are reducing, tooling holes may take up space on a compact PCB that the designer could have used for routing or placing components. As we fabricate small compact PCBs in panels, we place the tool holes along the tooling strips on the panel. Tooling strips are extensions of the panel, and the operator removes them when de-paneling the individual boards once the fabrication is over.
Placing tool holes on the tooling strips allows the designer to make full use of the space on the board. It frees up precious PCB space from tooling holes, and the designer may use it for routing traces or placing components more effectively.
Rush PCB recommends using a PCB CAD design tool to place tool holes in PCBs during design. You may consult us for any design queries regarding tool holes, and our experts will be happy to assist you in any way possible.