What are the tips for specifying PCB hole tolerance

One of the more forgotten topics in PCB design is the holes for mounting components. Tolerances on hole dimensions are specified in PCB manufacturing to ensure proper fit of Plated Through Hole (PTH) components.

Altium Designer PCB design software can add hole tolerance properties to your pads and vias, which will be passed to the manufacturer through inclusion in the drill table. Here are five tips to help you quickly specify the size of holes in your next PCB design.

Tolerance

Component data sheets list plus/minus tolerances to accommodate changes in aging, wear, temperature, plating, materials, machining, etc. For example, a 1/4 watt resistor manufacturer's data sheet specifies a wire diameter of 0.022 ± 0.003. Therefore, the actual diameter can vary from 0.019 to 0.025.

Usually PCB manufacturers specify a hole tolerance of ±0.004. The wire must always fit in the hole, whether on the big or small end of the tolerance zone. Therefore, the minimum hole size must accommodate the maximum resistor lead plus tolerance (0.022 resistor lead + 0.003 resistor lead tolerance), plus 0.004 PCB hole tolerance. Therefore, 0.022 + 0.003 + 0.004 = 0.029 inches, the smallest hole size allowed in the board.

When drilling, the drill wears less. Alternatively, the drill may vibrate or wobble slightly in the hole, resulting in a slightly larger hole. The holes are then plated, and for each batch or location on the board, the plating can be thicker or thinner. You must also take into account the thermal expansion or contraction of the PCB substrate as it undergoes processing. Therefore, hole tolerances are critical in the design process to ensure proper placement of PTH parts. A rule of thumb is that you should make PCB holes 0.007" larger than the part wire diameter to accommodate all tolerances, drill wear or wobble, and plating variations. There is no default hole tolerance value in Altium Designer. You can adjust the hole tolerance properties in the Pad and Properties dialog. Hole tolerances and default values can also be established in the Pad Via Library panel and Footprint Library.

5 Tips for Specifying PCB Hole Tolerance

Hole tolerances in Altium Designer can be accessed and edited using several different methods, which we will examine below. In each method, you can set the minimum (-) and maximum (+) hole tolerance properties.

Hole tolerances in Altium Designer can be accessed and edited using several different methods, which we will examine below. In each method, you can set the minimum (-) and maximum (+) hole tolerance properties.

Tip 1 - Set Hole Tolerance Properties for Specific Pads and Vias

You can quickly set pad/via tolerances based on individual properties.

Right-click a pad or via and select Properties. In the Pad Properties dialog (Figure 1), edit the hole tolerance at Hole Information.

In the Through Hole Properties dialog, edit the hole tolerance at the upper left corner "Tolerance" in Figure 2.

Figure 1 Setting the hole tolerance in the Pad Properties dialog Figure 2 Setting the hole tolerance in the Via Properties dialog

Tip 2 - Use Pad or Through Hole Templates to Set Hole Tolerances

You can also specify hole tolerances using pad or through-hole templates.

Right-click Pad Via Library and select Add Via Template or Add Pad Template. Hole tolerance can be set at "Hole Information".

Tip 3 Set Hole Tolerance Properties for Multiple Pads or Vias at once

Conneniently, you can set the hole tolerance for multiple pads or vias at the same time.

Open the "PCB Inspector" panel, as shown in Figure 3. Select the pads or vias on the PCB to set and enter the necessary hole tolerance values in the right column under the "Object Specific" panel.

Table 1 Example table showing all hole tolerance columns

You can also group them by hole tolerance. From the Drill Symbols dialog (click Configure Drill Symbols in the Drill Table dialog), click Grouping, and select Hole Tolerance.

Note that when adding hole tolerance information to pads and vias, unless all pads or vias grouped under the "Count column" column have the same hole tolerance attribute, the hole tolerance value will be displayed as * (Asterisk).

Note: After adding information to the drill table, you must click OK to exit the Drill Table dialog or your changes will not be saved.

In conclusion

Ensuring proper hole tolerance is critical for proper mounting of PTH components to the PCB. Altium Designer simplifies the documentation of your hole size tolerances so that your tolerance specifications can be easily communicated to your manufacturer.