Background Theory > PCB Design > Understanding Manufacturing Tolerances on a PCB

When fabricating a printed circuit board (PCB), a manufacturer must follow certain tolerances to ensure that the final product works as intended. Tolerances are the allowable amount of variation in a physical dimension, which is inherent in all manufacturing processes. In PCB manufacturing, these tolerances are defined by a range of acceptable measurements rather than an exact number.

Here are some of the key manufacturing tolerances in PCB production.

Drilling tolerances

Drilling tolerances are crucial in PCB manufacturing because they can affect the fit of the components and the alignment of the layers. There are two types of drilling processes in PCB manufacturing: mechanical drilling and laser drilling, each having its own tolerances.

Mechanical Drilling Tolerances

This is the conventional method of drilling holes in PCBs, and it's commonly used for through-holes and larger vias. For standard PCB manufacturing processes, the typical drilling tolerances for mechanically drilled holes might be as follows:

Hole size: ±0.003" to ±0.006" (±0.075mm to ±0.15mm)

Hole location: ±0.003" to ±0.005" (±0.075mm to ±0.125mm)

Laser Drilling Tolerances

Laser drilling is used for smaller holes, often in high-density interconnect (HDI) PCBs. The tolerances for laser-drilled vias (often called microvias) are usually much tighter. Typical tolerances might be:

Hole size: ±0.0005" (±0.013mm)

Hole location: ±0.001" (±0.025mm)

It's important to note that these values are approximate, and the specific tolerances can vary based on the manufacturer and the specific processes they use. As a PCB designer, you should always check with your manufacturer for the exact specifications they can meet.

In addition, keep in mind that tighter tolerances often mean higher manufacturing costs, so it's a good idea to design with the maximum tolerances that your design can afford. Lastly, make sure that your design software is set to use the same units (imperial or metric) that your manufacturer uses to avoid any conversion errors.

Track width/spacing tolerances

The track (or trace) width and spacing tolerances refer to the variation allowed in the width of the conductive tracks and the space between them. Depending on the manufacturer and the specific production methods, typical tolerances can be around ±10-20%.

The track width and spacing tolerances on a PCB are essential for ensuring that the board performs as intended. They directly impact the board's electrical performance and reliability.

Track width is the width of the conductive paths on the PCB, and it's crucial for determining the maximum current that the track can carry. On the other hand, track spacing is the distance between two adjacent tracks, and it must be maintained to avoid electrical shorts and meet the requirements for electrical clearance.

The tolerances on track width and spacing can vary depending on several factors, including the manufacturing capabilities, the PCB material, the copper thickness, and the complexity of the design.

Here are some general values for track width/spacing tolerances:

Track Width Tolerance

For standard PCB manufacturing processes, a typical tolerance on track width could be around ±10% to ±20%. For example, if a track width is specified to be 0.010" (10 mils), a tolerance of ±20% would mean the actual track width could vary between 0.008" and 0.012".

Track Spacing Tolerance

This also might be in the range of ±10% to ±20% depending on the manufacturing capabilities.

It's important to keep in mind that these are just general values, and actual tolerances can vary from manufacturer to manufacturer. Manufacturers with more advanced capabilities may be able to achieve tighter tolerances. Moreover, tighter tolerances can often lead to higher manufacturing costs. Therefore, when designing your PCB, it's essential to understand the tolerances that your chosen manufacturer can achieve and design your board accordingly.

Layer-to-layer registration tolerances

PCB layer-to-layer registration refers to the alignment of the different layers within a multilayer PCB. In other words, it ensures that each layer of the board lines up correctly with all other layers. This is important for the proper functioning of the PCB, as any misalignment could lead to shorts, opens, or other failures.

Tolerances for layer-to-layer registration can vary significantly depending on the specific manufacturer, design complexity, the board's size, the number of layers, and the technology used in the fabrication process. However, to give you a general idea, here are some typical values:

For standard multilayer PCBs (4-16 layers): around ±4 to ±6 mils (0.1 to 0.15 mm)

For high-density interconnect (HDI) PCBs: as tight as ±2 to ±3 mils (0.05 to 0.075 mm)

Remember that these are general values and the specific tolerances can vary. In any case, if you are designing a PCB, it is recommended to check with your chosen PCB manufacturer to get the exact specifications.

Finally, it's important to note that more precise registration (lower tolerances) often implies higher manufacturing costs. Therefore, when designing, it's recommended to allow the maximum possible tolerance that your design can afford to keep the production costs reasonable.

Impedance tolerances

Impedance control is essential for high-speed digital, RF, and microwave circuits. The impedance of the traces must be controlled to ensure proper signal integrity. Typical impedance tolerances range from ±10% to ±15%.

Impedance control in PCB manufacturing refers to the process of controlling the impedance of signal paths to ensure signal integrity, particularly in high-speed digital, RF, and microwave circuits. If the impedance isn't matched properly between the source, transmission line (the trace on the PCB), and the load, it can result in signal reflections, leading to data loss and degraded performance.

Impedance control is typically important for traces that are longer than 1/6 to 1/10 of the signal's wavelength. For digital signals, it's generally necessary when the signal's rise time is less than half the time it takes for the signal to propagate down the length of the trace.

PCB impedance is affected by several factors, including the width and thickness of the traces, the thickness and dielectric constant (Er) of the substrate material, and the distance to the reference plane (usually a ground or power plane).

When it comes to tolerances, typically manufacturers may offer:

Impedance tolerance

Generally, a standard tolerance range for impedance control in PCB manufacturing is ±10%. However, more advanced manufacturers may offer tighter tolerances, such as ±7.5% or even ±5%.

Keep in mind, achieving controlled impedance with tight tolerances often requires more advanced manufacturing techniques and therefore can increase the cost of the PCB.

The specific values depend heavily on the manufacturer's capabilities and the specific technology they use in their manufacturing process, so it's always a good idea to check with your manufacturer for their specific capabilities.

If you are designing a PCB that requires impedance control, be sure to specify this in your design files and communicate this requirement clearly to your manufacturer. You should also provide them with the target impedance value(s) and any specific tolerances required.

PCB thickness tolerances

The overall thickness of a PCB, as well as the thickness of individual layers, can also have specified tolerances. A typical tolerance on overall PCB thickness might be around ±10%.

PCB thickness tolerances refer to the allowable variation in the thickness of the PCB. This is important as the PCB thickness can influence a variety of factors, including the rigidity of the board, the size of the vias that can be used, and the space available for traces.

The overall thickness of a PCB, as well as the thickness of individual layers, can have specified tolerances. The actual values of these tolerances can vary depending on the manufacturer and the specific manufacturing processes used, but some general values might include:

Overall PCB Thickness Tolerance

For standard PCBs, the overall thickness tolerance is typically around ±10%. This means that if your specified PCB thickness is 1.6mm, the actual thickness could vary between 1.44mm and 1.76mm. For more precise applications, some manufacturers might offer tighter tolerances, such as ±5%.

Copper Layer Thickness Tolerance

The copper layer thickness, often specified in ounces (oz), can also have tolerances. For example, a typical 1 oz copper layer should have a thickness of 1.4 mils (or about 0.035mm), but the actual thickness might have a tolerance of ±10%.

Dielectric Layer Thickness Tolerance

The thickness of the dielectric layer (the non-conductive layer that separates the copper layers) can also have specified tolerances. This tolerance can impact the impedance of the board and is typically around ±10%.

Again, these values are just typical examples, and the specific tolerances can vary based on the manufacturer and the specific processes they use. As with all aspects of PCB design, it's important to check with your manufacturer for their specific capabilities and to make sure that your design can accommodate these tolerances.

Solder mask tolerances

This refers to the alignment and dimensions of the solder mask, which covers and protects most of the circuitry. Tolerances in the solder mask can affect component assembly and soldering processes.

Solder mask is an insulating protective layer applied to the copper traces of a printed circuit board (PCB) to prevent solder bridges from forming between closely spaced solder pads. It helps to prevent corrosion and electrical shorts, and also provides a professional appearance to the PCB.

Tolerances on solder mask application can have an impact on how accurately the solder mask aligns with the pads and how precisely it covers the traces and spaces. Misalignment or inconsistency can cause issues during the soldering process, leading to poor connection or potential electrical shorts.

Here are some typical tolerances related to solder mask:

Solder Mask Alignment Tolerance

This refers to how accurately the solder mask aligns with the copper features on the board. A typical tolerance might be around ±2 to ±3 mils (0.05 to 0.075 mm), but this can vary depending on the manufacturer and the specific processes they use.

Solder Mask Clearance Tolerance

This refers to the gap or clearance between the solder mask and the solder pad. This is typically specified in the design with a solder mask expansion or solder mask swell. The tolerance on this can be around ±1 to ±2 mils (0.025 to 0.05 mm).

Solder Mask Thickness Tolerance

The thickness of the applied solder mask layer can also have tolerances. The typical range might be around 0.8 to 1.2 mils (0.02 to 0.03 mm), but this can vary depending on the specific solder mask material and the application process.

Again, these are just general values, and the specific tolerances can vary based on the manufacturer and the specific processes they use. It's important to check with your manufacturer for their specific capabilities and to design your board with these tolerances in mind.

Copper weight/foil thickness tolerances

This refers to the thickness of the copper layers in the PCB. Higher copper weights allow for larger current carrying capacity but can be more challenging to manufacture with tight tolerances.

It's important to consult with your PCB manufacturer to understand the specific tolerances they can achieve, as this can significantly affect the performance, reliability, and cost of the final PCB. Different manufacturers may also have different capabilities when it comes to achieving tight tolerances, so it's worth shopping around and finding a manufacturer that can meet the specific needs of your design

The copper weight or foil thickness on a PCB refers to the thickness of the copper layer used on the board. This is important because it can affect the electrical performance of the board, including the current-carrying capacity and impedance.

Copper weight is typically specified in ounces (oz), representing the weight of copper spread evenly over an area of one square foot. Common copper weights used in PCBs include 0.5 oz, 1 oz, 2 oz, and even up to 4 oz for high-power applications.

Each of these weights corresponds to a specific thickness:

0.5 oz ≈ 0.0007 inches ≈ 18 µm

1 oz ≈ 0.0014 inches ≈ 35 µm

2 oz ≈ 0.0028 inches ≈ 70 µm

4 oz ≈ 0.0056 inches ≈ 140 µm

The tolerance on the copper weight or foil thickness refers to the allowable variation in this thickness. This can vary depending on the manufacturer and the specific manufacturing processes used. However, some general values might include:

Copper Weight/Foil Thickness Tolerance

The copper foil thickness tolerance can typically be around ±10% to ±20%. However, with advanced manufacturing processes, some manufacturers may be able to achieve tighter tolerances.

So, if you specified a 1 oz copper weight, the actual thickness could vary between 0.00126 inches and 0.00154 inches (or between 32 µm and 38 µm) at a ±10% tolerance.

These are just general values and actual tolerances can vary. It's important to check with your specific PCB manufacturer for their capabilities, as achieving precise copper weights may involve additional costs. As with all aspects of PCB design, it's important to ensure that your design can accommodate these tolerances.