What is Track width (TW)?

Track width (TW) is one of the most important design parameters in printed circuit board (PCB) layout. It refers to the width of the copper tracks on a PCB that connect components and transmit electrical signals. Choosing the right track width is essential for proper functioning of a circuit and to avoid issues like overheating, signal loss, electromagnetic interference etc. This article provides a detailed overview of PCB track width - what it means, how it is calculated, what factors affect it, and how to select the optimal width for a given design.

What is Track Width?

The track width specifically refers to the width of the copper tracks or traces on the PCB that provide electrical connections between pads and components mounted on the board. It is usually denoted by the symbol 'W' or 'TW'.

PCB tracks of different widths connecting components

These tracks are typically made by etching away unwanted copper from a copper clad laminate sheet, leaving only the desired connections. The track width depends on the amount of current the trace needs to carry, tolerable voltage drop, track impedance and other constraints. Typical PCB track widths range from 0.2 mm to 2 mm. High current tracks can be wider than 2mm.

Some key points about PCB track width:

• It is usually specified in mm (or mils - 1/1000th of an inch)
• Thicker tracks can carry more current than thinner tracks safely.
• Thinner tracks allow more routing flexibility but have higher resistance.
• Matching track widths allow impedance control for high speed signals.
• Minimum width is set by manufacturability constraints.
• Very wide tracks may need split planes or voids to manage heat dissipation.

Why Track Width Matters

The width of copper tracks on a PCB determines a number of important parameters that impact the functionality, quality and reliability of the board. Some key factors influenced by track width are:

Current Carrying Capacity

• Thicker tracks can carry higher current as per ampacity tables. Thinner tracks may overheat and burn out when current exceeds safe limit.

Voltage Drop

• Thinner tracks have higher resistance, leading to larger voltage drops for long traces. Thicker tracks minimize resistive losses.

Track Impedance

• With high speed signals, track widths must be tuned to get characteristic impedance typically 50Ω or 75Ω.

Safety Spacing

• Minimum spacing between tracks required for given voltage rating. Wider tracks need larger spacings.

Manufacturability

• Very thin tracks prone to etching flaws. Very thick tracks affect etch uniformity.

Routing congestion

• Thinner tracks allow more routing flexibility in congested PCBs.

Signal Integrity

• Controlled width tracks reduce reflections, crosstalk issues in high speed digital circuits.

Cost

• Board with thinner tracks needs less copper area, reducing raw material cost.

As seen above, track width impacts many vital electrical and physical characteristics of a PCB. An optimal width must be selected after considering various design constraints.

How to Calculate Track Width

The required track width for a given current carrying capacity can be calculated using the ampacity table for common PCB copper weights. Here is the typical process:

1. Determine Required Current - Based on the power requirements of components being connected by the track (load current).
2. Determine Copper Weight - Thickness of copper on PCB, usually 1 oz (35 μm) or 2 oz (70 μm) generally.
3. Determine Maximum Allowable Temperature Rise - Based on the temperature rating of board and components, generally 30°C to 50°C rise over ambient.
4. Lookup Ampacity value - For the above copper weight and temp rise, lookup the max current density it can support (A/cm^2 or A/mm^2)
5. Calculate Minimum Cross Section - Divide required current by current density to get minimum copper cross section needed (in mm^2 or cm^2).
6. Calculate Track Width - For a given copper thickness, the width = cross section/thickness. Apply safety margin of 20%.

This table provides common ampacity values:

Copper WeightTemp RiseCurrent Density1 oz (35 μm)20°C1.4 A/mm^21 oz (35 μm)40°C1.15 A/mm^22 oz (70 μm)20°C1.9 A/mm^22 oz (70 μm)40°C1.6 A/mm^2

Let's take an example. Suppose we need to calculate the track width to carry a load current of 3A. The PCB uses 1 oz copper and allowable temperature rise is 30°C.

• Current required = 3A
• From table, for 1 oz Cu and 30°C rise, current density is 1.3 A/mm^2
• Cross section = Current / Current density = 3 / 1.3 = 2.31 mm^2
• For 1 oz Cu, thickness is 0.035 mm
• Therefore, width = Cross section / thickness = 2.31 / 0.035 = 66 mm
• Applying 20% margin, final width ~ 79 mm (~ 8 mils)

So for the given requirements, a minimum 79 mm (8 mil) track width is required. Similar calculations can be done for any other design parameters.

Factors That Affect Track Width

There are several factors that must be considered while determining appropriate track widths for a PCB design.

Current Carrying Capacity

The foremost consideration is the amount of current that needs to be carried by the track without overheating. Thicker tracks are needed for higher currents, as governed by the ampacity tables. Power traces generally need large widths.

Voltage Level

Higher voltage tracks require larger spacing between adjacent tracks as per safety regulations. This may necessitate thicker tracks despite lower currents, to maintain spacing.

Length of Conductor

Long traces tend to need wider widths to minimize resistance and voltage drops. Shorter connections can use thinner tracks. Length impacts heating as well.

Temperature Rise

The max temperature rise used for ampacity calculations affects current rating. A higher permitted rise allows thinner tracks. Ambient temperature must be considered.

Impedance Control

For high speed signals like clocks, characteristic impedance matching requires track widths to be set based on calculations. This often overrides current capacity considerations.

Number of traces

For parallel bus signals carrying same data, matched track widths are necessary for equal signal propagation. Similar width power and ground tracks are also common.

Manufacturing Capability

PCB fab shops have minimum track and spacing rules. Very thin or very thick tracks may not be practical or cost effective.

Routing Congestion

If routing space is limited, thinner tracks allow more options for routing connections between components.

EMC Considerations

For electromagnetic compatibility, certain critical signals may need thicker tracks to reduce interference. Other tracks can be thinner.

Thermal Management

Extremely thick tracks function like heat spreaders. But they may need voids to prevent delamination during soldering process.

Signal Integrity

For high speed digital systems, matched track widths help reduce reflections and enforce controlled impedances.

Cost

Thinner tracks need less copper area, reducing board fabrication cost. But very thin tracks have lower yield and may need thicker gold plating.

How to Select the Right Track Width

To determine the optimal track width for a given PCB design, follow these guidelines:

1. List critical design constraints - These may include target impedance, required current, voltage levels, EMC needs, routing congestion, manufacturability etc.
2. Classify tracks based on function - Power traces, low speed nets, high speed signals, single-ended, differential, impedance control needs etc.
3. Calculate minimum widths based on current - Use ampacity table for each power or data net to get minimum TW to carry max expected current.
4. Refine based on other constraints - Such as impedance needs, voltage drops, EMC, thermal issues, manufacturing capability and signal integrity.
5. Iterate with impedance calculator - For critical traces like clocks, use calculators to tune TW and maintain target impedance.
6. Review effects on overall design - Evaluate routing congestion, thermal issues, design rules and manufacturability with preliminary widths.
7. Optimize selectively - See if thinning non-critical traces can improve routability, cost and spacing margins. Confirm key constraints are still met.
8. Document rules - Create a TW table for different nets with guidance on which parameters were considered for selection.
9. Verify in circuit simulations - Perform thermal, power integrity and signal integrity simulations using assigned widths to validate performance.
10. Incorporate feedback during layout - Refine widths progressively during PCB layout based on constraints discovered during actual routing.

By carefully optimizing track widths based on all design considerations, PCB layout engineers can maximize the performance and reliability of the board while minimizing cost.

Track Width Effects on Signal Integrity

For high speed digital PCBs, track widths have a significant impact on signal integrity. Some key effects are:

Impedance Control

• Track widths must be calculated to achieve target impedance values typically 50-100Ω. This controls impedance matching and minimizes reflections.

Propagation Delay

• Wider tracks have slightly lower resistive losses, leading to faster rise/fall times and lower delay for signals.

Cross Talk

• Closely spaced same width tracks have increased capacitive and inductive coupling, leading to more cross talk noise.

Signal Losses

• Thinner tracks have higher resistive losses leading to attenuation, especially at higher frequencies.

Jitter

• Improper track widths and spacing can increase jitter due to reflections and coupling.

EMI

• Controlled width tracks help confine electric fields and reduce EMI emissions.

Ringing

• Uncontrolled impedances cause increased overshoot, undershoot and ringing, degrading signal quality.

Eye Diagrams

• Poor track width control leads to distorted eye patterns affecting timing margins and bit error rates.

Tuned Stubs

• Short track stubs connected to wider main traces can be tuned to control rise/fall times when needed.

Proper PCB stackup design along with intelligent track width tuning minimizes discontinuities and improves signal transmission over traces. This is critical for multi-gigabit digital systems.

Conclusion

In summary, PCB track width has a major influence on the functionality and reliability of a printed circuit board. Track widths must be carefully selected based on required current levels, acceptable voltage drops, target characteristic impedance, signal integrity, EMI control, manufacturing constraints and other parameters. Both thick and thin extreme widths can cause issues if not designed properly.

Calculation of appropriate track widths requires Electrical and PCB Design engineers to analyze the specific requirements of each connection, and make trade-offs between various considerations before arriving at optimal values. As PCB technology moves towards higher speeds and densities, precise track width selection will become even more critical.

FAQ

Here are some frequently asked questions about PCB track widths:

Q1. What is a typical track width for general board level interconnections?

A typical all-round track width for general routing is around 8-10 mils (0.2mm to 0.25mm). This provides a good balance of current capacity, manufacturability and routing flexibility for many standard designs.

Q2. When should I use wider track widths?

Use wider tracks for high current nets like power supplies, voltage regulators, power ground planes. Also for tracks spanning longer lengths to minimize resistive voltage drops. High voltage nets may need wider spacing, necessitating wider traces.

Q3. What is the typical track width range for controlled impedance designs?

For impedance controlled traces like clocks, typical widths are 4 mils to 8 mils (0.1mm to 0.2mm) when using common FR-4 PCB material. This provides 50Ω to 75Ω impedance needed for many high speed digital ICs when properly matched with dielectric thickness.

Q4. What are some typical very high current and very low current track widths?

Some typical examples of specialized track widths include:

• Power bus - 500 mils (12.5mm)
• Voltage regulator output - 300 mils (7.5mm)
• IC power pad - 100 to 300 mils (2.5mm to 7.5mm)
• Fine pitch QFN trace - 4 to 6 mils (0.1mm to 0.15mm)
• IC signal track - 8 to 10 mils (0.2mm to 0.25mm)

Q5. How can I calculate the exact track width needed for my design requirements?

You can use an online PCB track width calculator to calculate the exact width based on parameters like current, copper weight, temperature rise, trace length and impedance. A calculator helps optimize the width for your specific PCB design needs.