Determining the correct trace width is a critical aspect of printed circuit board (PCB) design. Trace width has a major impact on current carrying capacity, voltage drop, thermal performance and signal integrity. Insufficient trace widths can cause overheating or failure of the circuit. Excessively large traces waste valuable board space and cost. This comprehensive guide provides designers a detailed understanding of how to accurately calculate optimum PCB trace widths for their systems.
Factors Affecting Trace Width
Several key factors determine appropriate trace widths for a given PCB design:
Current Level
- Higher currents require larger traces to avoid overheating
- Trace width sized to carry max expected current without damage
Allowable Temperature Rise
- Trace heating limited to avoid damaging PCB laminate
- Typically limit temperature rise to 10°C or 20°C
Copper Thickness
- Thicker copper can handle more current
- 1 oz (35 μm) and 2 oz (70 μm) most common weights
Trace Length
- Longer traces increase resistance and voltage drop
- Maximum trace length factored into width calculation
Ambient Temperature
- Higher ambient temps reduce current capacity
- Often use 40°C ambient for calculations
Voltage Drop Limits
- Traces sized to avoid excessive IR voltage drops
- Max 5% drop often acceptable for power traces
Insulation Layers
- More copper layers in board increase heat spreading
- Allows narrower traces for given current
Trace Width Calculation Methods
There are several standard methods to calculate appropriate PCB trace widths:
IPC-2221 Formula
- Accounts for temperature rise and copper weight
- Conservative method ensures sufficient width
Circular Mils Method
- Uses circle diameter in mils to represent trace size
- Simple method but does not account for all factors
Trace Width Calculator Tools
- Sophisticated calculators consider multiple parameters
- Built into PCB design software for easy use
- Automates complex width optimization
Manufacturer Guidelines
- Many PCB fabricators provide trace width guides
- Based on experience and testing
- May provide overly conservative widths
Rule of Thumb
- Very approximate guidelines like 1A per 20 mils width
- Not accurate but used for rough estimation
For precision trace sizing, the IPC-2221 formula or trace width calculators are recommended.
IPC-2221 Trace Width Formula
The IPC-2221 formula is the industry standard method for calculating trace width based on current and temperature rise.
Where:
- W = Trace width (mm)
- I = Current through trace (A)
- ΔT = Temperature rise (°C)
- T0 = Ambient temperature (°C)
- K0 = Constant based on copper thickness (see table)
IPC-2221 K0 Values
| Copper Thickness | K0 |
|---|---|
| 1 oz (35 μm) | 0.024 |
| 2 oz (70 μm) | 0.048 |
This formula gives an accurate trace width when key parameters are known.
Circular Mils Trace Width Calculation
The circular mils method uses the diameter of a circle in mils to represent trace size.
Where:
- CM = Circular mils area
- I = Current through trace (A)
- K = Constant based on temperature (see table)
Circular Mils K Values
| Temperature Rise | K |
|---|---|
| 30°C | 700 |
| 40°C | 875 |
| 50°C | 1000 |
| 60°C | 1100 |
Once CM is found, it must be converted to trace width in mils based on copper thickness.
This method is easy to calculate but less sophisticated than IPC-2221.
Trace Width Calculator Tools
Trace width calculators automate width optimization based on multiple parameters. They are built into most PCB design software packages.
Typical inputs include:
- Current
- Temperature rise
- Copper thickness
- Ambient temperature
- Trace length
- Voltage drop %
- External layers
Outputs provided:
- Recommended trace width
- Percent utilization of trace
- Resistance, current density and temperature metrics
Using these tools simplifies determination of ideal trace widths.
PCB Manufacturer Guidelines
Many PCB fabrication companies provide their own trace width guidelines based on experience.
For example, for 1 oz copper with 10°C rise:
| Current | Width |
|---|---|
| 1 A | 25 mils |
| 2 A | 32 mils |
| 5 A | 50 mils |
| 10 A | 80 mils |
These values are often conservative but offer a reasonable starting point.
Trace Width Rules of Thumb
While not highly accurate, simple rules of thumb are useful for rough trace width estimation.
- Allow 1 A per 20 mils of trace width as a starting point.
- Size power traces at 30 mils minimum for robustness.
- Use 10 mils minimum for signal traces.
- Limit traces to 100 mils maximum to conserve space.
Rules of thumb provide general guidance but more rigorous methods are needed for final design.
Trace Width Guidelines by Current
Recommended minimum trace widths for common current levels:
| Current | Minimum Width |
|---|---|
| ≤ 0.5 A | 10 mils |
| 1 A | 15-20 mils |
| 2 A | 25-35 mils |
| 5 A | 40-60 mils |
| 10 A | 60-90 mils |
| 15 A | 90-120 mils |
| 20 A | 120-150 mils |
Higher temperatures require wider traces for the same current.
Trace Width Guidelines by Thickness
Suggested trace widths for standard copper weights:
| Copper Thickness | Typical Trace Width |
|---|---|
| 1⁄4 oz (8 μm) | 75 mils |
| 1⁄2 oz (16 μm) | 50 mils |
| 1 oz (35 μm) | 25 mils |
| 2 oz (70 μm) | 15 mils |
| 3 oz (105 μm) | 10 mils |
Thicker copper allows narrower traces for equivalent current.
Maximizing Trace Width Utilization
To optimize trace widths:
- Use maximum recommended % utilization for width. Many tools display this.
- Leverage thicker copper layers where possible.
- Ensure adequate clearance between traces.
- Keep traces short as possible with direct routing.
- Use multiple vias to distribute current across layers.
- Take advantage of internal power planes.
- Increase allowable temperature rise, if acceptable.
This achieves maximum current capacity within space constraints.
Effects of Inadequate Trace Width
Undersized trace width can lead to:
- Overheating causing laminate damage or lifting traces
- Excessive voltage drop impacting circuit performance
- Intermittent functionality due to expansion/contraction
- Eventual failure due to electromigration over time
- Lower product reliability and field failure rates
Adequate trace widths are critical for robust, reliable PCB performance.
High Current Trace Design Guidelines
For high current traces:
- Use wide traces up to maximum of 100 mils
- Utilize thick copper weights up to 3 oz (105 μm)
- Place vias frequently when changing layers
- Avoid narrowing traces when changing layers
- Use relief cutouts in plane layers for thermal venting
- Add cross-hatches along trace for heat spreading
- Keep sensitive components away from hot traces
Careful design is needed for traces carrying over 5-10 A to avoid overheating.
Frequently Asked Questions
How are internal power planes factored into trace width?
Power planes provide significant heat spreading which allows using narrower trace widths. Many width calculators ask for number of copper layers to account for this.
Can I adjust the acceptable temperature rise used?
Yes, you can alter the temperature rise criteria between 10-20°C based on materials used and allowable operating temperatures. This impacts resulting width.
Do I need to derate trace width at higher temperatures?
Often trace widths are derated by a percentage at elevated temperatures above 40°C ambient. For example, reducing width 5% per 10°C increased.
How much clearance should I provide around traces?
As a guideline, allow at least 3-5 mil clearance between signal traces and around traces passing planes or pads. Wider spaces needed at high voltages.
Can I use thinner trace widths if the current is pulsed, not continuous?
Yes, you can reduce width up to around 30% for intermittent pulses vs. continuous current. But average current over time should still be used.
Conclusion
Accurately calculating PCB trace widths is vital to avoid both undersized traces that overheat and oversized traces that waste board space. By considering key parameters including current, temperature rise, copper thickness, and ambient temperature, PCB designers can determine optimum trace widths. Using the IPC-2221 formula, trace width calculators, or PCB vendor guidelines provides precision trace sizing tailored to your specific design needs. Proper trace width helps ensure your boards perform reliably through their product lifetimes.
