Introduction
Designing reliable PCBs requires a clear understanding of how much current a trace can safely carry. While standards like IPC-2152 provide detailed guidance, engineers often rely on simple thumb rules during early design stages.
This blog breaks down practical insights, experimental observations, and when to use rules of thumb versus standards — helping you make better design decisions.
Understanding Current Carrying Capacity
Based on IPC-2152 estimates and practical test results:
- A 6 mil trace can safely handle ~1 A DC current
- A 20 mil trace can safely handle ~3 A DC current
These are conservative and safe design values validated through testing .
Design Guideline
- Use 6 mil traces for signals (<1 A)
- Use 20 mil traces for moderate power (~3 A)
Unless you have strong constraints, these values are reliable starting points.
Via Current Handling
For vias:
- A 13 mil diameter via can handle approximately 3 A
- For higher currents, use multiple vias in parallel
Example:
- For 10 A current, use at least 4 vias (3 A per via assumption)
Real-World Experiment Insight
A test board was built with different trace widths (6, 8, 10, 20, 100 mil) .
Key observations:
- 3 A through 20 mil trace → No noticeable heating
- 10 A → Trace heated rapidly and failed
This confirms:
- Practical limits align closely with conservative estimates
- Overdesign = reliability
Is the Thumb Rule Still Valid?
Yes — absolutely.
The “40 mil per Amp” Rule
- Use 40 mil width per 1 A (especially for internal layers)
- Designed for:
- High ambient temperature
- Long-term reliability
- Minimal risk
These rules aim for a 10°C–20°C temperature rise, ensuring safe operation .
Rule of Thumb vs IPC-2152
- Use Rule of Thumb When
- Early prototyping / rough routing
- Plenty of board space available
- Safety-critical power rails
- Want minimal voltage drop
- Use IPC-2152 When
- Space-constrained designs (BGA, wearables)
- Thermal performance is critical
- High-power processors (SoCs)
- Certification/documentation required
Comparison Summary
| Feature | Rule of Thumb (40 mil/A) | IPC-2152 |
| Philosophy | Conservative | Physics-based |
| Temp Rise | ~10–15°C | User-defined |
| Space Usage | High | Optimized |
| Complexity | Easy | Requires tools |
Practical Design Strategy (Recommended)
- Start with Rule of Thumb → fast and safe
- Refine using IPC-2152 / calculators → optimized design
- Always validate:
- Thermal rise
- Voltage drop
- Current density
Conclusion
There is no single “correct” method — both approaches are valuable.
- Rule of Thumb → speed + safety
- IPC-2152 → precision + optimization
The best engineers know when to use each.
Final Takeaway
If you have space → go conservative
If you don’t → trust physics and calculate
