Analyse de la relation entre l'épaisseur du cuivre, Largeur de trace, et capacité de transport de courant dans la conception de PCB
In printed circuit board (PCB) conception, the matching of copper thickness, largeur de trace, and current carrying capacity is a key factor that determines circuit reliability. Improper parameter selection may lead to overheating, burnout of copper traces, or even circuit failure, while excessive design increases cost and wastes valuable board space. This article systematically analyzes the relationship among these three factors and provides engineers with a scientific basis for Conception de PCB.
Basic Definitions of Core Parameters
1. Épaisseur de cuivre
PCB copper thickness is typically measured in ounces (Oz). One ounce of copper is defined as a copper weight of 1 ounce spread over an area of 1 square foot, corresponding to a thickness of approximately 35 µm (1.378 mil). Common specifications include 0.5 Oz (17.5 µm), 1 Oz (35 µm), 2 Oz (70 µm), et 3 Oz (105 µm). In special applications, heavy copper designs of 4 OZ or greater may be used.
Copper thickness directly determines the current carrying capability per unit area. The thicker the copper, the higher the current it can carry at the same trace width.
2. Largeur de trace
Trace width refers to the actual width of a PCB conductor, typically measured in millimeters (MM) or mils (1 mil = 0.0254 MM). Standard PCB trace widths range from 0.1 MM (4 mil) à 3 MM (118 mil), while fine-pitch designs may use widths below 0.05 MM (2 mil).
The selection of trace width requires balancing current requirements and routing space. High-density PCBs must allocate trace width resources efficiently within limited board area.
3. Current Carrying Capacity
Current carrying capacity refers to the maximum current that a conductor can continuously carry under stable operating conditions without exceeding the maximum allowable temperature rise (typically 60°C).
This parameter is affected by copper thickness, largeur de trace, ambient temperature, cooling conditions, and conductor length, among which copper thickness and trace width are the most critical factors.
Relationship Between Copper Thickness and Current
The effect of copper thickness on current carrying capacity is approximately linearly proportional. Under the same trace width and environmental conditions, doubling the copper thickness typically increases current carrying capability by about 80%–90% (not perfectly linear due to diminishing heat dissipation efficiency).
Using a 1 mm trace width as an example, the typical current capacities are as follows (ambient temperature 25°C, temperature rise 60°C):
| Épaisseur de cuivre | Current Capacity |
|---|---|
| 0.5 Oz (17.5 µm) | ~1.8 A |
| 1 Oz (35 µm) | ~2.5 A |
| 2 Oz (70 µm) | ~4.2 A |
| 3 Oz (105 µm) | ~5.8 A |
It should be noted that when copper thickness exceeds 3 Oz, the improvement in current carrying capacity gradually decreases. This is because heat dissipation in thick copper designs relies primarily on thermal conduction through the PCB substrate, whose thermal conductivity (approximately 0.3–0.8 W/m·K) is far lower than that of copper (401 W/m·K), becoming the primary thermal bottleneck.
PCB Trace Width and Current Quick Reference Table
In practical PCB design, engineers frequently refer to recommended trace widths for specific current levels. The following table provides reference values for 1 OZ copper thickness, 25°C ambient temperature, and a permissible temperature rise of 20°C.
PCB Trace Width vs. Current Table (1 OZ Copper)
| Current (UN) | Recommended Trace Width (MM) | Recommended Trace Width (mil) |
|---|---|---|
| 1UN | 0.25 | 10 |
| 2UN | 0.50 | 20 |
| 3UN | 0.75 | 30 |
| 5UN | 1.30 | 50 |
| 8UN | 2.00 | 80 |
| 10UN | 3.00 | 120 |
| 15UN | 5.00 | 200 |
| 20UN | 8.00 | 315 |
It should be noted that these values are provided as engineering references only. Actual PCB designs should also consider copper thickness, cooling conditions, ambient temperature, and PCB layer count.
Relationship Between Trace Width and Current
The relationship between trace width and current carrying capacity follows a square-root trend. With copper thickness fixed, current carrying capacity is approximately proportional to the square root of trace width.
Par exemple, avec 1 OZ copper:
- 0.5 mm largeur de trace: environ 1.8 UN
- 1 mm largeur de trace: environ 2.5 UN (doubling width increases current by only 39%)
- 2 mm largeur de trace: environ 3.6 UN (doubling width again increases current by 44%)
The primary reason for this nonlinear relationship is that:
- Heat dissipation area increases linearly with trace width.
- Joule heating (P = I²R) decreases as trace width increases because resistance decreases.
When trace width exceeds approximately 2 MM, the increase in heat dissipation area can no longer fully compensate for the quadratic increase in heat generated by higher currents, resulting in reduced current carrying efficiency.
En outre, trace length indirectly affects current carrying capacity. For the same width and thickness, longer traces have higher total resistance and accumulate more heat. It is generally recommended that when trace length exceeds 50 MM, the allowable current should be reduced by approximately 5%–10% for every additional 50 mm of length.
Current Design for Multilayer PCBs
For 4-layer, 6-couche, and higher-layer-count PCB designs, high-current paths are typically shared across multiple layers.
Les méthodes courantes incluent:
Parallel Routing Across Multiple Layers
Current is distributed across:
- Top Layer
- Couche intérieure(s)
- Couche inférieure
simultaneously.
Current Sharing Through Vias
Multiple vias are used to connect traces on different layers.
Par exemple:
- 5 A current: recommended 4–6 vias
- 10 A current: recommended 8–12 vias
Large Copper Pours
Using solid copper planes for power distribution can significantly reduce current density and temperature rise.
Compared with single-layer routing, multilayer parallel routing can increase current carrying capacity by approximately 30%–100%.
Optimization Strategies in Practical Design
1. Dynamically Adjust Parameter Combinations
When PCB space is limited, such as in high-density boards, flexible combinations such as “thin copper + wide traces” or “thick copper + narrow traces” may be used.
Par exemple, if routing constraints allow only a 0.6 mm largeur de trace:
- 1 OZ copper: environ 1.5 UN
- 2 OZ copper: environ 2.8 UN
Using 2 OZ copper increases current capacity by approximately 87% without increasing trace width.
2. Consider Ambient Temperature Compensation
For every 10°C increase in ambient temperature, copper current carrying capacity should be reduced by approximately 10%–15%.
Par exemple, in high-temperature environments such as automotive electronics operating at 85°C:
- 1 OZ copper
- 1 mm largeur de trace
Current carrying capacity should be reduced from approximately 2.5 A at 25°C to about 1.6 A to prevent overheating.
3. Reinforce Critical Areas
For power loops and high-current device pins, consider:
- Using localized thick copper (2–3 OZ)
- Parallel trace design (two identical traces in parallel can increase current capacity by approximately 70%; trace lengths and impedances should be matched)
- Adding thermal vias to copper traces (un 0.3 mm via per additional 2 mm of trace width can improve current capacity by approximately 15%)
4. Validate with Simulation Tools
For complex PCB designs, thermal simulation tools such as:
- ANSYS Icepak
- Cadence Celsius
are recommended to simulate copper temperature distribution under various current loads.
Simulation helps accurately identify thermal hotspots and reduces risks associated with relying solely on empirical design rules.
Current Carrying Capacity Table for Different Copper Thicknesses
The following table shows typical current carrying capacities for a 1 mm largeur de trace.
| Poids du cuivre | Épaisseur (µm) | Recommended Current (UN) |
|---|---|---|
| 0.5 Oz | 17.5 | 1.8 |
| 1 Oz | 35 | 2.5 |
| 2 Oz | 70 | 4.2 |
| 3 Oz | 105 | 5.8 |
| 4 Oz | 140 | 7.2 |
The table shows that:
- Increasing copper thickness from 1 OZ to 2 OZ improves current carrying capacity by approximately 68%.
- Increasing thickness from 3 OZ to 4 OZ provides significantly smaller gains due to limitations imposed by PCB substrate heat dissipation capability.
Skin Effect in High-Frequency PCBs
As current frequency increases, current tends to concentrate near the conductor surface. This phenomenon is known as the Skin Effect.
Skin Depth Formula
δ = √(2r / ωμ)
For copper conductors:
| Fréquence | Skin Depth |
|---|---|
| 1 MHz | 66 µm |
| 10 MHz | 21 µm |
| 100 MHz | 6.6 µm |
| 1 Ghz | 2.1 µm |
Depuis 1 OZ copper thickness is approximately 35 µm, increasing copper thickness provides limited benefit in reducing AC resistance once frequencies reach tens of MHz and above.
Donc:
For high-frequency PCB designs, increasing trace width is generally more effective than simply increasing copper thickness.
Common Misconceptions and Considerations
Idée fausse 1: Thicker Copper Is Always Better
Some engineers assume that thicker copper is always beneficial, overlooking the fact that:
- Thick copper increases Fabrication de PCB coût (2 OZ copper typically costs about 30% plus que 1 OZ copper).
- Thick copper may increase PCB warpage due to larger thermal expansion coefficient mismatches between copper and substrate.
Copper thickness should therefore be selected according to actual current requirements.
Idée fausse 2: Ignoring the Relationship Between Trace Width and Spacing
When trace width increases, spacing between adjacent traces should also be increased accordingly.
A common recommendation is:
Trace spacing ≥ 50% of trace width
This helps prevent creepage and electrical leakage, especially in high-voltage applications.
Important Consideration: Skin Effect at High Frequencies
When current frequency exceeds 1 MHz, current becomes concentrated near the copper surface, with a skin depth of approximately 20–30 μm.
Under these conditions, increasing copper thickness beyond 1 OZ provides significantly reduced benefits in terms of current carrying capacity.
Improving current handling should therefore focus primarily on increasing trace width.
Conclusion
Dans la conception de PCB, the relationship between copper thickness, largeur de trace, and current carrying capacity forms a dynamic system of interdependent constraints. Engineers must select the optimal parameter combination based on current requirements, space limitations, ambient temperature, et considérations de coûts.
The fundamental principle is:
While meeting current carrying requirements, minimize trace width and copper thickness as much as practical, and verify thermal reliability through simulation tools to achieve the best balance between performance and cost.
Understanding the interactions among these three factors is essential for improving PCB design quality and preventing circuit failures.
