1. What is copper foil?
Copper foil is a thin, continuous metal sheet. It is an electro-deposited, negative-type material. The foil is plated onto the base of the PCB. It acts as the conductor on the board. It bonds well to insulating layers. It takes solder mask and other protective layers. After etching, the remaining copper forms the circuit pattern. In early production, people used rolled copper foil. That means copper blocks are flattened into thin sheets.
So, how thick is the copper foil on a PCB?
2. Unit for copper thickness: oz
PCB copper thickness is usually given in oz (ounce). Ounce is a weight unit. The relation between ounce and gram is:
1 oz ≈ 28.35 g.
In the PCB industry, 1 oz means the weight of 1 oz of copper spread evenly over 1 square foot (ft²). That even layer has a certain thickness. That thickness is about 35 μm. Using a formula:
1 oz = 28.35 g / ft².
Below I show the calculation step by step so the result is clear:
- Take 1 oz mass: 28.35 g.
- Copper density: 8.93 g/cm³.
- 1 ft² = 929.03 cm².
Thickness in cm = mass / (density × area)
= 28.35 / (8.93 × 929.03) cm
= 28.35 / 82973.558 ≈ 0.00341721 cm.
Convert to micrometers (μm): 0.00341721 cm = 0.00341721 × 10,000 μm = 34.17 μm.
So 1 oz copper foil ≈ 34.17 μm. This value is usually rounded and quoted as 35 μm. In imperial units, 34.17 μm = 0.03417 mm. One mil = 0.0254 mm, so thickness ≈ 1.345 mil. People often say 1 oz ≈ 35 μm ≈ 1.35 mil.
3. Common copper thickness values
Common copper thickness values used in PCBs are:
- 0.5 oz ≈ 17.5 μm
- 1 oz ≈ 35 μm
- 2 oz ≈ 70 μm
- 3 oz ≈ 105 μm
Typical single- and double-sided PCBs use about 35 μm (1 oz) copper. Some boards use 50 μm eller 70 μm copper too. For multilayer boards, the outer layers are often 35 μm (1 oz). Inner layers are often 17.5 μm (0.5 oz).
Thick-copper boards start at about 3 oz and above. These boards are used in high-current or high-voltage products, such as power supplies.
4. Current carrying capacity for different copper thicknesses
Below is a practical table showing typical current capacity for copper strips of different thickness and width. The table lists current (A) and required width (mm) for copper thicknesses of 70 μm, 50 μm, og 35 μm. The test thickness parameter in the table is t = 10 (this is a sample reference value used in the source table).
Bemærk: When you use copper foil as a conductor for high current, it is common to derate the table values by 50% for a safe choice. That means pick a width that corresponds to about half the listed current if you want a safety margin.
| Current (A) / Width (mm) for 70 μm | Current (A) / Width (mm) for 50 μm | Current (A) / Width (mm) for 35 μm |
|---|---|---|
| 6.00 A — 2.50 mm | 5.10 A — 2.50 mm | 4.50 A — 2.50 mm |
| 5.10 A — 2.00 mm | 4.30 A — 2.00 mm | 4.00 A — 2.00 mm |
| 4.20 A — 1.50 mm | 3.50 A — 1.50 mm | 3.20 A — 1.50 mm |
| 3.60 A — 1.20 mm | 3.00 A — 1.20 mm | 2.70 A — 1.20 mm |
| 3.20 A — 1.00 mm | 2.60 A — 1.00 mm | 2.30 A — 1.00 mm |
| 2.80 A — 0.80 mm | 2.40 A — 0.80 mm | 2.00 A — 0.80 mm |
| 2.30 A — 0.60 mm | 1.90 A — 0.60 mm | 1.60 A — 0.60 mm |
| 2.00 A — 0.50 mm | 1.70 A — 0.50 mm | 1.35 A — 0.50 mm |
| 1.70 A — 0.40 mm | 1.35 A — 0.40 mm | 1.10 A — 0.40 mm |
| 1.30 A — 0.30 mm | 1.10 A — 0.30 mm | 0.80 A — 0.30 mm |
| 0.90 A — 0.20 mm | 0.70 A — 0.20 mm | 0.55 A — 0.20 mm |
| 0.70 A — 0.15 mm | 0.50 A — 0.15 mm | 0.20 A — 0.15 mm |
Again, choose a safe margin. A common rule is to reduce these table values by 50% when you design for production.
5. Other practical notes on copper as a conductor
- If you use copper foil as a long strip conductor, you must check its current capacity. For example, take a typical thickness of 0.03 mm (30 μm). If the copper strip has width W (mm) and length L (mm), the DC resistance can be approximated by: R ≈ 0.0005 × L / W (ohm) This formula gives a quick estimate for design checks.
- Copper current capacity also depends on the parts on the board, number and types of parts, and cooling. So real current capacity depends on both copper geometry and thermal conditions.
- A practical rule of thumb is: current capacity ≈ 0.15 × W (A). This is an empirical estimate used in some cases. It is simple and conservative for many boards.
6. Example: area and current density
Take a common case: copper thickness 35 μm and trace width 1 mm. The cross-sectional area is:
- Area = thickness × width = 0.035 mm × 1 mm = 0.035 mm².
If you use a current density rule of 30 A/mm², then current per 1 mm width ≈ 30 × 0.035 = 1.05 A. So roughly 1 A per mm trace width under that rule of thumb.
7. IPC formulas for more accurate current and temperature rise
IPC-2152 og IPC-D-275 give more accurate models. The text includes IPC-D-275 formulas in a common form:
- For internal traces: I = 0.0150 × (ΔT^0.5453) × (A^0.7349)
- For external traces: I = 0.0647 × (ΔT^0.4281) × (A^0.6732)
In these formulas:
- I is current in amps.
- ΔT is allowed temperature rise in °C.
- A is cross-sectional area in mil² (or other units depending on how you apply the formula). Use consistent units when you apply the formulas.
Use IPC methods if you need accurate allowed current for a given temperature rise.
8. Copper thickness and dielectric matching
In PCB design, “copper thickness — dielectric matching” means you choose copper thickness and the board dielectric together. This is to meet electrical needs, thermal needs, and mechanical needs. Key points:
8.1 Copper weight (copper thickness)
- Unit: oz/ft² (for example, 0.5 oz, 1 oz, 2 oz, 3 oz).
- 1 oz ≈ 35 μm ≈ 1.35 mil.
- Effects:
- Current carrying: thicker copper carries more current.
- Loss: at high frequency, skin effect matters. Thicker copper can reduce conductor loss in some cases.
- Thermal: thicker copper helps heat spread.
- Etching: thicker copper makes fine etch work harder. Minimum trace width and spacing may increase.
- Cost: thicker copper costs more.
8.2 Dielectric material
Key dielectric properties:
- Dielectric constant (Dk or εr): affects signal speed and impedance.
- Loss tangent (Df): affects high-frequency loss.
- Thickness (H): with copper thickness, H decides impedance and capacitance.
- CTE and Tg: thermal reliability.
Common materials: fr4 for general use, high-Tg fr4, and special high-frequency materials (e.g., Rogers).
8.3 Matching rules
- For impedance control, a common microstrip formula is: Z0 ≈ (87 / sqrt(εr_eff + 1.41)) * ln(5.98H / (0.8W + T)) where Z0 is impedance, εr_eff is effective dielectric constant, H is dielectric thickness, W is trace width, T is copper thickness.
- If copper thickness goes up, impedance goes down for the same width and dielectric. So you must increase width or increase dielectric thickness to keep the same impedance.
- For high-frequency signals, skin depth matters. Copper thickness should be at least several times skin depth at the highest frequency of interest. For very high frequencies, surface roughness also matters. Use low-roughness copper if you need low loss.
- For power planes and heavy current, use thicker copper (≥ 2 oz) and consider thermally conductive dielectric or metal-core boards for heat management.
8.4 Manufacturability
- Thick copper (≥ 3 oz) needs larger trace/spacing rules to avoid etch problems.
- Thin dielectrics require tight control over copper thickness. Variations affect impedance.
9. Selection table (short guide)
| Application Scenario | Recommended Copper Weight | Recommended Dielectric Material | Matching Reason |
|---|---|---|---|
| High-speed digital (>5 Gbps) | 0.5 oz – 1 oz | low-Df FR4 / Rogers RO4000 | Fine routing, low loss, easier impedance control. |
| Power modules / high current | 2 oz – 6 oz+ | FR4 High-Tg / high-thermal-conductivity materials | Higher current capacity and better heat dissipation. |
| RF / microwave (>10 GHz) | 0.5 oz (low roughness) | Rogers RO3000 / Teflon (PTFE) | Ultra-low loss and optimized surface effects for RF. |
| General consumer electronics | 1 oz | standard FR4 | Balanced cost and mature process. |
| High-density HDI boards | 0.5 oz – 1 oz | FR4 High-Tg / low-CTE materials | Fine traces and reliable laser via performance. |
10. Practical advice
- State your needs first: current, signal speed, impedance, heat.
- Plan stackup with proper tools.
- Do impedance simulation with material Dk and copper thickness.
- Ask the board house about its copper thickness tolerances and dielectric options.
- For high-frequency cases, measure material Dk and Df if possible.
Bemærk: The nominal copper weight (like 1 oz) is the starting thickness before etch. After etch the trace may have tapered sides. For real impedance checks use either average thickness or board house guidance.
