PCB Copper Weight and Thickness Explained

Copper weight is a fundamental PCB specification that directly impacts current capacity, heat dissipation, and cost. This guide explains what copper weight means, how it relates to thickness, and when to choose different copper weights.

What Is Copper Weight?

Copper weight refers to the weight of copper per unit area, expressed in ounces per square foot (oz/ft2). This somewhat unusual unit comes from the traditional copper foil manufacturing process.

Conversion: Weight to Thickness

Formula: Thickness (um) = Copper weight (oz) x 35

Standard Copper Weights

0.5 oz (17.5 um)

• Use case: Fine-pitch HDI designs, high-density routing
• Minimum trace width: 2-3 mil achievable
• Current capacity: Limited; typically for signal traces only
• Cost: Slightly less than 1oz base copper

1 oz (35 um) — Industry Standard

• Use case: Most general-purpose PCBs
• Minimum trace width: 3-4 mil standard process
• Current capacity: A 10mil (0.25mm) trace carries ~1A with 10°C rise
• Cost: Baseline — this is the default copper weight

2 oz (70 um)

• Use case: Power electronics, LED drivers, motor controllers
• Minimum trace width: 4-5 mil (wider than 1oz due to etching limitations)
• Current capacity: Roughly doubles the capacity of 1oz for the same trace width
• Cost: ~15-25% more than 1oz

3 oz (105 um)

• Use case: High-current applications, industrial power supplies
• Minimum trace width: 5-6 mil
• Cost: ~30-50% more than 1oz
• Note: Etching precision decreases; trace edges become less defined

Heavy Copper PCB (4oz and Above)

Heavy copper PCBs use 4oz or more copper per layer. Some extreme applications use up to 20oz copper. These boards require specialized manufacturing processes.

Manufacturing Differences

• Etching challenges: Thick copper requires longer etching times, leading to more lateral etching (undercut). Trace widths must be wider to compensate.
• Plating uniformity: Achieving uniform plating in holes becomes difficult with thick copper.
• Lamination pressure: Higher pressure needed to bond thick copper to the substrate.
• Registration: Layer-to-layer alignment tolerances are larger.

Heavy Copper Design Rules

Applications

• EV/HEV power electronics: Battery management, motor drives, DC-DC converters
• Power supplies: Server power, telecom rectifiers, industrial UPS
• Welding equipment: High-current control boards
• Military power systems: Ruggedized power distribution
• Planar transformers: PCB-integrated magnetics using heavy copper windings

Current Carrying Capacity

The IPC-2152 standard provides the most accurate guidance for trace current capacity. Key factors:

External (Outer) Layer Traces — 1oz Copper, 10°C Rise

Effect of Copper Weight on Current Capacity

For the same trace width (20 mil / 0.5mm) and 10°C temperature rise:

Key insight: Doubling copper weight does NOT double current capacity. The relationship is approximately: I ∝ (copper area)^0.725

Impact on Board Thickness

Copper weight directly affects total board thickness. For a standard 4-layer board:

Custom stackups can maintain 1.6mm board thickness with heavier copper by adjusting prepreg and core thicknesses.

Thermal Considerations

Copper is an excellent thermal conductor (385 W/m·K). Heavier copper improves:

1. Heat spreading: Wider, thicker traces spread heat away from hot components
2. Thermal vias: Heavier copper in via barrels improves vertical heat transfer
3. Copper pour areas: Large copper fills act as heat sinks

Thermal Resistance Comparison

For a 10mm x 10mm copper area:

Cost and Lead Time Impact

Conclusion

For most designs, 1oz copper is the default choice. Move to 2oz when you need more current capacity or better thermal performance. Consider 3oz+ only for dedicated power designs. Heavy copper (4oz+) is a specialized capability for high-power applications where the extra cost is justified by the performance requirements. Always consult your PCB manufacturer for their specific heavy copper capabilities and design rules.