PCB copper weight is one of those specifications that engineers inherit from decades-old convention without questioning the underlying logic. The term causes confusion because we specify a weight measurement — ounces — to describe what we actually care about: thickness.
The convention originates from the copper foil manufacturing process. One ounce of copper spread over one square foot yields a foil thickness of approximately 35 micrometers (1.37 mils). When you specify "1oz copper," the manufacturer knows you want 35 µm of base copper thickness.
Complete Copper Weight Conversion Table
| Copper Weight | Thickness (µm) | Thickness (mils) | Thickness (mm) |
|---|---|---|---|
| 0.5 oz | 17.5 | 0.7 | 0.0175 |
| 1 oz | 35 | 1.4 | 0.035 |
| 2 oz | 70 | 2.8 | 0.07 |
| 3 oz | 105 | 4.2 | 0.105 |
| 4 oz | 140 | 5.5 | 0.14 |
| 5 oz | 175 | 6.9 | 0.175 |
| 6 oz | 210 | 8.3 | 0.21 |
Real-World Applications by Copper Weight
Half-ounce (17.5 µm) serves signal-only applications: smartphone HDI boards, memory modules, and high-density FPGA breakout boards where 3-mil traces on inner layers are needed.
One-ounce (35 µm) remains the default for ~60% of PCB production. Consumer electronics, IoT, computing hardware, and communication equipment overwhelmingly use 1oz uniformly.
Two-ounce (70 µm) enters when designs need >1.5A per trace: power supplies, LED drivers, motor control, telecom power distribution, and automotive body electronics.
Three-ounce (105 µm) marks the "heavy copper" threshold where manufacturing processes change significantly. Industrial power supplies, EV charging, BMS for large packs.
Four-ounce and above (140+ µm) enters specialized territory: welding equipment, solar inverters, railway traction electronics.
Base Copper vs Finished Copper: The Common Trap
When you specify 1oz copper, outer layers go through electroplating during pattern plating, depositing an additional 20-30 µm. Your "1oz" outer layer actually measures 55-65 µm after plating — approaching 2oz effective thickness.
Inner layers do NOT receive this plating. They stay at base weight.
Why this matters:
- Impedance calculations must use finished copper thickness on outer layers
- A common 3-5 ohm impedance deviation happens when engineers model with base copper instead of finished
- Tolerance on finished copper: ±20% of plated thickness
Minimum Trace/Space Per Copper Weight (Real Production Data)
This is where most competitor guides fail — they never give you actual DFM numbers. From production data across thousands of boards:
| Copper Weight | Min Trace (mil) | Min Space (mil) | Notes |
|---|---|---|---|
| 0.5 oz | 3 | 3 | Finest achievable geometry |
| 1 oz | 3.5 (pref 4) | 3.5 (pref 4) | >95% yield at 4/4 |
| 2 oz | 6 | 6 | Some fabs require 8/8 |
| 3 oz | 8 (pref 10) | 8 (pref 10) | Alkaline etch preferred |
| 4 oz | 10 (pref 12) | 10 (pref 12) | Modified etch chemistry |
| 5 oz | 12 | 14 | Alkaline etch required |
| 6 oz | 14 | 16 | Specialty production |
The physics: chemical etchant attacks sideways (undercut) as well as downward. Thicker copper = longer etch = more undercut. The etch factor is typically 2.5:1 to 3.5:1 for standard acid etching.
Current Carrying Capacity (IPC-2152 Data)
Using IPC-2152 methodology with 10°C temperature rise above 25°C ambient:
Outer layer, 1oz base copper (finished ~1.7oz):
- 10-mil trace: ~1.2A
- 20-mil trace: ~2.0A
- 50-mil trace: ~3.8A
- 100-mil trace: ~6.2A
Outer layer, 2oz base copper (finished ~2.7oz):
- 10-mil trace: ~1.8A
- 20-mil trace: ~3.0A
- 50-mil trace: ~5.5A
- 100-mil trace: ~9.0A
Inner layers carry significantly less (30-40% reduction) because they can't dissipate heat as effectively, being sandwiched between insulating dielectric.
Critical insight: temperature rise is not linear with current — doubling allowable rise from 10°C to 20°C increases capacity by only ~40%, not 100%.
Mixed Copper Weight Stackups: The Cost-Performance Sweet Spot
Rather than uniform heavy copper (expensive), put heavy copper only where needed:
Most common: 2oz outer + 0.5-1oz inner. Enhanced current capacity where components connect, fine-pitch routing on inner signal layers. Cost: only 15-25% premium over all-1oz.
Aggressive hybrid: 3oz inner power planes + 0.5oz signal layers + 1oz outer. Great for power converters where heavy current flows through internal planes.
Typical 4-layer example:
- L1 (top signal/power): 2oz
- L2 (ground plane): 1oz
- L3 (power plane): 1oz
- L4 (bottom signal/power): 2oz
What Heavier Copper Actually Costs (Real 2026 Data)
Based on production cost data for a 4-layer board at 500-2000 pieces:
| Configuration | Cost Multiplier | Premium |
|---|---|---|
| All 0.5oz | 0.95x | -5% |
| All 1oz | 1.0x | Baseline |
| 2oz outer + 1oz inner | 1.10-1.18x | +10-18% |
| All 2oz | 1.15-1.25x | +15-25% |
| All 3oz | 1.40-1.60x | +40-60% |
| All 4oz | 1.80-2.20x | +80-120% |
| All 5-6oz | 2.50-3.50x | +150-250% |
Best optimization: If using uniform 2oz but only 2 layers need it, switch to mixed stackup and save 5-10% immediately.
Heavy Copper Manufacturing: What Changes at 3oz+
The transition isn't just "longer etching." It's fundamentally different:
Etching chemistry: Standard production uses acid copper chloride (CuCl₂) at ~50°C. Heavy copper switches to alkaline ammoniacal etchant with a better etch factor (3:1 to 4:1 vs 2.5:1 to 3:1), enabling tighter geometries despite thicker copper.
Lamination: 3oz traces stand 105µm proud, creating deep channels needing more resin to fill. Requires higher resin-content prepregs, modified pressure profiles, and copper balancing to prevent resin starvation.
Drilling: Thicker copper entry surfaces accelerate drill bit wear. Drill hit counts reduced 30-50%, aluminum entry material required, reduced spindle speeds on initial penetration.
Plating uniformity: Deep channels between traces create plating distribution challenges. Heavy copper boards show 15-20% more plating thickness variation.
Decision Framework: Choosing the Right Copper Weight
Under 1.5A per trace (outer, 15+ mil): 1oz is sufficient. No benefit from heavier copper.
1.5-4A per trace: 2oz on relevant layers. Use mixed stackup.
4-10A per trace: 3oz needed unless you can use 50-100+ mil traces. Mixed stackup with 3oz power + 1oz signal.
Above 10A per trace: 4oz+ required. Consult manufacturer early — geometry constraints are significant.
Beyond electrical: Heavier copper = stiffer, heavier boards. Flex/rigid-flex circuits almost always use 0.5-1oz because heavier copper cracks in bend zones.
Based on production data from PCB fabrication across multiple copper weight classes. Current capacity values derived from IPC-2152 methodology.
If you're designing a board with specific copper weight requirements, the full article on our blog includes additional detail on impedance interactions and manufacturing process specifics.
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