PCB copper weight describes the nominal copper thickness produced when a specified mass of copper is spread over one square foot. For engineers, the useful question is not weight alone but what that thickness does to trace geometry, current handling, etching and the finished stackup. RLX Solutions, a contract electronics manufacturer in Concord, Ontario, reviews copper requirements against the complete board design before fabrication.
PCB Copper Weight to Thickness Conversion
The common conversion is approximately 35 µm of copper thickness per 1 oz/ft². That makes 2 oz copper about 70 µm and 4 oz copper about 140 µm before considering how the fabrication process affects the finished copper. For the full picture, see our PCB Manufacturing Guide.
| Copper Weight | Approx. Thickness | Approx. Thickness in Mils | Typical Design Implication |
|---|---|---|---|
| 0.5 oz | 17.5 µm | 0.7 mil | Fine geometry, lower copper cross-section |
| 1 oz | 35 µm | 1.4 mil | Common starting point for signal and general-purpose layers |
| 2 oz | 70 µm | 2.8 mil | More copper cross-section, wider minimum geometry |
| 3 oz | 105 µm | 4.1 mil | Heavy-current geometry requires more fabrication allowance |
| 4 oz | 140 µm | 5.5 mil | Heavy copper changes etching, spacing and stackup decisions |
| 6 oz | 210 µm | 8.3 mil | Very heavy copper requires fabrication review |
These figures are nominal weight-to-thickness conversions, not a promise that every finished conductor measures exactly that thickness. On outer layers, plating and processing have to be considered when defining the finished copper requirement.
That distinction matters when an electrical calculation assumes one thickness while the fabrication drawing communicates another.
Why 1 oz, 2 oz and 4 oz Copper Behave Differently
Increasing copper weight increases conductor cross-sectional area. For the same trace width and length, that reduces electrical resistance.
Resistance follows the basic relationship:
R = ρL / A
where ρ is copper resistivity, L is conductor length and A is conductor cross-sectional area. If trace width stays fixed while copper thickness doubles, the cross-sectional area approximately doubles and the conductor resistance approximately halves.
That does not mean you should simply double copper whenever a trace carries more current. Trace width, temperature rise, ambient conditions, external versus internal layers, copper distribution and available routing area all influence the electrical and thermal result.
Fabrication also pushes in the opposite direction. Thicker copper is harder to etch into narrow, closely spaced features. The designer gains conductor cross-section but gives up some routing density.
A Simple 1 oz Versus 2 oz Trace Example
Consider a hypothetical 100 mm power trace that is 1.0 mm wide.
With 1 oz copper at approximately 35 µm, its nominal cross-sectional area is about:
1.0 mm × 0.035 mm = 0.035 mm²
At 2 oz:
1.0 mm × 0.070 mm = 0.070 mm²
The second trace has roughly twice the copper cross-section. All else equal, its resistance is approximately half that of the 1 oz version.
But the fabrication question has changed too. A 2-layer board with 1 oz copper can support finer trace and space geometry than the same basic construction using 2 oz copper. Copper weight therefore belongs in the routing discussion before layout is frozen, not as a late fabrication-note change.
1 oz Copper Is Often the Routing-Friendly Starting Point
One-ounce copper gives the layout engineer a useful balance between conductor thickness and feature definition. It is well suited to designs where routing density matters and the current requirement can be satisfied with the available trace widths and planes.
On a 2-layer, 1 oz circuit board, 3 mil / 3 mil trace and space can be fabricated within the applicable manufacturing envelope. Moving that 2-layer design to 2 oz changes that geometry to 6 mil / 6 mil.
That difference can affect an entire layout.
Imagine a hypothetical 2-layer controller with several dense connector breakouts. At 1 oz, the routing plan uses 5 mil traces with 5 mil clearance in its crowded regions. Changing the fabrication note to 2 oz without rerouting creates a mismatch because the original geometry was built around thinner copper.
The problem is not that 2 oz is inherently difficult. The problem is changing copper weight after routing decisions have already consumed the available space.
2 oz Copper Trades Routing Density for Conductor Cross-Section
Two-ounce copper becomes attractive when current, voltage drop or thermal behaviour requires more conductor cross-section than a 1 oz layout provides.
There are two ways to add conductor area: make the trace wider or make the copper thicker. Often the practical design uses both.
A wide 1 oz power trace may be electrically acceptable but consume routing space needed for other nets. Moving to 2 oz can increase cross-section without doubling every trace width. The trade-off is that the thicker copper itself needs more lateral allowance during fabrication.
This becomes especially important around dense pads, narrow neck-downs and copper pours. A designer may specify 2 oz because of a high-current path, then discover that an unrelated dense section of the same layer is what controls manufacturability.
Board A Versus Board B: Same Four Layers, Different Copper Decisions
Consider two hypothetical 4-layer, 1.6 mm boards.
| Decision Factor | Board A | Board B |
|---|---|---|
| Layer count | 4 | 4 |
| Board thickness | 1.6 mm | 1.6 mm |
| Outer copper | 1 oz | 2 oz |
| Inner copper | 1 oz | 1 oz |
| Dense outer routing | Yes | Yes |
| Main design priority | Routing density | Higher outer-layer conductor cross-section |
| Fabrication consequence | Finer outer geometry is easier to preserve | Etch compensation and spacing become more restrictive |
Both boards have the same layer count and overall thickness, but they should not be expected to behave identically in fabrication.
Board B puts more copper into the outer-layer etching process. The extra thickness increases the amount of copper that must be removed between conductors, which changes how tightly features can be resolved. Board A has less copper cross-section, but gives the designer more freedom where outer-layer routing is dense.
This is why layer count alone is a poor predictor of PCB fabrication difficulty or cost.
4 oz Copper Changes the Fabrication Problem
At 4 oz, copper is no longer just a thicker version of a conventional signal layer. The etching process becomes a major design constraint.
Etchant attacks copper vertically to clear unwanted material, but lateral material loss also matters. As copper becomes thicker, creating a deep, narrow channel between two conductors becomes more difficult without affecting their final width.
This is why heavy copper and very fine trace spacing work against each other.
Four-ounce outer copper sits within heavy-copper fabrication territory and should be confirmed against the actual artwork, stackup and conductor spacing. Outer copper can extend up to 6 oz for suitable builds, while inner copper requirements have different limits.
Worked 4 oz Example
Suppose a hypothetical 6-layer power-control board uses:
- Layer 1: 4 oz copper for high-current distribution
- Layer 2: 1 oz ground plane
- Layer 3: 1 oz signals
- Layer 4: 1 oz signals
- Layer 5: 1 oz power plane
- Layer 6: 4 oz copper for high-current distribution
- Finished board thickness: 2.0 mm
- Smallest outer trace gap in the initial layout: 4 mil
The engineer cannot assume that a 4 mil gap appropriate to a thinner-copper design remains suitable simply because the layer count and board outline have not changed. The heavy outer copper changes the etching condition.
The practical fix may involve increasing spacing, changing copper distribution or moving dense routing away from the heavy-copper layers. That decision is cheaper to make during stackup and layout planning than after the Gerbers are released.
Outer and Inner Copper Weight Are Separate Stackup Decisions
A PCB does not need the same copper weight on every layer.
Outer layers pass through processing that differs from internal copper layers, including plating associated with plated holes and outer-layer conductor formation. Inner layers are patterned before lamination into the multilayer structure.
That difference is why a stackup should identify copper by layer rather than say only “2 oz board.”
For example, a 6-layer printed circuit board could be specified conceptually as:
- Layer 1: 2 oz
- Layer 2: 1 oz
- Layer 3: 1 oz
- Layer 4: 1 oz
- Layer 5: 1 oz
- Layer 6: 2 oz
This may satisfy an outer-layer power requirement without imposing thick copper on every internal signal layer.
The fabrication envelope supports 0.5, 1 and 2 oz inner copper, while outer layers support 0.5, 1, 2 and 3 oz as standard choices, with heavier outer copper available for suitable designs.
The manufacturer therefore needs the required copper for each layer, not just one value attached to the whole board.
Copper Thickness Affects Trace Width and Spacing
The electrical engineer tends to see copper thickness as conductor area. The fabrication engineer also sees it as etch depth.
That second perspective explains why increasing copper weight can force a layout change even when the electrical schematic stays exactly the same.
Consider a dense 2-layer board:
| Design Variable | 1 oz Copper | 2 oz Copper |
|---|---|---|
| Copper thickness, nominal | ~35 µm | ~70 µm |
| Supported trace / space | 3 / 3 mil | 6 / 6 mil |
| Relative conductor cross-section at same width | 1× | ~2× |
| Routing-density pressure | Lower | Higher |
The mechanism is physical. A narrow space between thick copper features has a larger depth relative to its width. Maintaining conductor shape while clearing that channel becomes harder as copper thickness rises.
If a board combines high-current nets and dense digital routing, separating those functions across layers can be more effective than specifying heavy copper everywhere.
Copper Weight Also Changes the Finished Stackup
Copper occupies physical thickness, so changing copper weight changes the geometry the stackup engineer has to accommodate.
On a simple board, that may be straightforward. On a multilayer controlled-impedance design, however, conductor thickness interacts with dielectric spacing and trace geometry.
Controlled impedance is produced by the electromagnetic relationship among the conductor, reference plane and dielectric system. Changing conductor thickness while freezing everything else can therefore move the impedance away from its target.
This is one reason copper weight should be locked before controlled-impedance geometry is finalized.
For controlled-impedance fabrication, the stackup, target impedance and relevant fabrication information need to travel together. Controlled impedance can be verified by time-domain reflectometry (TDR) using a coupon, with a test report supplied with the order.
Heavy Copper Affects Vias as Well as Traces
A high-current design often combines thick copper planes with substantial via structures. Treating those as independent specifications can create avoidable problems.
The via barrel has its own conductive cross-section, while the surrounding pads and planes determine how current enters and leaves it. Increasing surface copper does not automatically make a small via appropriate for the same current path.
Board thickness also matters because it establishes drilling aspect ratio. A 0.20 mm finished hole through a 2.0 mm board represents a 10:1 thickness-to-hole relationship. If the board becomes 2.4 mm thick with the same hole diameter, the ratio becomes 12:1.
That geometric change affects drilling and plating even though the copper weight did not change.
For high-current paths, review trace width, copper thickness, via geometry, pad geometry and layer transitions as one conductive system.
Do Not Confuse Base Copper With Finished Copper
Base copper is the copper foil used as the starting material. Finished copper is the conductor condition after the relevant fabrication processes have been completed.
For inner layers, the relationship is comparatively direct because the copper is patterned and laminated. Outer layers go through additional processing, so a requirement for finished outer copper should be communicated clearly rather than inferred from an ambiguous note such as “2 oz PCB.”
That ambiguity matters when a calculation relies on conductor resistance or when a procurement team compares quotes from different fabrication interpretations.
A useful fabrication package identifies the copper requirement layer by layer and includes the stackup rather than forcing the manufacturer to reconstruct design intent from the Gerbers.
Choose Copper Weight From the Electrical Requirement Backwards
Do not begin with “1 oz or 2 oz?” Begin with the current path and routing constraints.
- Identify the current requirement. Determine which traces, planes and layer transitions actually need additional conductor area.
- Estimate the required cross-section. Consider trace width and copper thickness together rather than treating either one as fixed.
- Check available routing width. A wider 1 oz conductor may solve the problem without forcing thicker copper across a dense layer.
- Check fabrication geometry. Make sure the proposed copper weight is compatible with the smallest traces, spaces, pads and clearances on that layer.
- Review the stackup. Confirm whether outer and inner layers genuinely need the same copper weight.
- Revisit impedance structures. Any copper-thickness change can require controlled-impedance geometry to be recalculated.
- Review vias and transitions. A high-current trace is only as useful as the structures connecting it to the next layer.
This approach prevents one demanding power net from dictating unnecessary copper thickness across the entire circuit board.
What to Put in the Fabrication Package
Copper weight should be explicit enough that purchasing, engineering and fabrication are all ordering the same board.
A useful release package includes the manufacturing data plus the stackup and fabrication notes. Accepted manufacturing formats include Gerber RS-274X, ODB++, IPC-2581 and NC drill data. Stackup PDFs and impedance targets can accompany the package where applicable.
For copper specifically, identify which layers carry which copper requirements and whether the specification refers to a required finished condition. If heavy copper sits beside dense routing or controlled impedance, make that interaction visible before the board reaches production.
Get the Copper Stackup Reviewed Before Release
Copper weight affects electrical resistance, routing density, etching, vias and impedance at the same time. If you are specifying 2 oz, 4 oz or another mixed-copper stackup, RLX can review the manufacturing files and stackup as part of the engineer-reviewed design for manufacturability (DFM) process before the build is released.
Frequently Asked Questions
Is 1 oz PCB copper 35 µm thick?
Approximately. One-ounce copper is commonly converted to about 35 µm, or 1.4 mil, as a nominal thickness. That conversion is useful for electrical calculations, but engineers should distinguish nominal copper weight from the finished conductor condition, particularly on outer layers where fabrication includes additional processing.
How thick is 2 oz PCB copper?
Two-ounce copper is approximately 70 µm, or 2.8 mil, based on the nominal weight-to-thickness conversion. Compared with 1 oz copper at the same trace width, it provides approximately twice the conductor cross-sectional area. It also requires more fabrication allowance, so minimum trace and spacing requirements can change.
How thick is 4 oz copper on a PCB?
Four-ounce copper corresponds to approximately 140 µm, or 5.5 mil, nominal thickness. At that thickness, etching becomes a significant layout consideration because deep copper features are harder to resolve at fine spacing. Heavy-copper artwork should therefore be reviewed against the actual trace, gap and stackup geometry.
Can a PCB have different copper weights on different layers?
Yes. A multilayer PCB can use different copper weights on outer and inner layers when the fabrication stackup supports them. This is useful when power distribution needs thicker copper but dense signal layers benefit from thinner copper. Specify the requirement by layer rather than describing the entire board with one copper-weight value.
Is 2 oz copper always better than 1 oz for high current?
No. Current handling depends on conductor geometry and thermal conditions, not copper weight alone. Increasing trace width also increases cross-sectional area. A design may use wider 1 oz conductors, 2 oz copper, planes or a combination. The fabrication consequence matters because thicker copper can require larger trace spacing.
Does copper weight affect controlled impedance?
Yes. Conductor thickness is one of the geometric variables involved in controlled impedance. Changing copper weight without recalculating the trace and dielectric geometry can alter the impedance. For an impedance-controlled board, define copper requirements as part of the complete stackup rather than changing them independently after routing.
