Trace Width and Current Capacity in Flex PCB Design

Relationship between copper thickness, trace width, and current in flex PCB design

In flex PCB design, understanding the relationship between copper thickness, trace width, and current-carrying capacity is a critical skill every engineer must master. Whether you're working on power delivery networks or routing sensitive signal lines, selecting the wrong parameters can lead to excessive heat, reduced performance, or even flex circuit failure.

1. Basic Concept: Where Does the "Copper" in Flex PCBs Come From?

The conductive layers in flexible printed circuit boards (FPCBs) are mostly made of copper foil. Copper thickness is typically specified in oz/ft², which can be converted into μm or mm. In PCB terms, 1 oz of copper refers to its thickness when spread over one square foot:

1 oz copper foil = 35μm = 1.4 mil

Common Copper Thickness	Equivalent Thickness
0.5 oz	≈17.5μm (0.0175 mm)
1 oz	≈35μm (0.035 mm)
2 oz	≈70μm (0.070 mm)

While heavier copper (up to 6 oz or more) increases conductivity and current capacity, it reduces etching precision and flexibility, which are vital in flex PCB design.

2. Core Formula: How Trace Width Determines Current Capacity

Trace width refers to the width of a copper trace on the PCB, typically measured in mil or mm (1 mil = 0.001 inch ≈ 0.0254 mm). A wider trace carries more current, produces less heat, but takes up more space.

Current-Carrying Capacity

Trace width, usually measured in mil or mm, directly influences how much current a trace can safely carry. Wider traces reduce heat buildup but consume more board space.

The current-carrying capacity of a trace is determined by: Trace Width × Copper Thickness

Allowable temperature rise (usually 10°C or 20°C)

The IPC-2152 standard provides formulas and charts, but a commonly used estimate for external PCB layers is:

I = k ⋅ (W ⋅ T)^0.44

Where:

• I = Current (A)

• W = Trace width (mil)

• T = Copper thickness (oz)

• k = Constant (≈ 0.048 for external layers, ≈ 0.024 for internal layers)

This formula is an estimate. For accuracy, consult IPC-2152 or use simulation tools.

Copper thickness	Trace Width (mm)	Current Capacity (A)
0.5oz	0.25	0.5
0.375	0.7
0.5	0.7
0.625	0.9
1oz	0.25	1
0.375	1.2
0.5	1.3
0.625	1.7
2oz	0.25	1.4
0.375	1.6
0.5	2.1
0.625	2.5

Note: Flex PCBs require special consideration due to their structure (thinner insulation, different thermal behavior, etc.).

3. Special Considerations:

Due to their structural differences, flexible circuits require unique design strategies:

• Flexibility vs conductivity: Thick copper improves current flow but reduces flexibility.

• Avoid narrow traces in dynamic or bend zones.

• For high-current traces, consider placing them near the ground plane and using widened or copper-filled paths.

• Manufacturing constraints, such as etching limits affect minimum trace width and spacing.

These are essential flex PCB layout guidelines that ensure mechanical reliability and electrical performance.

4. Copper Thickness vs. Current—Conversion Tips

Many designers assume doubling the copper thickness doubles the current capacity, but thermal limitations prevent a linear increase.

Copper Weight	Relative Current Capacity
0.5 oz	≈ 0.7 × I
1 oz	I
2 oz	≈ 1.6 × I
3 oz	≈ 2.1 × I

This knowledge is key when optimizing flexible PCB current paths.

5. Common Misconceptions in Flex PCB Current Design

Myth 1: Wider traces are always better.

Overly wide traces can cause impedance discontinuities that degrade signal integrity.

Myth 2: Thicker copper is always safer.

While thicker copper improves current handling, it complicates etching and reduces trace resolution.

Myth 3: DC designs don’t need thermal management.

High continuous current can still cause copper oxidation and delamination in flex PCBs.