Etch Factor in Flex PCB Manufacturing

The etch factor is one of the most important parameters in flex PCB manufacturing. It determines how accurately copper traces can be formed during the etching process. For flexible printed circuits, where thin copper and fine-line features are common, understanding the etch factor is essential for stable product quality and long-term reliability.

What Is Etch Factor in Flex PCBs?

The etch factor describes the relationship between vertical etch depth and horizontal undercut.

Etch Factor = Vertical Etch Depth ÷ Horizontal Undercut

During etching, copper is not removed only downward. It also dissolves sideways. This side removal is called undercut. A higher etch factor means less undercut and more accurate trace formation.

Why Etch Factor Matters More in Flexible PCBs

Flex PCBs commonly use very thin copper foils such as 1/3 oz, 0.5 oz, and 1 oz. Thin copper is easier to over-etch, so the etch factor directly affects manufacturing capability.

Key reasons:

Flex PCBs often require fine-line routing (35–75 μm).

Thin copper has different etching rates between vertical and horizontal directions.

RA copper and ED copper respond differently to chemicals.

Poor etch factor leads to unstable impedance and reduced reliability.

Because of these factors, controlling the etch factor is critical for every flex PCB fabrication line.

Typical Etch Factor Values in Flex PCB Production

The actual number varies among factories, but the following ranges are common:

Copper Thickness	Typical Etch Factor	Notes
1/3 oz (12 μm)	1.2 – 2.0	Best for fine-line manufacturing
0.5 oz (18 μm)	1.0 – 1.5	Standard copper for flex PCBs
1 oz (35 μm)	0.8 – 1.0	Higher undercut, harder to control

A lower etch factor means heavier undercut and larger variations in finished trace width.

How Etch Factor Affects Trace Width and Impedance

1. Line Width Compensation

Fabricators must enlarge the artwork trace width to offset expected undercut. The amount of compensation depends directly on the etch factor.

2. Impedance Stability

More undercut makes the final trace narrower. Narrower traces lead to higher impedance. Designers should work closely with the flex PCB manufacturer to confirm achievable tolerances.

3. Minimum Line/Space Capability

If the etch factor is low, the factory cannot produce very fine lines. This limits the minimum L/S value on the flex circuit.

How to Improve Etch Factor in Flexible PCB Manufacturing

A fabrication line can optimize the etch factor through:

• Proper control of etching chemistry

• Balanced spray pressure and nozzle angle

• Stable etching temperature

• Using thin copper foils for fine-line designs

• Optimized cleaning and micro-etch before patterning

• High-quality dry film or LDI imaging

Improving etch factor directly improves production yield, especially for high-density flex circuits.

Design Tips for Engineers

• Ask the factory for the minimum trace width and spacing for each copper thickness.

• Use thinner copper if fine-line routing is required.

• Confirm line width compensation values during DFM review.

• For impedance traces, include a tolerance based on the factory’s real etching performance.

These steps help ensure predictable results during mass production.

Conclusion

The etch factor is a key parameter in flex PCB fabrication because it influences trace accuracy, impedance stability, and overall manufacturability. Good control of the etch factor allows manufacturers to produce fine-line flexible circuits with reliable performance. Understanding this parameter helps both designers and engineers build better flex PCB products.

If you want to explore more about materials and processes, visit our flex PCB fabrication page.