Flex PCB DES Process: Developing, Etching & Stripping Guide

The DES process—Developing, Etching, and Stripping—is one of the most critical stages in flex PCB manufacturing. It directly defines the final circuit pattern by removing unwanted copper and leaving only the designed traces.

As a subtractive process, DES determines the minimum line width, line spacing, and overall quality of flexible circuits. This guide explains the entire DES flow, including Mylar peeling, key process controls, and important concepts such as developing point and etching factor.

Mylar Peeling Before DES

For dry film imaging, the photoresist is laminated onto the copper foil with a Mylar (PET) protective layer.

After exposure and before entering DES, this Mylar must be removed.

• Dry film process → requires Mylar peeling

• Wet film/LPI process → no Mylar, no peeling step

 Mylar can be peeled manually or using an automated peeling machine. Proper peeling prevents film damage, residue, or wrinkles that affect imaging quality.

What the DES Process Does?

In flex PCB manufacturing, a DES line is traditionally an integrated, automated machine that performs:

• Developing (D) – removing unexposed photoresist

• Etching (E) – removing exposed copper 

• Stripping (S) – removing the remaining photoresist

The purpose is simple:

use photoresist to protect the lines you want, remove the copper you don’t want, and finally strip the resist to reveal the finished circuit.

DES Process Flow (Step-by-Step)

Step	Function
Loading	Boards enter the DES line automatically.
Developing	Removes unexposed dry film/wet film and exposes the copper that will be etched.
Water Rinse	Cleans chemical residue to avoid contamination of later stages.
Etching	Chemically removes copper exposed during development.
Stripping	Removes the remaining photoresist from protected copper areas.
Acid Cleaning	Neutralizes alkaline stripping solution and stabilizes copper surface.
Drying	Removes moisture from the board surface.
Unloading	Boards are collected for the next process.

 This structure is standard in both rigid and flexible PCB factory using the subtractive copper process.

Step Details

Developing

The developer removes all areas of photoresist that were not exposed during imaging.

A clean, sharp developing result is essential. Poor developing causes:

• Residual resist

•  Incomplete copper exposure

•  Over-etching due to chemical attack

Etching

Etching removes the unwanted copper revealed by development.

The remaining resist protects the designed traces. Etching quality determines:

• Minimum line/space capability

• Sidewall shape

• Dimensional accuracy 

Uniform spray pressure, chemistry balance, and conveyor speed are key to stable etching.

Stripping

After etching, the resist on the top of the remaining copper must be removed.

Incomplete stripping causes:

• Poor adhesion in later processes

• Solder mask defects

• Rework or scrap

 A clean copper surface is essential for subsequent processing stages. 

Key Technical Concepts

Developing Point (Break Point)

The developing point represents where copper first becomes visible in the developing section.

Formula:

Developing Point (%) = (Break Point Distance / Total Developing Length) × 100

Example:

If the developing length is 1.5 m and copper appears at 0.6 m: 0.6 ÷ 1.5 × 100% = 40%

 Typical control range: 40%–60%

•  Too early → exposed resist stays too long in developer → undercut / side attack

•  Too late → unexposed resist develops incompletely → residual resist defects

Etching Factor (EF)

 Etching Factor evaluates the difference between the top and bottom widths of a trace.

•  H: Copper thickness

•  Wt: Top line width

•  Wb: Bottom line width

 Etching Factor (F) = 2H / (Wb – Wt)

 A higher etching factor means:

•  Less side etching

•  More vertical sidewalls

•  Better fine-line capability

•  Stronger etching capability from the manufacturer

 This index is crucial for HDI flex PCB and fine-pitch applications.

DES Challenges in Flex PCB Production

Compared to rigid boards, flex PCBs require tighter control due to:

•  Material stretch and deformation during processing

•  Tension control on the conveyor

•  More sensitive photoresist behavior on PI film

•  Higher difficulty maintaining uniform etching

•  Greater tendency for line-width variation

Advanced flex PCB factories optimize pressure, nozzle flow, temperature, and dwell time to ensure consistent DES performance. 

Why DES Quality Matters ?

A stable DES process provides:

• Better line width/spacing accuracy

• High etching uniformity 

• Less copper residue 

• Stronger impedance control 

• Higher production yield 

• Better long-term reliability of flexible circuits

For products in consumer electronics, automotive, medical and wearables, DES quality directly impacts final performance.

Conclusion

DES is the core subtractive process that defines the real circuit pattern on a flex PCB. From Mylar peeling to developing, etching, and stripping, each step requires careful control to achieve fine-line accuracy and stable production quality.

Understanding DES helps designers and engineers make better choices in materials, tolerances, and manufacturability. For complete manufacturing knowledge, be sure to explore our flex PCB fabrication resources.