What Chemicals Are Used in Flexible PCB Etching?

The main chemicals used in flexible PCB etching are ferric chloride (FeCl₃), cupric chloride (CuCl₂), hydrochloric acid (HCl) for acid etching, and ammoniacal etchants such as cupric ammine complexes (Cu(NH₃)₄Cl₂), ammonium chloride (NH₄Cl), and ammonia (NH₃) for alkaline etching. These chemicals remove unwanted copper from the circuit surface to form precise conductive traces on polyimide flexible substrates. Acid etching is most commonly used in standard flexible PCB production, while alkaline etching is applied for thicker copper layers, multilayer, or rigid-flex designs.

Flexible PCB etching workshop

1. Acid Etching Process

Acid etching is the most common method for etching flexible PCB circuits. It mainly uses ferric chloride (FeCl₃) or cupric chloride (CuCl₂) as the primary etchant. These chemicals dissolve exposed copper areas while keeping protected traces intact, achieving clean and controlled etching results.

Ferric Chloride (FeCl₃) for Flex PCB Etching

Ferric chloride was once a commonly used etchant for PCBs. In recent years, however, manufacturers have shifted to more modern etching solutions, such as ammoniacal etchants and cupric chloride. These chemicals are effective in the subtractive PCB manufacturing process, where copper is selectively removed from a substrate to form the required circuit patterns.

Ferric chloride offers excellent line definition and stable etch rates, making it ideal for thin copper foils used on polyimide flexible substrates. Typical concentration: 38–42° Baumé, working temperature: 45–55°C.

Advantages:

• Produces uniform etched lines

• Cost-effective and easy to handle

• Suitable for polyimide-based FPCs

Disadvantages:

• Generates sludge that needs filtration

• Limited regeneration capability

Cupric Chloride (CuCl₂) for Flexible PCB Etching

Cupric chloride is a widely used chemical in flexible PCB etching. As a powerful oxidizing agent, it effectively dissolves copper and other metals, making it an efficient etchant for printed circuit boards. Many flex PCB manufacturers favor cupric chloride due to its fast etching speed, high efficiency, and ease of preparation and use.

Advantages:

• High precision for micro circuits

• Can be regenerated with oxygen or hydrogen peroxide

• Stable for continuous etching lines

Formula: CuCl₂+ HCl + O₂(or H₂O₂)

Supporting Chemicals

Hydrochloric acid (HCl) maintains the acidity and helps remove copper oxides, while oxidizers such as hydrogen peroxide (H₂O₂) assist in regeneration. Waste treatment involves lime or sodium hydroxide (NaOH) to neutralize and precipitate copper residues before discharge.

2. Alkaline Etching Process

In contrast, alkaline etching uses ammoniacal etchants, primarily based on ammonium chloride (NH₄Cl), ammonia (NH₃), and cupric ammine complexes (Cu(NH₃)₄Cl₂). Ammoniacal etchant is a chemical solution commonly used in the flexible PCB etching process. It is an effective etchant for copper-based PCBs. Ammoniacal etchant is preferred over other etchants for its high selectivity and low attack rate on copper. It is also environmentally friendly, as it can be safely disposed of and does not harm the environment.

This process is commonly applied to rigid PCB and thicker copper layers, but some flexible PCB manufacturers use it for advanced multilayer or rigid-flex designs.

Typical Composition:CuCl₂ + NH₄Cl + NH₃ + H₂O

Advantages:

• Produces extremely sharp and clean edges

• Easier to control etch rate for fine-line circuits

• Fully regenerable, reducing chemical waste

Disadvantages:

• Higher process cost

• Not ideal for very thin copper foils or single-layer flexible circuits

• Requires precise control of temperature and ammonia concentration

  
  

3. Acid Etching vs. Alkaline Etching

Aspect	Acid Etching	Alkaline Etching
Main Chemicals	Ferric chloride, cupric chloride, HCl	Cupric ammine, ammonium chloride, ammonia
Etching Type	Acid-based	Ammonia-based (alkaline)
Best For	Thin copper foils, single-layer FPC	Thicker copper, multilayer or rigid-flex boards
Etch Quality	Good uniformity, smooth sidewalls	High precision, cleaner edges
Regeneration	Limited, needs filtration	Fully regenerable with air or ammonia
Cost Level	Lower	Higher
Common Use	Flexible PCB manufacturing	Rigid or rigid-flex PCB production

Conclusion

The main chemicals used in flexible PCB etching include ferric chloride, cupric chloride, hydrochloric acid, and oxidizers for regeneration. While acid etching remains the industry standard for most flexible circuits due to its simplicity and cost-efficiency, alkaline etching offers superior precision and sustainability for complex or multilayer PCB designs.

At Flex Plus, we are committed to delivering high-quality flexible PCBs through advanced etching techniques, ensuring precise and reliable copper patterns for every project.