Introduction to the PCB Wet Process
In PCB (Printed Circuit Board) manufacturing, the complete production flow includes both dry processes and wet processes. Dry processes typically involve drilling, lamination, and exposure, while wet processes are carried out in chemical solutions and include steps such as through-hole plating, copper electroplating, imaging, stripping, and etching.
Compared with dry operations, the PCB wet process has a more direct impact on circuit accuracy, copper adhesion, solderability, and long-term reliability. In this article (Part 1), we introduce the key terminology and process concepts used in PCB wet processing, helping engineers, purchasers, and quality teams better understand this critical stage of circuit board manufacturing.
1. Abrasives in PCB Surface Preparation
Abrasives refer to materials used to clean and micro-roughen the copper surface before subsequent PCB processes. Common abrasives include:
- Polymer nonwoven fabrics
- Nonwoven fabrics embedded with emery
- Various sand-based materials
- Pumice slurry
These materials are widely applied in PCB surface treatment to remove oxidation and contaminants. However, abrasive brushes containing sand particles may scratch the copper surface, causing micro-grooves. These defects can negatively affect the adhesion of photoresist films or electroplated copper layers, leading to potential reliability issues.
The original structure of abrasive fibers mixed with sand particles is illustrated in the referenced schematic.
2. Air Knife in PCB Wet Processing Lines
An air knife is commonly installed at the exit of wet process equipment. It uses high-temperature, high-pressure air to rapidly remove residual water from the PCB surface.
Key benefits include:
- Faster drying of circuit boards
- Reduced oxidation risk
- Improved handling efficiency between process steps
Air knives are essential for maintaining consistency and cleanliness in high-volume PCB production lines.
3. Anti-Foaming Agents in PCB Chemical Baths
During processes such as dry film imaging, organic residues may dissolve into the solution. Combined with spray action and air mixing, excessive foam can form, disrupting process stability.
To control this, anti-foaming agents such as:
- Octanol (octyl alcohol)
- Silicone-based defoamers
are added to reduce surface tension and suppress foam generation.
Important note: Silicone resins containing silica-based cationic surfactants are not recommended for metal surface treatment. Once they contact copper surfaces, they are difficult to remove and may cause poor adhesion or reduced solderability in subsequent PCB coating processes
4. Bondability of PCB Copper Surfaces
Bondability refers to the ability of a PCB surface to form strong and stable adhesion with subsequent layers, such as photoresist, plating, or solder mask.
To achieve high bondability:
- The copper surface must be thoroughly cleaned
- Organic residues and oxidation must be removed
- Surface energy must remain stable throughout processing
Proper bondability is critical for ensuring PCB reliability and yield.
5. Banking Agent in PCB Etching
A banking agent is an organic additive used in PCB etching solutions. Its primary functions are:
- Forming a temporary protective film on weak line edges
- Reducing the impact of lateral chemical attack
- Minimizing side etching (also known as undercut)
The use of banking agents is especially important in fine-line PCB etching, and their formulations are often proprietary know-how of chemical suppliers.
6. Bright-Dip Treatment
Bright-dip is a controlled, light chemical etching process applied to metal surfaces. Its purpose is to:
- Smooth the copper surface
- Improve brightness and appearance
- Enhance uniformity before further processing
Bright-dip treatments are commonly used in PCB surface finishing stages.
7. Chemical Milling (Chemical Etching / PCM)
Chemical milling, also known as chemical blanking or Photo Chemical Machining (PCM), is a wet chemical process used to selectively remove metal material.
Applications include:
- Surface roughening
- Deep or partial etching
- Precision pattern formation without mechanical punching
Compared with traditional punching, chemical milling offers:
- Lower tooling cost
- Shorter preparation time
- No residual mechanical stress
This makes PCM technology highly suitable for high-precision PCB and metal component manufacturing.
8. Coating and Surface Coating in PCB Manufacturing
In PCB terminology, coating usually refers to the final or intermediate surface layers applied to the board.
In a broader sense, coating includes:
- Copper plating
- Protective surface finishes
- Any functional surface treatment layer
These coatings directly affect PCB corrosion resistance, solderability, and electrical performance.
9. Conversion Coating
Conversion coating is a process where a metal surface reacts chemically with a solution to form a protective conversion layer.
Common examples include:
- Phosphating on steel surfaces
- Chromating on zinc surfaces
- Zincating on aluminum surfaces
In PCB-related metal processing, conversion coatings can:
- Act as a base (striking layer) for further plating
- Improve adhesion
- Enhance corrosion resistance
10. Degreasing vs. Cleaning in PCB Processes
Traditionally, metal parts are degreased before electroplating to remove oils from mechanical processing. Typical methods include:
- Vapor degreasing using organic solvents
- Emulsion-based soaking degreasing
In PCB manufacturing, however, oil contamination is minimal compared to traditional metal plating. Therefore, instead of degreasing, a cleaning process is used to remove residues and prepare the board surface for subsequent wet processes.
Although different in concept, this cleaning step remains essential for stable PCB quality control.
11. Etch Factor in PCB Etching Quality
During etching, copper is removed not only vertically but also laterally beneath the resist, a phenomenon known as undercut. Excessive undercut can cause mushroom-shaped line profiles and fine-line defects.
The etch factor is a key indicator of PCB etching quality:
- In the IPC (U.S.) definition, etch factor = vertical etch depth ÷ lateral etching
- A higher etch factor indicates better etching performance
Note: Some European standards define the etch factor differently (inverse relationship), which can easily cause confusion. Since IPC standards dominate global PCB specifications, the IPC definition is now widely accepted in the PCB industry.
Optimize Your PCB Wet Process with the Right Partner
The PCB wet process plays a decisive role in circuit accuracy, surface quality, and long-term product reliability. From abrasives and etching chemistry to bonding and coating control, every detail matters in modern printed circuit board manufacturing.
If you are looking for a reliable PCB manufacturer or PCBA partner with deep expertise in wet process control, material selection, and quality assurance, we are here to help.
Contact us today to discuss your PCB or PCBA project, or send us an inquiry to get professional support, competitive pricing, and fast turnaround for your circuit board needs.
