The primary purpose of pretreatment in each PCB manufacturing process is to achieve two key objectives: cleaning and surface roughening.
Pretreatment is designed to remove oxidation and oil contamination, ensuring a clean board surface; simultaneously, it increases surface roughness through physical or chemical means to ensure adequate bonding strength, adhesion, and electroplating results in subsequent processes.
This article focuses on explaining the specific pretreatment steps involved in each major PCB manufacturing process and their respective purposes.
Common Pretreatments and Their Functions
1. Inner Layers
Chemical Cleaning / Mechanical Brushing / Micro-etching
Cleaning: Removes oil, dust, and oxide layers from the copper foil surface
Roughening: Increases the surface roughness of the copper to improve the adhesion of dry and wet films
2. Laminating
Brown Oxide Treatment / Black Oxide Treatment
Roughening: Forms an oxide layer (honeycomb structure) on the copper surface to enhance mechanical bonding with the prepreg (PP sheet) and prevent delamination
3. Drilling
Desmearing & Deburring
Cleaning (Inner Holes): Removes molten resin residues generated by high-temperature drilling, cleans the hole walls, and lays the foundation for subsequent through-hole plating
4. Through-Hole Plating
Degreasing / Micro-etching / Pre-soaking / Activation / Hole Finishing
Cleaning (Inner Holes): Improves the cleanliness and adsorption capacity of the hole walls, ensuring precise preparation for chemical copper deposition, which directly impacts the quality of the copper plating
5. Outer Layer Circuits
Chemical Cleaning / Microetching / Rinsing and Drying
Similar to “Inner Layer Circuit Pretreatment,” this cleans the copper surface to enhance dry film adhesion and ensure pattern accuracy
6. Solder Mask
Degreasing / Microetching / Rinsing and Drying (Acid Washing, Board Polishing, etc.)
Cleaning: Removes surface contaminants, improves solder mask ink adhesion, and prevents ink peeling (delamination)
7. Surface Treatment
Micro-etching / Acid washing and other functional pretreatments
Cleans and micro-etches the copper surface to ensure coating adhesion for processes such as Hot Air Solder Leveling (HASL) and Electrolytic Nickel-Immersion Gold (ENIG)
8. Others
Plasma Treatment
Multifunctional treatment: Removes drilling residues, activates the surface, roughens hole walls, and improves material bonding; efficient and environmentally friendly (commonly used for flexible boards and HDI boards)
Every pretreatment step lays a solid foundation for the subsequent “bonding” process.
What Defects can Pretreatment Prevent?
1. Pretreatment of Inner Layers (chemical Cleaning/brushing/micro-etching)
Defects caused by inadequate pretreatment—residual oxidation, oil contamination, or insufficient surface roughness on the copper surface, leading to poor dry film/wet film adhesion → circuit detachment, plating penetration, incomplete etching (short circuits), or over-etching (open circuits)
2. Browning/Blackening Treatment Before Laminating
Defects caused by improper processing: Uneven or too-thin roughened layer on the copper surface, resulting in insufficient bonding strength between the PP film and copper foil → Delamination, bubbling, thermal shock failure (board rupture during reflow soldering or hot oil testing)
3. Resin Residue Removal and Deburring After Drilling
Defects caused by improper processing: Residual molten resin (resin residue) on hole walls or burrs at hole openings; rough hole walls; lack of copper in holes.
Broken copper in holes (poor adhesion during subsequent plating); burrs may even cause short circuits in outer layer circuits
4. Pre-treatment for Through-Hole Metallization (Degreasing/Micro-etching/Activation/Hole Cleaning)
Defects: Contaminants not fully removed from hole walls or poor surface activity, preventing chemical copper deposition → No copper in holes, low backlight grade, voids in hole walls, or uneven plating thickness due to inconsistent activation
5. Pre-treatment for Outer Layer Circuits
Defects caused by improper preparation: Similar to inner layers, a dirty or rough copper surface results in poor dry film adhesion → Plating bleed during pattern transfer, jagged traces, short circuits/open circuits, and simultaneously affects the etching factor, leading to insufficient line width
6. Solder Mask Pretreatment (degreasing/micro-etching/brushing)
Defects caused by improper preparation: Fingerprints, oxidation, or hydrophobic residues on the copper surface result in poor solder mask ink adhesion → Ink peeling (delamination), bubbling during soldering, solder mask penetration into vias, or poor via coverage
7. Surface Treatment Preparation (micro-etching/pickling)
Defects caused by improper preparation: Contaminated copper surface or inappropriate surface roughness.
For HASL (Hot Air Solder Leveling), this results in poor tin coverage and uneven tin layer thickness;
For ENIG (Electroless Nickel Immersion Gold), it leads to poor nickel plating deposition, black pads, poor solderability, or post-soldering cracking
8. Plasma Treatment (for FPC/HDI/high-frequency boards)
Defects caused by inadequate treatment: Insufficient activation of hole walls or surfaces; incomplete removal of drilling residue;
Hydrophobic polyimide surfaces → resulting in copper-free plated-through holes, delamination, and poor adhesion
Every pretreatment step is designed to achieve “cleanliness + surface roughening”—if not performed properly, subsequent processes will sequentially result in issues such as adhesion failure, poor via conductivity, pattern defects, solder mask delamination, and poor solderability.
Strict adherence to process requirements can prevent the vast majority of these defects at the source.
Conclusion
In PCB manufacturing, pretreatment is a critical foundation process that directly affects bonding strength, plating quality, circuit accuracy, solder mask adhesion, and long-term reliability.
Whether applied to inner layers, drilled holes, outer layers, solder mask surfaces, or final surface finishes, every pretreatment step serves the essential purposes of cleaning contaminants and increasing surface roughness.
Proper pretreatment helps prevent many common PCB defects, including delamination, poor copper deposition, open circuits, short circuits, solder mask peeling, uneven plating, and poor solderability.
As PCB products continue to evolve toward high density, high frequency, miniaturization, and high reliability, the requirements for pretreatment processes are becoming increasingly stringent.
By strictly controlling pretreatment parameters and maintaining stable process quality, manufacturers can significantly improve PCB yield, product reliability, and overall production consistency while reducing downstream defects and manufacturing costs.
