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At present, the mainstream technologies used for manufacturing multilayer PCBs in the electronics industry can be broadly divided into two categories: the Plated Through-Hole (PTH) Process and the High-Density Interconnect (HDI) Build-Up Process. Both technologies create complex multilayer interconnections through a series of highly controlled manufacturing steps, enabling PCB production for different layer counts, precision requirements, and application scenarios.
Among them, the Plated Through-Hole (PTH) process remains the most widely adopted and versatile manufacturing technology in the PCB industry. After more than six decades of continuous development and refinement, it has evolved into a highly mature process supported by well-established equipment, optimized raw materials, and standardized production controls. The technology has also fostered a large-scale, standardized manufacturing ecosystem, offering exceptional reliability, process stability, and compatibility compared with alternative approaches.
One of the major advantages of the PTH process is its flexibility. It can be used for both conventional double-sided PCBs and complex multilayer circuit boards. Most of the core equipment and manufacturing procedures are shared between double-sided and multilayer PCBproduction, significantly reducing capital investment and manufacturing costs.
The fundamental principle of the PTH process is straightforward: plated-through holes are created within the insulating substrate, and conductive copper is deposited on the hole walls through electroplating. These metallized vias electrically connect conductors on different layers, enabling the formation of complete multilayer circuits.
Base Material Cutting
PCB manufacturers generally do not produce their own laminate materials. Core raw materials such as copper-clad laminates (CCLs), prepregs, and copper foils are sourced from specialized upstream material suppliers.
These materials are typically delivered as large standardized panels, commonly measuring 1 m × 1 m or 1 m × 1.2 m, which are not directly compatible with production equipment.
Therefore, the first manufacturing step is panel cutting. Large laminate sheets are precisely cut into production panels according to equipment capacity and product dimensions. Once cutting is completed, multilayer PCB production proceeds with inner-layer fabrication, followed by lamination. Subsequent processes such as drilling, plating, and outer-layer imaging largely follow the same workflow used for double-sided PCB manufacturing. After multiple inspection and finishing steps, the boards are packaged and shipped.
Inner Layer Circuit Formation
The inner layers of a multilayer PCB are typically fabricated using ultra-thin double-sided copper-clad laminates. This process directly determines the electrical performance and dimensional accuracy of the internal circuitry.
The manufacturing sequence begins with laminating a photosensitive dry film onto both sides of the copper-clad laminate. A phototool containing the predefined inner-layer circuit pattern is then aligned and exposed using ultraviolet light. During exposure, the circuit areas of the dry film are hardened through a photochemical reaction.
After exposure, the panel undergoes development, where the unexposed dry film is removed, leaving only the desired circuit pattern. Chemical etching is then performed to remove unwanted copper, exposing the predefined inner-layer circuitry.
Following etching, the remaining dry film is stripped away, leaving the finished copper traces fully exposed.
To identify defects such as opens, shorts, nicks, or missing traces, all inner layers are inspected using Automated Optical Inspection (AOI) systems. Defective panels are removed before further processing.
Qualified inner layers then undergo a brown oxide treatment, a critical preparation step for multilayer lamination. This treatment serves three important purposes:
Increasing the bonding surface area between copper and resin to improve interlayer adhesion.
Enhancing copper surface wettability, allowing molten resin to flow into fine circuit gaps and cavities.
Forming a stable passivation layer on the copper surface to prevent chemical reactions and moisture generation during lamination, thereby reducing risks such as delamination and blistering.
Multilayer Lamination
After oxide treatment, the inner layers are stacked according to the specified layer structure.
The standard stack-up sequence follows:
Inner Layer + Prepreg + Outer Copper Foil
Each layer is carefully aligned to ensure proper registration and to prevent contamination or misalignment.
The stack is then placed into a lamination press, where heat and pressure are applied under tightly controlled conditions. The prepreg softens, flows, and cures, bonding all layers into a single integrated PCB structure.
After lamination, the multilayer board exhibits excellent interlayer adhesion, dimensional stability, and structural integrity without delamination or void formation.
Precision Drilling
Following lamination, the conductive patterns on each layer remain electrically isolated. To establish electrical connections between layers, holes must be drilled through the board.
High-precision CNC drilling equipment is used to create vias and through-holes at designated locations. These holes provide pathways for subsequent metallization and interlayer electrical connectivity.
Drilling is therefore one of the most critical preparatory steps in multilayer PCB manufacturing.
Electroless Copper Deposition and Panel Plating
Because freshly drilled hole walls consist primarily of insulating resin materials, they must be metallized before they can conduct electricity.
This process consists of three key stages:
Desmearing
Electroless Copper Deposition
Panel Plating
Desmearing
Drilling generates resin smear on the hole walls. These residues can interfere with copper adhesion and reduce plating reliability.
Specialized chemical treatments remove the smear while simultaneously roughening the hole walls to improve copper-to-resin bonding strength.
Electroless Copper Deposition
Electroless copper deposition is a chemical plating process that deposits a thin conductive copper layer onto the non-conductive hole walls through oxidation-reduction reactions.
This copper seed layer serves as the conductive foundation for subsequent electroplating.
Panel Plating
Panel plating electrochemically thickens the copper deposited during the electroless process until the required copper thickness is achieved.
Because no circuit pattern has yet been formed on the outer layers, the entire panel surface is plated uniformly, hence the term panel plating.
Outer Layer Imaging and Pattern Plating
The purpose of this stage is to transfer the predefined outer-layer circuit pattern onto the PCB surface.
A dry film is laminated onto the panel and exposed using the outer-layer artwork. After development, the circuit areas are accurately defined.
Pattern plating is then performed on the exposed copper traces and plated through-holes. Copper is electroplated to increase conductor thickness, reduce resistance, and improve current-carrying capacity.
A protective tin layer is subsequently deposited over the plated copper to protect it from chemical attack during later etching operations.
Dry Film Stripping and Etching
After pattern plating, the remaining dry film is removed using a sodium hydroxide (NaOH) stripping solution.
The unwanted copper areas are then exposed and removed through chemical etching. Only the protected circuit pattern remains, forming the final outer-layer circuitry.
Solder Mask Application
The primary function of a PCB board is to support electronic components and provide reliable signal transmission.
Once the circuit pattern is completed, all non-soldering areas require protection. This is achieved through the solder mask process.
Using screen printing and photolithographic techniques, a protective solder mask coating—typically green, but also available in blue, black, red, white, and other colors—is applied to designated areas.
The solder mask serves several purposes:
Protecting copper traces from oxidation and corrosion.
Preventing accidental short circuits.
Improving insulation performance.
Extending service life.
Preventing solder bridging during component assembly.
Silkscreen Printing
To facilitate assembly, testing, maintenance, and product identification, various markings are printed on the solder mask surface.
Typical markings include:
Component reference designators
Product model numbers
Manufacturing batch codes
Polarity indicators
Functional symbols
Clear and accurate silkscreen markings improve manufacturing efficiency and product usability.
Surface Finishing
Surface finishing protects exposed copper areas such as pads and contact points that cannot be covered by solder mask.
Without protection, these copper surfaces can oxidize rapidly, resulting in poor solderability and unreliable electrical contact.
Common surface finish technologies include:
ENIG (Electroless Nickel Immersion Gold)
HASL (Hot Air Solder Leveling)
Immersion Tin
Immersion Silver
OSP (Organic Solderability Preservative)
These finishes provide oxidation resistance while maintaining excellent solderability and electrical performance.
Profile Routing
After all circuitry and surface treatments are completed, the PCB must be machined into its final shape according to the product design.
Additional features such as slots, cutouts, and mounting holes are also created during this stage.
Common profiling methods include:
CNC routing
Die punching
Manual routing
Manual cutting
Among these methods, CNC routing and die punching offer the highest precision and consistency and are preferred for high-volume production.
Electrical Testing and Final Inspection
Electrical testing verifies the functional integrity of the finished PCB.
Using dedicated testing equipment, each board is inspected for:
Open circuits
Short circuits
Excessive resistance
Connectivity defects
This ensures that defective products are identified before shipment.
The final quality inspection focuses on cosmetic and dimensional quality, including:
Surface scratches
Solder mask defects
Silkscreen misalignment
Burrs and edge defects
Plating imperfections
Minor defects may be reworked when possible. Only products that fully comply with design specifications and industry standards proceed to packaging and shipment.
With its mature process control, proven reliability, and excellent scalability, the Plated Through-Hole (PTH) process remains the cornerstone technology for mass-producing multilayer PCBs. It continues to provide a robust manufacturing foundation for modern electronic products across telecommunications, computing, automotive, industrial, and consumer electronics applications.