As one of the most widely used multilayer boards in electronic products today, the manufacturing process of 4-layer PCBs is significantly more complex than that of single-sided or double-sided boards. However, compared to high-layer-count boards (6 layers and above), they offer excellent cost-performance and manufacturability. A standard 4-layer PCB consists of a top layer, a bottom layer, and two inner layers (power/ground planes). These layers are laminated together under high temperature and pressure using prepreg (pre-impregnated) sheets and copper foil. The entire manufacturing cycle encompasses dozens of processes—from raw material preparation to final product inspection—each critically impacting the electrical performance, mechanical strength, and reliability of the final product. Below is a detailed introduction and analysis of the complete manufacturing workflow for 4-layer PCBs.

The manufacturing of a 4-layer PCB begins with the inner layers, which form the foundation of the entire multilayer board. The purpose of inner layer image transfer is to create the required circuit patterns on the copper-clad laminate (CCL). The main steps include:

First is cutting, where full-size CCL sheets are cut into production panels according to design dimensions. Precision must be maintained during cutting to avoid burrs and dimensional deviations. After cutting, the panels enter the pretreatment stage, including scrubbing and chemical cleaning. Scrubbing removes surface oxides and micro-contamination from the copper foil, enhancing adhesion between the copper surface and the dry film photoresist; this typically involves micro-etching using weak acidic or alkaline solutions. The cleaning stage removes residual abrasives and chemicals to ensure surface cleanliness.

Following pretreatment, a dry film photoresist is laminated onto the copper surface. This photosensitive polymer material is evenly attached via thermal lamination. During lamination, three parameters—temperature, pressure, and speed—must be strictly controlled. Temperature is generally maintained between 90°C and 110°C, and pressure must be evenly distributed to prevent bubbles and wrinkles. Once laminated, the panel proceeds to exposure. The designed inner layer film (artwork) is placed over the dry film, and ultraviolet (UV) light is applied. In exposed areas (transparent on the film), the dry film undergoes a photo-polymerization reaction and hardens, while unexposed areas remain soluble.

After exposure, the panel enters the development stage, where a sodium carbonate solution dissolves the unexposed, soluble dry film, revealing the copper areas intended for etching. Post-development inspection checks whether the circuit pattern is clear and intact, ensuring no defects like open circuits or short circuits exist. If confirmed, the panel moves to etching, where copper not protected by the hardened dry film is removed using cupric chloride or acidic copper chloride solutions to form the inner layer circuitry. After etching, a stripping process uses sodium hydroxide to completely remove the remaining dry film, exposing the finished inner copper traces. Finally, Automated Optical Inspection (AOI) is performed to verify that the trace width, spacing, and positional accuracy meet design specifications.

Once the inner layer circuits are formed and pass inspection, the copper surfaces undergo a surface treatment—most commonly black oxide or brown oxide treatment. Black oxide treatment forms a black oxide or tin oxide film on the copper surface. This film enhances the bonding strength between the copper surface and the prepreg, ensuring the inner copper foil adheres firmly to the semi-cured sheets during lamination. Brown oxide treatment forms an organic brown film on the copper surface, serving the same purposes of improving interlayer adhesion and preventing oxidation. Although this step may seem simple, it is crucial for lamination quality; improper treatment can lead to severe issues such as delamination or blistering. After treatment, the panels are washed and dried to ensure a clean, moisture-free surface before proceeding to the next stage.

Lamination is one of the most critical core processes in 4-layer PCB manufacturing and is the fundamental distinction between multilayer and double-sided boards. The purpose of lamination is to bond two processed inner cores with prepreg and outer layer copper foil under high temperature and pressure to form the complete 4-layer structure. The typical stack-up of a 4-layer board from top to bottom is: Outer Copper Foil → Prepreg → Inner Copper Foil (Layer 2) → Core Material → Inner Copper Foil (Layer 3) → Prepreg → Outer Copper Foil. Here, the core is the double-sided board with completed inner layer graphics, and prepreg is partially cured fiberglass cloth impregnated with epoxy resin, which flows and fully cures under heat and pressure to create a solid bond.

Before lamination, a pinning/riveting process aligns and temporarily fixes the layers using rivets to prevent misalignment during handling and press loading. Special attention must be paid to alignment holes and layer-to-layer registration marks to ensure interlayer accuracy. After pinning, the assembled stack is loaded into a vacuum laminator. The lamination process is divided into multiple temperature stages:

The entire lamination cycle typically takes 2–4 hours, depending on the board thickness and material properties. Post-lamination, X-ray inspection is conducted to confirm interlayer alignment accuracy and check for internal defects such as delamination or voids.

After lamination, holes must be drilled to achieve electrical interconnection between layers. 4-layer PCBs utilize through-holes, blind vias, and buried vias, with through-holes being the most common as they penetrate all four layers. CNC mechanical drilling machines or laser drilling machines are typically used; laser drilling is often employed for high-density interconnect (HDI) designs. Tungsten carbide or diamond-coated drill bits rotating at high speeds are used, with diameters ranging from 0.2 mm to 6.3 mm depending on design requirements. A drill life management system monitors bit wear in real-time, replacing worn bits promptly to guarantee hole wall quality.

Post-drilling, deburring is required to remove resin smears and copper debris from the hole walls and edges, typically via brushing or chemical methods. For high-reliability products, backlight inspection is also performed to confirm that each hole wall is smooth and intact, free from deviation or breakage. Drilling quality directly impacts the uniformity of subsequent copper plating and the ultimate reliability of the product, making this a vital process.

After drilling, a layer of copper must be deposited on the hole walls and board surface; this is achieved in two steps: electroless copper deposition (Plated Through Hole, PTH) and electroplating. First, electroless copper deposits a thin copper layer (approx. 0.5–1 μm) on the non-conductive resin surfaces of the hole walls, rendering them conductive for subsequent electroplating. This chemical process uses formaldehyde or other reducing agents to reduce copper ions onto the surfaces. Strict control of bath temperature, concentration, and pH is required, along with agitation, to ensure uniform coverage.

Next, panel plating (full-board electroplating) adds a thicker layer of copper. Typically, the hole wall copper thickness must reach 20–25 μm, and the surface copper thickness 17.5–25 μm. Acidic copper sulfate plating baths are used, with copper ions depositing on the cathode (the PCB) under direct current. Additives such as brighteners, levelers, and carriers are added to obtain a dense, bright, and uniform copper layer. Upon completion, both the hole walls and surface copper meet the required thickness, providing a solid base for subsequent pattern transfer.

The outer layer image transfer process is similar to the inner layer process but includes key differences. It begins with pretreatment (scrubbing and cleaning) to remove surface oxides and contaminants. Dry film photoresist is then laminated, followed by exposure and development to transfer the outer layer circuit pattern onto the resist. Unlike the inner layers, the outer layers require pattern plating. After development, the areas corresponding to the circuit traces are plated with additional copper and a protective layer of tin-lead or pure tin, while areas outside the traces remain protected by the dry film. Pattern plating increases the copper thickness in the trace areas to 45–75 μm or more, meeting high-current carrying requirements.

After pattern plating, the dry film protecting the non-circuit areas is stripped. The panel then undergoes etching, which removes the unprotected copper foil, leaving only the desired outer layer traces. Following etching, the tin or tin-lead alloy layer used as an etch resist is stripped away, revealing the copper circuitry. Finally, outer layer AOI verifies the integrity and precision of the traces.

The solder mask process applies a layer of ink to cover areas not requiring soldering, exposing only pads and vias. Liquid Photoimageable (LPI) solder mask ink (typically green, though blue, red, black, and white are available) is standard. The process starts with pretreatment to clean the surface and prepare it for better ink adhesion. The solder mask ink is then uniformly applied via screen printing or spraying, with a typical thickness of 15–30 μm.

After application, the panel is exposed and developed. Exposure uses a solder mask film; UV light passes through openings in the film, hardening the ink in those areas. The unexposed, soluble ink is then removed in a developer solution (dilute alkali), revealing the pads. A thermal curing process follows, typically baking at 150–160°C for 30–60 minutes to fully cure the ink, providing good heat and chemical resistance. Final inspection confirms the accuracy of the openings and checks for defects like insufficient or excess ink coverage.

Surface finish deposits a protective layer on exposed copper pads to prevent oxidation and ensure good solderability. Common finishes for 4-layer PCBs include Hot Air Solder Leveling (HASL), Organic Solderability Preservative (OSP), Electroless Nickel Immersion Gold (ENIG), Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), and Immersion Silver. HASL is traditional and economical, involving immersion in molten tin-lead alloy followed by hot air leveling to create a uniform coating. ENIG deposits 3–6 μm of nickel followed by 0.05–0.1 μm of gold, offering excellent flatness and long-term solderability, making it suitable for high-reliability products. OSP applies an organic protective film; it is simple and low-cost but has a shorter shelf life. The choice of finish depends on application requirements, reliability needs, and budget.

After surface finish, silkscreen printing applies text and symbols (component designators, polarity marks, logos, batch numbers, etc.) using epoxy ink. The ink is cured at approximately 150°C. Next is profiling, which defines the board outline. Methods include V-scoring, routing (milling), and punching. V-scoring creates V-shaped grooves between individual boards for easy separation later. Routing mills the PCB to its exact contour. Punching may be used for irregular shapes.

Upon completing all processing, electrical testing validates connectivity and checks for shorts or opens. Flying probe testing (suitable for low-volume, high-mix production) uses moving probes to test points individually. High-volume production typically uses dedicated bed-of-nails fixtures for simultaneous testing of all networks. After passing electrical tests, final visual inspection (including manual checks and AOI) is performed to detect surface defects such as scratches, contamination, blisters, or exposed copper.

For high-reliability 4-layer PCBs, a series of reliability tests may be required, including thermal stress testing, solderability testing, insulation resistance testing, and plating adhesion testing. Thermal stress testing subjects the board to multiple temperature cycles to check for delamination or hole wall cracks. Solderability testing verifies that pads meet IPC standards for solder wetting. These tests ensure stable long-term operation in the intended environment.

The above outlines the complete manufacturing process for 4-layer PCBs from raw materials to finished goods. Strict process control and quality management at every stage are essential to producing high-quality multilayer PCB products.