What a multilayer PCB is
A multilayer printed circuit board has three or more layers of copper circuitry, separated by insulating material and bonded into one solid board. Most boards use an even number of copper layers, such as 4, 6 or 8, though the exact stack depends on the design. Extra layers give designers more room to route signals, and they allow dedicated power and ground planes, which help with signal quality and electromagnetic interference.
The simplest multilayer board has four copper layers. If you want the details of that specific case, see our guide to 4 layer PCB manufacturing.
The materials: cores, prepreg and copper foil
Multilayer boards are built from a few basic ingredients:
- Core: a fully cured, rigid laminate (usually glass-fiber reinforced epoxy, often called FR-4) with copper on one or both sides. Inner circuit layers are etched into cores.
- Prepreg: thin glass-fiber sheets pre-impregnated with partially cured resin. It works as both the glue and the insulation between cores. Under heat and pressure the resin flows, then cures. Prepreg comes in different glass weave styles and thicknesses, which affect the spacing between layers.
- Copper foil: used for the outermost layers of many builds.
Manufacturers keep their own stock of cores and prepregs, so an unusual stack-up can add lead time. It is worth confirming availability with the fabricator before you finalize a design.
Step 1: Design data and manufacturing review
The process starts with data from the designer. The usual package includes Gerber files for each copper, mask and silkscreen layer, a drill file (commonly in Excellon format), and a fabrication note describing the stack-up, board thickness, finish and any impedance requirements.
The manufacturer’s engineers review this data, check it against their process capabilities (a design for manufacturability, or DFM, check) and may adjust things such as hole sizes or trace compensation. Problems are ideally caught here, since they are far cheaper to fix before production starts.
Step 2: Inner-layer imaging and etching
Each inner core is cleaned and coated with a photosensitive film called photoresist. The film is exposed to UV light through artwork of the circuit pattern. Depending on the process, either the exposed or unexposed areas harden. After developing, the unprotected copper is chemically etched away, and the remaining resist is stripped, leaving the inner-layer traces.
Step 3: Inspection and oxide treatment
Inner layers are usually checked by automated optical inspection (AOI), which compares the board to the design and flags opens, shorts and similar defects. This matters because inner layers cannot be repaired or inspected visually after lamination.
Before stacking, the copper surfaces are often treated (commonly called a brown or black oxide, or an equivalent alternative) to roughen them so the resin bonds well.
Step 4: Lamination
The layers are stacked in the order the stack-up specifies: copper foil, prepreg, an etched core, more prepreg, and so on. Pins or other alignment features keep the layers registered to one another. The stack is then pressed in a heated press, often under vacuum. The resin in the prepreg melts, flows around the copper features, and cures, fusing everything into a single panel.
Exact temperatures, pressures and cycle times depend on the material and the factory, so treat any single published figure as one example rather than a rule.
Step 5: Drilling and plating the holes
Holes for vias and component leads are drilled through the laminated panel on CNC drilling machines. Holes are then cleaned (to remove resin smear) and coated with a thin layer of copper, first by an electroless process and then by electroplating. This plating turns the bare hole wall into a conductor that connects the copper layers it passes through. Without it, the inner and outer layers would not be electrically linked.
Poor plating can leave voids in the hole wall, which is one of the manufacturing problems discussed in our article on circuit board failure.
Step 6: Outer-layer circuits
The outer layers are imaged in a similar way to the inner layers, but many factories use a pattern-plating approach: copper is built up in the circuit areas and covered with a protective metal such as tin, the unwanted copper is etched away, and the protective metal is then removed. The result is the finished outer traces and pads.
Step 7: Solder mask, surface finish and silkscreen
- Solder mask: a protective polymer layer, most often green but available in other colors, is applied and imaged so that pads stay open. It helps prevent solder bridges and protects the copper.
- Surface finish: exposed copper pads get a finish to prevent oxidation and help soldering. Common options include HASL (hot air solder leveling), lead-free HASL, ENIG (electroless nickel immersion gold) and OSP.
- Silkscreen: reference designators and logos are printed on top.
Step 8: Profiling and testing
Boards are made on large panels, so they are routed or scored (V-cut) to their final shape. Electrical testing, by flying probe or fixture-based test, checks that nets are connected as the netlist says and that there are no unwanted shorts. Further quality checks and packaging follow.
Common questions
How are PCB boards made?
In short, copper-clad laminate is patterned with photoresist and etched, layers are laminated with prepreg, holes are drilled and plated, and the board is finished with solder mask, surface finish and silkscreen, then tested. Single-sided and double-sided boards skip lamination.
What is a type 3 PCB?
The term is not used consistently. In IPC board-type naming it is generally associated with a multilayer board without blind or buried vias, but it is also easy to confuse with IPC performance classes (Class 1, 2 and 3), which describe quality and reliability expectations rather than construction. If you see “type 3” in a specification, check which standard it refers to.
Is JLCPCB a Chinese company?
JLCPCB is a PCB fabrication and assembly service that is generally understood to be based in Shenzhen, China. Many hobbyists use it for low-cost prototype orders, but you should check the current terms, shipping times and customs costs for your region before ordering.
What this means if you order a board
You do not need to master every process step to order a multilayer board, but a few habits help:
- Provide a clear stack-up and name your layers consistently in the Gerber package.
- Ask whether the stack-up you want uses materials the fabricator keeps in stock.
- Review any DFM feedback or production draft carefully before approving it.
- Start with a standard design, such as a four-layer board, before moving to higher layer counts.
For a closer look at the most common starting point, read our article on 4 layer PCB manufacturing.