In our age defined by electronic devices, whether it’s a data center server performing trillions of operations per second or the powerful smartphone in your pocket, at its core, they all share a common feature: a highly integrated, incredibly sophisticated multilayer printed circuit board (PCB).

If a double-sided board is a two-way street connecting point A to point B, then a multilayer PCB is a modern metropolis complete with overpasses, underground tunnels, expressways, and dedicated emergency lanes. It represents the pinnacle of PCB design and manufacturing technology and is an indispensable cornerstone of high-end electronic devices. Today, let’s demystify multilayer PCBs and explore the complexity and artistry behind them.

I. What is a Multilayer PCB? A marvel of engineering beyond dimensions

1.1 Basic Definition

A multilayer board is a complex PCB consisting of three or more layers of conductive patterns (copper foil) laminated together with prepreg (prepreg, PP) and interconnected through plated through-holes (PTHs). Common layer counts include 4, 6, 8, and even 100 or more, as used in supercomputers and large communications switches.

1.2 The Core Structure

A vivid analogy: imagine a “dimensional pocket” in a science fiction novel.

Top/Bottom Layers: the entrance and exit of the pocket, used to place the most important components and external connections.

Internal Planes: the multi-dimensional space inside the pocket. They are always assigned to Power Plane and Ground Plane, and offer a stable, low-noise energy supply and return route for all components.

Internal Signal Layers: hide in the secret path in the pocket, specially used for setting high-speed, sensitive signal lines, and avoiding external interference.

Prepreg: the magical adhesive to glue every dimension, it acts as both an insulator and a structural support.

Vias: A “portal” connecting different dimensions, including through-holes that run through the entire structure, blind vias that only go from the surface to the inner layer, and buried vias that are completely hidden in the inner layer.

This three-dimensional structure solves core challenges such as high-density interconnect (HDI), signal integrity (SI), power integrity (PI), and electromagnetic compatibility (EMC) that double-sided PCBs cannot cope with.

II. Why Multilayer PCBs? 4 Irresistible Advantages

Upgrading from double-sided boards to multi-layer boards isn’t just about increasing wiring space; it offers a significant leap forward:

1. Unparalleled wiring density and high-density interconnects (HDI)

This is the most obvious advantage. By adding inner layers, designers gain exponentially more wiring space, enabling the design of extremely complex ICs (such as large BGA packages with over 1,000 pins). Combined with blind and buried via technology, this allows for more precise interconnects, meeting the trend toward miniaturization and lightweighting in modern electronic products.

2. Excellent signal integrity

High-speed digital signals (such as PCIe, DDR, and USB 3.0+) are extremely sensitive to transmission paths. Multi-layer boards allow the use of stripline routing (signal lines sandwiched between two reference planes). Compared to the microstrip routing of double-sided boards (signal lines on the surface), this provides better shielding, reduces crosstalk and external radiation, and ensures clean, distortion-free signals.

3. Strong Power Integrity

Dedicated power and ground planes provide extremely low-impedance power supply paths. This effectively reduces power supply noise, prevents circuit logic errors caused by voltage fluctuations, and supports the transient high current demands of high-speed chip switching. A stable power distribution network (PDN) is the cornerstone of system reliability.

4. Excellent Electromagnetic Compatibility (EMC)

Tight power/ground planes create an effective Faraday cage effect, shielding the electromagnetic fields generated by high-speed signals within the board. This not only reduces electromagnetic interference (EMI) to the outside world but also enhances the board’s inherent resistance to external interference. This is crucial for products that must pass strict EMC certifications such as CE and FCC.

III. Unveiling the Multilayer Board Manufacturing Process—A Symphony of Precision and Collaboration

The manufacturing of multilayer boards is an extremely complex process, far more sophisticated than double-sided boards. Mistakes in any step can render an entire batch of boards scrapped.

1. Inner Layer Core Fabrication

Cutting: Cutting large sheets of CCL(copper clad laminate) into production sizes.

Inner Layer Pattern Transfer: The inner layer circuit pattern is transferred to the copper foil through coating, exposure, and development.

Inner Layer Etching: Etch away unnecessary copper to form inner layer circuits.

AOI (Automated Optical Inspection): This uses a high-precision camera to scan inner-layer circuits, comparing them to the original design to detect any breaks, shorts, or defects. This is critical to ensuring the quality of inner layers.

2. Lamination—The Magic Moment

This is the core step unique to multilayer boards.

Lay-up: The prepared inner core layer, prepreg, and copper foil (for the outer layer) are precisely aligned and stacked like a layer cake.

Lamination: Under high temperature (170-180°C) and high pressure, the prepreg melts and flows, filling the gaps between the lines, and solidifies after cooling, firmly bonding all layers into a solid whole.

3. Drilling

Using an extremely fine drill (as small as 0.1mm) or a laser, through holes, blind vias, and buried vias are drilled into the laminated board. Hole accuracy and wall quality are crucial for subsequent copper deposition.

4. Plated Through Holes (PTH) and Secondary Plating

Although similar in principle to double-sided PCBs, the larger hole aspect ratio requires extremely high copper deposition and plating uniformity. This ensures sufficient copper deposits on both the top and bottom of the hole walls to avoid the fatal defect of no copper (hole breakout).

5. Outer Layer Pattern Transfer and Plating

The process is similar to that of the inner layers, but pattern plating is used to thicken the copper on the traces and hole walls to withstand subsequent processing steps.

6. Solder Mask, Silk Screen, and Surface Treatment

Similar to the double-sided PCB process, but with higher alignment accuracy requirements.

7. Flying Probe/Tester Testing and Final Inspection

Due to the large number of nets, 100% electrical performance testing must be performed using a high-channel-count tester to ensure that all interconnects are correct.

IV. Layer Stackup Design—The Art of Performance

Stackup design is the soul of multilayer PCB design. A good stackup design maximizes performance, while a poor design, even with perfect routing, will not achieve the desired result.

Here is a classic 8-Layer Stack-up Design Plan Example

Layer Count	Layer Type	Purpose
Layer 1	Signal Layer(Top)	Place main components and high-speed signal lines
Layer 2	Ground Plane(GND Plane)	Provides a complete reference return path for Layer 1, shielding radiation
Layer 3	Signal Layer	High-speed signal routing
Layer 4	Power Supply Plane(PWR Plane)	Core voltage(eg +1.2V) distribution
Layer 5	Power Supply Plane(PWR Plane)	Auxiliary voltage(eg +3.3V, +5V) distribution
Layer 6	Signal Layer	Low-speed signal layout
Layer 7	Ground Plane(GND Plane)	Provides reference ground for Layer 8 and Layer 6
Layer 8	Signal Layer(Bottom)	Place main components and low-speed signal lines

Design Principles:

-. Each signal layer should be adjacent to a reference plane (power or ground). This is the golden rule for controlling impedance and ensuring signal integrity.

-. Power and ground planes should be tightly coupled. This means using a thin dielectric (e.g., 4 mil) to separate adjacent power and ground layers to form an effective decoupling capacitor.

-. High-speed signals should be preferentially routed on inner layers (striplines) for better EMC performance.

V. Application Areas—Carriers of High-End Technology

Multilayer boards are the absolute workhorse in the following fields:

-. Computers and Data Centers: Motherboards, graphics cards, servers, and SSDs.

-. Communications Equipment: 5G base stations, core routers, and fiber optic switches.

-. Consumer Electronics: Smartphones, tablets, smartwatches, and high-end game consoles.

-. Automotive Electronics: Autonomous driving domain controllers, smart cockpits, and in-car entertainment systems.

-. Aerospace and Defense: Radar systems, navigation equipment, and flight control systems, which place extremely high demands on reliability and layer count.

-. Medical Equipment: High-end imaging equipment (CT, MRI) and life monitoring systems.

VI. How to Estimate the Manufacturing Cost: Understand the Factors that Influence the Cost

The cost of multilayer PCBs is much higher than that of double-sided PCBs; the cost is influenced by the following factors together;

Layer Count: As the number of layers increases, the material, processing time, and difficulty increase nonlinearly.

Board Size: The larger the board size, the higher the cost.

Material types: high-frequency & high-speed material, like Rogers, Taconic, the high tg materials are much more expensive than normal FR-4

Board Thickness and Aspect Ratio (Aspect Ratio): The thicker the board, the larger the aspect ratio (board thickness/hole diameter) of the drilling, the greater the challenge to the drilling and electroplating processes, and the higher the cost.

HDI Process: Whether to use blind buried vias, laser vias, stacked vias, staggered vias, and other processes is the main factor that drives up costs.

Minimum Trace/Space: The finer the requirements (e.g., 3/3 mil), the higher the cost.

Copper Weight: Copper thickness requirements for inner and outer layers, especially when copper thickness of 2 oz or thicker is required.

Surface Finish: ENIG, ENEPIG, hard gold, and other finishes are more expensive than HASL.

Technical Requirements: Impedance control (controlling tolerances and number of channels), backdrilling (eliminating stubs), via-in-pad, and other special requirements.

Order quantity: Large volumes can significantly reduce NRE and tooling costs.

The best way to get an accurate quote: Provide your Gerber files, stackup diagram, and technical specifications (impedance requirements, special processes, etc.). We will perform a DFM analysis and provide a detailed quote. Jerico PCB has been working in professional multilayer PCBs fabrication for nearly 20 years. With our dedicated engineering team and modern facilities, we can make the board just as you want with reasonable price and unbelievable quality. Welcome to send over your projects at any time, Jerico team is always standby for you.

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