1、 DEFINITION OF MULTI LAYER PCB (MLB)

A multi-layer circuit board is a printed circuit board (PCB) made by alternately stacking three or more conductive layers (copper foil layers) and insulating substrates (prepreg), and bonding them together through high-temperature and high-pressure lamination processes. Its core features are:

The number of layers is ≥ 3 (conventional multi-layer boards usually have 4-16 layers, and high multi-layer boards have ≥ 10 layers);

Three dimensional interconnect structure: adjacent conductive layers are electrically connected through vias (including through holes, blind holes, and buried holes);

Precision alignment: Each layer of graphics needs to be controlled for offset (usually ≤± 25 μ m) through interlayer alignment (Registration) to ensure circuit connectivity.

2、APPLICATION AREAS OF MULTI-LAYER CIRCUIT BOARDS

Multilayer circuit boards, with their high integration, reliability, and multi-dimensional design flexibility, are widely used in scenarios that require strict electrical performance, space constraints, or environmental adaptability. Typical fields include:

1. Consumer Electronics

Smartphones/tablets: using HDI (high-density interconnect) multilayer boards (usually 8-16 layers), supporting high-density wiring (line width/line spacing ≤ 50 μ m) for screen drivers (LCD/AMOLED), processors (SoC), and camera modules (multi camera), while reducing EMI (electromagnetic interference) through a shielding layer (ground layer).

Laptop/server: The motherboard is mostly a 10-20 layer board, integrating high-speed interfaces such as CPU, memory (DDR), graphics card (PCIe), etc. It is necessary to control impedance (such as 50 Ω/100 Ω differential lines) and power integrity (PI, Power Integrity).

2. Industrial Control

PLC (Programmable Logic Controller): 8-12 layer board, needs to withstand industrial environment vibration and temperature fluctuations (-40 ℃~85 ℃), achieves signal isolation through multi-layer structure (digital/analog/power layer separation), and improves anti-interference ability.

Robot control system: 12-16 layer board, supporting servo motor drive, high-speed data transmission of sensor arrays (such as vision/force sensing) (such as CAN FD, Ethernet/IP), requiring optimized thermal management (copper pillars/heat dissipation pads).3. Automotive Electronics

ECU (Electronic Control Unit): 10-14 layer board, used for engine control, ADAS (Advanced Driver Assistance System), requires AEC-Q100 certification (temperature resistance -55 ℃~125 ℃, anti vibration), and integrates high-frequency radar (77GHz) antenna layer.

In car entertainment system (IVI): 8-12 layer board, supports multi screen interaction (central control/instrument/HUD), needs to control EMC (electromagnetic compatibility) to meet CISPR 25 standard (radiation ≤ 40dB μ V/m).

4. Telecommunications equipment

5G base station: 16-20 layer high-frequency high-speed board (such as Rogers RO4350B+FR4 hybrid), supporting millimeter wave (28GHz/39GHz) signal transmission, requiring low Df (dielectric loss factor ≤ 0.003) and impedance matching (± 5%), and reducing signal loss through back drilling.

Optical Module: 10-14 layer board, high-speed interface integrating laser (VCSEL) and detector (PD) (such as 100G/400G), requiring control of crosstalk (≤ -30dB) and coefficient of thermal expansion (CTE ≤ 10ppm/℃).

5. Medical Devices

Medical Imaging Instruments (MRI/CT): 12-18 layer high reliability board, requiring ISO 13485 certification (biocompatibility) and maintaining signal stability in strong magnetic field environments (non-magnetic substrates such as polyimide).

Portable medical equipment (blood glucose meter/heart rate monitor): 8-12 layer HDI board, supporting miniaturization (size ≤ 50mm × 50mm) and long battery life (low-power design).

6. Aerospace&Defense

Avionics: Special plates with more than 20 layers (such as polytetrafluoroethylene substrate) must withstand extreme temperatures (-55 ℃~200 ℃) and radiation (total dose ≥ 100krad), and be certified according to MIL-PRF-55110 standard.

Satellite Payload: Multi layer high-frequency board (such as PTFE+ceramic filling), supporting Ka band (26.5-40GHz) communication, requiring control of dielectric loss and phase stability.

3、 THE CORE ADVANTAGES OF MULTI-LAYER CIRCUIT BOARDS (COMPARED TO SINGLE/DOUBLE-LAYER BOARDS)

The structural characteristics of multi-layer circuit boards endow them with multiple improvements in electrical performance, mechanical performance, and design freedom. The specific advantages are as follows:

1. High Density Interconnect (HDI) wiring

The multi-layer structure allows for an increase in the number of wiring layers (for example, a 10 layer board can allocate 4 signal layers+2 power layers+2 ground layers+2 spare layers), and the line width/line spacing (L/S) can be reduced to 3/3mil (for conventional single-layer and double-layer boards, ≥ 8/8mil), meeting the pin density requirements of pin density components such as BGA (Ball Grid Array) and QFP (Quad Flat Package).

2. Optimize Signal Integrity (SI)

Layered shielding: By setting independent ground layers (GND) and power layers (VCC), an "electromagnetic shielding cavity" is formed to reduce signal crosstalk and radiated interference (EMI);

Impedance control: Multilayer boards can accurately design microstrip or stripline structures to achieve characteristic impedances such as 50 Ω/100 Ω (tolerance ≤± 5%), matching the transmission requirements of high-speed signals (such as PCIe 5.0, USB4).

3. Improve reliability

Mechanical support: Multi layer structure disperses stress (such as thermal expansion and mechanical impact), reducing the risk of plate warping (warpage ≤ 0.5%, single and double layer plates ≤ 1.0%);

Heat dissipation optimization: Heat conduction (thermal resistance ≤ 1 ℃/W) is achieved through metal cores (MC) or multi-layer copper pillars to avoid local overheating (applicable to LED drivers and power modules).

4. Adapt to complex functional requirements

Multi power/ground plane: Multi layer boards can be divided into multiple independent power domains (such as 3.3V/1.8V/1.2V) and ground planes to reduce power noise (PSRR ≥ 60dB);

Embedded Component: High multilayer boards can embed resistors and capacitors into the inner dielectric layer (such as embedded passive devices, BIP), reducing packaging size (reducing volume by 20% -30%).

5. Reduce overall cost efficiency

Although the cost of single board materials is higher than that of single and double layer boards, reducing the number of connectors and cables (such as replacing multiple single and double layer boards with multi-layer boards), lowering the risk of assembly labor and interconnect failure (reducing connection points by more than 50%), and reducing the overall BOM (Bill of Materials) cost by 15% -30%.

SUMMARIZE

Multilayer circuit boards are the core carrier for the development of modern electronic devices towards miniaturization, high-speed, and high reliability. Through multi-layer stacking and three-dimensional interconnect structures, they break through the limitations of single/double-layer boards in terms of wiring density, signal integrity, and environmental adaptability, and are widely used in high-end fields such as consumer electronics, industrial control, and automotive electronics. With the evolution of 5G/6G, AI servers, and autonomous driving technology, multi-layer circuit boards are moving towards higher layers (≥ 20 layers), finer lines (≤ 20 μ m), and lower Df (≤ 0.002), continuously driving the performance upgrade of electronic systems.