8 lyaer pcb
Industrial servers, precision medical instruments (such as ultrasonic equipment), automotive domain controllers, and mid - to - high - end FPGA development boards feature a design with multiple power layers and multiple ground layers. This design supports high - density wiring (the line width / line - to - line distance can reach 2mil/2mil) and ensures high signal integrity.
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A Multilayer Printed Circuit Board (MLB for short) is a printed circuit board formed by alternately stacking and pressing multiple independent circuit layers (signal layers, power layers, ground layers) and insulating substrates. The core principle is to solve the problems of "insufficient space, signal interference, and poor power stability" in single - layer or double - layer boards through "layer - by - layer wiring + inter - layer interconnection", achieving high - density and high - performance circuit connections. Its principle can be disassembled into four dimensions: core structure, inter - layer interconnection, signal/power management, and manufacturing logic, as follows:
I. Core Structure: Alternate Stacking of "Insulating Layers + Circuit Layers" to Form a Three - Dimensional Circuit
The essence of a multilayer printed circuit board is the "three - dimensional stacking of planar circuits". The most basic structure is alternately composed of two types of materials: "conductors (copper foils)" and "insulators (substrates)". A typical structure (taking an 8 - layer board as an example) is as follows:
Top layer (signal layer) → Insulating layer (pre - preg) → Inner layer 1 (power layer) → Insulating layer → Inner layer 2 (signal layer) → Insulating layer → Inner layer 3 (ground layer) → Insulating layer → Bottom layer (signal layer)
Top layer (signal layer) → Insulating layer (pre - preg) → Inner layer 1 (power layer) → Insulating layer → Inner layer 2 (signal layer) → Insulating layer → Inner layer 3 (ground layer) → Insulating layer → Bottom layer (signal layer)
Each layer has a clear function and can be divided into three categories:
- Signal Layer
- Function: Lays signal transmission lines between components (such as data lines between the CPU and memory, analog signal lines of sensors), serving as the "information channel" of the circuit.
- Characteristics: The top layer and the bottom layer are outer - layer signal layers where components can be soldered. The inner - layer signal layers in the middle are only used for wiring and do not directly contact the outside world.
- Power Layer
- Function: Provides a stable power supply (such as 5V, 12V), equivalent to a "three - dimensional power line" to power the entire circuit.
- Characteristics: Usually, it is a complete copper - foil plane (instead of scattered wires), which can reduce power impedance and minimize current fluctuations (avoiding voltage drops caused by thin wires).
- Ground Layer
- Function: Serves as the common "ground" of the circuit, providing a signal return path, and also shielding external electromagnetic interference (EMI) and inter - layer signal crosstalk.
- Characteristics: It is also a complete copper - foil plane. Working with the power layer and signal layers, it forms a "shielding cavity" to improve signal integrity. For example, the return current of high - frequency signals can directly flow through the ground layer, shortening the path.
- Insulating Layer (Dielectric Layer)
- Function: Isolates adjacent circuit layers to prevent inter - layer short - circuits and maintains the structural strength of the PCB.
- Materials: The mainstream is FR - 4 pre - preg (epoxy - glass - cloth substrate with stable dielectric constant and high - temperature resistance). In high - frequency scenarios, materials with a lower dielectric constant (such as PTFE) are used to reduce signal attenuation.
II. Key Principle 1: Inter - layer Interconnection - "Vias" Connect the Three - Dimensional Circuit
The circuit layers of a multilayer board exist independently and need to be electrically connected between layers through vias. This is the core technology that distinguishes multilayer boards from single - layer or double - layer boards, equivalent to the "three - dimensional wires" of the circuit. The types and principles of vias are as follows:
| Via Type | Structural Principle | Application Scenarios |
|---|---|---|
| Through - Hole | Runs from the top layer to the bottom layer, connecting the circuits of all layers. The hole wall is plated with copper to form a conductive path, and the hole can be filled with resin (to prevent solder from flowing in). | Scenarios that require connection across multiple layers (such as power supply from the power layer to components on the top layer). |
| Blind Via | Only extends from the outer layer (top layer / bottom layer) to a certain inner layer without penetrating the entire PCB, avoiding occupying the space of other inner layers. | Connection between the outer layer and a specific inner layer (such as from the top - layer signal layer to the inner - layer ground layer). |
| Buried Via | Completely hidden inside the PCB, connecting only two or more inner layers without being exposed on the top or bottom layer. Drilling and copper plating need to be completed before lamination. | Connection between inner layers (such as from inner - layer 1 power layer to inner - layer 3 signal layer). |
The core principle: The hole wall of the via forms a conductive layer through "electroless copper plating + electro - plating", connecting the copper - foil circuits of different layers, thus enabling the transmission of current and signals in a three - dimensional space and solving the problem in single - layer boards where "wiring crossovers can only be achieved by jumpers".
III. Key Principle 2: Signal and Power Optimization - Improving Performance through "Layered Planning"
The core advantage of a multilayer board is not only "space utilization" but also solving the performance bottlenecks of single - layer or double - layer boards through layered design. Its optimization principle is based on electromagnetics and signal integrity theory:
- Signal Interference Suppression: "Ground - wire Enclosure + Inter - layer Shielding"
- Problem Root: In a single - layer board, signal lines are prone to crosstalk (Crosstalk) with power lines and different signal lines due to "electromagnetic coupling", and high - frequency signals (such as USB3.0, HDMI) suffer severe attenuation.
- Multilayer - board Solution:
- "Pair" a ground layer with the signal layer: For example, place "signal layer 1" adjacent to the "ground layer". The return current of the signal line can directly flow through the underlying ground layer (the shortest path), reducing electromagnetic radiation.
- Isolate sensitive signals: Separate the analog signal layer (such as sensor signals) from the digital signal layer (such as CPU clock signals), and isolate them with a ground layer in the middle to prevent digital signals from interfering with analog signals.
- Power Stability Enhancement: "Planar Power Supply + Low - impedance Path"
- Problem Root: In a single - layer board, the power is supplied through thin wires. When the current increases, "voltage drop" (IR Drop) is likely to occur, resulting in unstable chip power supply.
- Multilayer - board Solution:
- Use a complete copper - foil plane as the power layer: The impedance of the copper - foil plane is much lower than that of thin wires (under the same current, the voltage drop is only 1/10 - 1/100 of that of the wire), which can provide a stable voltage.
- Place the power layer adjacent to the ground layer: Form a "parallel - plate capacitor" (the insulating layer is the dielectric, and the power layer and the ground layer are the two plates), which can filter out high - frequency noise in the power supply and further stabilize the power supply.
IV. Manufacturing Principle: "Layer - by - layer Fabrication + High - temperature Pressing" for Integrated Molding
The manufacturing process of a multilayer board is an assembly from "plane to three - dimension". The core steps reflect its "layer - by - layer stacking" principle. Taking a 4 - layer board as an example:
- Fabricating Inner - layer Circuits
- Take two double - sided copper - clad laminates (substrate + copper foils on both sides), and fabricate inner - layer circuits (such as the power layer and the ground layer) on the copper foils through "photolithography + etching".
- Conduct "AOI inspection" (Automated Optical Inspection) on the inner - layer circuits to ensure there are no short - circuits or open - circuits.
- Stacking and Pressing
- Stack materials in the designed order: Top - layer copper foil → Pre - preg (insulating layer) → Inner layer 1 (power layer) → Pre - preg → Inner layer 2 (ground layer) → Pre - preg → Bottom - layer copper foil.
- Press at high temperature and high pressure (temperature 170 - 180°C, pressure 30 - 50kg/cm²): The pre - preg melts and then solidifies, bonding all layers into a single entity to form a solid PCB substrate.
- Drilling and Copper Plating
- Drill through - holes / blind - holes / buried - holes with a CNC drilling machine.
- Form a thin copper layer on the hole wall through "electroless copper plating" (to ensure conductivity), and then thicken the copper layer through "electro - plating" to achieve inter - layer interconnection.
- Fabricating Outer - layer Circuits and Solder Mask
- Perform photolithography and etching on the outer - layer copper foils to fabricate the signal circuits on the top and bottom layers.
- Cover the solder - mask layer (green ink) to protect the circuits from oxidation and prevent soldering short - circuits, and finally print the silk - screen (component labels).
Multilayer Board Materials and Manufacturing Process Introduction
Materials for Multilayer Boards
- Copper Foil
- Copper is used as the conductive material for circuit patterns. High - purity copper foil is preferred as it has excellent electrical conductivity. It comes in different thicknesses, and the choice depends on the current - carrying requirements of the circuit. For example, in power - intensive areas, thicker copper foils are used to reduce resistance and power loss.
- Insulating Substrate
- FR - 4: This is the most commonly used insulating material in multilayer boards. It is made of epoxy resin and fiberglass cloth. FR - 4 offers good mechanical strength, heat resistance, and electrical insulation properties. It can withstand the high temperatures during the manufacturing process, such as lamination and soldering.
- PTFE (Polytetrafluoroethylene): In high - frequency applications, PTFE is used. It has a very low dielectric constant and low dissipation factor, which helps to minimize signal loss and distortion at high frequencies. However, it is more expensive and has lower mechanical strength compared to FR - 4.
- Pre - preg (Semi - cured Sheet)
- Pre - preg is an intermediate layer between the copper - clad layers. It consists of fiberglass impregnated with epoxy resin in a semi - cured state. During the lamination process, when heat and pressure are applied, the pre - preg fully cures, bonding the copper - clad layers together and providing insulation between them.
Manufacturing Process of Multilayer Boards
- Inner - layer Circuit Fabrication
- First, copper - clad laminates are prepared. Photolithography is then carried out. A photoresist is applied to the copper - clad surface, and then exposed to ultraviolet light through a mask with the desired circuit pattern. The exposed (or unexposed, depending on the type of photoresist) areas are then developed.
- After that, etching is done to remove the unwanted copper, leaving behind the inner - layer circuit patterns. Finally, the photoresist is stripped off, and the inner - layer circuits are inspected for any defects like short - circuits or open - circuits.
- Stacking and Lamination
- The pre - pregs and the fabricated inner - layer boards are stacked in the correct order according to the design. For example, power layers, ground layers, and signal layers are arranged with pre - pregs in between to separate and insulate them.
- The stacked layers are then placed in a laminator. High temperature (around 170 - 180°C) and pressure (about 30 - 50 kg/cm²) are applied. The pre - pregs fully cure under these conditions, bonding all the layers together to form a single, solid multilayer board.
- Drilling and Plating
- Holes are drilled through the laminated board using a CNC (Computer Numerical Control) drilling machine. These holes can be through - holes, blind - holes, or buried - holes, depending on the circuit design requirements.
- After drilling, the holes are processed through electroless copper plating. This deposits a thin layer of copper on the hole walls to make them conductive. Then, electro - plating is used to thicken the copper layer to the required thickness, ensuring reliable electrical connection between different layers.
- Outer - layer Circuit Fabrication and Finishing
- Similar to the inner - layer circuit fabrication, photolithography and etching are performed on the outer - layer copper - clad surfaces to create the outer - layer circuit patterns.
- A solder - mask layer is then applied. This layer protects the circuit from oxidation and helps to prevent short - circuits during soldering. Usually, it is green in color, but other colors are also available.
- Finally, silk - screening is done to print component designations, reference marks, and other information on the board.
