The concept, characteristics, application and manufacturing process of multilayer PCB

The structure of a PCB is not singular; it consists of single-layer, double-layer, and multilayer types. Generally, the more layers a PCB has, the more complex the corresponding product becomes. High-end products can have PCBs with thirty or forty layers, or even more. This article will comprehensively introduce the concept, features, and applications of multilayer PCBs

 

What is a Multilayer PCB?

A multilayer PCB is formed by stacking multiple independent circuit layers together, with electrical connections (such as copper foil inner layers, blind vias, buried vias, etc.) linking these layers. This design enables multilayer PCBs to achieve more functionality and higher performance within a limited space. The basic structure of a multilayer PCB typically includes the substrate, conductive layers, insulating layers, and vias or blind vias connecting each layer.

 

 

Features of Multilayer PCBs

Compared to single-layer or double-layer boards, multilayer PCBs have several distinct advantages:

1. High Density and Compact Size: Multilayer PCBs can accommodate more circuits in a limited space and provide higher integration. This makes them particularly suitable for applications requiring numerous components and complex routing.

2. Low Interference and Noise: The internal layers of a multilayer board can provide effective isolation, reducing signal interference and electromagnetic noise. This is crucial for high-frequency applications and sensitive circuits.

3. Excellent Electrical Performance: The ground and power layers of a multilayer PCB can effectively provide stable power and ground connections, minimizing power noise and voltage drops. This helps ensure the reliability and performance of the circuit.

4. Ease of Manufacturing and Maintenance: Multilayer PCBs can be mass-produced through automated processes with high consistency. Additionally, their compact structure makes maintenance more convenient.

1. Zoning: Divide the circuit into different functional areas, such as analog, digital, and power zones. This helps prevent interference between circuits and improves the clarity of the overall layout.

2. In-layer Routing and Vias: The inner layers of a multilayer PCB are typically used for signal transmission, so in-layer routing must be carefully planned. Vias are used to connect nodes across different layers, ensuring electrical connections. During routing, it is essential to minimize the impact of vias on signal integrity.

3. Differential Signal Routing: For high-speed data transmission or applications with strict noise suppression requirements, differential signal routing can effectively reduce interference and enhance signal integrity. In differential signal routing, the length and path of the two differential signal lines must be kept equal to ensure synchronous signal transmission.

4. Ground and Power Plane Planning: Multilayer PCBs typically include ground and power planes. Properly planning these planes provides stable power and ground connections, reducing voltage drops and power noise. Ground and power planes also offer effective shielding, minimizing signal interference.

5. Signal Integrity Considerations: During routing, attention must be paid to signal integrity. Avoid crossovers and parallel signal traces to reduce crosstalk and interference. Additionally, the impedance matching of signal traces must be considered to ensure signal quality and stability.

6. Thermal Management: High-power components on a multilayer PCB can generate heat, so thermal management should be considered during routing. By strategically planning heat dissipation areas and adding thermal traces, heat can be effectively dissipated, preventing component overheating and damage.

7. Simulation and Optimization: Before actual routing, electromagnetic simulation software can be used to simulate and optimize the design. Simulation allows for performance analysis of different routing schemes and adjustments based on results to achieve optimal routing performance.

 

Multilayer PCB Manufacturing Process

 

Due to the increased opportunities for cross-connections, overlaps, and traces in copper areas, manufacturing multilayer PCBs is a meticulous process that requires patience and special precautions. The entire process must be carried out in an electrostatic discharge (ESD) safe cleanroom environment. PCBs with more than 8 layers require special manufacturing equipment and devices.

Here are the basic steps involved in the multilayer PCB manufacturing process:

1. Design: The process begins with designing the PCB layout using PCB design software or CAD tools (such as Altium Designer, Eagle, or OrCAD).

2. Inner Layer Core Production: The inner layer core is made using copper foil, dry film resist, and UV-treated laminates to the required thickness.

3. Lamination: This process involves stacking inner layer cores, prepreg boards, and copper foil sheets. These materials are aligned using vias when stacked together. For a 4-layer board, the stack is as follows: bottom copper foil, prepreg board, inner layer core, additional prepreg board, and finally the top copper foil sheet.

4. Pressure Application: Using a heated hydraulic press, the stack is heated and vacuumed. The vacuum ensures there is no air between the layers. This process can take over two hours, depending on the number of layers.

5. Formation: After curing, the resin in the prepreg connects the layers, core material, and foils together, forming the multilayer PCB.

6. Cutting: The original copper-clad laminate is cut into boards that can be processed on the production line.

7. Inner Dry Film: The process of transferring the inner circuit pattern onto the PCB board.

8. Black Oxidation: Before lamination, the inner layer board with wiring must be coated black or brown. This process oxidizes the copper surface, providing mild corrosion resistance, improved appearance, and reduced light reflection.

9. Thin Lamination: This process uses B-stage prepreg to bond circuit layers into a unified whole.

10. Mechanical Drilling: Mechanical drilling uses high-speed drills to create vias in the PCB.

11. Via Metallization: Vias are metallized to solve the issue of ensuring uniform copper coverage on the cross-section for thermal shock resistance.

12. Dry Film and Pattern Electroplating: The process of transferring external layer patterns is similar to inner layer pattern transfer. Both use photosensitive dry film and photographic techniques to print the circuit pattern on the board.

13. Solder Mask Layer: The solder mask process involves adding a protective layer to the board’s surface to prevent oxidation and avoid solder bridges between dense pads.

    The specific process involves: pre-treatment → coating → pre-baking → exposure → development → UV curing → heat curing.

14. Silkscreen Printing: Silkscreen printing is used to mark components, test points, PCB parts, warning symbols, logos, and labels on the board using ink.

15. Surface Polishing: The main purpose of surface treatment is to ensure good solderability or electrical performance.

16. Profile Cutting: The process of cutting the PCB profile from the working panel or individual PCB, according to the customer’s requirements. The most common methods are milling (also called routing), V-cut, and punching.

17. Electronic Testing: Electronic testing is the electrical performance test of the PCB, often referred to as the “open” and “short” tests.

18. Final Inspection: Inspection methods typically include optical inspection, target check confirmation, and visual inspection to confirm or eliminate potential defects.

19. Packaging: Common packaging types include anti-static packaging, moisture-proof packaging, and vibration-proof packaging. Typically, vacuum packaging is used.

• Wireless Communication: Supports high-frequency signal processing, improving communication efficiency.
• Medical Devices: Provides high precision and reliability for portable monitors and medical imaging devices.
• Aerospace: Maintains signal accuracy and device reliability in extreme environments.
• Automotive Electronics: Used in advanced driver assistance systems (ADAS) and electric vehicle control units.
• Consumer Electronics: Ideal for portable devices such as smartphones and tablets.

Multilayer PCBs, with their multi-layered structure, high-density wiring, high performance, and design flexibility, play an increasingly important role in modern electronic engineering and are extensively used in various high-tech sectors. For more information on multilayer PCBs, you can consult SHINELINK, a trusted manufacturer specializing in various types of PCB fabrication and assembly, with high quality and reliability.