Common 4 Layer PCB Structures

1. H OZ Copper + 7628 + 1.1mm Core + 7628 + H OZ Copper
   7628 System: the dielectric thickness is about 0.2mm, The cheapest, widely used in consumer electronics.
2. H OZ Copper + 3313 + 1.3mm Core + 3313 + H OZ Copper
   3313 System: the dielectric thickness is about 0.09mm, Offers better dielectric performance, stable impedance, and greater design flexibility.
3. H OZ Copper + 2116 + 1.3mm Core + 2116 + H OZ Copper
   2116 System: the dielectric thickness is about 0.1.1mm, Preferred for high-reliability applications such as automotive and medical electronics.
   

   4 Layer PCB Lamination Structures

What is a 4 Layer PCB Stackup, and 4 Layer PCB Structures?

A PCB stackup refers to the arrangement of conductive copper layers (signal, power, and ground) and insulating dielectric layers (usually FR4 or high-frequency material) in a printed circuit board.

A 4 layer PCB consists of 4 conductive copper layers and 3 dielectric layers, with the following typical structure:

Top Layer (Signal): Main signal layer, used for component placement and routing.

Inner Layer 1: Often used as a ground plane (GND).

Inner Layer 2: Typically serves as a power plane.

Bottom Layer (Signal): Additional signal layer, also used for soldering.

This structure provides a good balance between signal integrity, EMI suppression, and thermal performance, making it one of the most popular choices for medium- to high-complexity electronic products.

Typical 4 Layer PCB Stackup Designs

The stackup of a 4 layer PCB is not fixed and can be customized based on specific circuit requirements. Below are two common stackup configurations:

Table 1: Common 4 Layer PCB Stackup Designs

Configuration A: Best for high-speed signal designs, as signals are tightly coupled with ground planes to reduce EMI.

Configuration B: Suitable for power-sensitive circuits, ensuring strong power and ground coupling to minimize noise.

Advantages of 4 Layer PCB Stackup

Why do engineers often select a 4 layer PCB in their designs? Here are the major benefits:

Improved Signal Integrity
Inner ground and power planes reduce crosstalk and noise, ensuring stable high-speed signal transmission.

Better EMI Performance
Optimized stackup structures effectively suppress electromagnetic interference.

Higher Density and Space Utilization
Compared to 2 layer PCBs, 4 layer boards support denser layouts and complex routing.

Design Flexibility
Power and signals are separated into dedicated layers, making routing easier and more reliable.

When it comes to ensuring these advantages in production, manufacturer capability is key. SQPCB is equipped with advanced LDI automatic exposure machines for circuit and solder mask layers, along with automated silkscreen printers. These investments significantly improve yield, eliminate film-related shrinkage and misalignment, reduce manual errors, scratches, and rework, while maintaining excellent solder mask gloss and surface quality.

4 Layer PCB Fabrication Process

The fabrication of a 4 layer PCB is more complex than that of a 2 layer PCB. The general steps include:

Inner Layer Circuit Formation – Patterning copper on the base laminate through exposure and etching.

Inner Layer Inspection – Using AOI (Automated Optical Inspection) to detect defects.

Lamination – Stacking copper foils and dielectric prepregs under heat and pressure to form a solid board.

Drilling and Copper Plating – Creating vias and plating them with copper for inter-layer connections.

Outer Layer Circuit and Solder Mask – Patterning outer copper layers, then applying solder mask for protection.

Surface Finish – Finishes like HASL (Hot Air Solder Leveling) or ENIG (Electroless Nickel Immersion Gold) are applied for solderability.

One key differentiator is that SQPCB owns in-house lamination lines, HASL, and ENIG processes, allowing full control of quality and lead time. This avoids risks associated with outsourcing and ensures consistent delivery.

Applications of 4 Layer PCBs

Thanks to their balance of performance and cost, 4 layer PCBs are widely used across industries:

Consumer Electronics: Smartphones, tablets, gaming consoles.

Automotive Electronics: ADAS, infotainment systems, ECUs.

Industrial Equipment: PLCs, motor drivers, power management.

Medical Devices: ECG monitors, portable diagnostic devices.

Comparison: 2 Layer vs 4 Layer vs 6 Layer PCBs

Table 3: PCB Comparison by Layer Count

Conclusion

The 4 layer PCB stackup plays a vital role in modern electronics, offering the right balance between complexity, cost, and performance. Proper stackup design not only ensures signal integrity and EMI suppression but also provides the reliability needed for demanding industries.

Choosing the right manufacturing partner is equally important. SQPCB ensures high yield and reliability through LDI exposure, automated text printing, in-house lamination, HASL, and ENIG processes, delivering consistent quality and on-time delivery.

Frequently Asked Questions (FAQ)

Q1: What is the standard thickness of a 4 layer PCB?
A1: The typical thickness is 1.6mm, but it can be customized from 0.8mm to 3.2mm.

Q2: How is impedance controlled in a 4 layer PCB?
A2: By designing the stackup, dielectric thickness, and trace widths precisely, combined with the manufacturer’s process capability.

Q3: What is the minimum trace width/spacing in 4 layer PCBs?
A3: Normally 4mil/4mil, but advanced equipment allows 3mil or even 2.5mil.

Q4: Are 4 layer PCBs suitable for high-speed signals?
A4: Yes, with proper stackup they reduce crosstalk and reflection, making them suitable for USB 3.0, HDMI, and other high-speed interfaces.

Q5: What should I consider when selecting a 4 layer PCB manufacturer?
A5: Focus on advanced equipment (like LDI exposure), in-house lamination and finishing capabilities, delivery reliability, and strict quality systems.