Why 4-layer PCBs are more advantageous than double-sided PCBs

Article By : Suresh Patel, Mer-Mar Electronics

Four-layer PCBs best double-layer PCBs when it comes to high-speed operation and compact design possibilities.

To meet the demands of the electronics industry, the PCB layers are increasing with circuit complexity. But a basic PCB starts with a single layer board, having one conductive layer. A general PCB stack-up includes a substrate layer, a copper layer, a solder mask layer, and a silkscreen layer. But when we refer to the number of PCB layers, it is the number of copper layers present in the stack-up.

A substrate layer is made up of fiberglass in rigid PCBs and flexible plastic in Flex PCBs. A double-layer PCB will have two copper layers whereas a 4-layer PCB will include four copper layers in the stack-up. The copper layer thickness depends on the power requirement of the circuit. A solder mask is used to insulate the copper layers, removing the chances of any metal component coming in contact with the copper layer. The silkscreen layer has the reference designators printed for all the components and symbols on the top and bottom surfaces of the PCB. This increases the readability of the components, switch settings, test points, to name a few, on the PCB.

A double-layer PCB, also known as double-sided PCB, consists of a substrate layer sandwiched between two copper layers. In this board, components can be mounted on both sides (top and bottom) and connected across the board using vias. This type of double-layer PCB is used in basic circuitry where the design complexity is minimum.

A 4-layer PCB is one of the commonly used multilayer PCBs. Here, the sheets are laminated with alternate substrate layers in multilayer PCB fabrication. In the stack-up, two inner layers are dedicated as ground (GND) and power (PWR) planes. This reduces electromagnetic interference and provides a short return path for the signals.

Comparatively, a double-layer PCB is simple in construction and has a shorter lead time. But it may not be suitable in many applications like a 4-layer PCB. Signal integrity and immunity to electromagnetic interference are some of the significant advantages of a 4-layer PCB.

Choosing either a double-layer or 4-layer PCB depends on various factors like maximum board dimension, required functionalities, expected robustness, build time, and cost budget. Starting from the PCB design to the production stage, there are many steps in which a comprehensive comparison can be outlined between a double-layer and 4-layer PCB. In general, a 4-layer PCB offers many advantages compared to a double-layer PCB and is definitely a better choice in most applications.

Starting with the PCB stack-up, a double-layer PCB will have a core layer in between two signal conductor layers which are known as Top and Bottom layers. The thickness of these signal layers depends on the circuit design. It is quite evident that the stack-up is very simple in a double-layer PCB. The components are placed on one or both sides. Traces are routed on both the layers and are connected using vias. A 4-Layer PCB will have a slightly complex stack-up. Apart from the top and bottom layers, there are two internal layers for routing signals. This provides a scope to include more functionality on a 4-layer PCB.

From the design perspective, a double-layer PCB can be used only in low-complexity circuits. Since there are only two conductive layers, routing dense traces is not possible in such a design. Though the component placement area is the same, the extra signal layers in a 4-layer PCB will surely improve the circuit density and also durability. More layers assist in making the PCB strong and long-lasting.

A double-layer PCB may not display propagation delay as the microstrip traces can be easily routed in this simple stack-up. But a 4-layer PCB can provide better signal integrity as the return paths can be routed effectively with the advantage of extra signal layers in the stack-up. Also, allocating distinct power and ground layers can reduce the electromagnetic interference in the circuit. A better thermal design can be incorporated in the 4-layer PCB with good heat dissipation strategies. This will make a 4-layer PCB more suitable in products used for long durations.

The PCB prototyping process has advanced with the latest automation techniques. This has enabled making both double-layer and 4-layer PCBs with almost the same effort and time. Usually, building prototypes are outsourced to a contract manufacturer, and hence choosing the right service provider is essential. The cost of fabricating a double-layer PCB is comparatively lesser due to the simplicity of the circuit and reduced material. A 4-layer PCB may be of a slightly higher price than a double-layer PCB but is worth the rate with added features and other advantages.

Advantages of a 4-layer PCB as compared with a double-layer PCB:

  1. They are smaller in size and lightweight compared to a double-layer PCB since the increased layer count provides scope for compact routing strategies.
  2. They are of better quality with improved signal integrity and noise reduction. This is possible due to the dedicated PWR and GND planes in a 4-layer PCB stack-up.
  3. The power capability of a device increases with the number of layers in a PCB. Hence a 4-layer PCB can handle more power requirements than a double-layer PCB.
  4. With a constant need of adding new features to any electronic product, the complexity of the circuit also increases. A 4-layer PCB is truly a better option against a double-layer PCB to accomplish this requirement.
  5. A 4-layer PCB can support designs operating at higher speeds. The routing of high-speed signals has to follow certain design guidelines. With increased signal layers, it becomes convenient to segregate and route high-speed signals effectively.
  6. The mechanical strength of a PCB also increases with a higher count of its layers. This can avoid geometrical distortions like board warps and twists.
  7. The increased layer count assists in a more distributed and uniform copper balance in the stack-up. This will enhance the overall PCB performance.

Though the cost and lead time may be slightly higher in a 4-layer PCB, the other benefits offered are surely worth the difference. High-speed operation and compact design possibilities are the advantages that will drive the usage of a 4-layer PCB as compared to a double-layer PCB.


About the Author

Suresh Patel has worked as a Sales Engineer and other management roles at Mer-Mar Electronics. He brings 25 years of experience in printed-circuit-board sales and technical client service and managing business. You can connect with him on Twitter.


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