Four-layer PCBs are something that makers have long known about but almost never gotten involved with. However, with the introduction of PCB prototyping facilities around the world, and widely available CAD packages, four-layer boards are now within the reach of makers. But what are four-layer boards and how can they help you with your next project?
Since their early days, makers have mainly been limited in circuit construction techniques. Breadboards provide makers the capability to reuse components, while stripboards allow for permanent fixtures. Printed circuit boards have also been available to makers, but mostly in the most crudest of ways (using the toner transfer method). These circuit boards remove the need for wires (especially double-sided boards) but they are lacking in features such as through-hole plating and silkscreen. However, new fabrication houses have led to high-quality prototyping PCBs at low prices (as low as $2 for 5 pieces).
The result is that maker projects can now integrate SMD devices, including QFN and BGA packages, which allows for miniaturized designs and more complex circuits. But not only are double-sided PCBs available to makers, so are four-layer PCBs! Most designs by makers will be suitable for two-layer boards, which may leave many to think, “Why would I want to use a four-layer board?”.
To understand the advantages of four-layer boards, it’s important to first understand the difference between two layers and four-layer boards. As the name suggests, four-layer boards have four separate copper layers that can be used for routing and power, whereas two layers only have two copper layers. Therefore, the first and most obvious advantage of four-layer boards is that there are two additional routing layers for signals, which allows for reduced PCB sizes (as well as the ease of integrating complex devices such as BGA that may have as many as 200 connections).
Four-layer boards are also advantageous for makers who have an interest in selling their designs. Electronic products that are sold commercially are legally required to be certified by either FCC or CE regulations, and these regulations include emissions that essentially require that circuits to not emit radio energy over a specified value. Two-layer designs can struggle with emissions control, but four-layer PCBs can utilize power planes and ground planes shielding to absorb .emitted mission from traces.
Four-layer PCBs also allow for signals to be routed inside the PCB stack and have ground planes on the top and bottom layers, but it is more common for the two inner layers to be power and ground. The four-layer arrangement can also make signal routing easier, in that power and ground connections can be removed entirely from the signal routing layers and therefore free up space for signals.
Four-layer PCBs do have some drawbacks that can make using them difficult. Firstly, four-layer PCBs are more expensive, which may make them impractical for simpler designs. Secondly, four-layer PCBs can hide traces in their inner layers which can make circuit debugging very difficult. In the event that a circuit requires debugging, it would be more practical to rely on the CAD drawings of the designs instead of following traces on the PCB, but this can also work as an advantage. The use of four layers can be a deterrent against engineer's reverse-engineering your design, as hidden layers are virtually impossible to see (only x-rays can be used to see these inner layers).
Four-layer PCBs can help miniaturize your design, improve its EMI performance, and allow for more complex routing. While they are rather advanced (thanks to the low cost of PCB fabrication currently available to makers), it would not be a bad idea to test them out to see how they can be beneficial to your next project.