Multilayer PCB – How to Route a Multilayer PCB?
Multilayer PCBs like a 4 layered PCB, 8 layered PCB, 16 layered PCB and so on, have become very common among PCB manufacturers. Nowadays, many computer and mobile phone microprocessors and motherboards consist of 8 layered, 12 layered, and even 16 layered printed circuit boards; although, this can be quite expensive, it is very essential in high density PCBs like the ones found in smartphones.
Simply put, designing a multilayer PCB often involves a few complex techniques. However, routing becomes incredibly easier in multilayered PCBs. Here are certain ways to effectively route a multilayered printed circuit board.
Perform Routing in Blocks
When it comes to routing a PCB with many layers, there is no standard rule to be followed. However, there is an effective method of doing so. You need to carry out the routing in ‘blocks’; for example if you have a PCB with Flash, RAM, FPGA, and an analog component, route the critical analog component first, then the ADC/DAC component, followed by routing the traces for the high speed signals.
Routing the digital components like flash, USB, and RAM should be the next step, after which you can finally route the minor components, power supply, and the ground plane.
Dedicating the Layers for the Supplies
On 2 layered PCBs, it is not a good practice to dedicate an entire layer for the ground or the power supply. Nevertheless, in very simple designs you can use this configuration. But in a multilayer PCB like a 4 layered board, usually the bottom layer is reserved for the ground, while the top layer is reserved for the power supply (VCC). The remaining two layers are dedicated for other forms of routing.
When talking about 6 and 8 layered PCBs, it is a common practice to isolate two layers for the power supply (VCC) and two layers for the ground. In some cases, 8 layered boards may have three or even more layers reserved for the power supply; this is mostly when your design consists of multiple input voltages, such as 1.8 V, 3.3 V, and 5 V. For example, if you have a microvia on a BGA (ball grid array) package, you can utilize the second layer for the supply voltage.
Locating the Components
If you want to make your printed circuit board much smaller in size, you can place the various electronic components on the bottom. As an example, suppose you are designing a multilayered BGA PCB and you have to place decoupling capacitors; you can easily do so by situating them towards the bottom of the board. Additionally, you can place RAM IC’s underneath the board as well. This will ensure that the traces are short, because of the close proximity of the RAM and the BGA.
Furthermore, you should never place connectors and buttons in inaccessible areas of the board. For example, never locate an Ethernet IC next to an Ethernet connector as it is quite large in size.