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4 Layer PCB - Stackup & Prototype
● What is a 4 layer PCB ● Why are Power and Ground Planes Inner Layers ● 2 Types of 4 Layer PCB Stackup ● 4 Layer PCB Prototype Service
4 Layer PCB - Stackup & Prototype
What is a 4layer PCB
4 layer PCB means there are 4 layers to rout electrical signals: Top Layer,Inner Layer1, Inner Layer2 and Bottom Layer. TOP and bottom layers are the outer layers where we are placing components and routing. Inner layer 1 and inner layer 2 are in the core, and normally used as power planes or used for signal routing. Therefore, 4 layer PCB = 3 Signal layer + GND layer or 4 layer PCB = 2 signal layers + a VCC layer + a GND layer.
4 Layer PCB Stackup
For a 4 layer PCB, why are the power and ground planes usually inner layers, while traces are on the outer layers?
Solid power and ground planes are good to reduce EMI emissions and can enhance the quality of the signal on the traces. Moreover, connecting components to the planes is much simpler than to route the power and ground trees with traces. The stackup with grounds on outer layers is said to be best from EMC perspective.
Connections between components tend to be clustered locally, and those local connections will have a fairly good chance of being routable on just one layer. If you put all the traces on the inner layers, you will require a via for all the signals to go from the component to the signal layers.
For example, when using SMD components, the connections are on the same layer as the component, so that there is not need for a via or through-hole pad. In space-constrained designs, you are able to put more circuitry on the other side of the board.
4 Layer PCB Board
SMT circuity is always on the surface, if your power and GND were outside, you have to continuously break it up for the parts being placed. Besides, it is really difficult to debug, if you put the power and GND planes on the outside of a board. For RF layout, the rules are different for a trace on the outer layer of a board than on the inner layers of a board. It is possible to use fewer layers if you're doing microstrip routing.
2 Types of 4 Layer PCBStackup
The standard 4 layer pcb stackup is as following and GND and VCC can be switched depending on the layer with more signals.
If you don’t want to connect all ground pins through vias, there is a different stack-up and the power is being routed with wide traces on the signal planes.
This may be a better stack-up with a four layer PCB, for the following reasons:
1. Signal layers are adjacent to ground planes. A signal running over a reference plane, whose voltage happens to be at VCC will still return over that reference plane.
2. Signal layers are tightly coupled to their adjacent planes.
3. The ground planes can act as shields for the inner signal layers. You'll get better results keeping your trace to plane height as low as possible.
4. Multiple ground planes lower the ground (reference plane) impedance of the board and reduce the common-mode radiation.
When a high speed signal changes reference plane, there ought to be a nearby path for it's return current to move between the two reference planes. With two ground planes you can do that with a single via connecting the two planes directly. With ground and power planes the connection has to go via a capacitor which typically requires two vias and a capacitor. That means worse signal integrity and more board area taken up. On the other hand having a power plane reduces volt drop on your power rail and frees up space on your signal layers.
Here is the 4 Layer PCB Stackup from ALLPCB:
If you want to know more about other layer PCB Stackup, such as 8 layer pcb stackup, you can click this pdf for reference.