What is a 4-layer PCB?
A 4-layer PCB (Printed Circuit Board) is a type of circuit board that consists of four layers of conductive material separated by insulating layers. The four layers typically include two inner layers and two outer layers. The inner layers are often used for power distribution and ground planes, while the outer layers are mainly for component placement and connection.
4-Layer PCB Design
Why are the power and ground planes usually inner layers, while traces are on the outer layers?
Solid power and ground planes are good for reducing EMI emissions and can enhance the quality of the signal on the traces. Moreover, connecting components to the planes is much simpler than routing the power and ground trees with traces. The stack-up with grounds on outer layers is said to be best from the 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 there is no need for a via or through-hole pad. In space-constrained designs, you can put more circuitry on the other side of the 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 tough 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 PCB Stackup
The standard 4-layer pcb stackup is as follows and GND and VCC can be switched depending on the layer with more signals.
[Signals-Ground-VCC-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.
[Ground-Signals-Signals-Ground]
This may be a better stack-up with a 4-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 the reference plane, there ought to be a nearby path for its 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.
Routing principles for four-layer board design
1. Priority routing
- Prioritize key signals, such as clock signals, high-speed signals, etc. These signals have high requirements for routing, and their routing should be short, straight and impedance-matched.
- Prioritize the routing of power and ground networks to provide a stable reference plane for subsequent signal routing.
2. Signal layering
- High-speed signals and sensitive signals should be as close to the reference plane (power or ground layer) as possible to reduce loop inductance and ensure signal integrity.
- Signals of different rates and sensitivities should be layered to avoid mutual interference.
3. Avoid crossing the split
- Signal routing should avoid crossing the split area of the power or ground plane as much as possible, otherwise it will cause discontinuity in the return path and increase signal reflection and radiation.
4. Reduce loop area
- Routing should minimize the area of the signal loop to reduce electromagnetic radiation and the possibility of susceptibility to external interference.
5. Routing length control
- For high-speed signals, the routing length should be strictly controlled to make it as equal as possible to reduce signal delay and phase difference.
- The routing length of key signals such as clock lines should meet the timing requirements.
6. Spacing control
- Sufficient spacing should be maintained between different signals to prevent crosstalk. Especially between high-speed signals and low-speed signals, sensitive signals and noise signals.
7. Impedance matching
- According to the transmission rate and characteristic impedance requirements of the signal, the width, spacing and stacking structure of the routing should be reasonably designed to ensure impedance matching and reduce signal reflection.
8. Power and ground wiring
- The power routing should have sufficient width to carry current and avoid excessive voltage drop.
- The ground plane should be kept intact, with minimal divisions and breakpoints to provide a good ground loop.
9. Use of vias
- Control the number and size of vias to minimize the parasitic capacitance and inductance caused by vias.
- Vias for high-speed signals should be back-drilled to reduce the impact of residual piles.
10. Follow the manufacturing process
- The wiring should comply with the process capabilities and design rules of the PCB manufacturer to ensure manufacturability.
Standard 4 Layer PCB Stackup of ALLPCB
ALLPCB can recommend stackup structures according to the Board Thickness, Inner Copper Thickness, and Outer Copper Thickness. You can check our 4-layer laminated structure diagrams.
If you want to know more about PCB Stackup for different layers, you can select different layers on the above page for reference.
4 Layer PCB Prototyping
For customers who haven’t tried our service, we offer 5 pieces of 4-layer PCB prototyping for only $1. To enjoy this privilege, the PCB should be within the size of 100 mm*150 mm. Your order will be shipped within 48 hours. We ship globally via major shipping companies. You can get an instant online quote for 4 layers PCB, by simply uploading your PCB files or entering your PCB specifications, such as the pcb board dimensions, quantity, etc.