Typical Four Layers PCB Stack-up– How to Design a PCB Stack-up
Every PCB design project begins with a stack-up that includes signal and plane layers that are clearly specified. Make sure you pick the correct stack-up for your project and that you design it to meet your fabricator’s specifications. Although a four-layer PCB stack-up appears modest, it must be built with the proper layer layout to ensure proper fabrication and impedance. You should be able to use the proper PCB design tools to create the finest 4-layer stack-up possible.
Have you ever been fascinated by the way when a crystal rock changes its colors while rotating? I have. Of course, there are solid reasons behind this phenomenon. This can be included refraction, reflection, and the electromagnetic spectrum. It is so captivating how changes in wavelengths can define the color and how a simple change in orientation can give you a brand new perspective.
In the same way, designing a single or two layers PCBs can give you a typical view of the design from a two-dimensional perspective. Similarly, increasing demand for more compact electronic products can require PCBs to stack up in multiple layers giving you a three-dimensional perspective.
Multiple PCBs came into popularity with the increase in the development of modern electronics. This irreversible development demands PCBs have certain advanced abilities, which include,
- Better functionality
- High speed
- Longer lifetime
A single or double-sided PCB is stacked up to generate a multilayer PCB through a predefined mutual connection between them combined by a semi-solid adhesive known as “Prepreg.” There are usually more than two conductive layers in one multilayer PCB, with one layer synthesized in the insulation board and two layers outside.
There could be complexities and densities in multilayer PCBs due to the complex structure, and some issues can take place like stray capacitance and noise. To determine the electromagnetic compatibility (EMC) performance of a product, one of the most important elements is to plan optimal multilayer stack up.
A well-designed layer stack-up can help in minimizing the radiation and stopping the circuit from being interfered with by external noise sources. Also, it can help in reducing the signal cross talk and resistance mismatch issues.
On the other hand, a poor stack-up can get EMI (Electromagnetic interference) radiations to rise, and as a result, the impedance mismatch can lower the product’s performance and reliability.
What is Actually Stack-up?
Usually, it means an arrangement of copper layers and insulating layers that makes up a PCB preceding the board layout design. A layer stack-up gives the option to get more circuitry on a single board through several PCB board layers.
For a multilayer PCB, the general layers include a ground plane called a GND plane, a power plane (PWR plane), and the inner signal layers.
Is Stacking Up Your PCB any Useful?
It is actually beneficial for you to pursue a stacked PCB configuration for your printed circuit board (PCB) based applications as the structure of PCB stack-up design bestows many different advantages.
- It can help in minimizing your circuit’s vulnerability to external noise.
- It can also help minimize radiations and reduce impedance and crosswalk concerns on high-speed printed circuit board layouts.
- A well-designed layered PCB stack-up can help you in balancing your need for low-cost and productive manufacturing methods with integrity issues.
- A right PCB stack-up can also help in an increased chance of enhancing the Electromagnetic compatibility of your design.
How do I choose Stack-up for my PCB?
Regardless of the number of layers in your board design, your contract manufacturer must adjust the parameters for padding separation, clearances, trace width, copper weights, and drill hole size. If your design has many layers, extra manufacturing design (DFM) standards for signals, power, and ground transmission via Vías and the PCB stacking must be considered. These additions include verticality in the view of your design.
A three-dimensional approach to PCB stack-up design that covers both vertical and horizontal factors has a major influence on board fabrication and PCB assembly. The number, configuration, or stacking of the layers and the type of material must be determined for fabrication.
I will give you four important and basic tips to follow to get yourself a well-designed PCB stack-up.
Determining the Number of Layers
The first thing that you should consider for your PCB stack-up is how many layers do you need. It can be included considering signal layers (high or low speed), power layers for high power boards (or when power supplies are part of your circuit), and ground layers. It is suggested to keep in mind not to mix signal types on inner layers. You can also use pin density to get the accurate number of layers for your PCB stack-up design.
Knowing the Layer Arrangement
After determining the number of layers, the next step should include considering the arrangement of the layers. And for that, you can follow some rules which include,
- There should be a minimum place between power and ground layers.
- Placing the signal layers along with internal power layers for tight coupling.
- Routing high speed on minimum thickness of microstrips.
- It would be best to avoid two signal layers adjacent to each other.
- Making sure to form the stack-up symmetric from the bottom and top layers to inward.
What Type of Layer Material Should be Used?
It is important to know the thickness of each signal layer for your PCB stack-up. This should include, together with determining thickness for prepreg and core(s). There are standard thicknesses and other properties to consider for different circuit board material types.
And you should consider these different properties, including electrical, mechanical, and thermal properties, while selecting your circuit board material type.
Determining Routing and Vias
To complete a PCB stack-up design, the final step would be determining and routing the traces. This can be included determining copper weighs, what type of vias to imply or where to put them. For all such specifications for design, it is better for you to work with your CM (a contract manufacturer) in making these determinations as some CMs usually avoid certain vias types such as via-in-pads.
The rules and criteria for managing a good stack up
PCBs with many layers and a three-dimensional design viewpoint are required to meet the rising need for more compact electronic devices. SMD packaging and layer stack-up, for example, become more difficult with this design viewpoint.
With the manufacture of more sophisticated printed circuits with numerous layers, the stack-up, or stack of PCBs, has gained a lot of relevance in recent years. The early PCB designs were basic, serving merely as a foundation for connecting electrical components.
Simple stack-ups range from four-layer PCBs to more complicated stack-ups requiring expert sequential lamination. The more layers there are, the more freedom the designer has to unravel his circuit, and the less likely he is to come across “impossible” solutions.
The copper layers and insulating layers that make up a circuit are arranged in the PCB overlapping processes. The stack-up you select has a significant impact on the board’s performance in numerous ways.
There are countless rules and regulations for managing a good stack-up. Let’s discuss a basic few of them.
- You should make sure that a plane must always be next to a signal layer.
- Multiple ground planes are extremely beneficial since they cut the board’s ground impedance and help in consistently minimizing the radiations.
- There should be high-speed signals routed on intermediate layers which would be situated between the various levels.
- Ground planes can also act as a barrier and confine the radiation coming from the high-speed rails.
- Ground plane boards can allow signal routing in microstrip or maybe a stripline arrangement; therefore, they are much preferable.
- Ground planes can also help in decreasing ground resistance and ground noise as well.
- It is suggested that you should place the signal layers relatively close to one other, even in adjacent planes.
- Both the power and mass planes must be closely connected.
What is a Four Layer PCB Stack-up?
A four-layer PCB is a printed circuit board made of four layers of glass fiber. These four layers are wiring layers to route electrical signals: one top layer, two inner layers, and one bottom layer. The holes, hidden holes, and blind holes that link these levels are common. These buried and blind holes are more in four-layer boards than double-sided boards.
The outer layers needed to install components and routing are the top and bottom layers. At the same time, two inner layers are placed in the core used for signal routing or as power planes. A four-layer PCB can be made by three signal layers, one GND layer or two signal layers, one GND, and one VCC layer.
The layout of a Four-Layer PCB Stack-up
So now we know that a four-layer circuit board includes top and bottom layers and two middle layers. Both the external layers are laid out with signal lines.
The middle layer uses a command stack manager to add internal plane 1 and internal plane 2 with the add plane option as the most often used power and ground layers linking the corresponding network labels.
It is advised that you avoid using the add layer option since it will increase MIDPLAYER, which is used to put multilayer signal lines.
Plane one and two are two copper layers that connect the power supply (VCC) and ground supply (GND).
If there happen to be several power sources, such as GND2 and VCC2, the stronger and thicker wire should be used in planes 1 and 2. At this moment, the corresponding copper ground will not be visible, and you will be able to see the wire or filling against the light very clearly.
Delimiting the Power Plane
To delimit the ground or power plane, all you have to do is to use use a place/split plane to delimit the area in the corresponding area of internal plane 1 and internal plane 2.
Make sure that VCC2 copper and GND2 copper should not remain in the same plane as VCC. Also, make sure that the network surface layers in the same plane should not be overlapping.
Let us suppose it split 1 and split 2 overlaps in the same plane, then two pieces will automatically get separated according to split 2 order. Therefore, try to pay special heed to the vias of split 1 when overlapping and do not connect split 1 to the area of split 2.
The via holes of this area will get connected to the corresponding copper in the same layer. The footprint components and plug-in parts that pass through external boards (top and bottom) will automatically move away from this region’s plane.
The Electric Layer Setting of Protel 99
- There are usually two types of electrical layers present in Protel 99.
- When you try to open a PCB design file and press the shortcut key L, the layer setting window will eventually appear.
- The layer on the left side is the positive layer, also called a signal layer which includes the top layer, the bottom layer, and the mid-layer.
- The layer in the middle is the negative layer. It is also called the internal layer.
- Both of these layers have got their unique properties and usage methods.
- The negative layer is mostly used as a ground and power layer, and the positive layer is used for pure track lines, including the inner and outer layer.
The Division of the Internal Electric Layer of Protel 99
There is a whole piece or several large copper partitions for the circuit that the ground and power layers use in a multilayer PCB board. If you are suing mid-layer (the positive layer), then you must lay copper. Using copper will be able to make the entire design data volume very large. It will also affect the design refresh speed, and it is not helpful to data communication and data transmission.
And if you are going to use a negative layer, then you will only need to create a thermal pad at the intersection point of the inner layer and the outer layer. It would be actually good for design and data transmission.
How to Add and Delete Inner Layers
At some point in the design, the deleting or adding of layers is needed. This happens in cases such as when a double-layer board is changed to a four-layer board, or a four-layer board is needed to be upgraded to a six-layer board version.
So if you need to add or delete an inner electrical layer, you should follow the steps below.
A schematic diagram of the stack structure is present on the left side of the design layer stack manager.
You can click the layer where you want to put a new layer and then click add a layer or add plane on the right side. This will add a new layer to the previous structure.
If you add a new layer that is a negative layer, then a corresponding network should be assigned to that new layer. You can do this by double-clicking the layer name.
If you want to add a new network to this layer, such as a power layer, you’ll have to use internal segmentation in its implementation. For that, you must set up a network with a large number of connections.
How to Design a Standard Four Layer PCB Stack-up?
There are three main options that you can try to design the stack when designing a four-layer PCB board.
In the first option, there is one ground layer, one power supply layer, and two signal layers that you can arrange in the following manner:
- The top layer will be a signal layer.
- The first inner layer (L2) will be the ground layer.
- The second inner layer (L2) will be the power layer.
- Again, the bottom layer will be the second signal layer.
You are again given two signal layers, one power supply layer and one ground layer in the second option. You can arrange these layers in a different manner keeping their positions as:
- The power supply layer will be placed as the top layer.
- The first inner layer will be a signal layer.
- The second inner layer will also be a signal layer.
- The ground layer will go as a bottom layer.
This option is just as similar as the first option, and the arrangement of the layers would be like this:
- The top layer will be a signal layer.
- The L2 (first inner layer) will be a power supply layer.
- The L3 (second inner layer) will be a ground layer.
- The bottom layer will be a signal layer.
Advantages and Disadvantages of the Above Three Options
In option one
The design is the main sack-up design of the four-layer PCB board. Here, a ground plane is placed under the component surface. As far as the layer thickness setting is concerned, you should follow certain suggestions, including:
The ground to power supply layer thickness should not be too thick to ensure the power plane’s decoupling effect by reducing the distribution resistance of the power supply plane and the ground plane.
In option two
The power supply layer and the ground layer are placed at the top and bottom, respectively. This arrangement can help achieve a certain shielding effect, but this method can have some demerits.
The plane resistance will be too high because there is a large distance between the power supply and ground layers.
Both the power supply and the ground plane will remain incomplete because of the effect of the electronic pads. As a result reference layer remains incomplete, and the signal impedance will not be continuous.
In option three
The arrangement of layers is similar to option one. But this one applies when the core device is connected in the bottom layout or in the underlying signal.
Now, if you do not wish to connect all the ground pins through vias, there is another option for you to stack up differently. In this case, the power will be routed with wide traces on the signal planes.
In this option, you can design your PCB stack-up two ground planes and two signal layers. The two ground layers will be placed in the middle, and the two signal layers will go to the top and bottom layers.
This could be a better stack-up design for a four-layer PCB because of the following reasons.
- Signal layers are tightly connected alongside their planes.
- The ground planes can act as a protection for each inner signal layer. In this way, you will be able to get the desired result by keeping your trace to plane height as low as possible.
- Signal layers are placed close to the ground planes. Therefore, a signal running over a reference plane will be able to return to the same plane because its voltage is left to remain at VCC.
- More than one ground plane will lower the board’s ground resistance and help minimize the common-mode radiations.
- There should be a nearby way for a high-speed signal’s return current to flow between the two reference planes when it changes the reference plane.
You can achieve that with a single ground plane if you link the two planes directly.
- The connection between the ground and power planes must go through a capacitor, which generally needs two vias and a capacitor.
- As a result, signal integrity suffers, and more board space is consumed. On the other hand, a power plane lowers the volt drop on your power rail while also freeing up space on your signal layers.
HOW TO DESIGN AND FABRICATE A 4-LAYER PCB STACK-UP WITH ALTIUM DESIGNER
A four-layer PCB stack-up is the most common and seems like a simple multilayer stack-up existing in modern electronics. It looks somewhat simple and can be easily fabricated. The only thing matters here is that the basic layer thickness and layer arrangement requirements should be met rightly.
Instead of spending a lot of your time guessing at the right layer count or dielectric constant and the loss tangent to define the PCB, you should actually use the best PCB design tools provided by the Altium designer to carry out your fabricating requirements.
The Altium designer software offers the best tools that are essential and much needed to make a PCB four-layer stack-up in the standard rigid boards, multilayer board assemblies, and rigid-flex boards.
Defining a Standard Four Layer Stack-up for Your Printed Circuit Board
As a four-layer PCB stack-up is so common, the designers should know how to use and design a 4 layer stack-up. It becomes easy to design a four-layer PCB stack-up when you have the best design tools and the right guidance from your PCB fabricator.
A standard four-layer PCB stack-up should have the following qualities in it.
- There should be a thick core layer in the center of the board, and it should be covered with two thinner prepreg layers.
(the two thin layers are often used for ground and power nets. While the surface layers are usually used for signals and for mounting components.)
- Now, the outer layer should be covered with a solder mask that should include exposed pads. These pads are there to provide a spot to mount SMD and through-hole components.
- The through-hole vias, on the other hand, are often used to support connections between layers.
- For alternative four-layer stack-ups, there should be a prepreg sheet on the central layer. The outer layer should be covered with two core laminates of almost the same thickness.
- The above arrangement may vary in the thickness of layers, but this happens only when working with resistance-controlled traces.
- Overall, the above-mentioned four-layer stack-up is much versatile. It can be used in almost everything from digital boards to computer motherboards and fast DDR memory sticks.
Defining the Form Factor With Circuit Board Shape
When you are done designing the stack-up of your four-layer board, it is time to turn towards shaping your board before the component placement. You can define the shape of the board area available for components and routes with the help of Altium Designer’s PCB editor.
You can apply a couple of techniques to do this.
- One of the ways is importing a DXF file into a mechanical layer. This can be actually helpful for you to design your board shape.
- The other way is to use the board shape editor to customize your board’s dimensions and shape.
Besides, the board shape tool also offers you many other options to take benefit of.
- You can break the printed circuit boards into several multiple board regions and then place the bending lines on a flex board region.
- You can also break power planes by using the board shape and then define the power plane boundaries.
- If you are working with the rigid-flex design, you can balance the mechanical and electronic design areas with the help of an imported 3D step model and can redefine the board shape.
You should keep in mind some of the important points while working on designing your four-layer PCB stack-up.
- Try to make sure that you select the right dielectric material for your PCB.
- Choose the thickness that would go with your fabricator design to ensure that you are implying the design requirements.
- It is really important and the key point that you should be able to understand the difference between prepreg and core materials in any PCB stack-up.
- If you wish to use more advanced capabilities than you are having in FR4 laminates, you can use other options in PCB materials that you can use in your design.
Support For Rigid-Flex PCB Designs
There are four different types of stack-ups for flex and rigid-flex boards that an IPC-6013 can provide. They are as follows:
This is a single-layer flex. This flex uses a single conductive layer either uncovered on one side or maybe laminated between the insulating layers.
This one is a double layer flex, and it uses two conductive layers. These layers are plated through holes.
A multilayer flex that uses plated through holes between its layers. There is also a conductor present which is allowed on all layers.
This type includes a multilayer rigid-flex where flexible and rigid layers are combined in the rigid section. There is also an exposed flexible section present on the rigid layers with conductors as well.
Rigid-flex multilayer boards have different four-layer PCB stack-up requirements; multilayer stacks with the rigid or flexible zone areas of printed circuit board must be defined by design teams.
Advanced PCB Layer Stack Management with Altium Designer
Designers may customize the layers of a printed circuit board using Altium’s Layer Stack Manager. You may add, merge, remove, and customize layer stacks using the Layer Stack Manager.
Altium Designer utilizes the Layer Stack Table to graphically describe the layer configurations after the Layer Stack Manager sets the material and mechanical requirements of the layers, materials, thicknesses, and dielectric constants.
While speaking with your fabricator, you may determine the layer characteristics. The features of a layer applied to the whole layer as well as all related stacks. After the physical layers, all mechanical layers emerge. You may apply the Layer Stack Table documentation to make rigid-flex layer stack designs less complicated.
Only Altium Designer supports a range of layer stack design tools, allowing users to create a 4-layer stack up or more complex circuit boards.
All Altium Designer users can quickly produce documentation for their circuit boards and submit their design data for production with a single application.
No other PCB design software provider offers as many features for successful circuit board design as Circuit Board Wizard.