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PCB Layers: The Ultimate Guide

A printed circuit board (PCB) mechanically supports and electrically connects components with conductive tracks, pads, and other features etched from sheet layers of copper laminates. These layers define the overall functioning of the PCB. Therefore, it is a grave mistake to design a PCB without considering the PCB layers. 

Imagine constructing a house from the roof! How does it sound? Weird, of course. Usually, building a home starts from the foundation up, which calls for a well-structured building plan. A professional construction engineer focuses on the foundation first. Later on, the builder focuses on the walls, doors, windows, ceiling, and other things.   

The same principle applies to a PCB design. A PCB requires a firm and clear designing principle, and the layers act as the foundation of the design. This guide will teach you everything about PCB layers. Welcome, and let’s enrich our PCB knowledge.

What is a PCB Layer?

A PCB layer is made up of signal transmitting levels that are vertical to the board’s plane. For instance, a 2-layer board has top and bottom layers while a 4-layered PCB has a top, inner 1, inner 2, and bottom layer. 

There is an insulating substrate in the middle of every PCB layer – the FR4 (Fire Retardant Version4). This substrate is an epoxy saturated fiberglass with Teflon-based material. Generally, standard PCBs contain solder masks on the surface copper layers. Sometimes, the mask is present in both the upper and lower layers. These masks get applied as a liquid or sheet onto the board layers.

The Components of PCB Layers

Dielectric Materials 

Dielectric materials play two essential roles in PCBs: 

  • They help to separate signals on nearby PCB layers.
  • They monitor the constant impedance over a broad frequency range. 

Copper 

The role of copper in a printed circuit board is indisputable. Every designer acknowledges it as a good conductor of electricity, but it plays other important functions. For example, it affects the PCB’s capacity to receive the proper current flow and influences power loss. 

The quality of copper in a board determines the impedance and thermal conductivity of a ground plane. Besides, you can strengthen your PCB stability by merging the copper’s length and thickness. The quality of copper also affects the PCB’s integrity. 

Trace Dimensions

Dimension lines and markings play an essential role in a printed circuit board. Their primary function is to define the dimension and mechanical layers of aboard. Secondly, they offer the board foundational measurements. The assembly information is useful in location regulation and component placement.

Types of PCB Layers

1. Mechanical Layers  

There are several mechanical layers, but you need a Mechanical 1 layer to fabricate your PCB. Mechanical 1 is a primary mechanical layer that defines the physical dimensions of your PCB. Manufacturers use this layer to cut PCBs from stock materials. 

A mechanical layer outline can have a rectangular or customized shape with circular corners. Sometimes, extra mechanical layers are used to outline tooling specifications and other diverse mechanical information. However, most boards do not need these additional layers. 

2. Kept Out Layers

Earlier on, we said that mechanical layers define the physical boundaries of a PCB. Kept-out layers compliment them by specifying limits on the working area of a PCB design. For instance, you may want to draw back all the parts ½ inch from the PCB margin. A kept-out layer will help you to restrain your design appropriately. Kept-out layers are not sent to fabricators. You use them in your design software to check whether you have violated the set boundaries. 

However, you can use kept-outs to detect segments on the PCB interior, which should stay clear for mechanical reasons. In the diagram above, the kept-out confines our working area from the outer margins.   

3. Routing Layers

You can use routing layers to interconnect PCB parts. These layers consume a lot of time to design. They are found on the upper and lower surfaces and inside the board. You must send routing layers to your PCB service provider for PCB manufacturing. The illustration provided above is the upper PCB layer. It clearly shows the component interconnect routing and the pads for the actual parts. 

4. Ground Planes & Power Planes

Ground planes and power planes are compact copper layers shorted to a permanent potential. Ground planes are linked to the GND, while power planes are connected to onboard voltages. Just like the routing layers, these layers are found on the upper, lower, and inner parts of a PCB.   

Though ground planes and power planes look boring, they are highly convenient for conducting power and ground across the PCB components. They may significantly improve the PCB performance under certain conditions. You must provide fabrication files defining the planes of your design when ordering a customized board. In the above illustration, the green outline shows the ground plane – a solid copper pour across the entire layer. 

5. Split Planes 

You can easily change the power and ground planes bearing multiple copper segments to various potentials. The modification improves the plane’s convenience – think of a plane with different voltages transmitting +5 to one PCB segment and -5 to another part. This is precisely the distribution in the illustration of the split power plane shown above.  

The left-hand side segment of the line down the middle of the PCB is linked to +5V, while the right-hand side segment is at -5V. There are some performance aspects you need to consider when dividing planes, particularly in ground planes. We will discuss these aspects in our future article. Stay tuned for more updates. 

6. Overlay/Silkscreen Layers

Overlay/silkscreen layers provide PCB parts with text designators on the upper and lower layers. When using these layers, you should send separate files to your PCB service provider for the upper and lower overlays. Silkscreen layers are not useful in internal layers and when your PCB lacks text designators. Nevertheless, the text is important, especially when debugging PCBs in the laboratory.

For instance, if you want to confirm the potential at one end of resistor R2, you check the text “R2” printed on your PCB. This method is more accessible than the tedious method of going back to your design on the computer or paper prints. Silkscreen layers are also essential in indicating your business name, design name, revision number, and board serial number.  

7. Solder Mask Layers

A solder mask is the thin green protective covering on the upper or lower side of the board. It protects traces from shorting when dust accumulates on a board. However, not all PCBs contain solder masks, and there are scenarios where you may need a solder mask on one side of your PCB. 

If you want your board to contain a solder mask, you must indicate the layers where this covering should be applied. Another important thing about solder masks is that they are all labeled negatives. In a layman’s language, regions that won’t be covered will be filled in on the solder mask layers. In the image above, the purple plugs on the upper layer are for components pads. Do not apply a solder mask in regions where parts need to make electrical contact with the PCB. 

8. Solder Paste Layers 

A solder paste is a material used to solder-mounted parts aboard. The pads on the surface mount components you apply in your PCB design should show solder paste in the uncovered copper sections. If your PCB contains many surface mount components, a solder paste will make the solder flow better since it links the component pin to the copper on your board. 

You need to create solder paste files for the upper and lower sections of your design when using surface mount components on those layers.  

Finding the Number of Layers in a PCB

This section contains the steps to guide you in choosing the number of layers you require in your printed circuit board. 

Step 1: Determine the Number of Layers You Need

There are numerous types of PCB layers, which are classified into 6 classes:

  • 1 or 2 Layer PCB: If you require a simple PCB or some breakouts, it is advisable to choose one or two layers. However, do not select this option for interfaces with controlled impedance. 
  • 4 Layer PCB: This is one of the safest options to use. It is ideal for small boards, breakouts, and simple PCBs. Besides, you can use this layer PCB for some microcontroller boards and Baseboards. 
  • 6 Layer PCB: If you plan to make big boards, this layer PCB is suitable for use. The 6 layer PCB is also ideal for high-volume PCBs. In fact, most personal computer (PC) motherboards use the 6 layer PCBs. 
  • 8 Layer PCB: This layer PCB is suitable for compact gadgets with minimal spacing. They are primarily used in wearable watches, PC motherboards, and communication backplanes.
  • 10 Layer PCB: This is the highest number of layers you can apply comfortably in a 1mm thick SODIMM socket. The 10 layer PCB contains adequate space for power planes, and it has enough power domains. 
  • 12 Layer PCB: If you are handling multiple industrial designs, you should use a 1 layer PCB. It is cost-effective and contains four stable ground planes. Besides, it comes with two extra signal layers. 

Step 2: Start with the Layout

For your PCB layout, it is advisable to start with the upper and lower layers. If your design uses high-speed signals, integrate two extra internal layers into one. Besides, you can use this technique if your system contains multiple connections. 

However, before you embark on your layout, place it all into the signal layers. If you need more space, integrate your power plane layers with slow signals. You can also introduce more signal layers to create enough space. 

Step 3: Place Your Board Layers in Order

There are numerous options when you need to place the board layers in an orderly manner. The commonest option is using the GND as the second layer. GND acts as a strong shield against high-speed signals dashing within the board.  

Step 4: Establish the Signal Layers and Density You Need

You need to find out the number of layers you require to determine the necessary density and signal layers. For example, a 1.0 pin density requires two single layers. Generally, the number of layers increases as the pin density reduces.

PCB Layout Rules

Creating Adequate Space for the Power and Ground Plane

We all agree that the power and ground planes are important elements of a PCB. They link the circuit to the ground by transmitting onboard voltages to the PCB components. Therefore, you must create adequate space for them. 

Secure the Board and Traces

It is important to protect your PCB and traces. You need to mask your layers to protect your PCB well. 

Positives for Board Layer Stackup Association 

Nowadays, there is a heightened number of board layers. The reason behind this is to raise the PCB capacity to transmit power. A PCB with a high power transmission capacity supports high-speed signals and minimizes cross-talk. These boards also prevent electromagnetic interloping.

Final Thoughts

PCB layers are important in PCB manufacturing. Therefore, if you plan to design a functioning PCB, you need to understand PCB layers well as they act as the foundation of electronic design. You also need to understand the different board layers to design a product that best fits your PCB needs.

We believe this article has taught you everything you need to know about PCB layers. Ensure you utilize the information well to improve your PCB designing capacity.

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