The rapid development in the electronics industry has continued to increase the need for lightweight and compact boards. Also, the miniaturization of circuit boards has helped the production of compact and complex electronic devices. Therefore, this development has resulted in the popularity of multilayer boards. These boards involve the combination of two or more double-sided or single-sided boards stacked together.
Proper stackup is crucial in multilayer circuit boards. This is because stackup affects the performance and functionality of a printed circuit board. A multilayer circuit features three or more conductive layers with one layer placed on the insulation board and two layers outside. The increase in the densities and complexities of PCBs could result in issues like stray capacitance, cross-talk, and noise.
What is a Stackup?
A stackup refers to how copper and insulating layers are arranged to create a PCB before board layout design. Also, a stackup enables manufacturers to get more circuitry through different PCB board layers. A PCB stackup can help to reduce the vulnerability of a circuit to external noise. Also, it reduces radiation and crosstalk concerns on high-speed PCB layouts.
A multilayer circuit board comprises inner signal layers, power plane, and ground plane. Several layers in a circuit board help to increase the ability of a board to disperse energy. A PCB stackup is simply the arrangement of copper and insulating layers in a circuit board design. Manufacturers arrange these layers to achieve functionality in circuit boards.
Optimal stackup in multilayer circuits is one of the most crucial factors that determine electromagnetic compatibility (EMC) performance. A good layer stack-up can reduce radiation and can prevent interference in circuits caused by external noise sources. Also, a well-stacked circuit board substrate can minimize impedance mismatch and crosstalk issues. However, a poor stackup can result in EMI radiation since ringing and reflections in the systems can reduce PCB performance.
Common Types of PCB Layer Stackups
There are various PCB board layers. The EMC requirements of the board and the size of the circuit are factors that determine the number of layers in a board. After determining the number of layers your board needs, you will have to determine the board layout.
2 layer stackup
A 2 layer board which is also known as a double-sided board is a type of multilayer board that allows interconnects on both sides. These circuit boards comprise three material layers which are the signal layer, laminate core, and bottom layer. Two of these layers are functional.
Also, the signal layer is the top layer. It features copper within a particular range of thickness. The laminate core is the layer that separates the bottom and top copper layers. Therefore, it keeps the copper layers isolated. The bottom layer is similar to the signal layer. It is the second layer of copper.
4 layer stackup
A 4 layer PCB stackup features 4 layers to route electrical signals. Also, the PCB manufacturer sandwiches these layers together. These layers are the bottom layer, top layer, and two inner layers. The bottom and top layers are on the outside while the two inner layers are between them. Manufacturers place components on the top and bottom layers of a 4 layer PCB.
The inner layers can’t create external connections. Therefore, they serve as power planes for routing signals. This helps to improve trace signal quality and also minimize EMI emissions. It is not advisable to make the inner layers signal layers. If your PCB project requires four signal layers, you will need to consider the 6 layer PCB stack up.
The 4 layer PCB stack up comprises top layer-inner layer 1- inner layer 2-bottom layer. Also, the 4-layer PCB stackup offers enhanced functionality. However, the 4 layer PCB stack up may not be ideal for some applications. You can switch the ground layers of a 4 layer PCB stack up based on the layer having more signals. A 4 layer PCB stackup features the power and ground planes as the inner layer. A 4 layer stackup produces a board thickness of 0.062.”
6 layer stackup
A 6 layer stackup is a 4 layer stackup with two more signal layers. Therefore, 6 layer stack up comprise four routing layers and two internal planes for power and grounding. This means that a 6 layer stack up comprises two outer and two internal layers, one ground plane, and one power plane.
This stackup arrangement for a 6-layer PCB is very common. This is because this stackup arrangement provides a balance between high-speed signaling and EMI control. Also, this stackup runs high-speed signals via the two buried layers. However, this design isn’t ideal for all applications.
For instance, you will need to place the power planes and ground planes close to each other for high-speed designs. A 6 layer stackup feature board thickness between the range of 0.031 and 0.062
8-layer stackup
An 8 layer PCB stackup provides better options as it adds two more layers for performance or routing. Also, it is an advanced circuit stack up that sets to achieve the objectives of a multilayer PCB design. A typical 8 layer stackup comprises ground and power planes in the center. Therefore, this provides great capacitance between layers. Also, it protects signal integrity by separating the second and third signal planes.
An 8 layer stack up comprises four signal layers, two power planes, and two ground planes. Also, 8-layer stackup doesn’t include more than four signal layers. Rather than adding more signal layers, this PCB includes more ground and power planes. The board thickness for an 8 layer board is available in 0.093”, 0.0125”, and 0.062.”
10 layer stackup
If the requirement of your PCB design involves six routing layers, a 10 layer stack up is an ideal choice. A 10 layer PCB stackup comprises four planes and six signal layers. Therefore, it features a tight coupling between the return and signal planes. A 10 layer stackup offers board thickness that ranges from 0.062” and 0.125.”
This PCB stackup arrangement routes high-speed signals on the internal signal layers. Therefore, this stackup offers exceptional signal integrity when properly routed and stacked. Also, it offers excellent EMC performance. It isn’t advisable to replace any of the power or ground planes with signal layers. If you do so, it could lead to poor performance. A 10 layer PCB stackup features multiple ground planes.
Objectives of a Multilayer PCB Stackup
While creating a multilayer stack up, manufacturers set out to achieve some objectives. Multilayer stackups design must meet certain design requirements. Therefore, multilayer boards should achieve the following objectives:
- Signal layers must be adjacent to planes
- Multiple ground planes lower impedance and radiation
- Ground and power planes must be closely coupled together
- Route high speed signals through buried layers between planes
- Signal layers must be closely coupled to their planes
While achieving these objectives is crucial, it is crucial to know that not every PCB layer stack up will meet all these objectives. Therefore, it is important to work with professional engineers to determine the objectives you should prioritize.
Benefits of PCB Layer Stackups
A PCB stackup offers a lot of benefits in electronics devices. The introduction of multilayer boards has contributed to the development of advanced electronic devices. Also, multilayer boards comprise at least three conductive layers. Due to the number of conductive layers in these boards, they offer great functionality. While PCB layering may be more complex, PCB stackups offer these benefits:
Reduce vulnerability
Multilayer PCBs can shield an inner layer from external noise. Therefore, this makes them less vulnerable to exterior forces. Also, it can reduce radiation and impedance problems on high-speed systems.
Improve functionality
Multilayer PCBs can improve the functionality and speed of an electronic device. Also, these boards can enhance electromagnetic compatibility of a board. Also, solid ground planes and power planes can help to minimize EMI emissions. This can as well improve signal quality on the traces.
Minimize costs
A properly arranged PCB stack up can help to achieve lower production costs. This is because these stackups place multiple circuits on a single board.
Reduce radiation
A good stackup can reduce radiation from an electronic device, particularly in high-speed boards. Therefore, it is crucial to arrange PCB stackup with functionality in mind. This is because poorly stacked layers can cause impedance mismatches and this can lead to greater EMI radiation.
Factors to Consider when Creating a Multilayer Stackup
There are design factors you need to take into consideration when designing a multilayer stackup.
Core materials
For a normal single sided board, a manufacturer can use aluminum cores. Aluminum cores are not ideal for multilayer stackups. This is because multilayer aluminum boards are very difficult to fabricate.
Interlayer offset
PCB layers can be offset when fabricating a multilayer PCBs. Therefore, you can prevent this by using rivet and bowel method and hot melting for PCB design.
Bow and twist
Bow and twist can result from uneven distribution of copper in a multilayer stackup. Therefore, ensure you design all multilayer boards symmetrically.
The inner layer copper thickness is a crucial factor when arranging PCB stackups. This is because board thickness offers great functionality. With the right PCB layer stack, you can achieve great functionality. Also, the board thickness is crucial when creating a multilayer stackup.
Conclusion
To maintain signal performance in a PCB layer stackup, the power plane is usually close to the ground plane. Multilayer printed circuit boards offer great benefits. Therefore, a good layer stackup is critical in ensuring functionality in these boards. Multilayer PCBs can have a board thickness that ranges from 0.4mm to 3.2mm.
