What is a PCB Stackup?
A PCB stackup is the arrangement of distinct layers of a printed circuit board, including the ground layers, power layers, and signal layers. Precision and consideration is important when planning a PCB stackup.
You will need to look at different factors such as the power distribution, electromagnetic interference, and signal integrity. Typically, PCBs can either be single-layer PCB, double-layer PCB, or multilayer PCB.
Every layer has a unique role to play in the performance of the board. The requirements of the circuit design will highly determine the PCB stackup. The electromagnetic interference also has a role to play in the decision making.
Power distribution and signal quality are strongly pegged on the right placement of ground and power layers. Ordering of the layers can equally contribute to the performance of the printed circuit board. For instance, signal layers in the middle of the PCB stackup guarantee less interference and noise.
When determining the PCB stackup, it is also important to choose the right materials and thickness. The placement of the signal should also be optimized.
What Material is in a PCB Stackup?
Depending on the design needs, the materials for a PCB stackup vary. Some of the common materials manufacturers use in a typical stackup include the following:
- Substrate – this is the foundation of a printed circuit board. Hence, it offers the robust mechanical support that the circuit needs. FR-4 is a common choice for a substrate because of its low cost, impressive electrical properties, and high strength.
- Copper foil – the conductive traces on a printed circuit board employ the effectiveness of a copper foil. It is laminated to the substrate. Ultimately, this creates conductive paths that the circuit highly relies on.
- Dielectric material – dielectric materials simply insulate the conductive layers of a printed circuit board. This forms a separation between the layers. PTFE and polyimide are the most common dielectric materials.
- Solder mask – this is placed on the copper traces. It prevents oxidation and provides insulation between conductive paths.
- Surface finish material – a thin layer of material is applied to the exposed copper surfaces of a printed circuit board. It not only makes it easier for soldering but also prevents corrosion. Some of the typical materials used as surface finish materials include nickel, tin, gold, and silver.
How Thick is a PCB Stackup?
The number of layers will determine how thick a PCB stackup is. Other determining factors include the type of material used and the thickness that each layer has. A typical printed circuit board stackup for a 2-layer board ranges anywhere between 0.8mm and 1.6mm. A 4-layer board can be anywhere between 0.8mm and 2.4mm in thickness.
Based on the materials used, the thickness can vary too. For instance, when using an FR-4 material, the thickness can range between 0.1mm and 0.2mm. Polyimide, which is typically thicker, can swing the thickness between 0.025mm and 0.075mm.
Thickness can affect how well a printed circuit board carries out its role. Features such as the signal integrity and impedance depend on the thickness of the PCB. Ensure you have the right level of thickness for your specified application.
2 Layer PCB Stackup Vs 4 Layer PCB Stackup Vs 6 Layer PCB Stackup Vs 8 Layer PCB Stackup
To compare these four distinct PCB stackup formations, we will look at them individually below:
2 Layer PCB Stackup
The most common PCB stackup that’s typical for most basic electrical circuits. A copper layer is on top and at the bottom. These two layers are separated via an insulating material. Aside from being easy, these kind of PCBs are equally affordable to manufacture. Nonetheless, their routing options are limited. This makes them vulnerable to interference and noise.
4 Layer PCB Stackup
It has four copper layers, which are separated via three layers of insulating material. Average to complicated electronic circuits depend on this stackup. It is best for circuits that need a bit of signal isolation. It has more routing options courtesy of the extra copper layers. This cuts down on interference and noise.
6 Layer PCB Stackup
It has six copper layers separated via five layers of insulating material. It’s an ideal pick for high-density circuits that need a good amount of signal isolation. They are costlier but have better signal performance and additional routing options.
8 Layer PCB Stackup
From the name, you can already tell that this stackup has eight layers of copper that are separated via seven layers of insulating material. It is required for high-performance circuits that need multiple ground planes and power planes. Such kinds of circuits also need EMI shielding and top-speed signal routing. It is the most expensive of the four PCB stackup options here. However, with it, you get the best signal integrity and a top performance level.
How to Choose a PCB Stackup?
Selecting the right PCB stackup is important as it dictates reliability and performance. Some important tips when selecting a PCB stackup include the following:
- Consider the PCB assembly – the ease of assembly will depend on the PCB stackup.
- EMC/EMI – the stackup should reduce radiation from the PCB and any interference from external forces.
- Thermal management – the PCB stackup will affect its thermal management properties.
- The cost of manufacture – different PCB stackup formations have their distinct prices.
- Mechanical limitations – the stackup design will be affected by factors such as size, shape, and weight.
- Power integrity – there should be low impedance paths for ground and power planes.
- Signal integrity – this is an integral factor when selecting a PCB stackup. Ensure there’s reduced crosstalk and signal reflections.
How to Calculate the PCB Stackup?
This is all about determining the number of layers needed for the PCB. It is also about looking at the materials used for each layer and the thickness each layer has. The steps you need to follow include the following:
1. Find the number of required layers depending on the circuit’s complexity and signal routing.
2. Find out how thick each layer should be.
3. Choose materials.
4. Arrange the layers.
5. Determine how thick the entire PCB stackup is.
6. Countercheck the PCB stackup design.
How to Design a Stackup for PCB?
The steps to take when designing a stackup for a printed circuit board include the following:
1. Find out how many layers are needed.
2. Find out what the thickness of each layer should be.
3. Identify the routing needs.
4. Take note of manufacturability.
5. Use simulation to countercheck the design.
PCB Stackup Example
Here’s an example of a typical PCB stackup:
- Layer One: Top layer signal traces.
- Layer Two: Ground plane.
- Layer Three: Signal plane.
- Layer Four: Power plane.
- Layer Five: Bottom layer signal traces.