PCB Layer Stackup Guide

This Gold Phoenix PCB Knowledge Center article explains common PCB layer stackups, their design objectives, and typical configurations from 2-layer through 10-layer boards.

Printed circuit boards (PCBs) are growing increasingly complex, with more functions and circuits fitting into a single board. Single-layer PCBs themselves are becoming increasingly complicated, but PCB layouts need to provide even more features within compact spaces. As a result, many manufacturers are turning toward multilayer PCB stackups. What is a PCB stackup? How does it work, and is it needed for a given application? Here are some essential points about PCB stackups to help make an informed decision.

The Purpose of PCB Layer Stackups

A PCB stackup refers to the arrangement of copper and insulating layers that make up a PCB. These layers are arranged to accommodate multiple printed circuit boards on the same device. At their most basic, multilayer PCBs consist of at least three conductive layers. The bottom layer is synthesized with the insulation board, and each circuit board layer is connected to the next.

While PCB layering creates a more complex and space-saving board, PCB stackups also offer the following advantages:

• Maximize functionality: Multilayer PCBs can multiply the speed and functionality of the device, making for a more functional board.

• Minimize vulnerability: Layer stacks can help protect internal layers from external noise, making the board less vulnerable to damaging exterior forces.

• Reduce radiation: Well-designed PCB layer stacks can help minimize radiation from the device, especially in high-speed layouts. It is important to note, however, that poor designs with impedance mismatches can easily result in greater EMI radiation than a normal PCB.

• Decrease costs: Good layer PCB stackups can also help achieve lower-cost manufacturing by placing multiple circuits onto a single board. One board means streamlined manufacturing for the board, the part the board will be used in, and the packaging for the complete setup.

On top of these benefits, PCB stackups also reduce impedance mismatch and signal crosstalk problems.

Objectives When Creating a Multilayer PCB Stack

Multilayer PCB stacks are designed carefully to meet certain design needs. The most basic objectives multilayer PCBs should achieve include the following:

It is important to note that not every PCB stack will meet all of these goals — only 8-layer boards can meet all five objectives. The PCB engineering team should determine which goals are the highest priority for a given stackup.

Other Factors to Consider When Creating a Multilayer PCB Stack

On top of the above objectives, there are other design factors to consider when developing a multilayer PCB stack:

• Interlayer offset: Layers can easily be offset when creating a multilayer PCB stack. To prevent this, use a hot-melting and rivet-and-dowel method for board side design.

• Stackup measling: Delamination in the form of measling is common when stacking boards. To prevent this, add pads with an epoxy plate while arranging the boards together. These pads will help balance the pressure to eliminate measling.

• Core materials: Single-layer PCBs can be made with standard substrate materials or aluminum cores. However, aluminum core PCBs are not available for multilayer stackups, as multilayer aluminum PCBs are extremely difficult to manufacture.

• Material consistency: Multilayer PCBs should use the same manufacturer’s materials for both core and prepreg. Using materials from different manufacturers may result in problems during lamination.

• Bow and twist: Bow and twist issues may arise if copper is unevenly distributed in multilayer stackups. For this reason, PCB designers should design all multilayer PCBs symmetrically — copper weights, prepreg thickness, and core thickness should be symmetrical.

It is also essential to closely monitor and design for PCB emissions. The design should be optimized to minimize emissions and avoid common issues that can result in return loops.

Common Types of PCB Layer Stackups

There are five types of PCB layer stackups — 2-layer, 4-layer, 6-layer, 8-layer, and 10-layer. The number of layers needed is primarily determined by the size of the circuit, the size of the board, and the EMC requirements.

2-Layer

Often called double-sided PCBs, 2-layer PCBs are probably the most common type of multilayer PCB as they can accommodate interconnects on both sides. These PCBs consist of three material layers, two of which are functional:

• Signal layer: This top layer consists of copper with an approximate thickness of 0.0014 inches (1 oz.) to 0.0021 inches (2 oz.).

• Laminate core: The laminate core separates the top and bottom copper layers, keeping them isolated. Vias create connections between the sides of the board.

• Bottom layer: This second layer of copper is similar to the signal layer with an approximate thickness of 0.0014 inches (1 oz.) to 0.0021 inches (2 oz.).

These PCBs are inexpensive and relatively easy to produce, which is why they are often favored over more expensive designs. They are also highly functional as there are no propagation delays, and they tend to have fewer potential design issues compared to other stackup options.

4-Layer

4-layer PCBs are the second most common option in multilayer boards. These PCBs have four layers to route electrical signals. The layers are sandwiched together, with a top and bottom layer on the outside and two inner layers contained between them. The top and bottom layers are where components and routing are placed. The inner layers cannot make outside connections, so they are often used as power planes or for signal routing, which helps enhance the quality of trace signals and reduces EMI emissions. It is not recommended to make the inner layers signal layers — if four signal layers are needed, a 6-layer board should be considered.

The two inner layers are separated from each other with a core layer, and the top and bottom layers are separated from the inner layers by prepreg. This results in a layer stackup as follows:

• Top layer

• Prepreg

• Inner layer 1

• Core

• Inner layer 2

• Prepreg

• Bottom layer

While more expensive to prototype and produce than 2-layer PCBs, 4-layer PCBs provide greater functionality overall. With thoughtful design, they can also provide excellent signal integrity and EMC capabilities.

6-Layer

A 6-layer PCB is functionally a 4-layer PCB with two additional signal layers. In total, 6-layer stackups include four routing layers — two internal and two outer — and two internal planes for grounding and power. The layers are arranged as follows:

• Top layer

• Prepreg

• Ground plane

• Core

• Route layer

• Prepreg

• Route layer

• Core

• Power plane

• Prepreg

• Bottom layer

This design is the most common, as it balances high-speed signaling with EMI control. High-speed signals are routed through the two buried layers while low-speed signals are routed through the surface layers.

However, this design is not effective for all applications. High-speed designs keep ground and power planes next to each other. Designs made for low EMI emissions have extra ground planes to help with shielding.

8-Layer

The 8-layer PCB stackup offers more options than the 6-layer board by adding two more layers for either routing or performance. It is also the first board that can achieve all five objectives for multilayer PCB design.

Typical 8-layer PCB stackups feature the power and ground layers in the center, offering good capacitance between layers as well as separation between the second and third signal planes to protect signal integrity. A common 8-layer PCB stackup looks like the following:

• Signal layer 1

• Prepreg

• Ground plane

• Core

• Signal layer 2

• Prepreg

• Power plane

• Core

• Ground plane

• Prepreg

• Signal layer 3

• Core

• Power plane

• Prepreg

• Signal layer 4

These PCBs do not feature more than four signal planes — instead of added signal planes, they feature more ground and power planes to optimize EMC performance. While 8-layer PCBs are more expensive than 2-, 4-, and 6-layer options, the percentage increase in cost of an 8-layer board over a 6-layer board is less than the percentage increase of a 6-layer board over a 4-layer board. This smaller increase makes it easier to justify the cost for improved emissions performance.

10-Layer

If a design requires six routing layers, a 10-layer board should be considered. A 10-layer PCB stack features six signal layers and four planes, with tight coupling between the signal and return planes. Typical 10-layer designs are arranged as follows:

• Signal layer 1

• Ground plane

• Signal layer 2

• Signal layer 3

• Power plane

• Ground plane

• Signal layer 4

• Signal layer 5

• Power plane

• Signal layer 6

High-speed signals are typically routed on the internal signal layers. When stacked and routed appropriately, this setup can provide both excellent signal integrity and great EMC performance. It is not recommended to replace any of the grounding or power planes with additional signal layers, as this can result in poor performance.

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