PCB Stack Up Design: Principles, Types, and Examples

Introduction

The continuous advancement of modern electronics has been driving the demand for PCBs to meet various requirements, including smaller size, lighter weight, faster speed, enhanced functionality, improved reliability, and extended lifespan. This has led to the widespread use of multilayer PCBs. These multilayer PCBs are created by stacking two or more single and/or double-sided PCBs together using a special type of semi-solid adhesive known as “prepreg.” The layers are securely interconnected to form a multilayer PCB. Typically, there are three or more conductive layers in a multilayer PCB, with two outer layers and one layer embedded within the insulation board. As PCB complexity and density increase, certain issues may arise, such as noise, stray capacitance, and cross talk, especially when the layer arrangement is inefficiently designed.

The effective planning of a multilayer stack-up holds great significance in determining the Electromagnetic Compatibility (EMC) performance of a product. By employing a carefully engineered layer stack-up, it becomes possible to minimize radiation and prevent external noise sources from interfering with the circuit. Furthermore, a well-structured arrangement of PCB substrates can mitigate signal cross-talk and address impedance mismatch problems. Conversely, a subpar stack-up could lead to an increase in Electromagnetic Interference (EMI) radiation due to reflections and ringing caused by an impedance mismatch, thereby significantly compromising the product’s performance and reliability. This article focuses on defining different stack-up layers of PCB, outlining design guidelines, and highlighting essential considerations. 

Stack Up 

Each printed circuit board (PCB) typically incorporates a minimum of one layer made of copper. This is understandable, as it serves the essential purpose of managing power and signals within the PCB. However, the composition of the PCB can vary significantly based on design specifications, leading to variations in the appearance of the stack-up.

A PCB with only one layer, known as a single side PCB, comprises a substrate, copper layer, solder mask, and optionally a labeling silkscreen. In contrast, a PCB with multiple layers is composed of several layers of such elements, along with core and prepreg. The prepreg, a composite material of fused fiberglass with resin, serves as a crucial bonding agent for the layers. The core, on the other hand, comprises a hard material such as FR4, which has copper laminated on both surfaces.

For multi-layer PCB, particularly those with a high number of layers and demanding specifications, meticulous planning and consideration of the stack-up configuration become indispensable.

 

Types of PCB Stack Up

The selection of stack-up is of utmost importance as it significantly impacts the performance of the PCB in terms of electrical, mechanical, and signal integrity. Presented below are some widely used types of PCB stack-up:

Single-layer:

This stack-up, also called single-sided PCB, consists of a solitary layer of copper affixed to one side of the PCB substrate. Components are mounted on one side while traces are routed on the opposite side.

Multilayer:

These stack-ups comprise three or more copper layers, which are insulated from one another by materials such as core and prepreg layers. The layers are interconnected using vias. Multilayer PCBs facilitate the creation of intricate circuit designs and are commonly utilized in advanced electronic devices.

Metal Core:

The MCPCBs cater to the needs of LEDs with high current requirements and various other applications that demand efficient power delivery. The core plays a vital role in the board’s thickness, serving dual essential functions. Initially, it effectively disperses heat from the ICs or other components during their work. Additionally, it ensures a level and conductive bottom surface, thereby enabling easy attachment to the external heatsinks.

 Metal Core PCB Stack up

High-Density Interconnect:

These PCBs are designed to meet the requirements of compact digital design, offering advanced connectivity options. These designs often utilize powerful processors with fast data buses, such as memory and links to external devices like PCI interfaces. By incorporating small vias, controlling impedance in traces, utilizing HD footprints, and adhering to precise solder mask provisions, these PCBs can support a wide range of layer configurations.

High Frequency:

They are well-suited for applications that demand efficient signal transmission rates and broad frequency spectrums of up to 100 gigahertz (GHz). These circuit boards find extensive usage in applications that heavily rely on radio frequencies and might incorporate unique functionalities such as trace antennas. They exhibit precise requirements regarding the gap between conductive traces, the spacing between components, and the solder utilized in connectors.

Flex and Rigid-Flex:

These PCBs are specifically engineered for compact and adaptable implementations, including wearable technology and portable gadgets. These sophisticated “boards” can be composed solely of pliable circuits or incorporate both flexible and inflexible boards, resulting in a unified assembly of circuit boards with the capability to flex and conform to various shapes. The construction of flex circuits involves a flexible polymer, one or more copper layers, and an overlay. These circuits employ traces, planes, and vias, similar to conventional PCBs.

 4 Layer Flex PCB stack up

Layer Selection Principle

When determining the appropriate number of layers for a multi-layer PCB board, several important factors come into play. 

The impedance matching, crosstalk, and noise coupling are influenced by the number and layout of signal layers, power planes, and ground planes. By strategically selecting and arranging these layers, signal degradation can be minimized, and signal integrity can be preserved. Moreover, the number of layers required for the PCB design depends on its desired functionality and complexity. Incorporating more layers enables a greater routing density, facilitating the accommodation of a larger number of signals and components. However, it is important to note that an increase in the layer count raises both the cost and fabrication complexity. Therefore, the following points should be considered while selecting PCB stack-up layers

1. Number of Layers:

When designing your PCB stack up, one of the initial factors to consider is determining the required number of layers. This involves taking into account the nature of the signals (high speed or low speed), power requirements (especially for high power boards or circuits with integrated power supplies), and the inclusion of ground layers or planes. It is advisable to avoid mixing different signal types on inner layers.

To accurately determine the layer count for most boards, you can utilize the pin density measurement.

 

2. Arrangement of Layers:

Once you have determined the appropriate number of layers, the next step is to decide how these layers should be arranged or stacked. Here are some guidelines to follow:

• For high-speed signals, route them on minimum-thickness microstrips.
• Place signal layers adjacent to internal power layers to ensure tight coupling.
• Minimize spacing between power and ground layers.
• Avoid placing two signal layers next to each other.
• Maintain symmetry in the stack-up design, both from the top and bottom layers inward.

3. Types of Layer Materials:

Another crucial aspect of your PCB stack-up is selecting the thickness for each signal layer. This decision should be made in conjunction with determining the thicknesses of prepreg and core materials. Different circuit board material types have standard thicknesses and other relevant properties, such as electrical, mechanical, and thermal characteristics. When choosing materials, it is important to consider these properties.

4. Routing and Via Placement:

The final step in completing the PCB stack-up design involves determining the routing of traces and placement of vias. This encompasses decisions regarding copper weights, via locations, and the types of vias to be used. Just like other design specifications, it is recommended to collaborate with your CM to make these determinations, as certain CMs may have preferences or restrictions on specific via types, such as via-in-pads.

 

General Principles for PCB Stack Up

Once the number of circuit board layers is determined, the next task involves strategically arranging the circuit placement for each layer. This step considers two primary factors:

• Distribution of specialized signal layers
• Distribution of power and ground layers

As the number of circuit board layers increases, the available combinations for arranging signal layers, ground layers, and power layers become more diverse. Consequently, selecting the optimal combination method becomes more challenging. However, there are general principles to guide this process:

1. Signal layers should be positioned adjacent to internal power layers (referred to as inner power/ground layers) and shielded by the copper film of the inner power layer.
2. The internal power layer should tightly integrate with the ground layer. This means minimizing the thickness between the internal power layer and the ground layer to enhance the power supply capacitor and increase the resonant frequency. If the electric potential difference between the internal power layer and the ground layer is not significant, a thinner insulation thickness, such as 5mil (0.127mm), can be utilized.
3. Avoid directly adjacent signal layers to prevent crosstalk, which can lead to circuit failure. Placing a ground layer between two signal layers effectively mitigates crosstalk.
4. Employing multiple grounded internal power layers effectively reduces ground impedance. For instance, assigning separate ground planes to signal layer A and signal layer B effectively mitigates common-mode interference.
5. Ensure symmetry in the layer structure.

PCB Stack-Up Requirements

Numerous elements contribute to the stack-up requirements of a PCB, with various significant factors playing a pivotal role. During PCB lamination, inner layers, copper foil, and prepreg are combined through pressure and heat to form a multilayered stack, finalizing the production of the PCB. Two lamination methods exist foil construction and cap construction. The construction of foil is recommended due to its advantages. It offers better manufacturability, improved yield, and lower costs due to its ability to use fewer core laminate materials and achieve improved yields.

Foil construction

In the process of foil construction, a material called prepreg is placed between the outer layers made of copper. For instance, in a board with four layers, the prepreg is positioned between the first and second layers at its top, and between the third and fourth layers. The manufacturing channel surface is composed of an uninterrupted copper foil sheet. The layer arrangement of a four-layer board that employs foil construction consists of the following sequence, starting from the top: The outer foil layer made of copper (referred to as layer 1) is covered with a protective solder mask. This layer is followed by a layer of prepreg and an inner laminate PCB core (comprising layers 2 & 3). Another layer of prepreg is added, followed by a solder mask applied to the outer copper foil layer (known as layer 4).

 4 Layer PCB stack-up Lamination

Cap construction, conversely, employs pre-copper-clad laminate components, referred to as core material, for the outer layers. In the context of a 4-level board, the external layers commence with a core laminate and feature a single prepreg layer sandwiched in between. The cap is generally discouraged in contemporary PCB fabrication unless it pertains to specialized scenarios involving hybrid PCBs with specific laminates and limited drilling options such as laser drilling. The layer configuration for a 4-layer circuit board using cap construction includes the following layers, listed in a top-to-bottom arrangement: outer laminate layers (layers 1 & 2) with solder mask, prepreg layer, and outer laminate layers (layers 3 & 4) with solder mask.

4 Layer PCB stack up Lamination

Board Component Density (layers number & spacing)

The density of components on a circuit board has a notable impact, especially when working with memory modules or processors that have a high number of pins or are in Ball Grid Array (BGA) configurations. In such cases, it may be necessary to incorporate extra layers solely for routing signals from the inner regions of the footprint.  

Types of Components and Interfaces

The choice of component types and interfaces, such as digital, analog, mixed-signal, or high-speed, can impact the board design. While utilizing BGA components saves space, it introduces cost and complexity during production, as well as difficulties in reworking if necessary. Accessing each pad of a BGA component, as depicted in the provided image of a 324-pin BGA component, may necessitate multiple layers.

324 pins using 12 Layer BGA Component

Board Types & Applications

The type and purpose of the board can impact the specific requirements for its stack-up, especially concerning thermal management. When working with printed circuit boards (PCBs) that manage power components or high current pathways, it might be essential to incorporate additional layers specifically allocated for power planes/polygons and enhance the usage of copper to minimize resistance (thus reducing IR drop) and proficiently dissipate heat produced by component junctions.

 

Demonstration of Stack-Up Designs

Understanding the fundamental principles of PCB multilayer stack-up is crucial before delving into the various stack-up options and selecting the appropriate design for your specific requirements. PCB layer stack-ups consist of five distinct types: 2-layer, 4-layer, 6-layer, 8-layer, and 10-layer. The decision regarding the number of layers is primarily influenced by factors such as circuit size, board dimensions, and EMC specifications. These considerations enable you to determine the optimal board layout.

Further elaboration on 4-layer, 6-layer, and 8-layer stack-up, is given below:

4 Layers Stack-Up

When it comes to PCB stack-up, 4-layer printed circuit boards (PCBs) rank as the second most commonly utilized option among multilayer boards. These specific PCBs are designed with four layers that facilitate the transmission of electrical signals. The layers are positioned together, with outer layers on the top and bottom, and two internal layers nestled between them. The outer layers are designated for component placement and routing. Conversely, the inner layers are primarily utilized for power planes or signal routing, enhancing trace signal quality and minimizing electromagnetic interference emissions. It is advisable to refrain from utilizing the inner layers solely for signal purposes. If a 4-layer PCB with four signal layers is needed, exploring the implementation of a 6-layer board is recommended.

To ensure proper separation, the two inner layers are isolated from each other by a core layer, while the top and bottom layers are segregated from the inner layers by a prepreg material. As a result, the layer stack-up arrangement appears as follows:

Although the prototyping and production costs of 4-layer PCBs are higher compared to 2-layer PCBs, they offer superior overall functionality. With meticulous design, these PCBs can deliver exceptional signal integrity and electromagnetic compatibility capabilities.

 4 Layers PCB Stack up

6 Layers Stack-Up

A six-layer board offers enhanced electromagnetic interference (EMI) reduction compared to a four-layer board. It achieves this by incorporating two additional signal layers between the planes. These extra layers serve two purposes: accommodating high-speed signals within the buried layers and routing low-speed signals on the surface layers. To ensure optimal performance, the signal layers should be positioned near their adjacent planes while maintaining the desired board thickness. 

6 Layers PCB Stack up

Achieving this thickness involves utilizing a thicker center core. Determining the ideal trace impedance, trace width, and prepreg/core thickness is a trade-off. Differential pairs, which employ differential mode signaling, have become increasingly prevalent in high-speed designs as they effectively reduce noise.

 

8 Layers Stack-Up

In typical 8-layer PCB configurations, the power and ground layers are strategically positioned at the core. This arrangement facilitates effective capacitance between the layers and ensures signal integrity by creating separation between the second and third signal planes. The resulting 8-layer PCB structure has gained wide popularity. These PCBs are designed with no more than four signal planes, prioritizing additional ground and power planes to enhance electromagnetic compatibility (EMC) performance.

Despite the higher manufacturing cost of 8-layer PCBs compared to their 6-layer counterparts, the decision to opt for this configuration is well justified. The increase in cost from a 6-layer to an 8-layer PCB is proportionally lower than the increase from a 4-layer to a 6-layer board. As a result, the incremental cost is more manageable, making it easier to justify the expense for improved emission performance.

 8 Layers PCB Stack up

Layer 6, strategically positioned amidst two adjacent ground planes, represents the most suitable option for effectively routing high-speed signals while satisfying critical impedance control demands. The existence of encompassing ground planes around the inner layers significantly mitigates EMC concerns. When layer 6 is already in use, and layer 3 is linked to a power plane, outer layers 1 and 8 emerge as the preferred choices for facilitating high-speed routing.

 

PCB Stack up Design Issues and Prevention

• Apart from the mentioned goals, there are additional factors to consider during the development of a multilayer PCB stack. Achieving precise alignment between layers is crucial, and a recommended approach involves a combination of hot-melting, riveting, and doweling techniques for board-side design to prevent any offset.
• To address delamination issues while stacking boards, incorporating epoxy plate pads in the arrangement can help distribute pressure evenly, effectively eliminating measling.
• When choosing core materials for single-layer PCBs, standard substrate materials or aluminum cores are viable options. However, it’s essential to note that aluminum core PCBs are unsuitable for multilayer stack-up due to significant manufacturing challenges.
• Consistency in materials is vital for multilayer PCBs, both in the core and prepreg. It’s recommended to use materials from the same manufacturer to avoid complications during lamination.
• Uneven distribution of copper in multilayer stack-up can lead to bow and twist issues. To prevent this, PCB designers should strive for symmetrical designs, ensuring that the copper weights, prepreg thickness, and core thickness are balanced throughout the stack-up. Additionally, close monitoring and optimization for PCB emissions are essential. By optimizing the design to minimize emissions, common issues that can cause return loops can be avoided.

PCB Stack up Advantages

While PCB layering offers intricate and space-efficient boards, PCB stack-up presents several benefits that enhance functionality and performance:

• Enhanced functionality

  Multilayer PCBs significantly boost device speed and functionality, resulting in highly efficient boards.

• Improved protection

  Layer stacks act as a shield against external noise, protecting internal layers and minimizing vulnerability to damage from external forces.

• Radiation reduction

  : Well-designed PCB layer stacks effectively minimize device radiation, particularly in high-speed layouts. It’s important to note that inadequate designs with impedance mismatches may generate more EMI radiation than a standard PCB.

• Cost reduction

  Optimized layer PCB stack-ups enable cost-effective manufacturing by integrating multiple circuits onto a single board. This approach streamlines the manufacturing process for the board, its components, and the required packaging. Additionally, PCB stack-ups help mitigate impedance mismatch and signal cross-talk issues. These advantages make PCB stack-up a highly sought-after choice.

Conclusion

The design of PCB stack-up plays a crucial role in meeting the demands of modern electronics. Multilayer PCBs are widely used to achieve smaller sizes, faster speeds, enhanced functionality, and improved reliability. The stack-up configuration significantly impacts the electromagnetic compatibility (EMC) performance of a product, as well as its signal integrity and impedance matching. Various types of stack-up, such as single-layer, multilayer, metalcore, HDI, high frequency, and flex & rigid-flex PCBs, are available to cater to different requirements. When determining the number of layers and arranging them, factors like signal types, power requirements, and material properties should be considered. Following general principles such as positioning signal layers adjacent to power layers, minimizing crosstalk, and ensuring symmetry in the layer structure can contribute to optimal stack-up designs. PCB stack-up requirements also depend on board component density, component types, board type and application, and thermal management needs. Understanding these requirements helps in selecting the appropriate stack-up design. Lastly, examples of 4-layer, 6-layer, and 8-layer stack-up are provided, highlighting the arrangement of signal layers and reference planes for high-speed signals. It is important to address design issues, such as proper layer alignment and prevention of measling, during the PCB stack-up design process.

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