6-Layer PCB Stack-Up: A Comprehensive Guide to Design, Applications, and Manufacturing

 

Introduction

A 6-layer PCB is a kind of multilayer PCB with the extra 2 layers added between 4 routing layers. In simple terms, a 6-layer PCB consists of circuit board materials of 6 layers. This 6 layer includes two internal layers, two internal planes, and two external layers. The ground and power layers are also there for power management.

The top and bottom layers, three prepreg layers, two internal routing levels, one internal ground layer, one internal power layer, and two core layers make up a conventional 1.6mm 6-layer PCB board. This stacking technique can significantly reduce the EMI effect. This article discusses everything about the 6-layer PCB Stack up

6-layer PCB Stack up details
6-layer PCB Stack up details

Why is 6-Layer PCB Stack Up Used?

A 6-layer PCB offers numerous advantages for different applications. Firstly, it has a lightweight construction due to lightweight components, reducing the overall weight. Secondly, the simple layout of the PCB with clear labels makes it easy to navigate. Additionally, its smaller size makes it ideal for miniature devices. The connection process is simplified, saving space and materials. The electrical properties of the PCB are exceptional, ensuring high performance and speed in compact designs. Lastly, the use of insulation layers and bonding agents enhances its durability.

Applications of 6-layered PCB

6-layer PCB is widely used in various industries. Some of the applications of the 6-layer PCB stack-up include:

  • It supports multiple high-speed interfaces. Thus, it applies to high-end consumer electronics products, including smartphones, microwave ovens, wearable devices, etc.
  • It is used in telecommunications, like signal transmission, GPS, satellites, and base stations.
  • It is used in medical devices, including scanning equipment, heart monitoring, and X-ray machines.
  • Many industrial systems use this PCB layer because of its durability and stable nature, which are the basic requirements of industrial production systems.

10 Rules to 6-Layer PCB Stack-up Designs

  • The very first rule is the use of ground planes. They are the greatest option because they can route signals in strip lines. It is essential for lowering ground noise. The lower ground impedance results in a significant reduction in ground noise.
  • High-speed signals must be directed to an intermediary layer between various levels. In this way, the ground plane serves as a shield and muffles radiation leaving the orbit at its fastest rate.
  • Close to the plane is required for signal layers.
  • Power and ground planes need to be connected carefully.
  • Making sure the configuration is symmetrical.
  • The required signal impedance is present.
  • Each signal layer’s thickness must be taken into account. There are standard thicknesses with the properties of various types of PCB materials.
  • Additionally, it is crucial to consider the intended material’s qualities. Additionally, pay close attention to the materials’ mechanical, chemical, electrical, and thermal characteristics.
  • The return current can flow on an adjacent plane, lowering the inductance of the return path to a minimum if the signal levels are close to the levels of the plane (ground or power).
  • The insulation between a signal layer and its neighboring plane can be thinner to enhance noise and EMI performance.

Factors to Consider in the Design of 6-layer PCB Stack-up

Several factors need to be considered while designing the 6-layered PCB stack-up:

Signal Integrity Considerations

The electrical signal transmission through PCB is the result of signal integrity. Thus, trace lengths are planned carefully to prevent signal delays and distortions. On the other hand, impedance matching involves designing traces and terminations to match the characteristic impedance of the transmission lines, minimizing signal reflections. Additionally, minimizing crosstalk between adjacent traces is essential to avoid interference and ensure signal integrity. The design can maintain the desired signal quality and prevent data errors or signal degradation by addressing these factors.

Power and Ground Plane Design

A PCB’s overall performance depends heavily on the design of the power and ground planes. Several advantages to power and ground plane distribution can be realized. Noise reduction is one of the benefits. The planes serve as a shield, shielding the circuitry from outside noise. Another crucial element is stable power distribution, which guarantees that each component receives a steady supply of clean power. This helps prevent voltage swings and potential issues. Moreover, careful attention must be given to the placement and routing of power and ground traces to minimize the loop area, which reduces electromagnetic interference and improves signal integrity. These considerations collectively contribute to the efficient and reliable operation of the PCB.

Impedance Control and Routing Guidelines

Impedance control and routing guidelines are essential for maintaining consistent signal characteristics and preventing signal degradation. These guidelines dictate trace widths, spacing, and layer stack-up to achieve desired impedance values. Adhering to these guidelines helps minimize reflections and signal distortion.

EMI/EMC Considerations

EMI/EMC considerations are crucial for minimizing electromagnetic interference and ensuring compliance with electromagnetic compatibility standards. Shielding techniques, proper grounding, and strategic component placement are key to reducing EMI/EMC issues and ensuring the PCB functions reliably in its intended environment.

Materials

Standard substrate materials or aluminum cores are used to make single-layer PCBs. However, for the multilayer stack-ups, it should be clear that the aluminum core PCBs are not available. This is because multilayer aluminum PCBs are challenging to manufacture.

Thermal Management Techniques

Thermal management techniques are vital to prevent overheating and ensure the longevity and reliability of the PCB. This involves incorporating heat sinks, thermal vias, and proper component placement to dissipate heat efficiently. Thermal simulations and calculations can help identify potential hotspots and guide the selection of suitable cooling strategies.

Stacking Techniques and Layer Ordering

The choice of stack-up configuration for a 6-layer board depends on the specific design goals. Opting for 4 signal layers would be ideal if the route has numerous signals. Alternatively, if ensuring signal integrity for high-speed circuits is the priority, selecting the option that offers the best protection is crucial. Here are several configurations commonly used for 6-layer boards:

The original stack-up configuration is as follows:

– Top Signal

– Inner Signal

– Ground Plane

– Power Plane

– Inner Signal

– Bottom Signal

This configuration is generally considered poor since there is no shielding for the signal layers, and the two signal layers are not adjacent to a plane. This configuration was abandoned mainly as signal integrity and performance became more important. However, replacing the top and bottom signal layers with a ground plane would restore a good 6-layer stack-up. The downside is that it leaves only two internal layers for signal routing.

The most commonly used 6-layer configuration in PCB design places the inner signal routing layers in the middle of the stack up:

– Top Signal

– Ground Plane

– Inner Signal

– Inner Signal

– Power Plane

– Bottom Signal

This configuration provides better shielding for the inner signal routing layers typically used for higher-frequency signals. The stack-up can be further improved by increasing the distance between the inner signal layers with a thicker dielectric material. However, separating the power and ground planes reduces their planar capacitance, necessitating additional decoupling in the design.

The 6-layer stack-up that offers the best signal integrity and performance is rare. It reduces the signal layers to 3 to add an extra ground layer:

– Top Signal

– Ground Plane

– Inner Signal

– Power Plane

– Ground Plane

– Bottom Signal

6-layer PCB Stacking
6-layer PCB Stacking

 

Each signal layer is immediately adjacent to a ground plane in this stack-up, ensuring optimal return path characteristics. Moreover, having the power and ground planes beside each other creates a planar capacitance. However, one signal layer is sacrificed for routing purposes.

Classifications of 6-layer PCB Stack-up

The 6-layer PCB Stack-up refers to the PCB design with 6 layers of traces and insulating materials. Each layer has its purpose and thus works accordingly.

The types and classifications of the 6-layer PCB Stack up are listed below:

Top Layer or Signal Layer:

This is the topmost layer of the circuit board where components and circuit elements are placed. Most of the signal path lies in this layer, and thus it is mainly used for signal routing between components.

Inner Signal Layers (Layers 2 and 3): 

These are the inner layers sandwiched between the top and bottom layers. They also carry signal traces and are primarily used to provide routing space for complex circuits and high-speed signals. Signals between these two inner layers can be arranged in various ways, such as strapline, microstrip, or mixed configurations, depending on the specific design requirements.

Power and Ground Layers (Layers 4 and 5): 

These layers are located between two signal planes. These planes mainly contributed to reducing noises, distortions, and interferences.

Bottom Layer (Signal Layer):

This is the last layer of the PCB. This also serves as a signal-routing layer.

Some types of 6-layer PCB stack-up are listed below:

Normal Stack-Up: This architecture offers better power distribution within the circuit board. Besides, it is also good for signal integrity problems, as it offers excellent signal integrity within the circuit board. It works by sandwiching internal power or ground planes between two signal layers.

Mixed Signal 6-layer Stack-Up: It separates the layers according to the signals they convey to reduce interference. Two signals, analog and digital, are placed separately.

High-Speed 6-layer Stack-Up: A ground plane shields the signal layer of high-speed type and an inner signal plane. It is designed for high-speed signals.

6-layer Stack Up
6-layer Stack Up

Power Integrity 6 -layer Stack-Up: It promotes power integrity by using an inner signal plane to enhance power distribution within the circuit board and lessen voltage drops, assuring a steady power supply.

Buried Capacitance 6-layer Stack-Up: This architecture integrates embedded layers of capacitance layers into the power layers. Thus, it improves power distribution within the circuit board.

Material Selection and Dielectric Properties

The printed circuit board must be flame-resistant and only be able to be softened, not ignited at a particular temperature. The glass transition temperature (Tg point) is the temperature at this point, and it concerns the PCB board’s dimensional stability.

The Tg values for common PCB materials are larger than 130 degrees, greater than 170 degrees for high Tg values, and less than 150 degrees for medium Tg values.

Printed circuit boards with Tg 170°C are typically considered high Tg PCBs.

High TG value plates are typically used when making multilayer circuit boards, such as 6-layer PCBs.

High TG Materials with over TG170

Manufacturer/Brand Material
ISOLA FR406/FR408
ARLON High Tg210 11N
GETEK High Tg180 ML200/RG200
NELCO High Tg175 N4000-6/N4000-11
NELCO High Tg190 N4000-12
NELCO High Tg210 N4000-13
ITEQ IT180

 

High-Frequency Material (RF Boards)

Prepreg Type Thickness (mm) Resin Content Dielectric Constant
2116 0.12 55% 4.5
7630 0.2 55% 4.7
7628 0.185 43% 4.7
1080 0.075 65% 4.2

 

Manufacturing of 6-Layer PCB Stack-up

By bonding PCB layers to the PCB core, a 6-layer PCB is created. The second and fifth layers are laminated on the core at high temperatures and high pressures after the circuit has been etched onto the PCB core, and the third and fourth layers have been completed. After that, the second and fifth layers are etched with the circuit. On the 4-layer board, the top and bottom layers are also laminated, and the circuit is etched. Additionally, the top and bottom layers are painted with the solder mask, and the PCB pads’ surface finish is polished.

The PCB layers can be etched using one of two approaches. Which approach to use depends on whether drilling HDI vias through the layer is necessary. The two techniques for circuit etching are:

  • Positive Plane Etching
  • Negative Plane Etching

Positive plane etching is used on PCB layers that don’t need HDI via drilling and uses lye as the etching fluid. A UV-sensitive film was used to print the circuit pattern on the copper layer, which is how it works. While the layer of undesirable copper hardens, it is still liquid underneath the circuit pattern shadow. The liquid film is then removed by washing. The exposed copper is then covered with a protective layer of tin. The dried film is removed next.

After lye is used to etch away the exposed undesired copper, just the copper used in the circuit layout is left.

The PCB layers that need HDI via drilling are treated using the negative plane etching technique, which uses acid as the etching liquid. The UV light creates a shadow of the circuit pattern on the copper layer after printing with a different kind of UV-sensitive film. While the layer of undesirable copper is still liquid, it solidifies on the copper behind the circuit pattern shadow. The liquid film is then removed by washing. And acid is used to remove the exposed undesirable copper. The dried film is finally peeled off, leaving only copper in the circuit layout.

Both circuit etching techniques can be utilized for the PCB core. However, only positive plane etching can be used for the top and bottom layers.

The PCB board is mechanically drilled after lamination if the 6-layer PCB only has PTH holes, and the PTH holes are then electroplated with copper for circuit layer connection.

Before lamination, the top, bottom, second, and fifth layers of the 6-layer HDI PCBs are laser-drilled and electroplated. Before lamination, the PCB core is laser-drilled and electro-plated.

Design for Manufacturability (DFM) Guidelines

DFM for PCB is the design rules or guidelines for ensuring its manufacturability. It identifies the PCB layout issues which could occur during the manufacturing of the PCB.

Some of the most common DFM guidelines required for the manufacturing of the 6-layer PCB stack-up are:

Avoid DFM issues in Drilled Holes

The drilling process is the base for the vias and the connectivity between the various layers. With the increase in holes, it is considered one of the most expensive and time-consuming processes in PCB manufacturing. Aspect ratio and drill-to-copper clearance are the two factors for this drilling process.

The aspect ratio can be calculated as follows:

Aspect Ratio = Board Thickness / Smallest Drilled Hole Diameter

It is always considered to have the least aspect ratio for this 6-layer PCB stack-up as it directly impacts the drilling process.

Similarly, the drill to copper is the next factor to consider in the drilling process. It is a challenging process, and for this 6-layer PCB stack-up, the tight drill to copper required X- rays into the inner layers to get the scaling information after the lamination process.

Designing Angular Rings without any breakouts

Angular Rings
Angular Rings

 

PCB manufacturer helps to get the perfect annular rings. The annular rings depend on the IPC spec. Thus, below are some of the design tips for the annular rings:

  1. Check for the presence of copper pads for plated drills.
  2. Ensure that the manufacturer can maintain the annular ring.
  3. The minimum internal annular ring can be at least 1 mil, while the maximum external annular ring can be at most 2 mil.

Efficient Trace Routing

Trace Routing
Trace Routing

 

Ensure the proper signal checks for the parameters like space requirements, conductor width, and hole registrations.

  1. Maintain proper line spacing between two traces to avoid flashover and tracking between the electrical conductors. Factors like voltage, application, and assembly types impact space requirements. For proper line spacing, maintain the high-voltage circuits at the top and low-voltage circuits at the bottom of the PCB.
  2. The parallel-sided notch, V groove, can solve the creepage issue. Adding a slot between the traces and a vertical barrier can avoid spacing errors.
  3. Make sure to calculate the PCB trace current carrying capacity. The minimum spacing can limit the excess loss of the current. The trace size on the outer layer should be at least 4 mils.

DFM Check for the Solder Mask

solder mask clearance in a circuit board
Solder mask clearance in a circuit board
  1. How to calculate the necessary solder mask clearance?
    Find out what the manufacturer considers to be the minimum solder mask clearance. This number typically depends on the PCB manufacturer’s production capabilities and technologies. Commonly, the clearance is described as a distance, such as millimeters or mils.
  2. Verify important areas: Determine the crucial PCB locations for the solder mask clearance. These regions frequently contain fine-pitch elements, dense areas, or regions with close trace-to-trace spacing. Ensure the clearance for the solder mask in these places complies with the manufacturer’s specifications.
  3. Verify approval for the via-to-solder mask: Pay close attention to the PCB’s vias. Verify that there is enough clearance around the vias in the solder mask openings to prevent any potential shorts or coverage problems. The manufacturer’s instructions will state the minimal clearance needed for vias.
  4. Use a DFM software tool to check the solder mask clearance: To check the solder mask clearance, use a DFM software tool or PCB design software with built-in DFM tests. These tools may automatically identify and highlight solder mask clearance violations, allowing you to fix them before submitting the production design. Examine the PCB layout carefully to find any spots where the solder mask clearance may be jeopardized. Ensure that the pads, visa, and other important features’ solder mask apertures, are the proper size and location.
  5. Consult the manufacturer: Consult the PCB manufacturer directly if you have any concerns or queries regarding the solder mask clearance or other DFM factors. Based on their unique strengths and requirements, they can offer insightful advice.

Difference between 8 Layer PCB Stack-up, 6 Layer PCB Stack-up, and 4 Layer PCB Stack-up

Aspect 8-Layer PCB Stack up 6-Layer PCB Stack up 4-Layer PCB Stack up
Number of Layers 8 layers 6 layers 4 layers
Signal Routing More layers allow for complex signal routing Fewer layers limit signal routing options Limited layers restrict signal routing options
Signal Integrity Better signal integrity due to additional layers Good signal integrity in a balanced configuration Signal integrity may be compromised
Power Distribution More dedicated power and ground planes Fewer dedicated power and ground planes Limited dedicated power and ground planes
Noise and EMI Improved noise reduction and EMI suppression Moderate noise reduction and EMI suppression Less effective noise reduction and EMI suppression
Design Complexity Higher design complexity due to more layers Moderate design complexity Simpler design due to fewer layers
Cost Higher cost due to additional layers and materials Moderate cost Lower cost due to fewer layers and materials
Application Suitability Suitable for complex and high-speed designs Suitable for moderate complexity designs Suitable for simpler designs and cost-sensitive applications

Conclusion

Four signal layers, one ground layer, one power layer, three signal layers, two ground layers, and one power layer can make up a six-layer PCB. There are also 4 different kinds of 6-layer PCB stack-ups. 

A 6-layer printed circuit board provides higher performance and longevity. A well-constructed 6-layer PCB stack-up guarantees signal integrity and a low dielectric constant (DK). A 6-layer PCB’s designing and manufacturing processes are difficult and time-consuming. However, a 6-layer PCB offers more possibilities for arranging the Signals, Power, and Ground tracks in a condensed area.

The particular specifications of each circuit determine a 6-layer PCB’s stack-up design. Therefore, choosing how to stack or organize the layers while considering the needs of the board results in improved electrical properties. For instance, crosstalk, electromagnetic interference (EMI), and the effect of outside noise are all decreased.

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