A printed circuit board (PCB) comprises various conductive and non-conductive layers laminated to form the basis for mounting and interconnecting electronic components. The number and arrangement of the different layers of PCB play a crucial role in determining its functionality and capabilities. This article provides an in-depth overview of the different types of layers found in PCBs.
Overview of PCB Layers
The layers in a PCB can be broadly classified into the following categories:
The substrate refers to the base insulating material on which the copper layers are laminated. The most commonly used substrate material is FR-4, a fire-resistant fiberglass epoxy laminate. Other substrate materials include CEM-1, CEM-3, polyimide, etc. The substrate provides mechanical support and electrical isolation between the copper layers.
Copper foils laminated onto the substrate form the conductive layers for routing signals and supplying power. The number of copper layers determines whether the PCB is single-sided, double-sided, or multilayer. The thickness of the copper foil (1oz, 2oz, etc.) impacts current carrying capacity and impedance control.
Solder Mask Layers
The solder mask is a coat of polymer layer applied on the outer copper layers, exposing the contact fingers and bonding pads. It acts as insulation against solder bridging and protects against corrosion and external damage.
The silkscreen layer prints text, symbols, and lines on the PCB for identification and orientation. It improves readability and provides crucial information for debugging and assembly.
The outer surfaces are coated with metallic and organic finishes to enhance solderability and avoid oxidation. Standard surface finishes are Immersion Tin, Silver, ENIG (Electroless Nickel Immersion Gold), OSP (Organic Solderability Preservative), etc.
In the following sections, we will explore the different categories of PCBs based on the number and arrangement of layers.
Single-sided PCBs have a single layer of copper laminated onto one side of the insulating substrate material. The opposite side of the PCB is used for mounting and soldering components. Traces, pads, vias, and other conductive features are patterned onto the single copper layer to form the circuits.
Advantages of Single-Sided PCBs
- Low Manufacturing Cost: Single-sided PCBs are the least expensive type of PCB to fabricate since they require only a single copper layer deposition step and one etching process. This makes them ideal for high-volume consumer electronics where cost is critical.
- Simpler Design: Routing and layout are straightforward, with access to only one conductive layer. There is no need for complex interlayer routing or via transitions. Design rules are also simplified without considering layer-to-layer spacing or registration.
- High Component Density: Components mounted on the opposite side can achieve a very high component density on the single copper layer. Though limited in interconnectivity, spacing components closely can maximize utilization of the available PCB area.
- Ease of Assembly: Assembly and soldering of leaded components is more straightforward, with access to only one side of the board. There are no components on the bottom side obstructing access during manual soldering. Automated assembly is also more straightforward without restricting component types to surface mounts.
- Faster Production: Single-sided boards can be fabricated, assembled, and tested faster, owing to the simplicity of the process. This allows shortened time-to-market for products requiring low-complexity PCBs.
- Weight and Space Savings: Single-sided PCBs help save real estate with components on only one side in space and weight-constrained applications. This maximizes area utilization in small handheld battery-powered devices.
Disadvantages of Single-Sided PCBs
- Limited Routing Space: The single copper layer severely restricts routing options for more complex boards. There is no way to crossover traces or change routing layers. High pin count components become challenging.
- No Internal Shielding: Without any ground or power planes between layers, there is no isolation between traces. External shielding becomes necessary for noise prevention in sensitive circuits.
- Component Placement: Components can only be placed on one side of the PCB, restricting optimization. Heat-generating parts cannot be isolated from temperature-sensitive components on the other side.
- Higher Layer Currents: With a single conductive layer, the layer currents are higher for a given trace width. This impacts voltage drop, thermal dissipation, and electro-migration limits.
- No Impedance Control: Lack of solid power/ground planes prevents controlled impedance traces critical for high-speed signals. Only high-tolerance traces are possible.
- Manual Rework Difficult: Diagnosing and reworking faults on single-sided PCBs is challenging, with limited access to just one side. Troubleshooting typically involves destructive methods.
- Not Scalable: Increasing complexity inevitably requires more board layers. Single-sided PCB construction is unsuitable for complex digital circuits or high-pin count devices.
Applications of Single-Sided PCBs
Some typical applications of single-sided PCBs based on their advantages include:
- Simple analog circuits such as amplifiers, filters, and oscillators.
- Low-frequency digital logic boards.
- Basic lighting systems with LED arrays.
- TV and radio receivers and transmitters.
- Battery-operated portable electronics and toys.
- Minimalist wearable devices and sensors.
- Analog control boards and data acquisition systems.
- Educational and hobbyist projects using microcontrollers.
Single-sided PCBs provide the most cost-effective and straightforward solution for applications requiring simple connectivity over a limited board area.
Double-sided PCBs contain a copper layer on the insulating substrate’s top and bottom. This allows components to be assembled on both sides of the board and enables more complex routing options.
Advantages of Double-Sided PCBs
- Increased Component Density: With components placed on both sides, the available surface area can be maximally utilized. Component densities double compared to a single-sided board of the exact dimensions.
- Space Savings: Smaller PCB sizes are possible with components on both sides. This allows more compact product designs due to reduced board space requirements.
- Complex Routing: Traces can be routed on top and bottom layers, with vias providing interconnection between layers. This facilitates complex routing, which is not possible on single-layer boards.
- Lower Layer Currents: Currents are halved for a given trace width since they are split between two layers. This reduces voltage drops across traces.
- Cost Effective: Double-sided PCBs only require two copper foil lamination steps, so incremental cost increase over single-sided is minor. Much cheaper compared to multilayer.
- Ease of Assembly: Manual soldering is simplified with open access to both sides of the board. Automated assembly only requires additional bottom-side capability.
- Flexibility: A mixture of through-hole and SMT components can be supported. Double-sided boards are compatible with low-tech production.
Disadvantages of Double-Sided PCBs
- No Internal Signal Layers: Lack of routing layers inside the PCB prevents isolation between critical traces. External shielding becomes necessary.
- Space Constraints: Though improved over single-sided, component crowding can still be an issue for dense double-sided boards. Multilayer PCBs perform better.
- Complex Layout: Routing on both layers makes the layout process more intricate. Engineering change orders are also more challenging to implement.
- Manual Rework: Fault diagnosis and reworking faulty components is challenging, with limited access to sandwiched layers.
- No Impedance Control: Impedance cannot be accurately controlled without internal reference planes. Only high-tolerance traces are supported.
- Not Scalable: Double-sided construction provides inadequate routing capability for very complex boards. More layers are necessitated.
Applications of Double-Sided PCBs
Some typical applications where double-sided PCBs are commonly used:
- Consumer electronics like home audio/video systems.
- Test and measurement equipment like oscilloscopes.
- Radio transmitters and receivers operating at HF/VHF bands.
- Industrial robotic arms and servo motor controls.
- Automotive engine control units (ECUs).
- Low-end computing peripherals and disk drives.
- Standalone medical diagnostic systems.
- IoT sensors and wireless communications boards.
Double-sided PCBs offer the optimal balance of capability and affordability for cost-sensitive products requiring moderate routing complexity.
Multilayer PCBs contain four or more conductive copper layers separated by insulating dielectric substrate layers. This enables highly complex routing for high-density boards with thousands of traces and vias.
Advantages of Multilayer PCBs
- High Component Density: Component densities exceeding 2000 pins/sq.in. are possible with components mounted on both sides and multiple routing layers.
- Complex Routing: Multiple internal layers accommodate very complex signal interconnections between components not feasible with double-sided boards.
- Shielding and Isolation: Ground and power planes between signals provide isolation and prevent coupling or crosstalk between sensitive traces.
- Controlled Impedances: Microstrip traces over ground/power planes give tightly controlled impedances for high-speed signals like clock distribution.
- Compactness: Far greater miniaturization is possible than conventional PCBs due to high component density and multilayer stack-up.
- Passive Integration: Passive components like capacitors and inductors can be formed within the PCB layers, eliminating surface mounts.
- Noise Reduction: In high-current devices, multiple ground planes reduce ground noise and supply voltage ripple.
Disadvantages of Multilayer PCBs
- Manufacturing Complexity: Require many lamination cycles and accurate interlayer alignment. This increases fabrication time and cost significantly.
- Simulation Overhead: Extensive signal and power integrity simulations are needed to verify that the layout meets speed, EMI, and isolation requirements.
- Complex Repair: Troubleshooting destructive components or opens/shorts within inner layers is extremely easy with X-rays and special tools.
- Layer Currents: High-layer currents can cause overheating unless adequate copper thickness is used. Thermal management is critical.
- Registration: Exact layer-to-layer alignment is essential to ensure vias connect correctly between layers.
- Advanced Design Tools: High-end CAD software with automated routing, 3D modeling, and signal analysis capabilities is indispensable.
Applications of Multilayer PCBs
Some examples of products using multilayer PCBs:
- Desktop and laptop computing systems
- High-performance embedded controllers
- Graphics cards and GPUs
- Gaming consoles and VR headsets
- 5G wireless infrastructure
- RADAR and electronic warfare systems
- MRI, PET, and CT scanners
- Spacecraft avionics and navigation
- Next-gen smartphones and tablets
- Automotive infotainment and telematics
Multilayer boards provide unmatched interconnectivity and functionality for products pushing the envelope in speed, density, and power.
Common Layer Stack-ups
While the layer count can range from 4 to 32 layers based on complexity, some standard multilayer PCB layer configurations are:
- 4-layer: Used in consumer electronics, medical devices, and IoT products. Layers are organized as Signal-Power-Ground-Signal.
- 6-layer: Used in industrial controls, telecom, and defense systems. Layers are organized as Signal-Power-Ground-Signal-Signal-Power.
- 8-layer: Used in high-speed computing systems and arranged as Signal-Power-Ground-Signal-Signal-Ground-Power-Signal.
- 10-layer or more: Used in high-end servers, network switches, and RADAR systems. It has multiple power and ground layers.
PCB Layer Importance
Now that we have explored the different types of PCB layers let’s understand why they are so crucial for the overall functioning of the PCB.
The foremost consideration is the number of conductive copper layers in the PCB. More layers allow greater routing complexity, accommodating higher component densities. High layer counts are essential for multilayer PCBs to meet intricate circuit requirements.
The sequence in which layers are stacked is also critical, especially the positioning of power and ground planes. This provides proper isolation and shielding between critical signals.
The thickness of the copper foil affects current capacity, thermal performance, and signal quality. Thicker copper layers are preferred for handling high-power applications.
Proper alignment between layers is necessary to ensure vias connect accurately between layers. Any mismatch can cause open-circuit faults.
The insulating dielectric thickness between adjacent copper layers must provide adequate isolation. Minimum separation is required to prevent arcing and parasitic capacitance.
Symmetric layer stacks with similar layer types on either side of the core help to minimize warpage and mechanical stress.
PCB Layer Design Guidelines
- Position power and ground layers adjacent to each other to form decoupling capacitors.
- Insert ground planes between signals that need isolation to prevent coupling.
- Route critical high-speed traces on outer layers adjacent to a ground plane.
- Assign one layer as a ground plane to provide a low-impedance return path.
- Use adjacent layer pairs for differential signaling to match lengths.
- Minimize the number of vias cutting across ground and power planes.
- Maintain clearance between copper and board edge to prevent short circuits.
- Allow sufficient spacing between adjacent copper regions to avoid bridging.
- Do not run traces under high pin count packages like BGAs.
- Verify trace widths can handle expected current levels.
Considering these guidelines during layout, the layers can be optimally utilized to meet design requirements.
Example 10-Layer PCB Stackup
To better understand the organization of layers in a multilayer PCB, let’s look at a sample 10-layer stack designed for a high-speed digital system.
Layer 1 (Top Layer): Used for low-speed signals, component pads, and silkscreen markings.
Layer 2: High-speed signal layer routed over a solid ground plane.
Layer 3 (Ground Plane 1): Provides shielding and low impedance return path.
Layer 4: Power plane for providing 3.3V supply to components.
Layer 5: Another layer for routing low-speed digital and analog signals.
Layer 6 (Ground Plane 2): Additional ground plane for shielding and noise isolation.
Layer 7: Power plane for 5V power distribution on the board.
Layer 8: High-speed bus signals requiring controlled impedance.
Layer 9 (Ground Plane 3): Final ground plane for EMI shielding.
Layer 10 (Bottom Layer): Used for low-speed signals and connectors.
This demonstrates how following proper guidelines allows the layers to be optimally leveraged for routing, shielding, and isolation in a high-density 10-layer board.
The unique advantages of different PCB layer types and arrangements provide designers with the flexibility to meet diverse circuit requirements. Understanding the role played by each layer enables electrical engineers to make informed decisions on layer count, stack up, and sequence. This knowledge helps utilize the layers optimally while following sound design practices. Rigorous testing and simulation of the layered PCB layout are imperative before progressing to fabrication and assembly, especially for mission-critical systems.
Frequently Asked Questions (FAQs)
What is the most common layer count for multilayer PCBs?
The most commonly used layer count for multilayer PCBs is 4 layers. 6-layer and 8-layer PCBs are also widely used in many applications. High-density boards can have 36 layers or more.
Why are multiple ground layers used in multilayer boards?
Using multiple ground layers provides several benefits:
- Lower ground impedance due to a larger copper area
- Reduces ground noise and ripple voltage
- Provides isolation between signal layers
- Limits radiated EMI emissions
- Allows decoupling capacitors to be integrated
Can traces be routed on the inner layers of a multilayer PCB?
Yes, traces can also be routed on the inner signal layers in a multilayer PCB in addition to the top and bottom layers. This helps accommodate more complex routing in boards with high component densities.
How are layers aligned in multilayer PCB fabrication?
Registration is used to precisely align the artwork on multiple layers during multilayer PCB fabrication using fiducials and optical techniques. This ensures interlayer connections match reliably.
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