A printed circuit board (PCB) forms the foundation for building electronic circuits by providing the base for mounting and interconnecting components. PCBs with multiple conductive layers enable increased component density and complex circuit routing. A 3 layer PCB refers to a board with three copper layers separated by insulating dielectric substrates.
This article provides a comprehensive overview of 3 layer PCB technology. We will cover the advantages of 3 layer boards, their construction, common design techniques, applications, and manufacturing considerations. With an understanding of their capabilities and limitations, engineers can effectively utilize 3 layer PCBs in many types of electronic designs.
Advantages of 3 Layer PCBs
Three layer boards provide the following benefits compared to simpler 2 layer PCBs:
- Added routing flexibility – The extra layer allows more interconnectivity and circuit optimization. Parts placement and routing are less constrained.
- Improved signal integrity – The middle layer can be a continuous ground plane. This provides controlled impedance signal routing on outer layers.
- Lower EMI/noise – Enclosing signals between power and ground layers reduces interference and crosstalk.
- Better power distribution – Additional layer allows power-ground plane pairs for each voltage domain.
- Higher component density – Components can be placed on both sides with vertical interconnects through the middle layer.
- Smaller board sizes – Miniaturization by utilizing both sides for SMT parts placement.
- Mixed signal designs – Analog and digital sections can be segregated across layers.
For these reasons, 3 layer construction provides excellent capability and cost-benefit for many electronic products.
Construction of 3 Layer PCBs
The fabrication of 3 layer printed circuit boards involves laminating conductive copper layers separated by insulating dielectric substrates:
- The center substrate is called the core. It provides mechanical support. The default material for core and prepreg layers is typically glass reinforced FR-4.
- A sheet of copper foil is laminated onto both sides of the core. These form the top and bottom conductive layers of the PCB.
- Photolithographic processing patterns the copper layers into the required circuit traces, pads, and features.
- Plated through holes and buried vias provide vertical interconnections between the layers.
- Soldermask selectively coats the copper layers for protection and isolation.
This creates a 3 layer board ready for component assembly. The sequence can be repeated to build multilayer boards.
Typical 3 Layer Stackups
While all 3 layers can be used for routing signals, some standard layer assignments provide good design starting points:
3 Layer With Ground Plane
- Layer 1 – Signals
- Layer 2 – Ground plane
- Layer 3 – Signals
This offers a continuous reference ground plane for controlled impedance routing and shielding.
3 Layer With Split Power Planes
- Layer 1 – Signals
- Layer 2 – Split power planes (VCC and VDD)
- Layer 3 – Signals
Separate power supply domains can be isolated between the split power planes.
3 Layer With Buried Signal Plane
- Layer 1 – Ground
- Layer 2 – Signals
- Layer 3 – Ground
The buried signal layer fully encompasses routing surrounded by ground planes.
The stackup can be customized based on signal isolation, thermal and EMI requirements.
Design Considerations for 3 Layer Boards
Here are some important design practices when working with 3 layer PCBs:
- Split power planes correctly for digital and analog domains based on current levels.
- Use a large number of via stitches to connect split power planes for lowest impedance.
- Assign critical signals to outer layers adjacent to the ground/power planes.
- Route opposing signal polarities on same layer to minimize crosstalk.
- Use diagonal routing over middle layer to change layers when needed.
- Enable thermal relief stitching for highest current paths.
- Maximize copper area on outer layers for best heat dissipation.
- Follow adequate design clearances between planes and traces.
- Model power and ground impedances to avoid resonance and coupling issues.
- Simulate signal integrity and radiated emissions to high frequencies.
Proper 3 layer stackup design and layout techniques result in schematics which transition smoothly through manufacturing.
Typical Applications of 3 Layer PCBs
Here are some examples of products where 3 layer boards are commonly used:
- Consumer electronics – IoT devices, smart home gadgets, wearable tech
- Vehicles – Auto infotainment panels, GPS display units
- Industrial – PLCs, motor controllers, sensors
- Instrumentation – Meters, analyzers, handheld testers
- Medical – Diagnostic equipment, body-worn monitors
- IT – Ethernet switches, modems, routers
- Communications – Radio transceivers, video gear
3 layer boards balance cost, complexity and performance for mid-range applications. The proliferation of digital electronics drives large volumes of 3 layer PCBs today.
3 Layer PCB Manufacturing Overview
3 layer PCB fabrication in volume involves the following key steps:
- Materials – Core substrates, prepreg, copper foils
- Imaging – Photoresists, direct laser/mechanical patterning
- Lamination – Stacking layers under heat and pressure
- Etching – Chemically etching away unwanted copper
- Drilling – Machines drilling holes for vias and mounting
- Plating – Electroplating copper over hole walls and surfaces
- Solder mask – Liquid photoimageable solder resist layers
- Silkscreen – Printed reference markings
- Testing – Electrical testing, quality inspection
- Assembly – SMT component placement and soldering
High-yielding fabrication lines enable cost-effective mass production of 3 layer boards.
With three conductive layers to work with, PCB designers have sufficient flexibility to route out interconnects and partition domains without undue complexity. 3 layer boards offer excellent capability per unit cost, enabling their ubiquity across industrial and consumer electronics. Advances in PCB materials, fabrication equipment and assembly technologies will continue to improve technical features, density and reliability while reducing manufacturing costs.
Frequently Asked Questions
Here are some common questions about 3 layer PCBs:
What are the typical substrate thicknesses used in 3 layer boards?
Standard cores are commonly 0.8mm, 1.0mm, 1.6mm while prepregs range from 0.1mm to 0.25mm. Overall thickness is usually between 1.6mm to 2.4mm.
What are the minimum track/spacing dimensions achievable on 3 layer boards?
With processes like direct imaging, trace/space down to 125um (5 mils) is routinely achievable on outer layers.
What are common 3 layer PCB sizes?
Smaller boards of around 50x50mm to 160x100mm are typical. Large boards up to 460x360mm area are also manufactured cost-effectively.
What are the limitations of 3 layer boards compared to 4+ layer ones?
Design constraints in routing congestion, inability to isolate multiple signals, lower component density and lack of flexibility for high pin-count parts.