The Structural Function of a PCB
One of the primary functions of a PCB is to provide a sturdy mechanical structure to securely mount electronic components. The board itself is typically made of a strong, insulating material such as fiberglass. This substrate material provides a stable platform onto which the copper traces and electronic components can be attached.
Common PCB Substrate Materials
| Material | Dielectric Constant | Dissipation Factor | Thermal Conductivity (W/mK) | Tg (°C) |
|---|---|---|---|---|
| FR-4 | 4.5 @ 1 MHz | 0.02 @ 1 MHz | 0.3 | 135 |
| Polyimide | 3.4 @ 1 GHz | 0.002 @ 1 GHz | 0.2 | 250 |
| Teflon | 2.1 @ 10 GHz | 0.001 @ 10 GHz | 0.25 | 327 |
The copper traces and pads are laminated or etched onto the substrate. Components are then soldered to the pads, creating a strong mechanical bond. For through-hole components, the leads are inserted through drilled holes and soldered to pads on the opposite side of the board. Surface mount components are soldered directly to pads on the surface of the PCB.
Multilayer PCBs use additional layers of substrate and copper to allow for more complex interconnections. The layers are laminated together under heat and pressure to create a single sturdy board structure.
The Electrical Function of a PCB
The other main function of a PCB is to electrically interconnect electronic components to create a functional circuit. The copper traces printed onto the PCB substrate act as wires to carry electrical current between components.
PCB Electrical Characteristics
| Characteristic | Description |
|---|---|
| Conductivity | The ability of the copper traces to carry electrical current with minimal resistance. High conductivity is desirable. |
| Insulation | The ability of the PCB substrate and solder mask to prevent current from flowing where it shouldn’t. High insulation is desirable. |
| Impedance | The combined effect of resistance, capacitance and inductance on AC signals. Controlled impedance is important for high-speed circuits. |
The layout of the copper traces is carefully designed to create the necessary interconnections while avoiding unintentional short circuits or crosstalk between signals. A solder mask layer covers the copper traces, exposing only the pads needed for soldering components.
For high frequency circuits, the shape and length of traces is also important to maintain appropriate impedance and avoid reflections. Ground and power planes are used to provide low-impedance current return paths and shielding.
Other Important PCB Functions
In addition to their structural and electrical roles, PCBs fulfill several other important functions:
Thermal Management
As electronic components operate, they generate heat which must be dissipated to avoid damage. The PCB plays a key role in thermal management by providing a path to transfer heat away from components.
Copper planes and wide traces can be used to spread heat across the board. Thermal vias conduct heat through the board to heat sinks on the opposite side. For high power components, metal core PCBs with a thick aluminum or copper substrate are used to enhance heat transfer.
Electromagnetic Compatibility
Electronic devices must avoid emitting electromagnetic interference (EMI) which could disrupt other devices, while also not being susceptible to EMI from external sources. PCB design is critical to ensuring electromagnetic compatibility (EMC).
Proper grounding and shielding techniques are used to contain EMI. Sensitive signals are routed away from potential sources of interference. Filters and shielded connectors also help achieve EMC compliance.
Protection and Safety
PCBs also serve to protect users from potentially hazardous voltages or currents inside the device. Proper layout and insulation techniques prevent users from accidentally coming into contact with dangerous electrical energy.
Fuses and other overcurrent protection devices are integrated into the PCB to safeguard against fault conditions. High voltage areas are segregated and marked to avoid unintentional contact during assembly or maintenance.
Advanced PCB Functions
As electronic devices become more complex and sophisticated, so do the functionalities that PCBs are called upon to perform. Some examples of advanced PCB functions include:
Embedded Components
To save space and improve performance, some components like resistors, capacitors and antennas can be embedded within the layers of the PCB, rather than mounting them externally on the surface. This requires advanced manufacturing processes but allows greater miniaturization and reliability.
Flex and Rigid-Flex
For devices with constrained space or movable parts, traditional rigid PCBs may not be suitable. Flexible PCBs can be bent and folded to fit into tight spaces. Rigid-flex PCBs combine the best of both, with rigid sections for mounting components and flex sections to make connections in between.
Optical Interconnects
As data rates continue to increase, the limitations of electrical signals over copper become more problematic. To overcome this, some advanced PCBs now incorporate optical interconnects using fiber optics or waveguides to transmit signals using light. This enables much higher bandwidth and resistance to EMI.
Frequently Asked Questions
What are the main functions of a PCB?
A printed circuit board serves two main functions:
1. To mechanically support electronic components.
2. To electrically interconnect those components to create a functional circuit.
What is a PCB made of?
A PCB is typically made of a insulating substrate material like fiberglass, with copper traces and pads laminated or etched onto the surface to create interconnections. Other materials like silkscreen, solder mask, and surface finishes are also applied.
What are some advanced functions PCBs can provide?
Some advanced functionalities that PCBs can incorporate include:
– Embedded components within the board layers
– Flexible or rigid-flex circuits for movable or constrained designs
– Optical interconnects for high-speed data transmission
– Thermal management for heat dissipation
– Electromagnetic shielding for interference protection
How does a PCB provide protection and safety?
A well-designed PCB protects users from hazardous voltages and currents in several ways:
– Proper insulation and spacing prevent accidental contact with energized conductors
– Fuses and other overcurrent devices are included to prevent fault conditions
– High voltage areas are segregated and clearly marked
– Grounding techniques safely direct any fault currents away from users
What is the difference between a through-hole and surface mount PCB?
In through-hole construction, component leads are inserted into drilled holes in the PCB and soldered to pads on the opposite side. In surface mount construction, components are soldered directly to pads on the same side of the board, without penetrating through holes. Surface mount allows for smaller components and higher density.
