A printed circuit board (PCB) is the foundation of electronics hardware, providing mechanical support and electrical connections for components. Understanding PCB design is key for anyone creating an electronic product, from hobbyists to professional engineers.
This guide covers PCB basics - what they are, how they work, and how to design them. Follow along to learn:
- PCB definition and composition
- PCB types and materials
- Creating schematics
- Laying out PCBs
- Design software options
- Manufacturing and testing
Let's get started demystifying PCB technology!
What is a Printed Circuit Board?
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A PCB mechanically supports and electrically connects electronic components using conductive copper traces etched from copper sheets laminated onto a non-conductive substrate.
Components - ICs, resistors, connectors and other parts are soldered onto pads/holes on the PCB traces to assemble the complete circuitry.
Traces - The conductive copper pathways transfer power and signals between components.
Substrate - The non-conductive PCB base, usually fiberglass or plastic, provides structural rigidity.
Here is a diagram of a simple PCB:
When components are soldered onto the pads and holes, a functioning electronic circuit board is created.
Functional Blocks in PCBs
Although simple PCBs may only route some power and signals, more complex boards are organized into functional blocks.
Common functions integrated:
Power - Converts AC to DC, regulates voltages, protects from spikes.
Processor - Microcontroller or microprocessor chip for programmable logic and control.
Memory - Temporary data storage like RAM and longer-term like flash memory.
Logic - Support chips handling inputs/outputs, computation, timing, peripherals.
User Interfaces - Displays, buttons, switches, touch/gesture sensors.
Communications - Wireless radios like WiFi/Bluetooth and wired ports like USB.
Sensors - Components that measure real-world parameters like temperature, location, images.
These blocks work together when coding brings the hardware functionality to life.
Why are PCBs Needed?
You may wonder why electronics systems don't just use loose wires to connect components instead of rigid printed circuit boards. While possible for simple prototypes, PCBs provide major benefits:
Compact Size - With components soldered closely together and lacking bulky wiring, products shrink to smallest form.
Reliability - The solid mass of solder joints withstands shock/vibration better than individual wires prone to loosening.
Repeatability - Manufacturing thousands of identical boards is easy compared to inconsistent hand-wired units.
Performance - Carefully routed traces optimize high speed signals compared to messy point-to-point cabling.
Safety - Insulated substrate prevents exposed electrical hazards and properly designed boards prevent failures.
For most electronics hardware, the PCB provides the foundation for transforming concepts into functional commercial-grade products.
Types of Printed Circuit Boards
While all PCBs perform the same role of interconnecting components, there is flexibility in materials and layouts. Common categories include:
Single-Sided
As the name suggests, copper traces and pads are only on one side of the substrate material. Often seen in simple low cost circuits.
Double-Sided
Conductive layers on both sides of the board allow more component density and trace routing flexibility. Two-layer boards are most common.
Multi-Layer
High complexity designs stack multiple conductive layers internally allowing intricate routing or separating digital/analog signals. Component density gets very high.
Rigid PCB
The standard type using a solid substrate material that is rigid and inflexible. FR-4 fiberglass is most prevalent but alternatives like ceramic exist.
Flexible PCB
Uses a flexible plastic substrate that can bend and twist. Lightweight for wearables and space savings in multi-axis devices.
Rigid-Flex
Combines both rigid and flexible substrates in a hybrid design. Enables dense components on rigid sections while still allowing flexing.
High-Frequency
Special substrate, dielectric layer, and trace geometry techniques optimize signal integrity at microwave and gigabit frequencies.
Adaptations like these enable PCBs to suit diverse electronics use cases with the best fit for mechanical and electrical needs.
Key Materials in Modern PCBs
Advancements in materials science continuously improve PCB functionality and capabilities. Here are primary constituents:
Substrate (Base) Material
The substrate forms the core foundation upon which all other layers build on. Key characteristics like thermal performance, dielectric properties, strength, and cost are highly dependent on base material choice. Here are popular options:
FR-4 Glass Epoxy – Thermoset plastic reinforced with woven fiberglass for rigid construction. Offers good balance of capability and affordability. Very common.
CEM-1 Cotton Paper - Woven cellulose fibers yield flatness and thermal stability suited to lead-free assembly. Low moisture absorption.
Polyimide – Flexible plastic material capable of high temperatures. Used in flex circuits and exotic applications.
PTFE - Synthetic fluoropolymers like Teflon with excellent electrical performance but very high material costs.
Alumina Ceramic – For high frequency and extreme environments. Cost is 5-10x standard PCBs but can survive over 250°C.
Dielectric Layers
Dielectric layers sandwich between copper foil traces insulating them from nearby signals while tightly controlling trace impedances. Key dielectrics include:
FR-4 Prepreg – Common glass weave prepreg becoming more controlled for faster edge rates.
Isola 370HR - Special laminates like Isola 370HR provide tight thickness tolerances for fine 4 mil traces.
Rogers 4350 – Very consistent impedances through rigid press cycles, critical for multi-GHz data signals.
Rogers RO3000 – Thin laminates suited for lightweight high layer count constructions.
Taconic RF-35 - Extremely low loss tangent material ideal for milimeterwave applications over 70GHz.
Copper Foil
The conductive copper foil bonded to dielectric cores handles power and signals. Varying copper weights suit needs:
1⁄2 oz (17 μm) Copper – Ultra-thin foils minimize rigid flex PCB thickness for compact folds.
1 oz (35 μm) Copper – Standard thickness for cost sensitive consumer boards. Avoid for high current.
2 oz (70 μm) Copper – Robust foil for power circuitry seeing higher amperages and heat levels.
Rolled Annealed Copper – Smooth tempered copper optimized for fine line widths down to 5 mils.
Creating Schematic Diagrams
With background on what PCBs are composed of, let's look at how PCB design starts. The first step is creating circuit schematics to define components and connectivity.
Schematic Capture Process
1. Layout Sheet – The blank canvas where symbols and wires come together.
2. Place Symbols – Drag component symbols like resistors, ICs, connectors onto the sheet.
3. Connect Symbols – Draw wires between symbols to define electrical connectivity.
4. Annotate Design - Add labeled nets, component values, reference designators.
5. Verify Circuit – Validate schematic for errors through designer review and electrical rules checks.
Ideally schematic capture is fast and smooth but designs often iterate by tweaking connectivity as the concept evolves.
Why Schematics Matter
While physically laying out PCB traces is crucial, quality schematics enable effective circuit boards through:
Specifying Components – Part numbers, values, ratings guide parts selection.
Communicating Concepts – Visualizing component interconnections allows discussion.
Facilitating PCB Layout – Coupling symbols electrically steers router autoplacement and autorouting.
Providing Documentation – Serves as the visual record for reworking boards and diagnosing issues.
For anything beyond a simple PCB, the schematic is vital to an efficient design process and clear communication.
Transitioning from Schematic to PCB Layout
Once schematic capture completes, the next phase of the design flow is transforming symbolic connectivity into physical place-and-route on PCB editor layers.
Netlist Extraction
The first step is using the EDA tool to read all component-to-component connections from the schematic and generate a netlist file consolidating this net connectivity data.
The netlist contains lines like:
U1 Pin 5 -> R3 Pin 1 U2 Pin 2 -> C6 Pin 1 -> J2 Pin 3
This nets file gets imported into the PCB editor.
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