What is the PCB design?

The Basics of PCB Design

At its core, PCB design is about connecting electronic components together to create a functional circuit. The main steps in the PCB design process are:

  1. Create a schematic diagram that defines the components and their connections
  2. Design the physical layout of the PCB, placing components and routing traces
  3. Generate manufacturing files for fabrication and assembly of the PCB

PCB design requires knowledge of electronics, material properties, manufacturing processes, signal integrity, and design for manufacturability (DFM) principles. The designer must balance competing priorities like performance, cost, reliability, and time-to-market.

PCB Layers and Materials

PCBs are constructed of multiple layers of copper and insulating substrate material laminated together. The copper layers are etched to form traces, pads, and planes for connecting components. Common PCB materials include:

Material Typical Applications
FR-4 Most common, low-cost PCB substrate
High Tg FR-4 Improved thermal stability vs. standard FR-4
Polyimide High temp, flexible circuits
Ceramic RF/microwave, high power
Metal core LED lighting, power electronics

The number of copper layers in a PCB can range from 1 to 50+ depending on the complexity of the circuit. More layers allow for higher component density but increase manufacturing cost. Typical low-cost PCBs use 1-4 layers, while high-density interconnect (HDI) PCBs may use 8-16 layers.

Component Selection and Placement

Selecting the right components is critical for meeting the electrical and mechanical requirements of the design. Key factors to consider include:

  • Electrical ratings (voltage, current, power, frequency, etc.)
  • Package type and dimensions
  • Cost and availability
  • Quality and reliability
  • Thermal characteristics
  • Manufacturability (e.g. ability to be machine-placed)

Placing components on the PCB is both art and science. A good placement achieves several goals:

  1. Minimizes distance between connected components to reduce trace length and improve signal integrity
  2. Groups related components together functionally
  3. Provides adequate spacing for assembly and rework
  4. Considers airflow and cooling needs of heat-generating parts
  5. Keeps sensitive analog/RF circuits away from noisy digital circuits
  6. Places connectors and controls for user accessibility
  7. Meets mechanical constraints and form factor

pcb layout tools provide automatic and interactive component placement features to help optimize the design.

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PCB Routing and Signal Integrity

Once components are placed, they must be connected together by routing traces on the copper layers. PCB routing is governed by design rules that specify parameters like:

  • Minimum trace width and spacing
  • Minimum drill hole size for vias
  • Pad and via geometries
  • Clearances to board edge and mounting holes
  • Copper pour (ground plane) clearances

Traces must be sized appropriately for the current they will carry. Wider traces have lower resistance but consume more board space. Calculations or simulation can determine the proper trace widths.

Signal integrity (SI) analysis ensures that signals arrive at their destination with sufficient quality and timing. SI problems like crosstalk, reflections, and electromagnetic interference (EMI) become more acute at higher speeds. Some signal integrity considerations in PCB routing include:

  • Controlling impedance of traces to match driver/receiver
  • Proper termination of transmission lines
  • Minimizing crosstalk by separating sensitive traces
  • Providing uninterrupted return paths for high-speed signals
  • Avoiding stubs and unterminated traces
  • Minimizing loop area of current paths to limit EMI

Length matching is important for routing certain nets like high-speed memory buses or differential pairs. All traces in a matched group must be the same length to ensure signals arrive simultaneously. Length matching can be accomplished through serpentine routing patterns or using manual/automatic tuning features in the PCB layout tool.

Power Integrity and Grounding

Power integrity ensures that clean, stable power is delivered to all devices on the PCB. Careful PCB layout is essential to minimize the impedance between the power supply and load. Typical techniques include:

  • Using large copper pour areas (planes) for power distribution
  • placing decoupling capacitors close to ICs
  • Sizing traces for acceptable voltage drop
  • Using multiple vias to reduce path impedance

Grounding is closely tied to power integrity. A low-impedance ground return path must be provided for every signal. Ground planes are preferable to traces as they minimize ground loops and provide consistent reference potential. On two-layer PCBs, a combination of ground pours and traces can be effective.

Split planes are sometimes used to provide separate analog and digital grounds to minimize coupling between them. The two ground regions should only be connected at a single point to avoid ground loops.

Thermal Management

Thermal management is the process of removing heat from components to keep them within safe operating limits. Excessive temperature can cause premature failure or degraded performance. Some PCB thermal management techniques include:

  • Providing adequate copper area for heat sinking
  • Using thermal vias to conduct heat into internal copper layers or planes
  • Specifying larger pad sizes or thicker copper for high-power parts
  • Placing hot components near board edges or away from temperature-sensitive parts
  • Considering airflow direction from fans or system enclosure
  • Using heatsinks, heatpipes, or metal core substrates for extreme cooling needs

Thermal simulation tools can predict component temperatures and identify hot spots that need additional cooling.

Design for Manufacturing (DFM)

DFM practices help ensure a design can be reliably and economically manufactured. Involving the PCB fabricator and assembly shop early in the design process is beneficial to assess manufacturing feasibility.

Some key DFM considerations:

  • Specifying reasonable minimum trace/space geometry for the pcb manufacturer’s process capability
  • Minimizing the number of unique hole sizes and sizes below 15 mils
  • Using appropriate solder mask clearances and expansions
  • Providing adequate space for assembly and test points
  • Panelizing boards for efficient assembly
  • Following reflow soldering guidelines and providing solder paste stencil data
  • Minimizing component variety to simplify inventory and assembly

Design rule checking (DRC) features in PCB layout tools help verify that manufacturing constraints are met before releasing the design to production.

Documentation and Manufacturing Outputs

pcb design software can generate the files needed for manufacturing once the layout is complete. Typical outputs include:

  • Gerber files for each layer (copper, solder mask, silkscreen, etc.)
  • NC drill files
  • Electrical test point files
  • Bill of materials (BOM)
  • Assembly drawings
  • 3D visualization files

Well-organized documentation is important for communicating design intent and requirements to the manufacturing team. Investing time to provide clear fabrication and assembly instructions can prevent costly mistakes.

Conclusion

PCB design is a complex, multi-disciplinary field that requires knowledge of electronics, manufacturing, signal integrity, and thermal management. By following good design practices and collaborating with manufacturing partners, designers can create high-quality PCBs that meet performance, reliability, and cost goals.

Frequently Asked Questions

What software is used for PCB design?

Popular PCB design software includes Altium Designer, OrCAD, Eagle, KiCad, and Fusion 360. These tools provide schematic capture, layout, routing, and manufacturing output generation features. Some are easier to learn while others offer more advanced capabilities.

How much does it cost to design and manufacture a PCB?

The cost of PCB design and manufacturing varies widely depending on factors like complexity, layer count, size, material, quantity, and turnaround time. Simple 2-layer PCBs might cost a few dollars each in volume, while complex multilayer boards can cost hundreds of dollars. NRE (non-recurring engineering) costs for design and setup can range from hundreds to thousands of dollars. Obtaining quotes from multiple PCB vendors is recommended.

What are some common PCB manufacturing defects?

Some common PCB manufacturing defects include:

  • Shorts or opens due to overetching, underetching, or debris
  • Drill misregistration or inaccurate hole sizes
  • Incorrect solder mask alignment or coverage
  • Insufficient copper thickness or trace width
  • Surface contamination or poor solderability
  • Delamination or blistering of layers

Following DFM guidelines, providing clear manufacturing data, and specifying adequate testing can minimize the occurrence of defects.

What is the typical turnaround time for pcb fabrication and assembly?

Standard lead times for PCB fabrication and assembly are 1-2 weeks, but this can vary depending on the supplier and job complexity. Rush services are available for an additional fee, with turnaround times as fast as 24 hours in some cases. Providing complete and accurate design data can help expedite manufacturing.

How do I choose the right PCB manufacturer?

When selecting a PCB manufacturer, consider factors like:

  • Technical capabilities (e.g. layer counts, materials, minimum features)
  • Quality certifications (e.g. ISO9001, UL, IPC)
  • Domestic vs. offshore location
  • Lead times and rush service availability
  • Prototype and volume pricing
  • Design support and customer service
  • Reputation and references

Obtaining quotes from multiple suppliers and asking for recommendations from other designers can help identify the best fit for a given project.