Printed circuit boards (PCBs) are essential components in most electronic devices. They provide the mechanical structure to mount and interconnect electronic components using conductive pathways or traces etched from copper sheets laminated onto a non-conductive substrate. PCBs allow complex electronic circuits to be condensed into much more compact forms than wiring components discretely.

The PCB design and manufacturing process consists of several steps – circuit schematic capture, PCB layout, design checking, manufacturing and assembly. In this comprehensive guide, we will go through the PCB making process step-by-step, from designing the board layout to sending gerber files for fabrication.

PCB Design Steps

The design phase is critical to creating a functional PCB and can be broken down into the following key stages:

Schematic Capture

This involves creating a circuit diagram containing the electronic components and their interconnections. Various electronic design automation (EDA) software tools are available for drawing schematics, with popular options being:

• Altium
• Eagle
• KiCad
• OrCAD
• Proteus

The schematic should contain all the components and nets required for the circuit, along with details like reference designators, values, ratings etc. It provides the connectivity map for PCB layout.

PCB Layout

This translates the schematic into the physical PCB layout, representing components as footprints/land patterns. EDA tools allow the PCB outline and dimensions, component locations, routing and other features to be defined. Some layout guidelines include:

• Partitioning the circuit into appropriate sections
• Placing components based on connectivity
• Routing priority for critical signals
• Maintaining clearance between tracks and pads
• Allowing sufficient board real estate for component placements and routing

Some layout tips:

• Group related components together
• Place connectors on board edges
• Ensure adequate spacing between conductors
• Use 90° angles on traces as much as possible

Design Rule Checking

This verifies the PCB layout against specified geometric constraints for manufacturability. Design rules may include:

• Minimum clearance between conductors
• Minimum annular ring size for pads
• Minimum trace width and spacing
• Aspect ratios on drilled holes

Design rule checks help avoid common errors like trace overlap or insufficient spacing between adjacent copper features. They ensure the board can be fabricated without electrical shorts or open circuits.

Generating Manufacturing Files

Once the PCB layout is finalized, the EDA tool is used to generate the files needed for board fabrication and assembly, including:

• Gerber files – for photo plotting copper layers
• Drill files – for numerically controlled (NC) drilling
• Bill of materials (BOM) – component list for assembly
• Centroid file – for automatic component placement

PCB Manufacturing Process

Professional PCB manufacturers typically go through the following sequence of steps for fabrication, depending on the board complexity and layer count:

Prototype vs Production

Low volume prototype PCBs may be fabricated individually using manual processes whereas volume production PCBs are manufactured in panel form for better efficiency.

Sourcing Materials

The laminate, copper foils, soldermask and other materials are sourced by the PCB company. Common substrates include FR-4 glass epoxy, Rogers, ceramic and polyimide.

Imaging

The gerber files are used to image or photo plot the land pattern artwork onto the copper layers for the various PCB layers. Photoresist is coated, exposed to UV through the gerber patterns and then developed. This forms a protective mask on the copper surface.

Etching

The exposed copper without resist coating is etched away chemically, leaving only the desired conductive pattern on the PCB layers. Common etching techniques include conveyorized spray etching and print/etch processes.

Lamination

The individually etched innerlayers are stacked up in the designed sequence along with pre-preg bonding sheets. This board stack is laminated under pressure and temperature to bond the layers together.

Drilling

Holes are precision drilled for mounting and interconnecting components. Excellent registration is critical to ensure vias connect the correct pads on each layer. CNC drilling machines with optical alignment are commonly used.

Plating and Coating

The walls of drilled holes are plated with copper to provide conduction between layers. Other coatings like solder mask, silkscreen, gold fingers etc. are applied as per the board finish requirements.

Routing and Scoring

Individual PCB outlines are routed from the panel using carbide bits or lasers. Profiling and V-scoring may be employed for boards requiring post-assembly depanelization.

Electrical Testing

Finished boards undergo testing to verify electrical connectivity and isolate any manufacturing defects before shipment. Testing methods include flying probe, fixture-based and ICT (in-circuit test).

Final Finish

A protective coating like ENIG (electroless nickel immersion gold) may be applied to exposed copper pads to prevent oxidation. The boards are then electrically tested, inspected and packed for delivery.

PCB Assembly Process

After receiving fabricated bare boards from the PCB manufacturer, electronic components are mounted and soldered to assemble the finished circuit board assembly (PCBA). This is either done manually or using automated assembly processes for volume production.

Some key SMT (surface mount technology) assembly steps are:

Paste Printing

Solder paste is stencil printed onto pads on the PCB to provide adhesive and metallization for surface mount device (SMD) attachment. Squeegeeing pushes paste through stencil apertures.

Component Placement

SMDs are accurately placed onto the solder paste deposits on the board using automated pick and place machines. High speed equipment can populate thousands of components per hour.

Reflow Soldering

The populated board passes through a reflow oven with temperature profiles designed to melt the solder paste and attach components. The solder joints then cool down and solidify to form both mechanical and electrical connections.

Cleaning and Inspection

Post reflow flux residue is cleaned using systems like batch immersion or inline spray. Assembled boards are then optically inspected for defects and touchup rework is done if needed.

Conformal Coating

A protective polymer coating may be applied over the assembled board to prevent corrosion or dendrite growth during operation. It also helps provide insulation and mechanical support.

Functional Testing

Electrical tests validate PCBA functionality. Continuity checks, in-circuit tests and functional panel tests are commonly implemented to catch assembly process defects.

Automated assembly offers high throughput, repeatability and quality for medium to high volume production. Critical, low volume or highly complex boards may use manual assembly techniques.

Key PCB Design and Manufacturing Considerations

Here are some key factors to consider during the PCB design and fabrication process:

Layer Stackup

The number of conductive layers, substrate materials and their sequencing in the layer stackup impacts cost, weight, thickness and other PCB characteristics.

Board Shape and Dimensions

The PCB outline format (circular, rectangular, polygonal etc), dimensions and number of boards per panel influence cost. Allow tolerance for manufacturing variations.

Component Density

High component densities affect board cost due to challenges like complex routing or requirement for thinner conductors and dielectrics.

Copper Weight

Copper thickness (oz/ft2 or thickness in microns) impacts conductor current capacity. Thicker copper requires longer etching.

Hole Sizes and Tolerances

Plated through hole reliability depends on hole walls having sufficient annular ring for copper contact. Tighter hole tolerances increase cost.

Fine Line Traces/Spaces

Narrow trace/space dimensions drive up cost. Match trace widths to current requirements since excess copper increases material cost.

Controlled Impedance

Maintaining designed trace impedance requires precise dielectric thicknesses, copper weights and lamination pressure. This increases fabrication complexity and cost.

High Frequency Boards

Special dielectric materials with tight thickness tolerances are needed to manage loss and skew for high speed signals. This affects PCB cost.

Flex/Rigid Flex

Specialized flexible substrates and adhesives are used. Careful process control is needed to avoid flex damage during handling and assembly.

Board Finishes and Coatings

ENIG, immersion silver, solder mask, silkscreen and other special finishes impact board price.

Fabrication Technology

Photochemical etching can support finer features than traditional print-and-etch but has higher costs. Laser direct imaging improves resolution.

Testing and Inspection

100% electrical testing of traces adds cost but ensures quality boards. Automated optical inspection can catch physical defects early.

Lead Time and Quantity

Low volume prototype boards typically have higher per unit costs and longer lead times compared to volume production runs.

PCB Design Tips and Best Practices

Here are some tips for optimizing PCB design:

• Simplicity – Avoid unnecessary complexity in the circuit, component placements and routing. Simpler layouts increase yield and reduce cost.
• Supply Decoupling – Use adequate local decoupling capacitors for each IC to suppress supply noise.
• Return Paths – Ensure every signal has an adjacent return path for controlled impedance and EMI control.
• Component Footprints – Follow datasheet recommendations but allow some tolerances in case larger/smaller substitutes are needed.
• High Speed Routing – Minimize discontinuities, stubs, right angles. Avoid adjacent vias. Match trace lengths.
• Thermal Management – Ensure components are not overloaded and have adequate copper area for cooling. Add thermal reliefs to pads if needed.
• Test Points – Include test/probe points to validate key nodes, supplies, signals.
• Board Outline – Allow sufficient clearance between board edge and components/traces. Add tooling holes for assembly.
• Fab and Assembly – Design for ease of manufacturability. Avoid placements that hinder PCB fab, component mounting or hand soldering.
• Documentation – Maintain schematics, BOM, assembly drawings, notes and other documentation to accompanies the design files.

Following best practices during PCB layout minimizes errors, avoids unnecessary cost and helps accelerate the design process.

Frequently Asked Questions

What are the different types of PCBs?

Some common PCB types are:

• Single sided – Conductors on one side only
• Double sided – Conductors on both sides
• Multilayer – Three or more conductive layers laminated together
• Rigid – Built on rigid fiberglass substrates like FR4
• Flexible – Uses flexible polymer substrates
• Rigid-flex – Combines both rigid and flexible substrates

What design software tools are used for PCB design?

Some popular PCB design EDA tools used for schematic capture and layout are Altium Designer, Cadence Allegro, Mentor Graphics Xpedition, OrCAD, KiCad, Eagle, DipTrace etc.

What are the typical PCB substrate options?

Common rigid PCB substrate cores include FR-4 glass epoxy, high Tg FR-4, Rogers, polyimide, Arlon, Isola, cyanate ester. Flex PCB substrates include polyimide, PET, PEN etc. Ceramic substrates are also sometimes used.

What are the different kinds of PCB finishes and coatings?

Common finishes include:

• HASL – Hot air solder leveling
• ENIG – Electroless nickel immersion gold
• Immersion silver
• OSP – Organic solderability preservative
• Liquid photoimageable solder mask
• Silkscreen print

How are PCBs electrically tested?

Some PCB electrical test methods include:

• Flying probe testing
• Fixture based board level testing
• Bed of nails testing
• ICT (in-circuit test)
• Functional panel testing
• Boundary scan

How are assembled PCBs protected from harsh environments?

Conformal coatings help protect assembled PCBs from moisture, dust, chemicals and temperature extremes. Typical conformal coatings include acrylic, polyurethane, silicone and parylene.

This covers the major steps and considerations when designing, fabricuring and assembling PCBs. With careful planning and execution, high quality boards suited for the target application can be successfully manufactured.