Electronic Manufacturing Terminology: What is the Difference Between PCB and PCBA?

PCB and PCBA are related to each other. They are both part of the electronic manufacturing process and while some may want to use them interchangeably, they are actually different from one another. A PCB is a printed circuit board, while a PCBA is the process of assembling the components for the board to function as needed for its intended application.

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What is a PCB?

PCBs are a crucial element of all electronic devices, and mechanically support and electrically connect the electronic components. The boards themselves are typically made of fiberglass or a composite epoxy. The boards contain the conductive pathways that connect the individual parts of the board and are printed onto the board according to the design. These printed circuits and conducting materials are typically made of copper and coated with tin-lead to avoid oxidation. PCBs are used in consumer electronics, heavy machinery, robotics, vehicles, and medical equipment.

Types of PCBs

• Single-Layer PCBs

Single-layer PCBs are the simplest type. They are coated with a single layer of conducting material and a layer of solder. The components of the PCBs are marked with a silkscreen. Because they are simpler, they are less expensive and easy to design and build. They can often be ordered in high volumes without issue. However, the single layer may not be sufficient for devices that require a higher number of components and connections.

• Double-Layer PCBs

These PCBs have a layer of conducting material on both the top and the bottom on the board, giving them more flexibility and a smaller size than a single-layer PCB. The conductive material on either side of the board is connected via through-holes or surface mounts.

• Multi-Layer PCBs

Multi-layer printed circuit boards have more than two conductive layers. Because of the increase in space due to the multiple layers, they are best used in complex applications. They typically contain an even number of boards to avoid problems such as warping. Because of the increased circuit density, multi-layer PCBs are more powerful, more durable, and can be smaller and more lightweight. However, the cost to produce a multi-layer PCB is higher, requiring more time and skill to design and build.

• Flexible PCBs

Flexible PCBs are defined by the flexible nature of their construction and can be single, double, or multi-layer. Flexible PCBs are built to withstand being flexed repeatedly on a regular basis and are found in so many modern conveniences like laptops, cell phones, and robotic arms.

• Rigid PCBs

Rigid PCBs are also able to be single, double, or multi-layer, but use a rigid material. Once a rigid PCB is manufactured it cannot be modified into any other shape.

• Flex-Rigid PCBs

The circuits consist of multiple layers in a combination of rigid and flex boards with a higher component density.

What is PCBA?

PCBA, which refers to Printed Circuit Board Assembly, is a process, not a product. PCBA refers to the process of assembling the PCB with the necessary components.

PCBA Process

There are two main methods for assembling PCBs.

• Surface-Mount Technology (SMT)

SMT is the highly-automated process of mounting electronic components right to the surface of a PCB, enabling manufacturers to place complex circuits into small components. The steps involved are preparing the PCB, placing the components, reflow soldering, and inspection.

• Through-Hole

Through-hole is an older method, yet creates a stronger connection between the board and the components. This assembly process involves drilling holes into a PCB to attach the electronic components or leads. The steps involve drilling the holes, placing the leads, soldering, and inspection.

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Introduction

Printed circuit boards (PCBs) and printed circuit board assemblies (PCBAs) are crucial components in most modern electronics. While they may seem similar at first glance, there are some key differences between PCBs and PCBAs. This article will examine what each is, how they are made, and why distinguishing between the two matters for designers and engineers. Well also look at some of the pros and cons of PCBs versus PCBAs and when each is the optimal choice. Read on to learn more about these integral parts of electronic devices and equip yourself to choose the right option for your next project.

What is a PCB?

A printed circuit board (PCB) is the physical structure that electrically connects electronic components using conductive tracks, pads and other features. The board is made from sheets of copper laminated onto a non-conductive substrate like FR-4 fiberglass. The copper layer is then chemically etched and processed to produce the traces and pads that connect the components. The components themselves are soldered onto the PCB to complete an electronic circuit.

PCBs provide both the mechanical structure to mount components and the electrical connections between them. They are the platform that powers everything from simple electronics like toys to complex devices like smartphones and computers. Good PCB design is crucial for the reliability, performance and manufacturability of an electronic product.

PCB Composition

A basic PCB consists of the following elements:

• Substrate  The base material that supports the copper layers. Usually a laminate like FR-4 fiberglass but could also be ceramic, Teflon or other materials for high frequency or flexible circuits.
• Copper Layers  Thin copper foil that is laminated onto the substrate, etched and plated to form the conductive parts of the PCB including traces, pads and vias. There can be single, double or multi-layer boards.
• Solder Mask  An epoxy based layer that coats the PCB protecting copper from oxidation and preventing solder bridges between pads.
• Silkscreen  Printed layer for labelling connections and components.
• Finish  Coating like tin, gold or nickel over pads and traces to facilitate soldering and prevent corrosion.
• Drill Holes  Holes drilled through the PCB to mount components and for vias between layers.
• Vias  Plated through holes that provide connections between copper layers.
• Edges  May have connections for inserting into an edge connector or slots/holes for mounting.

PCB Fabrication

PCBs are fabricated using the following general process:

1. Design  Create circuit schematics and layouts using CAD/CAE software. Performs electrical and thermal simulation.
2. Prototype  Initial small production run of the designed board for testing.
3. Fabrication  Panel sizes of raw PCB substrate are coated with copper then the circuit layouts are transferred onto them using a photolithographic process. The unwanted copper is etched away leaving just the desired traces/pads.
4. Assembly  Solder paste is applied to pads and components placed. The boards pass through a reflow oven melting the paste to solder components in place. May also involve automated assembly.
5. Testing  Quality testing performed including visual inspection, in-circuit test (ICT), and functional test.
6. Volume Production  Circuit boards are mass produced once the design is finalized and testing is complete.

There are many details to each step but this gives an overview of producing a finished PCB ready for electronic components to be attached.

What is a PCBA?

A printed circuit board assembly (PCBA) takes the PCB a step further by soldering electronic components onto the board. While the PCB forms the support structure and connections, the PCBA integrates real components to create a functional electronic circuit.

A PCBA consists of:

• PCB  The foundation circuit board described previously. This provides the base on which to assemble components.
• Components  The various electrical and electronic components that perform the desired functions. May include ICs, resistors, capacitors, LEDs, processors, connectors, switches, etc.
• Solder  The solder paste, joints and connections that electrically and mechanically fasten components to the PCB.
• Conformal Coating  A protective insulating lacquer layer over the entire assembly.
• Enclosure  The box or case that houses the PCBA along with any additional hardware or structural parts.

The PCBA takes on its electrical personality based on the specific components soldered onto the PCB. The same PCB design could be populated with different components for different applications. PCBAs encompass the entire functioning electronic circuit rather than just the board alone.

PCBA Assembly

There are two main approaches to assembling PCBAs:

• Through-hole  Leads are inserted through holes in the PCB then soldered on the other side to attach components.
• Surface-mount  Components are soldered to pads on the surface of the PCB without leads passing through holes. Higher density but more difficult assembly.

PCBA assembly involves the following general steps:

1. PCB Fabrication  Produce the starting bare printed circuit boards as described previously.
2. Stencil/Dispensing  Apply solder paste to the board at pad locations.
3. Pick and Place  Robotic machine picks components and precisely places them on the applicable pads.
4. Reflow  The board passes through an oven melting the solder paste to form joints and permanently attach components.
5. Conformal Coating  Entire assembly is coated with a thin lacquer layer to prevent short circuits.
6. Programming  Program any programmable components like microcontrollers with firmware code.
7. Testing  Perform electrical testing, burn-in and functional testing.
8. Final Assembly  May involve adding connectors, mounting into an enclosure, attaching heatsinks/hardware, etc.
9. Packaging  Unit packaging and shipping preparation.

This describes the general PCBA sequence. There are additional steps like AOI inspection, cleaning, and repair that may be involved depending on the specific assembly facility and process.

Differences between PCBs and PCBAs

Now that weve looked at what constitutes a PCB and PCBA separately, lets directly compare them:

Summarizing the differences:

• PCBs form the physical platform to mount and interconnect electronic components.
• PCBAs include soldered components to create a functioning electronic circuit.
• PCBs involve fabrication of the bare boards while PCBAs also require component assembly.
• PCBs are only part of the electronics package while PCBAs can operate as standalone devices.
• Designers focus on interconnect routing for PCBs and schematic/component selection for PCBAs.
• PCBAs take longer to produce but result in an assembled electronic product.

PCB vs. PCBA Design and Fabrication Considerations

Understanding the distinction between PCBs and PCBAs helps guide designers and engineers make the right decisions when developing electronics. Some key considerations include:

When to use PCBs

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• For base platforms to assemble components onto
• Building modular circuit blocks for larger systems
• High volume commodity boards without components
• Boards sold to consumers/hobbyists for customization
• When ease of fabrication is a priority

When to use PCBAs

• Creating functioning electronic devices and products
• Faster time-to-market is critical
• For high complexity boards with many components
• Embedding programmable controllers like microprocessors
• To control quality and component selection

PCB Design Factors

• Component placement accessibility
• Routability of high speed signals
• Impedance control and matching for traces
• Number of layers to reduce cross-talk
• Thermal design to dissipate heat
• Mechanical mounting points and stiffness

PCBA Design Factors

• Sourcing and selection of components
• Placement for efficient assembly
• Cost optimization of expensive components
• Thermal design with heat sinks
• Stress analysis of solder joints
• Conformal coating coverage

Fabrication Options

• Lower costs but longer lead times going overseas
• Domestic boards have quicker turns but cost more
• Hand assembly vs. automated assembly processes
• Level of post assembly testing from basic to intensive
• Different PCB substrate materials have pros and cons

PCBA Manufacturing Contracts

Many companies turn to electronics manufacturing services (EMS) providers to fabricate and assemble PCBAs rather than doing it entirely in-house. The stages of the typical EMS engagement include:

Design Create manufacturing-friendly PCBA designs working closely with the EMS engineering team.

Documentation Provide complete documentation including bill of materials (BOM), assembly drawings, test specifications, etc.

NRE Initial non-recurring engineering (NRE) costs for tooling, programming, stencils, etc.

Prototypes Low volume initial run used for validation before full production.

DFM Design for manufacturability suggestions from EMS to optimize yield and reduce cost.

Pre-Production Small pilot batch with full testing to confirm process and quality.

Volume Production Mass production runs shipped out to customer while monitoring quality.

Change Management Engineering change order (ECO) process to handle component changes, upgrades, etc.

Post-Production Support Handle replacements, repairs, and even end-of-life redesigns or last time buys.

Choosing the right EMS vendor depends on capabilities, location, costs, quality certifications, sophisticated prototyping vs. high volume production, and other factors.

Summary/Conclusion

While related, PCBs and PCBAs fulfill very different roles in electronic products. PCBs provide the physical base for mounting components while PCBAs deliver functioning assembled circuits. Understanding the composition, design, fabrication, testing and applications of PCBs versus PCBAs helps engineers make the optimal choice. Considering manufacturing factors like cost, quality, and lead time is also important when deciding between in-house or outsourced production. Whether you need bare boards or fully loaded devices, keep these differences in mind as you architect your next electronics project.

Frequently Asked Questions

Q: What are some key differences between through-hole and surface mount PCBA assembly?

A: Some of the main differences include:

• Component Size  Surface mount parts are generally smaller with leads on the bottom rather than through-hole pins. Allows higher density.
• PCB Design  SMT needs pads on outer layers while through-hole requires drilling plated through holes.
• Assembly  Through-hole is inserted by hand or with simple automatons vs SMT using precise pick-and-place machines.
• Soldering  Through-hole assembled with selective hand soldering compared to SMT reflow ovens.
• Inspection  Through-hole permits visual inspection of solder joints but SMT requires x-ray inspection.
• Rework  Replacing defective through-hole parts easier than removing and replacing SMT components.
• Cost  Through-hole assembly costs lower for small volumes but SMT is preferable for high volumes.

Q: What are the typical layers in multilayer PCBs?

A: Common layer stacks in multilayer PCBs include:

• 4 layers Top, GND, PWR, Bottom
• 6 layers Top, GND1, PWR, GND2, PWR, Bottom
• 8 layers Top, GND1, PWR1, GND2, SIG1, SIG2, PWR2, Bottom
• 10+ layers Additional GND/PWR planes and signal layers

Key considerations for layers:

• Signal layers for routing high speed traces
• Ground layers to create return paths
• Power layers for power distribution
• Layer order and adjacency to control EMI and cross-talk

High complexity boards may have 20+ layers to accommodate routing density.

Q: What design factors maximize PCBA manufacturability and yield?

A: Some key PCBA design choices to optimize manufacturability include:

• Component selection Prefer easily sourced parts with multiple suppliers
• Placement Group components to reduce pick-and-place machine movement
• Rotate polars Consistent component orientation for automated assembly
• Clearances Adequate spacing between parts and copper for soldering
• Thermals Prevent overheating delicate components during solder reflow
• Test points Include test pads/points for validation and troubleshooting
• Finishes Choose solderable PCB finishes like ENIG rather than hard gold
• Fiducials Add fiducial markers to align and reference components
• Copper relief Reduce large copper pours which can cause tombstoning

Paying attention to these design factors when creating the schematics and layout will maximize manufacturing yield and reduce long term production costs.

Q: What types of testing are performed on PCBAs?

A: Common electrical testing performed on assembled PCBAs includes:

• In-circuit Test (ICT) Checks shorts, opens, component values
• Flying Probe Tests connectivity between nodes
• Boundary Scan Tests interconnects between ICs with JTAG
• Functional Test Validates circuit performs intended function
• Burn-in Stresses boards at high temps over time
• Hipot High potential used to verify dielectric isolation

In addition to electrical testing, other checks include:

• Automated Optical Inspection Verifies component placement, polarity
• X-ray Inspection Looks for hidden defects in solder joints
• Visual Inspection Manual visual examination for any flaws

The specific testing methodology depends on the PCBA complexity, volume, acceptable quality level (AQL) and program budget.

Q: What are some best practices when outsourcing PCBA production?

A: Here are some tips for effectively outsourcing PCBA manufacturing:

• Do thorough due diligence in selecting the EMS vendor based on capabilities, quality, communications, experience, etc.
• Be highly responsive to EMS questions and requests this prevents delays on their end.
• Clearly define test specifications, AQLs, deliverables, etc in the production agreement.
• Make regular site visits to the EMS for process audits and to strengthen the relationship.
• Share long-term forecasts and plans even if not contractual so they can prepare.
• Provide detailed BOM, assembly drawings, fabrication/test documentation to the vendor.
• Seek DFM guidance from the EMS and incorporate recommendations.
• Plan for multiple supply chain contingencies and record last time buy statuses.

Good partnerships with contract manufacturers results in shared success producing high quality PCBAs cost effectively with short lead times.

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