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. We’ll 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:
- Design – Create circuit schematics and layouts using CAD/CAE software. Performs electrical and thermal simulation.
- Prototype – Initial small production run of the designed board for testing.
- 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.
- 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.
- Testing – Quality testing performed including visual inspection, in-circuit test (ICT), and functional test.
- 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:
- PCB Fabrication – Produce the starting bare printed circuit boards as described previously.
- Stencil/Dispensing – Apply solder paste to the board at pad locations.
- Pick and Place – Robotic machine picks components and precisely places them on the applicable pads.
- Reflow – The board passes through an oven melting the solder paste to form joints and permanently attach components.
- Conformal Coating – Entire assembly is coated with a thin lacquer layer to prevent short circuits.
- Programming – Program any programmable components like microcontrollers with firmware code.
- Testing – Perform electrical testing, burn-in and functional testing.
- Final Assembly – May involve adding connectors, mounting into an enclosure, attaching heatsinks/hardware, etc.
- 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 we’ve looked at what constitutes a PCB and PCBA separately, let’s directly compare them:
| Parameter | PCB | PCBA |
|---|---|---|
| Composition | Fiberglass substrate, copper traces, pads, drill holes, solder mask, silkscreen. | PCB, electronic components, solder connections, conformal coating. |
| Functionality | Provides mechanical structure and electrical connections. | Forms a functional electronic circuit. |
| Design | PCB layout done with CAD software focusing on trace routing, layer stackup, signal integrity. | Schematic capture for connectivity then component placement and routing. |
| Fabrication | Photolithographic copper patterning and etching. | Soldering components onto the PCB either through-hole or surface-mount. |
| Testing | Bare boards tested for continuity, shorts, opens, impedance. | Assembled boards undergo ICT, burn-in, functional test. |
| Uses | Used as the starting base for electronics projects and products. | Used to create functioning electronic devices and systems. |
| Standalone | Cannot function on its own as an electrical device. | Performs self-contained electronic functions. |
| Dependence | PCBA must be assembled onto a PCB. | Can operate independently of further assembly once components are soldered. |
| Cost | Lower cost due to being an unassembled board. | Higher cost due to inclusion of potentially expensive components. |
| Turnaround Time | Faster to fabricate just PCBs since no assembly is involved. | Slower due to the component sourcing, assembly, inspection and test. |
| Designer Role | PCB layout engineers focus on interconnect design. | PCB designers may also select components and develop schematics. |
| Engineer Role | Electrical engineers identify component requirements and connectivity. | Mechanical engineers integrate enclosure, fittings, thermal design. |
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
- 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.
