What Happens During PCB Assembly?

ElecSuppliers Electronics GuideAug 19, 2026

PCB assembly is the process of turning a bare printed circuit board into a working electronic assembly. Components are selected and prepared, solder paste is applied, surface-mount components are placed, soldering is completed, and the finished board is inspected and tested.

The exact process depends on the product, component types, board design and required production volume. A simple SMT board may pass through only a few major stages, while a more complex PCBA can require through-hole assembly, multiple soldering processes and several inspection steps.

1. PCB and Production Files Are Prepared

PCB assembly starts before any component is placed.

The assembly manufacturer normally works with the PCB design files, Gerber files, bill of materials (BOM), pick-and-place data and assembly drawings. These files define the board structure, component locations, part numbers and assembly requirements.

The BOM is particularly important because it identifies the exact components required for the assembly. A resistor with the correct resistance but the wrong package, voltage rating or tolerance may not be an acceptable substitute.

The manufacturer may also review the design for manufacturability before production begins.

2. Components Are Prepared

Components are then prepared for assembly according to the BOM and placement data.

For SMT production, components are commonly supplied on reels, trays or other automated packaging. The component package needs to match the footprint on the PCB.

This stage is also where component identification becomes important. Similar-looking parts can have different electrical specifications, so the manufacturer's part number should be checked against the approved BOM.

3. Solder Paste Is Printed

For most SMT assemblies, solder paste is applied to the PCB using a stencil.

The stencil contains openings corresponding to the solder pads that require paste. A printer places a controlled amount of solder paste onto these areas.

Paste volume matters. Too much solder paste can contribute to solder bridges, while insufficient paste can result in weak or incomplete solder joints.

Many production lines use SPI (Solder Paste Inspection) after printing to check paste position and volume before components are placed.

4. Components Are Placed

The PCB then moves to the pick-and-place machine.

The machine picks components from feeders or trays and places them onto the solder-paste-covered pads according to the programmed placement coordinates.

Small passive components can be placed at very high speed, while larger ICs and connectors require accurate orientation and positioning.

At this point, the components are not permanently attached. The solder paste temporarily holds them in position until the board reaches the soldering stage.

5. Reflow Soldering Creates the Electrical Connections

The populated PCB enters a reflow oven.

The board passes through controlled temperature zones that gradually heat the assembly. The solder paste melts during the appropriate stage and forms solder joints between the component terminals and PCB pads.

After reaching the required peak temperature, the board is cooled in a controlled manner.

The resulting solder joints mechanically attach the components and provide the electrical connection between the components and PCB.

6. AOI Checks the Assembly

After SMT soldering, the board may pass through Automated Optical Inspection (AOI).

AOI systems use cameras and image-processing software to inspect component placement and soldering conditions.

Typical problems that AOI can identify include missing components, incorrect component placement, reversed components, offset components and visible solder defects.

AOI is especially useful for high-volume production because it provides consistent inspection across large numbers of assembled boards.

7. Through-Hole Components May Be Added

Not every component can be mounted using SMT.

Connectors, transformers, large capacitors and certain mechanical components may use through-hole technology (THT). Their leads pass through holes in the PCB rather than being mounted entirely on the surface.

Depending on the design, through-hole components can be inserted manually or by automated equipment.

The board may then go through wave soldering or selective soldering to connect the through-hole leads to the PCB.

8. Additional Inspection and Testing

Inspection does not necessarily end with AOI.

More advanced assemblies may require X-ray inspection, especially when solder joints are hidden beneath components such as BGAs.

Electrical testing can also be performed.

ICT (In-Circuit Test) checks specific electrical characteristics and connections on the assembled PCB, while functional testing evaluates whether the completed board actually performs its intended function.

The appropriate test method depends on the product and the customer's requirements.

9. Programming and Final Inspection

Some PCB assemblies require firmware programming before they can be used.

The manufacturer may program the MCU, FPGA, memory device or other programmable component during production testing.

The board can then undergo a final visual inspection, cleaning if required, labeling and packaging.

For products that require traceability, production information may also be recorded against the individual PCB assembly.

Why the Order of PCB Assembly Matters

PCB assembly is not simply a matter of placing components on a board.

Each stage affects the next one.

Poor solder-paste printing can cause placement and soldering problems. Incorrect component placement can result in electrical or functional failures. Inadequate reflow conditions can produce unreliable solder joints.

For this reason, modern PCBA production relies on a combination of process control, automated placement, soldering and inspection rather than relying only on final testing.

A Typical PCB Assembly Flow

A typical SMT assembly process can be summarized as:

PCB preparation → solder paste printing → SPI → component placement → reflow soldering → AOI → THT assembly if required → soldering → testing → programming → final inspection

Not every PCB assembly requires every stage. The actual process is determined by the board design and production requirements.

For a simple SMT board, the process may be relatively short. A mixed-technology board with BGA devices, through-hole connectors and functional testing can require considerably more production steps.

What Determines the PCB Assembly Process?

The assembly process is mainly determined by the PCB design and component selection.

The number of SMT and through-hole components, BGA usage, component size, board thickness, soldering requirements, production volume and testing requirements can all affect the manufacturing process.

This is why two PCB assemblies with similar dimensions can require very different production methods.

Understanding these stages also helps when preparing a PCB assembly project because complete and accurate Gerber files, BOM data, pick-and-place files and assembly drawings give the manufacturer the information needed to build the board correctly.