The PCB assembly process, step by step
PCB assembly turns a bare board and a set of components into a working PCBA. Solder paste is printed onto the pads, surface-mount parts are placed by machine and soldered in a reflow oven, and the result is inspected. Through-hole parts are added, the board is cleaned or coated if required, and it is tested. This guide explains each step and what the assembler needs from you.
Inputs: what the assembler needs
Assembly starts with data. A PCBA job normally needs four things:
- Gerber files, including the paste layers used to make the stencil.
- A bill of materials (BOM) with reference designators, quantities, manufacturer names and manufacturer part numbers.
- A pick-and-place file (centroid file) with X/Y position, rotation and side for every placed part.
- An assembly drawing showing polarity, orientation, do-not-place parts and any special instructions.
If one of these is missing or inconsistent, the job stops at the first step until it is resolved.
The assembly steps
- DFA review and data preparation. The assembler checks design for assembly: footprints, component spacing, polarity marks, fiducials and panel format. The BOM is cross-checked against the pick-and-place file and Gerbers, and machine programs are prepared.
- Component sourcing and kitting. In turnkey assembly the assembler buys the parts; in consigned assembly you supply them. Either way, parts are counted and kitted per job. Long-lead or obsolete parts can be handled through component sourcing.
- Solder paste printing with a stencil. Solder paste is printed through a laser-cut stainless-steel stencil onto the SMT pads.
- Solder paste inspection (SPI). A 3D inspection system measures paste volume, height and position on each pad. Printing causes a large share of soldering defects, so catching them here is efficient.
- Pick-and-place. SMT machines pick components from reels, trays or tubes and place them on the paste using the centroid data, with vision systems correcting for position.
- Reflow soldering. The board passes through a reflow oven with a controlled temperature profile that melts the paste and forms the joints. Double-sided boards go through the process twice.
- AOI. Automated optical inspection checks for missing, shifted, rotated or wrong-polarity parts and visible solder defects.
- X-ray for BGA/QFN. BGA and QFN joints are hidden under the package, so X-ray inspection is used to check for bridges, opens and voiding.
- THT insertion and wave or selective soldering. Through-hole parts such as connectors and large capacitors are inserted and soldered by wave soldering, selective soldering or by hand.
- Cleaning. Flux residues are removed where the process, the flux type or the end application requires it.
- Conformal coating (optional). A protective conformal coating is applied for boards exposed to moisture, dust or chemicals, with keep-out areas masked.
- Testing. Depending on the specification: in-circuit test (ICT), flying probe or a functional test that powers the board and checks its behaviour.
- Final inspection to IPC-A-610, ESD-safe packing. Boards are inspected against IPC-A-610 for the agreed class and packed in ESD-safe packaging.
Turnkey, consigned or partial turnkey
How components reach the assembly line affects cost, risk and how much work stays with you.
With turnkey assembly, the assembler manages sourcing, storage and moisture-sensitive handling, and takes responsibility for the parts it buys. With consigned parts, you remain responsible for correct part numbers, packaging, quantities and condition.
Whichever model you choose, a clean BOM with manufacturer part numbers and approved alternates is the single most useful document you can provide. It lets the assembler check availability early and propose substitutes before a missing part holds up the build.
Moisture-sensitive components deserve special attention. Parts such as BGAs and many plastic ICs absorb moisture from the air and can crack during reflow if they are not stored and baked correctly. Assemblers manage this for the parts they buy; if you consign parts, keep them sealed in their original dry packaging until they are delivered.
What slows assembly down
Most assembly delays come from the inputs, not the machines:
- BOM errors: missing manufacturer part numbers, wrong packages, quantities that don't match the reference designators, or descriptions that allow several interpretations.
- Missing or wrong pick-and-place data: rotations that don't match the footprint library, parts on the wrong side, or a file that doesn't match the latest Gerbers.
- Unavailable or obsolete parts: a single part on allocation can hold up the whole build. Approved alternates on the BOM help, and so does checking availability before the design is frozen. If a part is going end-of-life, a last-time buy can secure stock for future builds.
- Unclear polarity marking and no assembly drawing, which leads to questions during first-article inspection.
A first article inspection on the first board of a new build catches remaining issues before the full batch runs.
Prototype vs volume assembly
Prototype and volume assembly use the same basic process, but the priorities differ.
Moving from prototype to production is a good moment to revisit DFA, panel format and test strategy.
Need boards assembled? Send your Gerber files and BOM for a free PCB assembly quote from the right assembly partner.
PCB assembly →FAQ
How do you assemble a PCB?
Solder paste is printed onto the bare board through a stencil and inspected. A pick-and-place machine then mounts the surface-mount parts, and the board passes through a reflow oven that melts the paste into solder joints. After optical and, where needed, X-ray inspection, through-hole parts are added by wave, selective or hand soldering. The board is then cleaned or coated if required and tested.
How long does PCB assembly take?
It depends mainly on component availability, the complexity of the board and the quantity. When all parts are in stock and the data is complete, the assembly itself is a relatively short part of the job. Sourcing long-lead or obsolete parts, double-sided placement, BGAs that need X-ray and extensive testing all add time.
Which files do I need for PCB assembly?
You need the Gerber files including paste layers, a BOM with manufacturer part numbers and reference designators, and a pick-and-place file with positions, rotations and board side. An assembly drawing showing polarity and special instructions is strongly recommended. If the board is fabricated in the same order, include the drill files, stack-up and fabrication notes as well.
Can I supply my own components?
Yes. This is called consigned assembly, and most assemblers accept it fully or partly, where you supply some parts and the assembler buys the rest. Parts should arrive correctly labelled, in packaging suitable for machine placement, with extra quantity for machine set-up losses. Responsibility for consigned parts stays with the customer.
Related glossary terms
- PCBA
PCBA (Printed Circuit Board Assembly) is the process of populating a bare PCB with electronic components — resistors, capacitors, ICs, connectors, and other parts — to produce a functional electronic assembly ready for testing or integration into a finished product.
- BOM
A BOM (Bill of Materials) is the structured list of every component needed to build a PCB, mapping reference designators on the board to specific manufacturer parts. It is one of the three core files — alongside Gerber and pick and place — required to quote and assemble a PCBA.
- Pick-and-place file
A pick and place file (also called centroid file, CPL, or XY data) is a machine-readable list of every component's position, rotation, and board side. It is one of the three core files — alongside Gerber and BOM — needed to quote and run PCB assembly.
- Solder paste
Solder paste is a mixture of tiny solder alloy spheres suspended in flux, printed onto PCB pads before surface-mount components are placed. During reflow the solder melts and forms the joints, while the flux removes oxides and helps wetting. Paste is specified by alloy, powder size and flux type.
- Reflow soldering
Reflow soldering is the process used to solder surface-mount components to a PCB. Solder paste is printed onto the pads, components are placed on the paste, and the board passes through an oven that follows a controlled temperature profile. The paste melts, wets the pads and leads, and forms solder joints as it cools.
- BGA and QFN
BGA and QFN packages are surface-mount IC packages whose solder joints sit underneath the component body. A BGA (ball grid array) connects through an array of solder balls on its underside, while a QFN (quad flat no-leads) has flat pads along its edges and usually a large exposed thermal pad, so joints cannot be fully inspected visually.
- Wave soldering
Wave soldering is a bulk soldering process in which an assembled PCB is passed over a standing wave of molten solder, soldering all exposed through-hole joints on the underside in one pass. It is mainly used for through-hole components and can also solder certain glued SMT components on the bottom side of the board.
- IPC-A-610
IPC-A-610 is the IPC standard that defines visual acceptance criteria for electronic assemblies. It describes what acceptable and defective solder joints, component placement, cleanliness and damage look like on a populated PCB, using photographs and illustrations, across three product classes. It is the most widely used inspection standard in PCB assembly.
- Turnkey assembly
Turnkey assembly is a full-service PCBA model where one manufacturer handles everything: bare board fabrication, component sourcing, assembly, inspection and testing. The customer supplies the design files — Gerber, BOM and pick-and-place — and receives finished boards. Because one manufacturer is responsible for the whole build, there is a single point of accountability for parts and workmanship.
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