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PCB Assembly: The Complete Guide for Hardware Startups
PCB assembly from stencil to functional test: paste, pick-and-place, reflow, AOI, X-ray, through-hole, IPC-A-610 classes, and what each step costs.
by the HardwareMap editors6 min read
Contents
PCB assembly is the step where a bare printed circuit board becomes a working circuit: solder paste goes on, components go down, an oven melts the joints, and machines and people check the result. This guide walks the process in factory order, shows what each step catches and costs, and sits under our how to start a hardware company playbook.
The PCB assembly process, step by step
A modern surface-mount (SMT) line runs the same sequence whether it builds five boards or fifty thousand. The steps below are in the order the board sees them.
1. Incoming inspection and kitting
Bare boards and components arrive, get counted against the bill of materials, and are loaded onto reels and feeders. Moisture-sensitive parts are tracked against their floor life and baked if they have been exposed too long; IPC/JEDEC J-STD-033 defines the handling. Most "my boards came back wrong" stories start here: a wrong part number, a reel of the wrong value, a part that sat in humid air. A clean BOM with manufacturer part numbers on every line is the cheapest quality step you control. Our bill of materials example shows the columns an assembler expects.
2. Stencil and solder paste printing
A stainless steel stencil, laser-cut with an opening for every pad, sits over the board. A squeegee pushes solder paste through the openings. Stencils for fine-pitch work are commonly around 0.1 to 0.15 mm thick, and aperture size sets how much paste lands on each pad. Paste is graded by particle size: Type 3 suits general work, Type 4 and finer suit small pads and tight pitch.
Paste printing is where most SMT defects are born. Too much paste bridges pins. Too little leaves open joints. Lines with solder paste inspection (SPI) measure the paste volume on every pad in 3D before any part is placed.
3. Pick-and-place
A pick-and-place machine reads your centroid file (X, Y, rotation and side for every part), picks components from feeders with vacuum nozzles, checks them with a camera and sets them into the wet paste. Fast machines place tens of thousands of parts an hour. Small parts are the limit: online services publish their minimum package and pitch. JLCPCB, for example, lists 0402 as the smallest package on its economic service and 0201 on its standard service (JLCPCB assembly capabilities).
4. Reflow soldering
The board rides a conveyor through a reflow oven with zones for preheat, soak, reflow and cooling. Lead-free SAC305 solder melts at roughly 217 to 220 °C, so peak temperatures sit above that, commonly in the 235 to 250 °C range. The profile has to suit both the paste and the most heat-sensitive part on the board. Double-sided boards go through twice, with the lighter side first.
5. Inspection: AOI and X-ray
Automated optical inspection (AOI) photographs every joint and flags missing parts, wrong polarity, tombstoning, bridges and skew. It cannot see under a part. Ball grid arrays (BGA), QFNs and LGAs hide their joints beneath the package, so assemblers use X-ray to check for bridges, opens and voids. JLCPCB states X-ray is a required step when it detects those packages on a board.
6. Through-hole (THT) assembly
Connectors, large capacitors, transformers and anything that takes mechanical load often stay through-hole. They are inserted after SMT and soldered by wave soldering, selective soldering or by hand. Hand soldering is fine for prototypes and a cost driver at volume: every through-hole part is a labor line on the quote. Converting a THT part to an SMT equivalent is one of the cheapest design for manufacturing wins.
7. Cleaning, coating and test
Boards may be washed to remove flux residue, and conformal-coated if they will live outdoors, near moisture or in vibration. Then they are tested:
- Flying probe test moves probes across test points to check for shorts, opens and component values. No fixture, so it suits prototypes and low volume.
- In-circuit test (ICT) uses a bed-of-nails fixture to test every net at once. Fast per board, but the fixture is a one-time cost.
- Functional test (FCT) powers the board and runs it: boot, flash firmware, read sensors, toggle outputs. You usually write it; the assembler runs it.
- Boundary scan (JTAG) tests connections on dense digital boards where probes cannot reach.
IPC standards and the three classes
Assemblers and their customers settle "is this joint good?" with IPC standards (IPC standards). Two matter most for PCB assembly. IPC-A-610 defines visual acceptability criteria for finished assemblies. J-STD-001 defines the requirements for the soldering process and materials.
Both use three product classes:
The class goes on your fabrication and assembly drawings. If you write nothing, ask which class the assembler builds to by default. A customer contract or a regulator may set the class for you, so check before you pay for Class 3 or build to Class 1. IPC also certifies operators and inspectors, which is a useful question when you audit an assembler.
Which PCB assembly service fits your stage
The assembler that suits a first prototype rarely suits a 10,000-unit run. Roughly:
Online services such as JLCPCB, PCBWay and MacroFab quote in a browser from Gerbers, a BOM and a centroid file. Seeed Studio also runs a small-batch assembly service out of Shenzhen. For the detail on early runs, see prototype PCB assembly. For who sources the parts, see turnkey vs consigned PCB assembly. For domestic options and when they are worth the premium, see PCB assembly in the USA. When the program outgrows online services, electronics contract manufacturers covers how to pick one.
What you send the assembler
A clean data package gets a quote in hours and a build with fewer phone calls. Send:
- Gerber or ODB++ files for the bare board, plus a fabrication drawing with stackup, finish and IPC class.
- Bill of materials with reference designators, quantity, manufacturer and manufacturer part number for every line, plus approved alternates.
- Centroid (pick-and-place) file with X, Y, rotation and side.
- Assembly drawing showing polarity marks, do-not-populate parts and anything special.
- Test instructions and firmware, if the assembler runs functional test.
Mark do-not-populate lines clearly in the BOM. Unmarked DNP parts are a classic source of boards that come back with parts you did not want.
Common defects and what prevents them
Most first-build failures fall into a short list. Each has a design-side fix that costs nothing once it is in the layout.
Two habits catch most of the rest. Ask for a first-article inspection: the assembler builds one board, checks it against your BOM and drawing, and holds the run until you sign off. And ask for the AOI and X-ray reports with the shipment, so a failure in the field can be traced to a lot.
What PCB assembly costs
The quote splits into setup, stencil, assembly labor, components and test. Components usually dominate at any volume above a few boards, and setup dominates on tiny runs. As an order of magnitude, a 10-board prototype run from an online service can land at $100 to $1,000est before parts, depending on part count, sides and packages. The full breakdown, with a worked example, is in PCB assembly cost.
Frequently asked questions
What is PCB assembly?
PCB assembly, or PCBA, is the process of mounting and soldering electronic components onto a bare printed circuit board so it becomes a working circuit. A typical line prints solder paste through a stencil, places parts with a pick-and-place machine, melts the paste in a reflow oven, inspects the joints, adds through-hole parts, and tests the finished board.
What is the difference between a PCB and a PCBA?
A PCB is the bare board: fiberglass laminate, copper traces, solder mask and silkscreen, with no parts on it. A PCBA is that board after assembly, with components soldered in place. Board fabrication and board assembly are separate processes, often done by separate factories, and quoted as separate line items even when one vendor sells both.
Which IPC-A-610 class should my product be built to?
Class 1 covers general electronics where function is what matters. Class 2 covers dedicated-service products such as most commercial and industrial devices, and is the usual default. Class 3 covers high-reliability products where downtime is not acceptable. Your customer contract or regulator may set the class. Higher classes tighten acceptance criteria and add inspection cost, so confirm the requirement before specifying it.
Do I need X-ray inspection on my boards?
X-ray matters when solder joints are hidden under the package, as with BGA, QFN and LGA parts. Optical inspection cannot see those joints, so voids and bridges go undetected without it. Some assemblers run X-ray automatically when they detect these packages. For boards with no hidden-joint parts, AOI and functional test usually cover the risk. Ask your assembler what it runs by default.
How long does PCB assembly take?
The assembly itself takes hours. The calendar time is driven by parts procurement, bare board fabrication and the queue at the factory. Quick-turn services advertise turnarounds measured in days once parts and boards are on hand. Long-lead components can add weeks or months. Check stock for every line on your bill of materials before you commit to a date.
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