PCB Assembly
Also: PCBA, printed circuit board assembly, circuit board assembly, SMT assembly, surface mount assembly, through-hole assembly, electronics manufacturing services, EMS, contract electronics manufacturing, turnkey PCB assembly, box build
PCB assembly places and solders surface-mount and through-hole components onto bare boards, then inspects and tests them, to IPC-A-610 and J-STD-001.
53 shops tagged PCB assembly among 843 electronics manufacturing companies in the Noramark directory
PCB Assembly shops by state
Surface-mount assembly is a line of four machines. A stencil printer squeegees solder paste through a laser-cut stainless stencil onto the pads. A pick-and-place machine sets components from tape, tray and tube onto the paste. A reflow oven takes the board through a controlled profile of preheat, soak, peak and cooling that melts the paste into solder joints. Automated optical inspection (AOI) then compares every part and joint with the program. Double-sided boards go through the line twice. Lead-free SAC305 solder melts at about 217-220 °C and is reflowed at a peak of roughly 235-250 °C; tin-lead solder (Sn63/Pb37) melts at 183 °C and is still used on exempt aerospace, defense and some medical products.
Through-hole parts such as connectors, large capacitors and transformers are inserted and soldered by wave soldering, selective soldering or hand. Boards that mix both technologies are the norm. Bottom-terminated packages such as BGAs, QFNs and LGAs hide their joints under the body, so they are checked by X-ray. After soldering, boards may be washed or left with no-clean flux residue, programmed, tested, conformally coated, and built into enclosures with cables and hardware, which the trade calls box build.
Workmanship is judged against IPC-A-610, Acceptability of Electronic Assemblies, and boards are built to J-STD-001, Requirements for Soldered Electrical and Electronic Assemblies. Both define three product classes: Class 1 for general electronics, Class 2 for dedicated-service electronics where long life is wanted, and Class 3 for high-performance products where failure is not acceptable, such as medical life support, aerospace and defense. Class 2 is the commercial default. Class 3 tightens the acceptance criteria and adds inspection and records. Moisture-sensitive components are handled per J-STD-033, and static-sensitive parts under an ANSI/ESD S20.20 control program.
Assemblers quote two ways. In a turnkey job the assembler buys the boards and components against the bill of materials; in a consigned (kitted) job the customer supplies them. Component availability, not assembly time, usually sets the schedule and much of the price. Testing ranges from AOI alone, through flying-probe or bed-of-nails in-circuit test (ICT) that checks nets and component values, to functional test on a fixture that powers the board and exercises it. How much test coverage is required is the buyer's decision and belongs in the RFQ.
At a glance
| Typical tolerances | Pick-and-place machines position parts within about ±0.025-0.05 mm (±0.001-0.002"), enough for 0.4 mm pitch QFPs and BGAs and 0201 passives (0.6 x 0.3 mm) at most contract assemblers; 0.3 mm pitch parts and 01005 passives are available at fewer shops and should be confirmed. Solder paste is printed through stencils about 0.10-0.15 mm (0.004-0.006") thick. Finished joints are accepted by IPC-A-610 class, not by a dimension: the standard sets limits for component offset, fillet height, wetting, voiding and through-hole barrel fill for each class. |
|---|---|
| Size limits | Most lines take boards or panels from about 50 x 50 mm (2 x 2") up to about 450-510 mm (18-20") long; longer boards such as backplanes and LED strips need shops with long-board equipment, and small boards are run in panels with tooling rails. Board thickness from about 0.4 to 3.2 mm (0.016-0.125"), with 1.6 mm (0.062") standard. Component height is limited by the placement head and the oven clearance, commonly 12-25 mm (0.5-1"); taller parts are placed by hand. Quantities run from a single prototype to hundreds of thousands, and shops tend to specialize either in prototype and low-volume, high-mix work or in volume production. Limits vary by line, so check the assembler's capability sheet. |
| Surface finish | A finished assembly is judged on its solder joints and its cleanliness. Lead-free joints are duller and grainier than tin-lead joints; that is normal, not a defect. No-clean flux leaves a thin clear residue that stays on the board; water-soluble flux is washed off, and cleanliness can be verified by ionic contamination testing where the drawing asks for it. Conformal coating (acrylic, urethane, silicone or parylene) protects the board from moisture and contamination, at about 0.03-0.13 mm (0.001-0.005") thick for acrylic and urethane; connectors, test points and other keep-out areas must be marked. Potting, underfill and staking are added for vibration and shock. |
| Lead time | Quick-turn prototypes in 1-5 business days once boards, parts and stencil are in hand. Turnkey prototype and small-lot orders in about 2-4 weeks including board fabrication and component purchasing; production orders typically 4-8 weeks. The longest-lead component sets the schedule: parts on allocation or at end of life can add weeks or months, so check stock and approve alternates early. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Substrates: rigid FR-4 boards are the bulk of the work. Assemblers also build on high-Tg and high-frequency laminates, aluminum-backed (metal-core) boards for LEDs and power, and flexible polyimide circuits and rigid-flex, which need carriers and baking before reflow. Solder: lead-free SAC305 for RoHS products, tin-lead Sn63/Pb37 where exemptions apply, and low-temperature or high-reliability alloys on request. Flux is no-clean, water-soluble (which must be washed off) or rosin based. The board's pad finish matters to assembly: ENIG, immersion silver and immersion tin are flat and suit fine-pitch parts; hot-air solder leveling (HASL) is cheaper and less flat; OSP has a shorter shelf life. Components arrive on tape and reel, in trays or in tubes; loose and cut-tape parts raise handling cost.
What drives the cost
- Bill of materials: components are usually most of the price of a turnkey assembly, and shortages and minimum buy quantities add to it
- Number of placements and of unique part numbers: each part number is a feeder to load and a line in the program
- Setup charges: stencil, programming, first article and test fixtures, which dominate small lots
- Through-hole and hand-soldered parts, which cost several times as much as a surface-mount placement
- Double-sided assembly, fine-pitch and BGA parts, and X-ray inspection
- Test level: AOI only, flying probe, an in-circuit test fixture, functional test, burn-in
- IPC class and documentation: Class 3, lot traceability, cleaning, conformal coating, first article reports
When to use it
- Any product with a populated circuit board, from one prototype to volume production
- Turnkey builds where one supplier buys boards and parts, assembles, tests and ships
- Fine-pitch, BGA and double-sided designs that cannot be hand soldered reliably
- Products that must be built and documented to IPC-A-610 Class 2 or Class 3
- Box builds: boards integrated with cables, enclosures, firmware and final test
When not to
- A one-off board with a few large parts, which a technician can hand solder faster than a line can be set up
- Designs not yet checked for manufacturability: footprints that do not match the parts, no fiducials, no test access
- Bare-board questions such as layer count, impedance and drilling, which belong to the board fabricator
- Wiring between boards and devices: that is wire harness and cable assembly
Design tips
- Send a complete package: Gerber or ODB++ data, a bill of materials with manufacturer part numbers and approved alternates, pick-and-place (centroid) data, and an assembly drawing with polarity marks.
- Add three global fiducials per side, and local fiducials for fine-pitch and BGA parts.
- Panelize small boards with tooling rails, or let the assembler do it, and say whether breakaway tabs or V-scoring is acceptable.
- Keep surface-mount parts on one side where the design allows; a second side is a second pass through print, place and reflow.
- Use footprints from the component data sheet or IPC-7351 land patterns, and check pin 1 and polarity marks against the silkscreen.
- Give test access: test points on the nets that matter, and a programming header or pads.
- State the IPC class, lead-free or tin-lead solder, cleaning, conformal coating with its keep-out areas, and the inspection and test you expect.
- Check the availability and lifecycle status of every part before releasing the bill of materials, and name substitutes for passives and connectors.
Frequently asked questions
- What is the difference between SMT and through-hole assembly?
- Surface-mount (SMT) parts sit on pads on the board surface and are soldered by printing paste and reflowing it in an oven; they are small, placed by machine and make up most of a modern board. Through-hole parts have leads that pass through plated holes and are soldered by wave, selective or hand soldering; they are used for connectors and large or heavily loaded parts. Most boards use both.
- What files does a PCB assembler need?
- The bare-board data (Gerber or ODB++), a bill of materials with manufacturer part numbers and reference designators, pick-and-place (centroid) data giving each part's position, rotation and side, and an assembly drawing. Add test and programming instructions, the IPC class and any coating or cleaning requirements.
- What is the difference between IPC Class 2 and Class 3?
- Both are product classes in IPC-A-610 and J-STD-001. Class 2 covers dedicated-service electronics such as industrial controls and most commercial products, where long life is wanted but uninterrupted service is not critical. Class 3 covers products that must work on demand, such as medical life support, aerospace and defense. Class 3 has tighter acceptance limits and usually more inspection and records, so it costs more.
- What is turnkey PCB assembly?
- The assembler buys everything: bare boards from a fabricator and components from distributors, against your files and bill of materials, then assembles, tests and ships. In a consigned or kitted job you supply the boards and parts and pay for assembly only. Partial turnkey splits the buying, usually with the customer supplying the hard-to-find parts.
- How are BGA solder joints inspected?
- By X-ray. The solder balls sit under the package where a camera cannot see them, so the assembler images them with 2D or 3D X-ray to find bridges, missing balls, misalignment and voids. X-ray does not catch every open joint, so electrical test backs it up.
