Assembly: Stencils, Reflow and Assembly Services
Hand, stencil and oven assembly, assembly services, costs and when each makes sense.
Four ways to assemble a board
| Method | Tools | Parts it suits | Boards |
|---|---|---|---|
| Hand soldering | Iron, flux, tweezers, magnifier | Through-hole; chips down to 0603 or 0402; fine-pitch with drag soldering | 1 to 3 |
| Hand placement with a stencil and a hot plate or oven | Stencil, paste, tweezers, toaster oven or hot plate | Any SMD, including fine pitch and QFN (not BGA without care) | 1 to 30 |
| Assembly service (prototype) | Their pick-and-place and reflow | SMD of all sizes; BGA with X-ray; through-hole hand or wave | 5 to 500 |
| Contract manufacturer | Full line with test | Everything; production runs | 100 and more |
The soldering itself (technique, temperatures, defects) is in the Soldering guide; this page is about choosing and organizing the assembly.
Hand assembly
Soldering by hand needs no extra tooling and gives the best control for one or two boards. The time depends on the parts: chip parts are quick, fine-pitch ICs and connectors slow. See Surface-Mount Soldering for the techniques.
Worked example: hand-assembling the sensor board
The 36-part board used in PCB Design and Layout, with typical times per part:
| Parts | Number | Time each | Total |
|---|---|---|---|
| 0603 resistors and capacitors | 22 | 45 s | 16.5 min |
| 0805 capacitors | 3 | 45 s | 2.3 min |
| 0603 LEDs | 2 | 45 s | 1.5 min |
| TQFP-32 microcontroller (drag solder) | 1 | 240 s | 4.0 min |
| SOT-223 regulator | 1 | 90 s | 1.5 min |
| SOT-23 TVS diode | 1 | 60 s | 1.0 min |
| Crystal | 1 | 90 s | 1.5 min |
| I2C sensor, LGA (hot air) | 1 | 300 s | 5.0 min |
| USB-C receptacle | 1 | 300 s | 5.0 min |
| 1 × 6 pin headers | 2 | 120 s | 4.0 min |
| Tactile switch | 1 | 60 s | 1.0 min |
- Total soldering time: 43 minutes (about 0.7 h), plus inspection and fixes: call it an hour.
- At $20 per hour of your time that is $14.42 per board: acceptable for one board, but not for ten.
Stencil, paste and oven
A stencil is a thin steel (or polyimide) foil with a hole for every pad. Paste wiped across it leaves the right amount on each pad. Parts are then placed with tweezers or a vacuum pen, and the board is reflowed in an oven or on a hot plate (see Reflow Profiles, Ovens and Hot Plates). Paste, powder types and aperture rules are in Solder Alloys, Flux and Paste.
- Order a stencil with the boards: a frameless 0.12 mm stainless stencil of the board outline is cheap; a framed one is easier for a printer.
- Tape the board and stencil to a flat surface, aligned over the pads (a printed jig of the board helps); tape or magnets hold the stencil.
- Wipe paste with a card or squeegee at a shallow angle in one or two passes; lift the stencil straight up.
- Place the parts, ICs first, then the chips; check alignment under magnification.
- Reflow the board; rework any bridges or tombstones.
Worked example: time and paste for the stencil route
- Printing 3 min, placing 36 parts at 20 s each 12 min, reflow 8 min and fixes 5 min: 28 minutes for the first board (against 43 by hand). Additional boards in the same batch are faster, as the stencil and oven are reused.
- Paste: 380 pads × 0.10 mm³ = 38 mm³ = 0.16 g per board; 20 boards (with 20 % waste) use 3.9 g: one 10 g syringe is enough.
- The area ratio of a 0603 aperture (0.9 × 0.8 mm) on a 0.12 mm foil is 1.76; for a 0.5 mm QFP (0.25 × 1.5 mm) it is 0.89 at 0.12 mm and 0.71 at 0.15 mm: both above 0.66, but the thinner foil is safer.
Assembly services
Many fabrication services also assemble boards, using their own stock of common parts or parts you supply. You upload the Gerbers, the bill of materials (BOM) and a pick-and-place file (also called centroid or CPL), check an online preview and receive finished boards. The result is neat, fast and repeatable.
| Model | How it works | Notes |
|---|---|---|
| Turnkey | The service buys all parts | Simplest; check the part numbers; extra parts fees; long lead for rare parts |
| Consignment / kitted | You send the parts | You control the parts; extra loss allowance (5 to 10 % or more) needed |
| Hybrid | They buy common parts; you send rare ones | Common for prototypes |
| Standard library parts | Parts from the service’s stock, with a fee for each unusual part | Cheapest; check the library before choosing parts (the design guide on schematic and parts) |
Worked example: the price of assembly
An illustrative model for the sensor board (not a quote): set-up $30, stencil $10, fine-pitch fee $15, $1.50 for each of 12 unique part lines, $0.02 per placed part (36 per board) and $8.00 of parts per board.
| Boards | Assembly service | By hand | Stencil and oven |
|---|---|---|---|
| 5 | $117 ($23.32 each) | $112 ($22.42 each) | $97 ($19.33 each) |
| 10 | $160 ($16.02 each) | $224 ($22.42 each) | $183 ($18.33 each) |
| 50 | $509 ($10.18 each) | $1 121 ($22.42 each) | $877 ($17.53 each) |
| 100 | $945 ($9.45 each) | $2 242 ($22.42 each) | $1 743 ($17.43 each) |
- The service has a fixed cost of about $73 and about $8.72 per extra board.
- Counting your time at $20 an hour, the service is cheaper than hand assembly from about 6 boards and cheaper than the stencil and oven route from about 8 boards.
- The service also gives a more repeatable result, and the only practical way for BGAs and for 0201 parts.
Which is cheaper?
| Situation | Best route |
|---|---|
| One or two boards, mostly through-hole or 0603 and larger | Hand soldering |
| A few boards, fine pitch or QFN parts | Stencil and oven, or a hot plate |
| Five or more boards, many parts | Assembly service |
| BGA or 0201 parts | Assembly service (with X-ray) |
| Single-sided boards with a few SMD parts | Hand assembly with paste in a syringe |
| A deadline of days | Hand or stencil at home: the service takes at least a week |
Preparing the design for assembly
- BOM: reference designators, quantity, value, footprint, manufacturer, manufacturer part number and supplier number; one line per unique part. Mark parts not to fit.
- Pick-and-place file: reference, X, Y, rotation and side. Rotation conventions differ between programs: compare the service’s preview with the board.
- Fiducials: two or three round copper marks on the board or panel, with a clear mask opening.
- Panel rails: 5 mm clear edges on two sides for the conveyor, and tooling holes (see Manufacturing Rules, Files and Design Review).
- Keep parts apart by the courtyard spacing the assembler specifies (0.25 mm or more), and keep tall parts away from small ones.
- One side if possible: double-sided assembly doubles the set-up.
- Through-hole parts are hand-soldered or wave-soldered at extra cost: choose SMD where you can.
- Pin 1 and polarity marks clearly on the silkscreen and in the assembly drawing.
Parts: sourcing, substitutes and counterfeits
- Check stock and lead time before you finish the design; a part that is out of stock for 40 weeks ends a project.
- Buy from authorized distributors (Digi-Key, Mouser, Farnell, LCSC and manufacturers’ own stores). Counterfeit parts appear on marketplaces.
- Buy extra of small parts: 10 % or a few spare (a reel strip costs little), and a minimum quantity for each part.
- Choose a second source for passives; resistors and capacitors have equivalents (check voltage and temperature rating, and the capacitor’s DC bias).
- Fix the BOM before ordering, with manufacturer part numbers, so that the assembler cannot substitute a different part without asking.
- Moisture-sensitive parts arrive in dry bags: keep them sealed until use (see Reflow Profiles, Ovens and Hot Plates).
Inspecting and testing the result
Worked example: how many joints will be bad?
- The board has about 400 joints. A service running at 200 defects per million joints gives (1 − 200×10−6)400 = 92.3 % of boards with no defect: about one in thirteen has a fault. Hand assembly is often worse, at 500 to 2 000 per million.
- So inspect every board (see Joint Quality, Inspection and Failure) and test it (see First Power-Up, Bring-up and Testing).
- Under magnification: bridges, tombstones, missing or rotated parts, polarity, solder balls, flux residue.
- Compare the board with the assembly drawing and the BOM; check pin 1 on every IC.
- Ask the service for photos or an inspection report (AOI, X-ray for BGAs) with the order.
- Clean the boards if the flux is not no-clean.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Parts placed rotated 90° or 180° | Rotation convention in the placement file | Check the service’s preview; add a rotation correction |
| Wrong part fitted | BOM ambiguity or substitution | Exact manufacturer part numbers; check before ordering |
| Missing parts | Out of stock or marked do not place | Check the order confirmation; hand-fit the part |
| Solder bridges on fine-pitch pins | Stencil too thick or aperture too large | Thinner foil; reduce apertures; rework (see Soldering) |
| Hot plate reflow leaves cold joints | Board heated too slowly or cooled early | Follow a profile; check the plate temperature with a thermocouple |
| Boards come back with flux residue | No-clean flux left | Clean with isopropyl alcohol if needed |
Related
- PCB Prototyping: the overview of this guide.
- Ordering from a Fabrication Service: ordering the bare boards.
- First Power-Up, Bring-up and Testing: testing the finished board.
- Soldering: techniques, paste and reflow.
FAQ
Should I assemble the boards myself or use a service?
By hand for one or two boards or for a deadline; with a stencil and oven for a few; with a service for five or more, fine pitch or BGAs.
Do I need a stencil?
For more than a handful of SMD joints, yes: it is cheap and saves much time. For a few chips, paste from a syringe works.
What does an assembly service need from me?
Gerber files, a BOM with manufacturer part numbers and a pick-and-place (centroid) file, plus special instructions.
Why was my part rotated wrongly?
Different programs define the zero rotation of a footprint differently. Check the placement preview and the rotation convention before ordering.
Can I use a toaster oven for reflow?
Yes, with a thermocouple controller and an open door for cooling; see Reflow Profiles, Ovens and Hot Plates.
References
- IPC-A-610, Acceptability of Electronic Assemblies; IPC-7525, Stencil Design Guidelines; IPC J-STD-001.
- Assembly services’ published capability and BOM guidelines.
- Lee, N.-C., Reflow Soldering Processes and Troubleshooting, Newnes.
Last reviewed: October 2026. Times and prices are an illustrative model: use real quotes and your own rate.