---
type: process
name: "PCB Assembly (SMT Reflow)"
category: "Joining"
subcategory: "Thermal"
materials: ["Metal", "Composite", "Plastic"]
tolerances: "Placement accuracy ±0.001–0.002 in (±25–50 µm) at 3σ; stencil foil 0.004–0.006 in (100–150 µm) with aperture area ratio above 0.66; board bow and twist ≤ 0.75% per IPC-6012"
volumes: "5 to 1,000,000+ boards"
lead_time: "1–3 minutes of process time per board once running; 1–3 weeks for a prototype build including stencil, programming and component procurement"
url: https://manufacturingprocesses.org/processes/joining/pcb-assembly-smt-reflow
---

# PCB Assembly (SMT Reflow)

PCB assembly prints solder paste, places components and reflows the board through a heated oven so every joint forms in one pass.

- **Category**: [Joining](https://manufacturingprocesses.org/processes/joining.md)
- **Family**: Thermal
- **Materials**: Metal, Composite, Plastic
- **Typical tolerances**: Placement accuracy ±0.001–0.002 in (±25–50 µm) at 3σ; stencil foil 0.004–0.006 in (100–150 µm) with aperture area ratio above 0.66; board bow and twist ≤ 0.75% per IPC-6012
- **Typical volumes**: 5 to 1,000,000+ boards
- **Lead time**: 1–3 minutes of process time per board once running; 1–3 weeks for a prototype build including stencil, programming and component procurement

## Overview

Surface-mount assembly prints solder paste through a laser-cut stencil, places components onto the wet paste, and carries the whole board through a reflow oven where every joint on the board forms in a single thermal pass. A modern line places 10,000–100,000 components per hour with a placement accuracy of ±0.001–0.002 in (±25–50 µm).

The process is defined by its thermal profile. Lead-free SAC305 solder melts at 217 °C (423 °F), and a standard profile ramps at 1–3 °C/s, soaks at 150–200 °C for 60–120 s, peaks at 235–250 °C, and holds 45–90 s above liquidus before cooling at up to 4 °C/s.

Solder joints are electrical connections first — SAC305 shears at roughly 5,000 psi (35 MPa), so mechanical loads belong in mounting hardware, not in the solder. Volumes span a five-board prototype to millions of units on the same equipment.

## How it works

1. **Stencil printing.** Solder paste is squeegeed through a laser-cut stainless foil, typically 0.004–0.006 in (100–150 µm) thick. Aperture design follows IPC-7525: the area ratio (aperture opening area divided by aperture wall area) must stay above 0.66 or the paste sticks in the aperture instead of releasing onto the pad. Paste type matters — Type 3 powder for general work, Type 4 or 5 for 0201 and finer.
2. **Solder paste inspection (SPI).** A 3D scanner measures deposited paste volume, height and offset on every pad. Most reflow defects are printing defects, so this is where they get caught.
3. **Placement.** Vision-centered pick-and-place heads set components from 01005 (0.016 x 0.008 in / 0.4 x 0.2 mm) up to large connectors, holding them in the tacky paste.
4. **Reflow.** The board passes through a multi-zone convection oven. Preheat drives off solvent and activates flux; the soak equalizes temperature between light and heavy thermal masses; the spike takes every joint above liquidus long enough to wet and form intermetallic; controlled cooling sets the grain structure. Peak temperature is bounded above by the component body limits in J-STD-020.
5. **Inspection and second side.** AOI checks placement, polarity and joint appearance; X-ray inspects hidden joints under BGAs and QFNs. Double-sided boards run the whole sequence again, with the first side's components held by their own solidified joints.
6. **Through-hole and finishing.** Any remaining leaded parts are wave or selective soldered to at least 75% hole fill per IPC-A-610, then the board is cleaned, conformal coated or potted as required.

## Design guidelines

### Design the panel, not just the board

Assembly equipment handles panels, not individual boards. Provide 0.2–0.5 in (5–12 mm) tooling rails on at least two edges, keep components clear of the rails, and add global fiducials — 0.040 in (1 mm) copper circles with 0.080 in (2 mm) of clear solder-mask-free area — at three corners, plus local fiducials next to fine-pitch and BGA footprints.

### Give the machines and the rework tech room

Keep at least 0.020 in (0.5 mm) between component bodies for placement accuracy and AOI discrimination, and 0.100 in (2.5 mm) of clear area around a BGA so a rework nozzle can seat. Tall components next to short ones create shadowing in both convection and inspection — group by height where the layout allows.

### Balance the thermal mass

A large ground plane, a heavy connector or a thick copper pour acts as a heat sink and pulls its joints below liquidus while neighboring joints overheat. Thermally relieve pads that connect to planes with spoke connections, and avoid putting a small chip resistor immediately beside a large thermal mass.

### Respect board flatness

IPC-6012 limits bow and twist to 0.75% for boards carrying surface-mount components. Beyond that, fine-pitch parts and BGAs cannot be co-planar with their pads. Board thickness (0.062 in / 1.6 mm is the default), copper balance across layers and panel routing all feed into this.

### Thermal and reliability limits

| Item | Value | Notes |
| --- | --- | --- |
| SAC305 liquidus | 217 °C (423 °F) | Lead-free default |
| Sn63/Pb37 liquidus | 183 °C (361 °F), eutectic | Lower process temperature, still used in high-reliability work |
| Peak reflow temperature | 235–250 °C for SAC305 | Bounded by component body limits in J-STD-020 |
| Time above liquidus | 45–90 s | Too short gives incomplete wetting, too long grows brittle intermetallic |
| Ramp rate | 1–3 °C/s | Faster cracks ceramic capacitors and lifts components |
| Cooling rate | Up to 4 °C/s | Faster refines grain but stresses joints |
| Stencil area ratio | Above 0.66 | Below this, paste stays in the aperture |
| BGA void area per ball | ≤ 25% commonly specified | IPC-7095 flags voiding above 30% |
| Plated through-hole fill | ≥ 75% | IPC-A-610 Class 2 and 3 |
| Board bow and twist | ≤ 0.75% | IPC-6012 for surface-mount assemblies |

### Inspection

Surface-mount assembly is the most heavily inspected joining process in manufacturing, and the inspection sequence is the design constraint. SPI verifies paste volume before anything is placed. AOI after reflow checks presence, polarity, alignment and fillet shape on every visible joint. Automated X-ray (2D or 3D) is the only way to see BGA, QFN and bottom-terminated joints, and is where voiding and head-in-pillow defects are found. In-circuit test or flying probe verifies electrical connectivity, and functional test closes the loop. Acceptance is defined by IPC-A-610 class: Class 1 general electronics, Class 2 dedicated service, Class 3 high-reliability.

## Cost drivers

Assembly cost splits into non-recurring setup and per-board processing. Setup means a stencil, a placement program, feeder loading and a first-article inspection — a fixed cost paid once per revision, which is why a five-board prototype run costs far more per board than a thousand-board run of the identical design.

Per-board cost is driven by placement count and by anything that breaks the single-pass flow: double-sided assembly doubles the line passes, through-hole parts add a wave or selective soldering operation and often hand labor, odd-form components need manual placement, and unique part numbers each consume a feeder slot.

Component procurement, not assembly, usually dominates total cost and lead time. Design decisions that constrain sourcing to a single supplier or an unusual package have far more cost impact than anything on the line.

1. **Put everything on one side** where the design allows — it halves the line passes and eliminates the adhesive or second-reflow constraints of double-sided assembly.
2. **Eliminate through-hole parts.** One remaining leaded connector adds a whole soldering operation to an otherwise fully surface-mount board.
3. **Reduce unique part numbers.** Consolidating resistor and capacitor values frees feeder slots and lowers setup time.
4. **Use standard package sizes.** 0402 and 0603 place faster and more reliably than 0201, and need only Type 3 paste.
5. **Panelize deliberately.** Board utilization within a panel directly sets the per-board share of bare board and processing cost.

## FAQ

### What is a standard lead-free reflow profile?

For SAC305: ramp at 1–3 °C/s to a 150–200 °C soak held 60–120 s, spike to a 235–250 °C peak, hold 45–90 s above the 217 °C liquidus, then cool at up to 4 °C/s. Peak temperature is bounded by the component body limits in J-STD-020, and the soak exists to equalize temperature between light chip parts and heavy thermal masses.

### What is the stencil area ratio rule?

Aperture opening area divided by aperture wall area must exceed 0.66, per IPC-7525. Below that, surface tension holds the paste against the aperture walls instead of releasing it onto the pad, and you get insufficient or missing deposits. It is the practical limit on how fine a pitch a given stencil thickness can print.

### How much voiding is acceptable in a BGA solder joint?

A void area of 25% or less per ball is the figure most commonly specified; IPC-7095 treats voiding above 30% as a concern requiring evaluation. Voids are found by X-ray inspection, since BGA joints are hidden under the package. Excessive voiding usually traces back to paste volume, flux activity or an overly aggressive reflow profile.

### Can solder joints carry mechanical load?

Not meaningfully. SAC305 shears at roughly 5,000 psi (35 MPa) and creeps at room temperature. Connectors that get plugged and unplugged, heavy components and anything subject to shock or vibration need mechanical retention — screws, standoffs, brackets or staking — with the solder carrying only the electrical connection.

### Why does a prototype PCB assembly cost so much per board?

Because setup cost is fixed per revision: a laser-cut stencil, a placement program, feeder loading and first-article inspection are paid once whether the run is five boards or five thousand. Component procurement adds its own minimum order quantities and lead time. Per-board processing itself is only 1–3 minutes of line time.

### What is the difference between IPC-A-610 Class 1, 2 and 3?

They are acceptance levels, not process differences. Class 1 covers general consumer electronics where function is all that matters; Class 2 covers dedicated service equipment where extended life is expected; Class 3 covers high-reliability applications where failure is not tolerable, and tightens criteria for fillet shape, voiding, cleanliness and hole fill. Specify the class on the assembly drawing.

## Alternative processes

- [Adhesive Bonding](https://manufacturingprocesses.org/processes/joining/adhesive-bonding.md): Adhesive bonding joins parts with a cured polymer layer that spreads load over the whole bond area instead of concentrating it at fastener points.
- [Mechanical Fastening](https://manufacturingprocesses.org/processes/joining/mechanical-fastening.md): Mechanical fastening joins parts with screws, inserts, clinch hardware or snap fits so the joint can be assembled and taken apart again.
- [Ultrasonic Welding](https://manufacturingprocesses.org/processes/joining/ultrasonic-welding.md): Ultrasonic welding applies high-frequency vibration through a horn to melt a thermoplastic joint locally in well under a second.

## Related processes

- [Photo Etching](https://manufacturingprocesses.org/processes/finishing/photo-etching.md): Photo etching masks a surface photographically and etches shallow detail into it for decoration, marking or texture.
- [Screen Printing](https://manufacturingprocesses.org/processes/finishing/screen-printing.md): Screen printing pushes ink through a stencilled mesh onto the surface below, laying down one opaque color per pass.
- [Soldering and Brazing](https://manufacturingprocesses.org/processes/joining/soldering-and-brazing.md): Soldering and brazing join metals with a molten filler that wets the joint by capillary action while the parent metal stays solid.
- [Laser Marking and Engraving](https://manufacturingprocesses.org/processes/finishing/laser-marking-and-engraving.md): Laser marking alters the surface with a focused beam — annealing, etching or engraving it — to make permanent, contact-free marks.

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*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/joining/pcb-assembly-smt-reflow)*

*Last updated: August 11, 2026*