SMT assembly is the process of mounting electronic components directly onto the surface of a PCB, soldered in place by a reflow oven. The board passes through five stations: solder paste printing, pick-and-place, reflow soldering, automated optical inspection (AOI), and optional rework — the standard way modern circuit boards are built.
What is the SMT assembly process
Surface-mount technology (SMT) assembly is the dominant method for building modern PCBs. The board moves through five stations on a conveyor: solder paste printer, pick-and-place machine, reflow oven, automated optical inspection (AOI), and — if needed — manual rework. A 250 mm board with 800 placements typically clears the full line in 8-12 minutes.
At Energetika-VDS we run a DDM Novastar SMT line in Strumica, North Macedonia: SPR-45 stencil printer, LS60 pick-and-place head, GF-120HT reflow oven. Founded 1992 by Vasko Stamboliev, the shop assembles 50 to 50 000 units per order at IPC-A-610 Class 2 default, Class 3 on request.
Step 1 — Solder paste printing
The stencil printer (DDM Novastar SPR-45) deposits Type 4 SAC305 solder paste through a laser-cut stainless-steel stencil — typically 100-150 µm thick — onto every pad on the PCB. Squeegee speed 20-80 mm/s, separation speed 0.5-3 mm/s. Print tolerance: ±25 µm.
Key inputs:
| Parameter | Typical range | Notes |
|---|---|---|
| Stencil thickness | 100-150 µm | 100 µm for 0402/0201, 150 µm for QFN/BGA |
| Paste type | T4 / T5 | T5 (15-25 µm) for 0201 and µBGA |
| Aperture ratio | ≥0.66 | Below this = poor release |
| Print speed | 20-80 mm/s | Slower = thicker deposit |
Solder paste inspection (SPI) — if equipped — measures volume, area, height of each deposit. 2D AOI alternative catches gross misprints.
Step 2 — Pick-and-place
The LS60 places components from tape, tube, or tray onto the wet paste. Vision-centred nozzles handle parts from 0201 (0.6 × 0.3 mm) up to 45 × 45 mm QFP and BGA. Cycle: ~0.15 s per chip placement, ~0.5 s per fine-pitch IC.
Placement accuracy: ±50 µm @ 3σ for chips, ±30 µm for fine-pitch. Component feeders pre-loaded by the operator; setup time per side is the throughput bottleneck on low-volume runs.
See our full SMT line specs for feeder counts and head configurations.
Step 3 — Reflow soldering
The GF-120HT 8-zone convection oven runs a profile matched to the paste data sheet. For SAC305:
| Zone | Temperature | Duration | Purpose |
|---|---|---|---|
| Preheat | 25 to 150°C | 60-90 s | Ramp 1-3°C/s |
| Soak | 150-200°C | 60-120 s | Flux activation |
| Reflow | 217-245°C peak | 30-90 s above 217°C | Solder melts and wets |
| Cooldown | 245 to 50°C | 60-120 s | Ramp ≤4°C/s |
Total dwell: 4-7 minutes. Time-above-liquidus (TAL) of 45-90 s is the sweet spot — too short = cold joints, too long = excessive intermetallic growth.
Step 4 — Automated optical inspection (AOI)
In-house AOI scans every board at 10-20 µm resolution post-reflow. The system flags:
- Missing components
- Tombstoned chips (0402/0201 vertical)
- Solder bridges (≥80 µm)
- Insufficient solder / lifted leads
- Polarity reversal (via top-marking OCR)
- Skew >25% of pad width
False-call rate on a tuned program: 1-3%. True-defect catch rate: 95%+ for visible joints. BGA and QFN bottom-pad joints need X-ray — see our BGA assembly guide for that workflow. Full inspection options on the inspection and testing page.
Step 5 — Rework (optional)
Manual rework station handles AOI escapes and engineering changes. Hot-air nozzle for QFP/QFN, infrared BGA rework station for ≥10 mm packages. Typical rework cycle: 5-15 minutes per joint, traceable to the operator and serial number.
Rework cost is the silent killer of margin. A well-tuned DFM pass — see our DFM checklist — cuts rework rate from 2-3% to under 0.3%.
SMT vs THT — when each wins
| Factor | SMT | THT |
|---|---|---|
| Component size | 0201 to BGA | Through-hole only |
| Density | 2-4× higher | Lower |
| Mechanical strength | Lower | Higher (good for connectors, transformers) |
| Cost per joint | €0.001-0.005 | €0.02-0.08 |
| Automation | Full | Wave or selective solder |
Most modern boards are mixed-tech: SMT on both sides, THT for power connectors and high-mass parts.
Cost and lead time
A 100-board run with 250 placements per side, IPC Class 2, runs €8-18 per board at Energetika-VDS depending on part count and side count. Lead time: 5-10 working days after kit-on-floor. Get a binding quote via the quote estimator or RFQ form.
Compared to JLCPCB or PCBWay (Asia, 2-4 week door-to-door including freight), or Eurocircuits and AISLER (EU, but Eurocircuits caps at 50 pcs / 5000 placements), we sit in the EU-shore mid-volume sweet spot — same continent, no tariffs, IPC Class 3 capable.
Frequently asked questions
What is the SMT process? Surface-mount assembly: solder paste is printed onto PCB pads, components are placed by a pick-and-place machine, the board is heated in a reflow oven to melt the solder, then inspected by AOI.
How long does SMT assembly take? Per board, 8-12 minutes through a 5-zone line for 500-1000 placements. For an order, expect 5-10 working days from kit complete to ship, depending on volume and side count.
What is SAC305? A lead-free solder alloy: 96.5% tin, 3% silver, 0.5% copper. Melts at ~217°C, peaks at 235-245°C in reflow. Industry standard since RoHS forced lead out in 2006.
What is reflow soldering? Heating a PCB with pre-deposited solder paste through a controlled thermal profile so the paste melts, wets the pads and leads, then solidifies — forming all joints simultaneously.
SMT vs THT — which is better? Neither — they solve different problems. SMT for density and cost; THT for mechanical strength on connectors, large capacitors, transformers. Most boards use both, with SMT being 90%+ of joint count.
What does SMT stand for? Surface-Mount Technology — the method of mounting components directly on the PCB surface rather than through drilled holes.
What is the difference between SMD and SMT? SMD (Surface-Mount Device) is the component; SMT (Surface-Mount Technology) is the process that places and solders it. You assemble SMDs using SMT. See SMD vs SMT vs THT explained.
Where can I get SMT assembly service in Europe? Energetika-VDS runs an in-house SMT line in Strumica, North Macedonia, shipping 1-2 weeks door-to-door across the EU. See our SMT assembly service.