What SMT is
Surface mount technology (SMT) is the method used to assemble electronic components onto pads on the surface of a printed circuit board, rather than inserting their leads through holes. The components are called surface mount devices (SMDs). Because SMDs are small and placed by machine, SMT allows dense, double-sided boards built at high speed.
Through-hole technology (THT) is still used for connectors, large capacitors, transformers and other parts that need mechanical strength. Many boards are mixed: SMT for most parts, then wave, selective or hand soldering for the through-hole parts. SMT is one stage in making a PCBA, not the whole of it.
How an SMT line works
| Step | What happens | Main control |
|---|---|---|
| Solder paste printing | Paste is printed onto pads through a metal stencil | Stencil design, printer setup, paste handling |
| Solder paste inspection (SPI) | A 3D system measures paste volume and position | SPI limits and response to alarms |
| Pick and place | Machines place components from reels and trays | Feeder setup, part verification, placement program |
| Reflow | The board passes through an oven that melts the paste | Temperature profile measured for each product |
| Automated optical inspection (AOI) | Cameras check placement, polarity and visible joints | AOI program tuning and review of flagged boards |
| X-ray (where needed) | Inspects hidden joints under BGAs and similar packages | Sampling plan and void criteria |
For a double-sided board the sequence runs twice, once per side.
Where SMT defects come from
Typical defects include solder bridges, insufficient solder, tombstoning (a small part standing on end), misaligned or missing parts, reversed polarity, voids under large pads and head-in-pillow joints on BGAs. Many SMT engineers trace a large share of soldering defects back to paste printing, which is why SPI and stencil control get so much attention on a well-run line.
Moisture is another source. Many IC packages absorb moisture from the air; IPC/JEDEC J-STD-020 classifies them by moisture sensitivity level (MSL), and J-STD-033 sets how they are stored, exposed and baked. A package that has absorbed too much moisture can crack or delaminate in reflow as the moisture turns to steam. Good lines track how long each opened package has been exposed and bake parts that exceed their floor life.
What to check on a supplier’s SMT line
- SPI and AOI actually in use. Machines sometimes run in bypass. Ask to see today’s data and how flagged boards are reviewed.
- Reflow profiles. A profile measured with thermocouples on your board, not a generic oven setting.
- Material verification. Reels scanned against the program at feeder setup, so the wrong part cannot go on the wrong feeder.
- Moisture control. Dry storage, exposure tracking and baking records.
- Stencil management. Identification, cleaning, inspection and replacement.
- ESD control. Grounded benches, wrist straps, ESD-safe packaging and regular checks.
- First-piece check. Verification of the first board after every changeover.
- Traceability. Which reel, lot and date code went into which board.
The factory audit checklist puts these into a form you can use on site.
Capacity and small batches
SMT capacity is often quoted as placements per hour per machine. Rated speed is not real output: changeovers, line balance and inspection all reduce it. For a new program, ask how many lines could run your board and how heavily they are loaded today.
Small batches cost more per board because setup is fixed: loading feeders, verifying the program, printing and checking the first board. Stencils and programming are usually charged once per board revision.
Design choices that affect SMT
Component size, pitch and density all affect yield. Very small passives and fine-pitch packages are routine on a modern line but leave less margin for error. Fiducials, panel design and clearance around tall parts help the line run well; these belong in DFA and DFM reviews before layout is frozen. Yield on the line shows up in first pass yield.
Common mistakes
- Missing or inaccurate centroid files and polarity markings.
- Assuming AOI catches everything; hidden joints need X-ray.
- No requirement for a product-specific reflow profile.
- Ignoring moisture sensitivity during small, slow builds where reels stay open for days.
- Comparing suppliers by machine brand instead of by process control.
Where Zefinity helps
An SMT line says a lot about a factory’s discipline. In capability assessments and factory audits, Zefinity looks at how a supplier controls paste, placement, reflow and inspection in daily practice, not just which machines are on the floor.
Frequently asked questions
What is the difference between SMT and SMD?
SMT (surface mount technology) is the assembly method. An SMD (surface mount device) is a component designed to be assembled that way. An SMT line places SMDs onto pads on the surface of a circuit board, as opposed to through-hole components, whose leads pass through holes in the board.
Why do SMT suppliers charge for stencils and programming?
Each board needs a stencil cut for its pad layout and a placement program for its components. Both are one-time costs for a given board revision, so suppliers often list them separately or within NRE. A revision that moves pads or components can require a new stencil and program.