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Surface-mount technology (SMT) is a method of assembling electronic circuits in which components sit directly on conductive pads on the surface of a printed circuit board (PCB), instead of having their leads pushed through drilled holes. The components themselves are called surface-mount devices (SMDs). SMT names the assembly process; SMD names the part that the process attaches.
How an SMT assembly line works
Most SMT lines follow the same basic sequence. The process resource published by Surface Mount Process in its introduction to SMT describes solder-paste printing, component placement, and reflow as the core stages. Individual factories add or change steps, so treat the sequence below as the common pattern rather than a fixed standard.
- Print solder paste onto the pads. A stencil deposits a controlled amount of solder paste on each pad where a component will sit.
- Place the components. A pick-and-place machine positions each SMD on its paste deposit. The paste holds the part in position until soldering.
- Reflow the assembly. The board passes through a controlled heating profile. The paste melts, wets the component terminals and pads, and solidifies into the electrical and mechanical joints.
- Inspect. Automated optical inspection is commonly used after reflow, though not every line includes it or uses it at the same point.
Reflow is the step that turns a loosely held assembly into a finished board, so the heating profile matters as much as the placement accuracy. A technical overview on ScienceDirect Topics also notes that eliminating component-lead holes can simplify board fabrication, and that components may be mounted on either side of the board where the design and process allow it. These are capabilities and common advantages, not guaranteed results for every design.
SMT compared with through-hole technology
Through-hole technology inserts component leads into drilled holes and solders them on the opposite side. SMT removes the holes for the component leads and solders parts to the surface. The differences show up in package size, density, automation, and mechanical behavior.
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| Factor | Surface-mount (SMT/SMD) | Through-hole |
|---|---|---|
| Attachment | Terminals soldered to pads on the board surface | Leads inserted through drilled holes and soldered |
| Board fabrication | No holes needed for component leads, which can simplify fabrication | Requires drilling for every lead |
| Typical package size and density | Associated with smaller packages and closer spacing | Larger packages; lower placement density |
| Automation | Well suited to high-volume automated placement and reflow | Automation possible, but often combined with manual or wave-soldering steps |
| Mechanical demands | Not suited to every part; some mechanical requirements favor through-hole | Retained for parts that need stronger mechanical anchoring |
| Repair and rework | Requires specialized soldering and desoldering skills or equipment; exact constraints not stated in the cited sources | Leads are generally accessible for hand soldering; specific rework effort not stated in the cited sources |
| Mixed boards | Commonly combined with through-hole parts on one board | Commonly combined with SMDs on one board |
The sources describe these as tradeoffs. Neither method is universally cheaper or better, and the sources do not establish a single cost ratio between them.
Advantages of SMT and their limits
- Compact layouts. SMD packages are generally smaller and can be spaced more closely, which helps when board area is limited. A smaller footprint does not automatically mean a smaller product, because enclosure, thermal, and connector requirements still apply.
- Automated assembly. Placement and reflow suit high-volume production. Low volumes, prototypes, and one-off repairs may not gain the same benefit.
- Simpler drilling. Omitting component-lead holes reduces drilling work. The overall fabrication saving depends on the board design and the number of holes it would otherwise need.
- Two-sided mounting. Components can go on both faces where the design and reflow process allow it. Parts on the second side must survive the heating of the first side’s reflow.
Choosing between SMT and through-hole
Use these questions to decide which method suits a given part or board:
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- Is a suitable SMD package available for the component, and does it meet the electrical and thermal requirements?
- Does the part carry mechanical load, such as connectors or parts that are pulled or pressed often? If so, check whether through-hole mounting is the better fit.
- Is board area or height the main constraint, favoring SMT?
- Will the assembly volume justify automated placement and reflow equipment?
- How often will the board need repair, and does the team have the tools and skills for surface-mount rework?
Many practical boards answer these questions differently for different parts, which is why mixed-technology designs are common.
Learning SMD soldering and desoldering
Readers who want to practice SMT by hand have a documented route. The Bharat Skills vocational material from the Government of India, titled Electronics Mechanic: Surface Mount Technology and dated 2022, covers SMD soldering and desoldering as part of its training. A 2009 arXiv paper, Reflow Soldering of Surface Mount Electronic Components in a Laboratory, describes reflow soldering and surface-mount prototyping in a laboratory setting. Both sources describe methods and training context rather than specific tools or settings, so check your equipment documentation for temperatures and times.
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What the sources do and do not establish
The definitions above are consistent across the technical overviews, vocational training material, and laboratory paper consulted. No industry-wide percentage of boards that use SMT was found in these sources, so this article does not give a market-share figure. Current capabilities of individual PCB assembly providers were also not verified, so check any provider’s stated SMT capability directly before relying on it.
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