Surface mount technology (SMT) is the dominant technology for producing electronic boards. It involves mounting SMD components directly onto the PCB surface — without inserting leads through holes. The result: smaller boards, higher component density, full automation and higher series repeatability.
This article explains how an SMT line works step by step, which components it handles and when SMT is the right choice for your project.
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What is SMT and how does it differ from THT?
SMT (Surface Mount Technology) is an assembly method in which electronic components — known as SMD (Surface Mount Device) elements — are placed and soldered directly onto the surface of a printed circuit board (PCB). There are no leads through holes — the component sits on pads, and the electrical connection is formed by the layer of solder paste melted in a reflow oven.
Unlike through-hole technology (THT), where component leads pass through the board and are soldered on the underside, SMT allows assembly on both sides of the board and the use of much smaller component packages.
The practical effects of this difference:
- Smaller board for the same functionality
- Assembly on both sides of the PCB possible
- Fully automated process — higher repeatability
- Lower unit cost in production series
- Better performance at high frequencies (shorter signal paths)
Most modern electronics projects use mixed assembly: SMT as the main technology + THT for connectors, transformers and high-power components.
Which components can be assembled with SMT?
SMT supports a huge range of component packages. Key categories:
SMD passive components
Resistors, capacitors, inductors in packages from 0201 (0.6 × 0.3 mm) to 2512. The smaller the package, the higher the requirements for pick&place machine accuracy and solder paste stencil quality.
Integrated circuits in QFP / SOIC / SOT packages
Classic IC packages with side leads. Clearly visible after assembly and easy to inspect with AOI.
BGA (Ball Grid Array) packages
Leads hidden under the package — solder balls invisible after assembly. Require X-Ray inspection to verify joint quality. Typical for processors, FPGA chips and memory modules.
QFN / LGA packages
Flat packages without visible side leads. Popular in power and RF circuits. Like BGA, they require X-Ray inspection under rigorous quality control.
LED components, SMD connectors, quartz, SMD transformers
A wide range of passive and active components available in SMD packages.
At Siltegro we handle components from 0201 upwards, including BGA and QFN — with offline AOI inspection and X-Ray on request or as a standard for selected project types.
The SMT assembly process — production line stages
Step 1 — Solder paste application
Solder paste is applied to the PCB's solder pads through a precision stencil. A laser-cut stainless steel stencil matches the pad layout on the PCB exactly. The thickness and evenness of the applied paste directly affect the quality of the solder joints.
Step 2 — Component placement (Pick&place)
The pick&place machine picks components from reels (tape & reel) or trays and places them on the board with accuracy down to a few micrometres. Modern machines place several thousand components per hour. At Siltegro we use Hanwha pick&place machines — valued for their precision and reliability.
Step 3 — Reflow soldering
The board with placed components passes through a multi-zone reflow oven. A controlled temperature profile — preheating, soaking, reflow and cooling phases — melts the paste and creates permanent electrical connections. The profile is adapted to the requirements of each board and component type.
Step 4 — AOI (Automatic Optical Inspection)
Every board passes through an AOI system after soldering — a camera scans the entire surface and compares the image with the design. It detects missing components, misalignment, solder bridges and cold joints. At Siltegro we use the Saki DMG Mori AOI system — inspection at an offline station, not random sampling.
Step 5 — X-Ray inspection — optional or standard
For BGA and QFN components, where connections are invisible after assembly, X-Ray inspection verifies the quality and completeness of the solder balls. Available at Siltegro as an option or as a standard for indicated project types.
Step 6 — Electrical testing
ICT (In-Circuit Test) or functional tests according to the client's specification. The test report is attached to the order documentation.
When to use surface mount technology (SMT)?
SMT is the right choice for the vast majority of modern electronics projects. Especially where:
- Miniaturization is key — portable devices, IoT, sensors, communication modules
- The project requires high component density — a lot of functionality on a small board
- Series production is planned — full automation means repeatability and lower unit cost
- The project uses BGA, QFN or other advanced IC packages — available only in SMT
- Good HF performance is required — short signal paths are an SMT advantage
When SMT is not enough and THT or mixed assembly is needed:
- Mechanical connectors exposed to pull-out forces (connectors, USB, audio)
- High-power components and large transformers
- Devices exposed to vibration and shock (automotive, heavy industry)
- Components requiring field servicing
→ When to use through-hole technology (THT)?
What sets us apart in SMT assembly?
AOI inspection for every board — not sampling
Every board passes through AOI after soldering. Not random checks, not sampling — every unit from every batch. Defects are caught on the line, not at the client.
IPC-A-610 — certification of the whole team
The IPC-A-610 standard applies to every operator on the line. The entire operator team holds current certification — not just the quality control staff.
Production against the golden sample
Every series is produced against the approved reference. Deviations from the golden sample are detected and reported before delivery.
Full batch traceability as a standard
Every series number, every component, every client has its own identifier. Traceability is built into every order — not a premium option. Documentation archived for up to 10 years.
Assembly from 1 piece — the same standard
Electronics production from a single piece on a dedicated prototype line. The same IPC-A-610 standard, the same AOI inspection, the same documentation as a multi-thousand series.
SMT assembly at Siltegro — what is worth knowing?
Our SMT machine park was chosen with flexibility and scalability in mind. We have several modern SMT lines, each adapted to handle a different type of order — from prototypes, through small and medium series, up to high-volume production on our newest, high-output line. This allows us to execute orders efficiently regardless of their size, while maintaining the highest SMD technology standards.
Machine park — Hanwha pick&place machines, offline Saki DMG Mori AOI inspection, X-Ray inspection available. Reflow oven with a profiled thermal process for every order.
Component range — we handle components from 0201 upwards: BGA, QFN, QFP, SOT and others. Single-sided and double-sided assembly.
Quality standard — the entire operator team holds IPC-A-610 certification. AOI inspection for every board — not sampling. X-Ray inspection available for BGA and QFN components. Production against the approved reference (golden sample).
Traceability — every production batch is marked with a unique ID. Documentation archived for up to 10 years. Full traceability from component intake to delivery.
Prototypes and series — electronic prototypes and series SMT electronics production from 1 piece on a dedicated prototype line — with the same quality standards. Contract electronics production for Polish and European manufacturers.
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Series SMT and THT production →
