SMT
Surface Mount Technology
SMT (Surface Mount Technology) is the dominant PCB assembly method where components are placed directly onto the surface of the board and soldered using reflow ovens. It enables smaller, denser, double-sided boards and supports nearly all modern component packages including BGAs, QFNs, and 0402 passives.
What it is
SMT assembly follows a fixed sequence. Solder paste — a mixture of tiny solder spheres in flux — is printed onto the board through a laser-cut steel stencil, depositing paste precisely on each pad. A pick-and-place machine then positions components onto the paste using vision-aligned heads, working from a pick-and-place file that specifies the location and rotation of every part. The populated board passes through a reflow oven that follows a controlled temperature profile, melting the solder paste and forming joints as it cools.
After reflow, automated optical inspection (AOI) checks for missing, misaligned, or poorly soldered components. Hidden joints under BGAs or QFNs may require X-ray inspection. Boards with components on both sides go through the SMT process twice, with the first side's joints protected by a higher-melting-point paste or careful thermal profiling.
SMT supports the full range of modern component packages: 0402 and smaller passives (down to 01005 in specialised assembly), fine-pitch QFPs, BGAs down to 0.4 mm pitch in standard production (0.3 mm and below available at specialised fabricators), QFNs, and modules. It is faster, denser, and cheaper at volume than through-hole assembly.
When it matters
Nearly all modern designs are SMT-dominant, with through-hole used selectively for connectors, large capacitors, and parts that need mechanical strength. The choice affects pick-and-place programming, stencil cost, and inspection requirements — for fine-pitch BGAs in particular, the surface finish, stencil design, and reflow profile all become critical and benefit from DFM review before assembly starts. Designs with mixed SMT and THT cost more per board than pure SMT because they require a separate soldering pass (wave, selective, or hand soldering).
At Nordic PCB
SMT assembly is the default at all our certified European partners, with capability for fine-pitch BGAs, 0402 passives, and double-sided population. Our DFM review covers SMT-specific concerns including pad geometry for BGAs, solder paste stencil requirements, and footprint compatibility with the selected surface finish — flagging issues before paste printing.
Related terms
- PCBA
PCBA (Printed Circuit Board Assembly) is the process of populating a bare PCB with electronic components — resistors, capacitors, ICs, connectors, and other parts — to produce a functional electronic assembly ready for testing or integration into a finished product.
- THT
THT (Through-Hole Technology) is an assembly method where component leads are inserted through plated holes in the PCB and soldered from the opposite side, typically using wave or selective soldering. Used for connectors, large capacitors, transformers, and parts requiring mechanical strength or high current capacity.
- AOI
AOI (Automated Optical Inspection) uses high-resolution cameras to inspect solder joints, component placement, and orientation after assembly. Modern 3D AOI measures joint height and volume volumetrically, catching defects like lifted leads and insufficient solder that 2D systems miss. Limited to visible surfaces — hidden joints under BGAs require X-ray inspection.
- Stencil
A stencil is a thin laser-cut steel sheet with apertures matching the SMT pads on the PCB, used to apply solder paste precisely before component placement. Typical thickness is 100-150 µm. Stencil aperture design affects solder paste volume and is critical for fine-pitch BGAs and QFNs to prevent insufficient solder or bridging.
- Pick and place file
A pick and place file (also called centroid file, CPL, or XY data) is a machine-readable list of every component's position, rotation, and board side. It is one of the three core files — alongside Gerber and BOM — needed to quote and run PCB assembly.