SMD vs SMT: what's the difference?

8 mm SMD component tape loaded in an SMT Caddy MTI manual SMT tape feeder

Short answer: SMD (surface-mount device) is the component. SMT (surface-mount technology) is the method of assembling those components onto pads on the surface of a PCB. "SMD resistor" and "SMT assembly" are both correct, and in everyday use the terms are interchangeable: an SMD tape feeder and an SMT tape feeder are the same thing.

SMD: the part

A surface-mount device has metal terminations or short leads that sit on top of copper pads, instead of wire leads that pass through holes in the board. Common SMD packages:

  • Chip resistors and capacitors, named by size: 1206, 0805, 0603, 0402, 0201 (imperial codes, in hundredths of an inch; 0402 is about 1.0 × 0.5 mm).
  • Transistors and diodes: SOT-23, SOD-123 and similar.
  • ICs: SOIC, TSSOP, QFN, QFP, BGA.

SMT: the process

Surface-mount technology covers the assembly steps:

  1. Paste: solder paste is applied to the pads, usually through a stencil.
  2. Pick and place: each part is taken from its packaging and set onto the paste, by hand, with a manual pick-and-place setup or with an automatic machine.
  3. Reflow: the board is heated so the paste melts and forms the joints.
  4. Inspection and any rework.

The opposite approach is through-hole technology (THT), where leads pass through drilled holes and are soldered on the other side.

How SMD parts are packaged: tape

Most SMD parts ship in carrier tape: a strip with one pocket per part, sealed by a clear cover tape, wound on a reel or sold as a short length of cut tape. Standard tape widths are 8, 12, 16, 24 and 32 mm; small passives use 8 mm tape. Automatic machines feed tape from reels. For prototypes, you usually work from cut tape, and that's where a manual SMD/SMT tape feeder helps: it advances the strip one pocket at a time and peels the cover tape, so the next part is always at the same pick point.

Which term should you search for?

Both. People tend to say "SMD" for parts and tape ("SMD tape", "SMD resistor") and "SMT" for equipment and process ("SMT feeder", "SMT line"), but sellers use both for the same product. If you're looking for equipment to hand-assemble prototype boards, try "manual pick and place", "SMD tape feeder" and "SMT tape feeder".

Related