SMD / SMT

What they mean

SMD (Surface Mount Device) is the component. SMT (Surface Mount Technology) is the assembly method that puts it on the board. In practice the two terms get used interchangeably — “SMD parts”, “an SMT assembly line”, and so on.

Both terms relate to the same basic approach: components are mounted directly onto pads on the PCB surface, with their terminals soldered to those pads. Component leads don't go through drilled holes. That's the contrast with through-hole packaging like DIP chips, where each pin passes through the board and solders to a pad on the far side.

Why the industry switched

• Density. SMD parts can sit on both sides of a board and get much closer to each other than through-hole parts can. Modern smartphone PCBs are overwhelmingly populated with surface-mount components for this reason.
• Cost. Surface-mount assembly reduces the number of component through-holes and the associated fabrication and assembly steps, while allowing greater component density. Vias for inter-layer connections still exist on an SMT board, but their count and cost are typically much lower than a comparable through-hole design.
• Automation. Modern pick-and-place machines can populate tens of thousands of components per hour under suitable conditions, with sub-millimetre accuracy. Through-hole insertion is slower and needs specialised heads for each package.
• High-frequency behaviour. At high frequencies the shorter connections and lower parasitic inductance of many SMD packages provide a significant advantage. Through-hole parts become increasingly hard to use as frequency rises, though they aren't categorically unsuitable above any particular frequency.

The main package families

• Chip resistors and capacitors — the little rectangles labelled by size (0805, 0603, 0402, 0201, 01005). These are normally imperial dimensions in hundredths of an inch, though metric package codes are also used — 0805 imperial is 2.0 × 1.25 mm, 0603 imperial is 1.6 × 0.8 mm, and so on. Always worth checking which convention a given datasheet means.
• SOIC / SOP / TSSOP — small-outline packages, essentially a shrunk DIP with gullwing leads bent sideways.
• SOT / SC-series — small transistor-style packages available in many shapes and pin counts.
• QFP (Quad Flat Package) — square package with gullwing leads on all four sides. TQFP and LQFP are the common thin variants.
• QFN / DFN — Quad or Dual Flat No-lead. The terminals are flush metal pads on the underside of the package rather than legs sticking out. Denser than QFP but harder to hand-solder.
• BGA (Ball Grid Array) — a grid of solder balls on the underside of the chip. Normally assembled by reflow; manual soldering and inspection are considerably more difficult because the joints are underneath the package.
• WLCSP (Wafer-Level Chip-Scale Package) — a very small BGA-style package whose footprint is approximately the size of the silicon die. Extremely dense and specialist.

How SMD parts are soldered

Production SMT assembly deposits solder paste through a stencil onto the pads, drops parts on top with a pick-and-place head, then runs the whole board through a reflow oven. The solder paste melts, wets to both pad and terminal, and solidifies as the board cools.

At the bench, with a fine tip, good flux, and suitable technique, many packages down to 0603 and fine-pitch ICs such as SOIC can be hand-soldered. Smaller passives and lead-under-body packages such as QFN are increasingly better suited to hot-air rework, a reflow hotplate, or a small oven. QFN and especially BGA packages are substantially harder to hand-solder and inspect, so reflow or specialised rework equipment is normally preferred.

Why breakouts exist

An increasing proportion of modern ICs are available only in SMD packages — particularly high-pin-count and highly integrated devices. Their terminal pitches — 0.65 mm on a TSSOP, 0.5 mm on a QFP, tighter still on a QFN or BGA — are nowhere near the 2.54 mm pitch of a solderless breadboard, and lead-under-body packages like QFN and BGA don't have pins to push into anything.

A breakout board carries the SMD chip on top and translates its terminals out to a DIP-format 2.54 mm pin header on the underside. The chip operates just as it would on a conventional PCB; the breakout primarily provides the mechanical and electrical translation to breadboard-compatible spacing. That's our whole product line.