SOT-223 PCB Footprint: Dimensions, Thermal DFM, Assembly

SOT-223 is a power package package used for surface mount assembly, also seen labeled SOT-223-3, SOT-223-4, TO-261AA. A dependable footprint follows the exact orderable-device drawing rather than the family name: nominal body About 6.5 × 3.5 mm, overall span About 6.5–7.0 mm including leads and tab, seated height Typically 1.6–1.8 mm, pitch 2.3 mm for the three small leads, pin count 3 small leads plus one large tab pad; tab is often electrically duplicated, and exposed pad Large gull-wing/tab terminal on the rear side.

Follow the device drawing and thermal test board; tab pad area is both electrical and thermal design input.

Typical uses include linear regulators, medium-power transistors, voltage references. TO-261/SOT-223 variants are similar, but lead, tab, and thermal recommendations must come from the exact MPN.

• The wide tab dominates heat flow and is commonly connected to the center lead internally, but the net depends on device function.
• Published power dissipation is inseparable from copper area, layer stack, airflow, and maximum junction temperature.

Connect the tab to a deliberate copper heat spreader on its actual net and add thermal vias only when another layer can safely share that net.

• Keep heat-sensitive feedback dividers and electrolytic capacitors outside the expected hot zone.

Hand assembly is rated moderate. Preheat, solder small leads first, then heat the tab with adequate flux. Watch for a cold tab joint and assuming the tab is ground when it may be output or drain.

• Segment or reduce paste on an unusually large tab land according to the assembler's stencil rules.
• Avoid narrow thermal-relief spokes when the tab must conduct significant heat or load current.
• Verify the courtyard includes the full lead and tab span, not just the molded body.

Inspection focus:

• The small leads are optically visible; tab wetting is less obvious and may need side inspection or thermal/functional evidence.
• Measure case temperature and output behavior at worst-case input voltage and load rather than relying on room-temperature continuity.

Common mistakes:

• Connecting the tab to ground by package habit instead of checking the device pin table.
• Claiming a regulator current rating without calculating package dissipation at the actual voltage drop.

1. Before approving SOT-223, compare the exact orderable-device drawing with the library item: body range (About 6.5 × 3.5 mm), terminal or lead span (About 6.5–7.0 mm including leads and tab), pitch (2.3 mm for the three small leads), pin count (3 small leads plus one large tab pad; tab is often electrically duplicated), height (Typically 1.6–1.8 mm), and exposed-pad definition (Large gull-wing/tab terminal on the rear side). Record the source drawing revision and every intentional courtyard, toe, heel, side, mask, or paste adjustment.
2. Treat the moderate hand-solder rating as a prototype-planning input, not proof of production yield. Review a cold tab joint and assuming the tab is ground when it may be output or drain with the assembler, confirm that preheat, solder small leads first, then heat the tab with adequate flux is compatible with the build, and require the S1 connectivity gate plus relevant S2 geometry checks to pass against the released footprint and selected fabrication profile.

Manufacturing gate checks:

1. S1Pad count, numbering, and schematic parity. The tab and center lead relationship must match the selected regulator or transistor.
2. S2Solder-mask web and pad opening. The tab opening and nearby small pads need controlled mask and paste geometry.
3. S2Courtyard and body clearance. The full tab span and thermal keepout must be represented.