The TO-92 package is a three-lead molded plastic body about 4.6 mm across, with leads on 1.27 mm centers and a flat face for orientation. JEDEC registers it as TO-226AA. Two things trip people up: the pinout is set by the case style code, not by “TO-92”, and the taped version is mechanically a different part from the bulk version. Dimensions, thermal numbers and footprint rules below.
Key takeaways
- TO-92 equals TO-226AA in JEDEC Publication 95, and SOT54 in Philips/NXP nomenclature.
- Straight leads sit on 1.27 mm centers. Formed leads spread to 2.40–2.90 mm, and taped parts are always formed.
- The pinout comes from the case style number. onsemi CASE 29 STYLE 1 is E-B-C; CASE 29-04 STYLE 17 is C-B-E.
- A 2N3904 in TO-92 dissipates 625 mW at 25 °C and 400 mW at 70 °C. The 1.5 W figure at TC = 25 °C is not reachable on a plastic body.
- The package is still current: TI ships AEC-Q100 Grade 0 Hall sensors in TO-92 rated to 150 °C ambient.
What is a TO-92 package?
The TO-92 package is a three-lead through-hole plastic package for low-power discrete semiconductors and small ICs, standardized by JEDEC as TO-226AA. The body is roughly 4.6 mm wide, 4.8 mm tall and 3.7 mm thick, with one flat face used as the orientation index and leads on 1.27 mm centers. It houses transistors, JFETs, small regulators, temperature sensors and Hall-effect sensors.
[IMAGE 1: TO-92 package photographed flat-face-on beside a SOT-23 part, with calipers spanning the 1.27 mm lead pitch | alt: “TO-92 package flat face and 1.27 mm lead pitch next to a SOT-23”]
TO-92 dimensions
Bourns publishes the outline as drawing MD-TO92-001-a (August 2005), compliant to TO-226AA in JEDEC Publication 95. These are limits, not typicals.
| Feature | Min | Max |
| Body height | 4.32 mm | 5.34 mm |
| Body width across the flat | 4.44 mm | 5.21 mm |
| Body thickness, flat face to round back | 3.17 mm | 4.19 mm |
| Lead cross-section, wide dimension | 0.40 mm | 0.56 mm |
| Lead cross-section, thin dimension | 0.35 mm | 0.41 mm |
| Lead length | 12.7 mm | not specified |
| Straight-lead pitch | 1.27 mm basic | 1.27 mm basic |
| Formed-lead spacing | 2.40 mm | 2.90 mm |
Table 1. TO-92 (TO-226AA) outline limits, per Bourns drawings MD-TO92-001-a and MD-TO92-002-a, August 2005.
Two consequences. The leads are rectangular, not round, so hole sizing works from the diagonal: 0.56 mm × 0.41 mm gives a 0.69 mm corner-to-corner maximum, and a 0.9 mm to 1.0 mm finished hole clears it with room for a fillet. And the body height tolerance spans 1.02 mm, which matters if the part sits under a shield or a stacked board.
The pinout is set by the case style, not by “TO-92”
This is the single most expensive mistake on this package. TO-226AA fixes the body and the lead pitch. It does not fix which lead is the collector. The case style number on the datasheet does.
| Device and source | Case or package code | Pin 1 | Pin 2 | Pin 3 |
| 2N3903 / 2N3904 (onsemi, Rev. 3, Nov 2000) | CASE 29, STYLE 1 | Emitter | Base | Collector |
| BC546 / BC547 / BC548 (Motorola BC546/D) | CASE 29-04, STYLE 17 | Collector | Base | Emitter |
| BC546 / BC547 (Philips spec, 15 Apr 1999) | SOT54 | Emitter | Base | Collector |
Table 2. Documented pin assignments for TO-92 devices. The outline is identical; the case style is not.
Read those last two rows carefully. The same type number, BC547, is documented with pin 1 as collector by one manufacturer and pin 1 as emitter by another. The drawings differ in which face they are viewed from, so both are internally consistent and neither is wrong. What is wrong is copying a footprint from the type number.
Pins are numbered left to right with the flat face toward you and the leads pointing down. Take the assignment from the drawing on the datasheet you are actually buying against, and record the case style code in the footprint library alongside the part number.
[IMAGE 2: two TO-92 outlines side by side, flat face forward, one labeled E-B-C and one labeled C-B-E, with the case style codes called out | alt: “TO-92 package pinout differences between CASE 29 STYLE 1 and CASE 29-04 STYLE 17”]
Footprint: 1.27 mm straight or 2.54 mm formed
Straight-lead parts arrive on 1.27 mm centers. Most boards use 2.54 mm, in a triangular pattern with the middle lead offset, which means the leads get bent either by the assembler or by the supplier.
Buying pre-formed solves the repeatability problem. The Bourns formed-lead drawing gives 2.40 mm to 2.90 mm outer spacing, so the pattern must accept that spread rather than assuming an exact 2.54 mm.
Bending in-house has a rule. IPC-A-610 and J-STD-001 require the bend to start at least one lead diameter or thickness, and never less than 0.8 mm, away from the body or lead seal, with a minimum inside bend radius of one lead thickness for leads under 0.8 mm. TO-92 leads are 0.35 mm to 0.56 mm, so both the 0.8 mm standoff and a tight inside radius apply. Bending at the plastic is what cracks the die attach.
Thermal limits, and one number that is not usable
Compare the same die in three packages, from the Fairchild 2N3904 / MMBT3904 / PZT3904 datasheet (Rev. B0, October 2011).
| Parameter | TO-92 (2N3904) | SOT-23 (MMBT3904) | SOT-223 (PZT3904) |
| Total device dissipation at 25 °C | 625 mW | 350 mW | 1,000 mW |
| Derating above 25 °C | 5.0 mW/°C | 2.8 mW/°C | 8.0 mW/°C |
| RθJA | 200 °C/W | 357 °C/W | 125 °C/W |
| RθJC | 83.3 °C/W | not specified | not specified |
| Board condition for the RθJA figure | free air | FR-4, 1.6 × 1.6 × 0.06 in | FR-4, 36 × 18 × 1.5 mm, collector pad 6 cm² min |
| Mounting | Through-hole | Surface mount | Surface mount |
Table 3. One die, three packages. The RθJA figures assume different test boards and are not directly comparable.
That last row is why the numbers are not directly comparable. The SOT-223 figure assumes a 6 cm² copper pad; remove it and the advantage disappears. The TO-92 figure assumes nothing at all, which makes it the honest one.
Worked example. A 2N3904 at TA = 70 °C in still air: 625 mW − (5.0 mW/°C × 45 °C) = 400 mW. The thermal path agrees, since (150 °C − 70 °C) ÷ 200 °C/W is also 400 mW.
Now the trap. onsemi also lists 1.5 W at TC = 25 °C with 12 mW/°C derating. The arithmetic is self-consistent, because 1.5 W × 83.3 °C/W is a 125 °C rise to the 150 °C junction limit. It is still unusable: there is no tab, no exposed pad and no practical way to clamp a molded plastic body to 25 °C. Budget from RθJA.
Compliance and current status
TO-92 is not a legacy-only package. Texas Instruments ships the DRV5013, DRV5053 and DRV5055 Hall-effect sensors in a TO-92 body designated LPG or LPE, measuring 4.00 mm × 3.15 mm, AEC-Q100 qualified, with Grade 0 parts rated to 150 °C ambient. Through-hole survives here because a Hall sensor often needs to sit on a lead frame away from the board, pointed at a magnet.
For moisture handling, a molded TO-92 falls under the usual plastic-package rules rather than the seal tests used for hermetic cans. Confirm the MSL and floor life on the specific datasheet before a reflow or wave process.
Ordering, packing and lead time
The orderable suffix decides the mechanical part you receive, and tape and reel forces formed leads, because the leads must be spread to hold the part in the tape.
Two current examples. onsemi ships 2N3903 and 2N3904 as 5,000 units per box in bulk, with the RLRM suffix giving 2,000 per ammo pack. TI’s DRV5013 nomenclature uses R for 3,000 per reel, T for 250 per reel, M for 3,000 per box in ammo pack, and a blank suffix for 1,000 per bag in bulk.
Lead times diverge by packing. At DigiKey in August 2026, onsemi’s 2N3904BU in bulk showed a 27-week manufacturer lead time, the taped 2N3904TA and 2N3904TAR showed 13 weeks, and a second-source 2N3904 showed 5 weeks. Same die, same outline, very different schedule risk.
That spread is also the counterfeit exposure. Loose TO-92 parts from surplus channels carry no reel label and a laser mark that is easy to fake. Buy through authorized distribution and keep the reel or ammo-pack label with the lot.
[IMAGE 3: TO-92 parts in ammo-pack tape with formed leads alongside a loose bulk bag of straight-lead parts | alt: “TO-92 package in ammo pack with formed leads compared with bulk straight-lead parts”]
Design mistakes that cause returns
- Taking the pinout from the type number instead of the case style code. BC547 is documented both ways.
- Assuming a footprint drawn for straight 1.27 mm leads will accept a taped, pre-formed part.
- Sizing the hole from a nominal round lead. The lead is rectangular, and the 0.69 mm diagonal governs.
- Bending leads at the body rather than 0.8 mm out, which stresses the lead seal and the die attach.
- Budgeting power from the TC = 25 °C rating on a package with no thermal interface.
- Ordering bulk when the schedule needs tape, or the reverse, without checking lead times per suffix.
Frequently asked questions
What are the dimensions of a TO-92 package?
Per the TO-226AA outline, the body runs 4.32 mm to 5.34 mm high, 4.44 mm to 5.21 mm wide across the flat face, and 3.17 mm to 4.19 mm thick front to back. Leads are rectangular, 0.40 mm to 0.56 mm by 0.35 mm to 0.41 mm, at least 12.7 mm long, on 1.27 mm centers.
What is the TO-92 pinout?
There is no single TO-92 pinout. Pins number left to right with the flat face toward you. The case style code sets the function: onsemi CASE 29 STYLE 1 is emitter, base, collector, while CASE 29-04 STYLE 17 is collector, base, emitter. Always read the drawing on the device datasheet.
Is TO-92 the same as TO-226?
TO-226 is the JEDEC family; TO-226AA is the standard three-lead TO-92 outline in JEDEC Publication 95. Philips and NXP call the same body SOT54. Distributors often list the package as TO-226-3 or TO-92-3, and add “formed leads” for the taped version.
How much power can a TO-92 package dissipate?
Around 625 mW at 25 °C ambient in free air for a typical small-signal transistor, derating at 5.0 mW/°C. That gives 400 mW at 70 °C. Junction-to-ambient resistance is about 200 °C/W, so the package, not the die, sets the limit.
What is the TO-92 lead spacing?
Straight leads are on 1.27 mm (0.050 in) centers as molded. Formed or taped parts are spread to 2.40 mm to 2.90 mm so they sit on a nominal 2.54 mm grid and stay retained in the carrier tape. A footprint should accept the formed range, not a single value.
What to do next
Before you place the footprint, get three things off the specific datasheet: the case style code, whether the orderable suffix is straight or formed, and the RθJA figure. Those three settle the pinout, the hole pattern and the power budget, and none of them is implied by the words “TO-92”.
If the design is new and the part will be machine-placed, price the SOT-23 version of the same die first. Move to TO-92 only when you need a through-hole lead frame, a sensor standing off the board, or a hand-assembled build.
Primary sources
- Bourns TO-92 and TO-92 (formed leads) package mechanical information — https://bourns.com/pdfs/TO-92pkg.pdf
- onsemi (Motorola) 2N3903 / 2N3904 datasheet, CASE 29 STYLE 1 — https://www.montana.edu/aolson/ee503/2N3903-D_onsemi.pdf
- Fairchild 2N3904 / MMBT3904 / PZT3904 datasheet, Rev. B0 — https://pdf.direnc.net/upload/2n3904-transistor-bjt-npn-to-92-datasheet.pdf
- Texas Instruments DRV5013 digital-latch Hall sensor datasheet (TO-92 LPG/LPE) — https://www.ti.com/lit/ds/symlink/drv5013.pdf
- Analog Devices 3-pin plastic header-style package [TO-92], TO-226-AA — https://www.analog.com/media/en/package-pcb-resources/package/pkg_pdf/to-92t/T-3-1.pdf
Proposed internal links
- [INTERNAL LINK: “SOT-23 footprint and land pattern” → SOT-23 package guide]
- [INTERNAL LINK: “TO-72 four-lead metal can” → TO-72 package reference page]
- [INTERNAL LINK: “through-hole lead forming to IPC-A-610” → lead forming and strain relief guide]
- [INTERNAL LINK: “reading thermal resistance figures” → junction-to-ambient thermal design guide]
- [INTERNAL LINK: “tape, reel and ammo pack formats” → component packaging and feeder guide]
