TO-can packages are hermetically sealed metal cylinders with radial leads, still specified where a plastic body cannot go: rad-hard parts, precision references, laser diodes, and anything that must survive vacuum or moisture. TO stands for Transistor Outline. The family map matters because JEDEC renamed most of it, so TO-18 is now TO-206AA and TO-39 is TO-205AD, and distributor filters use both sets of names.

Key takeaways

• JEDEC groups these by lead circle diameter: TO-206 covers the 2.54 mm circle (TO-18, TO-46, TO-52, TO-72), TO-205 the 5.08 mm circle (TO-5, TO-12, TO-33, TO-39).
• A metal can does not automatically dissipate more in free air. The TO-18 2N2222A is rated 500 mW at 25 °C ambient, less than a plastic TO-92 equivalent.
• The can wins when the case is heat-sunk: the same STMicroelectronics datasheet gives 83.3 °C/W junction-to-case against 300 °C/W junction-to-ambient for TO-18.
• Junction-to-case resistance for one outline varies by vendor — Philips specifies 146 K/W for TO-18 where ST specifies 83.3 °C/W.
• Lead times run 9 to 22 weeks on current TO-18 parts, and several classic numbers are already obsolete.

[IMAGE 1: TO-18, TO-39, TO-5 and TO-8 cans photographed at true relative scale with base diameters annotated | alt: “TO-can packages TO-18, TO-39, TO-5 and TO-8 compared at scale”]

What a TO-can package is

A TO-can package is a semiconductor housed in a drawn metal cap welded to a metal header, with leads passing through glass-to-metal seals in a circle on the underside. The cavity is sealed hermetically, usually with an inert fill. The case is frequently tied to one electrode, the collector on most transistors.

That construction sets everything else. Glass seals give hermeticity, the metal header gives a low-resistance thermal path out of the die, and the drawn-cap process gives the cost that plastic molding undercut decades ago.

The renaming: TO-205 and TO-206

JEDEC reorganized these outlines around the diameter of the lead circle rather than around individual case numbers. Two families absorbed most of the old designations.

Family	Lead circle	Old name	Current JEDEC variant
TO-206	2.54 mm (0.100 in)	TO-18	TO-206AA
TO-206	2.54 mm	TO-46	TO-206AB
TO-206	2.54 mm	TO-52	TO-206AC
TO-206	2.54 mm	TO-72	TO-206AF
TO-206	2.54 mm	(new, 1.40 mm cap height)	TO-206AD
TO-205	5.08 mm (0.200 in)	TO-5	TO-205AA
TO-205	5.08 mm	TO-12	TO-205AB
TO-205	5.08 mm	TO-33	TO-205AC
TO-205	5.08 mm	TO-39	TO-205AD
TO-205	5.08 mm	(new, 4.32 mm cap height)	TO-205AF

Datasheets carry both forms. Microchip’s rad-hard JANS_2N3057A, for example, is offered in “TO-46 (TO-206AB)” with alternates in “TO-39 (TO-205AD)” and “TO-18 (TO-206AA).”

International equivalents exist too. The TO-5 group maps to IEC 60191 codes such as C4/B4A and DIN 41873 codes such as 5A3, which is worth knowing if you receive European drawings.

[INTERNAL LINK: JEDEC package outline numbering -> JEP95 outline reference]

Sizes: what actually distinguishes the cans

Package	Base diameter	Cap diameter	Cap height	Lead circle
TO-18 (TO-206AA)	5.6 mm	4.70 mm	4.83 mm	2.54 mm
TO-72 (TO-206AF)	5.6 mm	4.70 mm	4.83 mm	2.54 mm, 4 leads
TO-39 (TO-205AD)	9 mm	—	up to 12.7 mm high-profile	5.08 mm
TO-8	15.24 mm	12.29 mm	7.62 mm	—

Two details from the TO-18 outline are easy to miss. Lead diameter is nominally 0.45 mm, and minimum lead length is 12.7 mm, so these arrive long and get trimmed. The orientation tab sits 45° from pin 1, which on a transistor is usually the emitter, so the tab is not aligned with any lead.

TO-72 shares the TO-18 body exactly and adds a fourth lead, normally bonded to the case for RF shielding. If a schematic shows a fourth pin on a can that looks like a TO-18, that is what it is.

What the can buys thermally, and what it does not

This is where the ranking pages mislead. STMicroelectronics published one datasheet covering the 2N2219A in TO-39 and the 2N2222A in TO-18, so the two packages are directly comparable.

Parameter	TO-18 (2N2222A)	TO-39 (2N2219A)
Total dissipation, Tamb ≤ 25 °C	0.5 W	0.8 W
Total dissipation, TC ≤ 25 °C	1.8 W	3 W
Rth junction-to-case, max	83.3 °C/W	50 °C/W
Rth junction-to-ambient, max	300 °C/W	187.5 °C/W
Max junction temperature	175 °C	175 °C

Work the derating at 70 °C ambient. In free air:

TO-18: (175 − 70) ÷ 300 = 350 mW

TO-39: (175 − 70) ÷ 187.5 = 560 mW

Now clip a heat sink to the TO-39 can and hold the case at 70 °C:

(175 − 70) ÷ 50 = 2.1 W

That is 3.75× the free-air figure. The metal buys nothing until you use it. An unheatsunk TO-18 in still air handles less power than a plastic TO-92 of similar rating, because free-air dissipation is set by surface area and lead conduction, not by the case material. Choose a can for the case path, hermeticity, or shielding, not for a vague expectation of better cooling.

[IMAGE 2: TO-39 with and without a clip-on heat sink, dissipation figures annotated for both | alt: “TO-can package dissipation with and without a clip-on heat sink”]

The same outline, different thermal resistance

Package geometry does not fix thermal performance. For the same TO-18 outline and essentially the same device, Philips Semiconductors specified 350 K/W junction-to-ambient and 146 K/W junction-to-case in its 1997 2N2222 product specification, where ST specified 300 °C/W and 83.3 °C/W.

The difference is die size, die-attach material, and header construction, none of which the outline drawing controls. Continental Device India’s 2N2221/2N2222 sheet adds a third framing entirely: 500 mW at Ta = 25 °C derating at 2.28 mW/°C, and 1.2 W at Tc = 25 °C derating at 6.85 mW/°C.

Never carry a thermal number across from a second source. Pull the figure from the datasheet of the part you are actually buying.

Hermeticity and where TO cans still win

The glass-to-metal seal is the reason these packages survive. Plastic bodies absorb moisture and outgas; a welded can does neither, which is why the format persists in four places.

Radiation-hardened parts. Microchip’s JANS_2N3057A ships in TO-46 with JAN qualification to MIL-PRF-19500/391. The original 2N2222 was itself a military part under MIL-PRF-19500/255.

Precision references and sensors. A sealed cavity holds a stable internal environment, and the metal shields the die electrically.

Optoelectronics. TO headers with window caps carry laser diodes and photodiodes, where the seal protects the facet and the cap holds the optic.

Vacuum and space. Anything that must not outgas into a chamber or a spacecraft.

Outside those cases, the plastic equivalent wins on cost and automation. That is the honest summary.

Lead counts, profiles and the 0.230 inch exception

TI’s metal can outline set, document SNOA033, catalogs more variants than most engineers expect: TO-3 in 2, 4, 8 and 15 lead versions; TO-5 in 6, 8 and 10 lead; TO-39 in low-profile and high-profile 3-lead forms plus a 4-lead version; TO-46 in 2, 3 and 4 lead.

It also documents a trap. TO-5 cans exist with two different lead circles, the standard 0.200 in and a widened 0.230 in, the latter allowing a larger die under an unchanged cap. TO-77 and TO-101 use the widened circle. A footprint drawn for one will not accept the other.

Profile matters as well. High-profile TO-39 reaches 12.7 mm tall against low-profile versions well under that, and datasheets often carry both under one part number family.

Cost, lead time and lifecycle

Current DigiKey listings tell the sourcing story for TO-18 parts:

• 2N2222A (Microchip): Active, zero stock, 22-week manufacturer standard lead time, $2.90 at qty 1 and $2.69 at 100.
• 2N2222A PBFREE (Central Semiconductor): Active, 9-week lead time.
• 2N2907A PBFREE (Central Semiconductor): Active, 12-week lead time.
• 2N2222 (onsemi): Obsolete.

Three consequences. Prices sit near $3 for a small-signal transistor whose plastic equivalent costs cents. Lead times of 9 to 22 weeks mean these are order-ahead parts. And obsolescence is uneven within a single part number: one supplier’s 2N2222 is dead while another’s 2N2222A ships.

For legacy repair, the JAN and 2N numbering helps, since multiple suppliers build to the same registered outline and military-qualified versions carry a slash sheet you can search against.

[INTERNAL LINK: component obsolescence and second-source qualification -> lifecycle management]

TO-can package FAQ

What is a TO can package?

A semiconductor in a hermetically sealed metal cylinder, with leads passing through glass seals in a circle on the underside. TO stands for Transistor Outline, a JEDEC drawing series. The case is often connected internally to one electrode, typically the collector on a transistor.

What is the difference between TO-18 and TO-39?

Size and lead circle. TO-18 has a 5.6 mm base with leads on a 2.54 mm circle; TO-39 has a 9 mm base with leads on a 5.08 mm circle. On one ST datasheet the TO-39 part is rated 0.8 W in free air against 0.5 W for the TO-18, and 50 °C/W junction-to-case against 83.3 °C/W.

Is TO-18 the same as TO-206AA?

Yes. JEDEC reorganized the metal can outlines by lead circle diameter, and TO-18 became TO-206AA. The same renaming made TO-46 into TO-206AB, TO-52 into TO-206AC, and TO-72 into TO-206AF. Datasheets and distributor filters use both forms, often together.

Why are metal can packages still used?

Hermeticity. Radiation-hardened parts, precision references, laser diode headers, and vacuum or space hardware all need a sealed cavity that plastic cannot provide. Microchip’s rad-hard JANS_2N3057A, qualified to MIL-PRF-19500/391, ships in TO-46 for exactly this reason.

Is the case connected to the collector?

On most metal can transistors, yes. That makes the can an electrical node, so an ungrounded heat sink clipped to it sits at collector potential. On TO-72 the fourth lead is bonded to the case instead, providing a shield connection independent of the device electrodes.

Are TO-5 and TO-18 packages obsolete?

Not as outlines, but individual part numbers are going. onsemi’s 2N2222 in TO-18 is listed obsolete while Central Semiconductor’s 2N2222A PBFREE remains active with a 9-week lead time. Check lifecycle per manufacturer part number, not per package.

What to do next

Identify the can by lead circle before anything else. Leads on a 2.54 mm circle put you in TO-206; leads on a 5.08 mm circle put you in TO-205. Then confirm the cap height, since several variants differ only in that dimension.

Decide whether you are buying hermeticity or a thermal path. If it is the thermal path, budget a clip-on heat sink and design to the junction-to-case figure. If it is neither, the plastic equivalent is cheaper, more available, and easier to place.

Pull thermal numbers from the datasheet of the exact part you will buy. The 63 % spread between two vendors’ junction-to-case figures for the same TO-18 outline is larger than most designers’ thermal margin.

[IMAGE 3: cutaway of a TO-can showing glass-to-metal lead seals, die attach to the header, and the welded cap | alt: “TO-can package cutaway showing glass seals, die attach and welded cap”]

Primary sources

• STMicroelectronics, 2N2219A / 2N2222A high speed switches datasheet, February 2003 — https://docs.rs-online.com/66ed/0900766b813647d2.pdf
• Texas Instruments / National Semiconductor, Metal Can Packages (TO-3/5/8/18/39/46/52/72), SNOA033, August 1999 — https://www.ti.com/lit/an/snoa033/snoa033.pdf
• Microchip, JANS_2N3057A radiation hardened low power NPN transistor, DS00005462A — https://ww1.microchip.com/downloads/aemDocuments/documents/HRDS/ProductDocuments/DataSheets/JANS_2N3057A_RADHARD.pdf
• Wikipedia, TO-18 (TO-206 renaming and outline dimensions) — https://en.wikipedia.org/wiki/TO-18
• Wikipedia, TO-5 (TO-205 renaming, IEC 60191 and DIN 41873 equivalents) — https://en.wikipedia.org/wiki/TO-5
• Wikipedia, TO-8 (outline dimensions) — https://en.wikipedia.org/wiki/TO-8