The TO-5 package is a hermetic metal can with leads on a 5.08 mm pin circle and a locating tab 45° from pin 1. JEDEC registers it as TO-205AA. The classic dimensions are an 8.9 mm base, an 8.1 mm cap and a 6.3 mm cap height — but the eight-lead cans sold as TO-5 are shorter than that.

Below are the controlled dimensions, the pin numbering, and the case connection that puts a supply rail on the outside of the package.

[IMAGE 1: a 3-lead and an 8-lead metal can side by side on a millimetre rule, tabs visible, with a plastic SOIC for scale | alt: “TO-5 package metal cans in 3-lead and 8-lead versions compared with a SOIC”]

Key takeaways

• TO-5 is TO-205AA. TO-12, TO-33 and TO-39 are the same body with different lead counts or lengths.
• An 8-lead “TO-5” is dimensionally TO-99. Analog Devices publishes the same outline under both names.
• The case is often a supply rail. On the OP77 in TO-99, pin 4 is V− and it is connected to the can.
• Junction-to-ambient is 150 °C/W for that package — and that figure assumes the part is in a socket.
• Metal cans now cost roughly 20× their plastic siblings, and the lines are thinning.

TO-5 is TO-205AA, and the name has drifted

JEDEC folded the 5.08 mm pin-circle metal cans into the TO-205 family: TO-5 became TO-205AA, TO-12 became TO-205AB, TO-33 became TO-205AC and TO-39 became TO-205AD. The old names stayed in circulation.

The family separates on lead count and lead length rather than body size.

Variant	Leads	Cap height	Distinguishing feature
TO-5 (TO-205AA)	3	6.3 mm	38.1 mm minimum leads, for point-to-point wiring
TO-39 (TO-205AD)	3	6.3 mm	Leads shortened to 12.7 mm minimum for through-hole boards
TO-12 / TO-33	4	6.3 mm	Fourth lead usually bonded to the case for RF shielding
TO-78 / TO-99	8	4.45 mm	Reduced cap; TO-78 may sit directly on the board, TO-99 requires clearance
TO-100	10	4.45 mm	Lead circle enlarged to 5.84 mm to fit a larger die

Fairchild’s 2N696 and 2N697, the first commercial silicon transistors, shipped in TO-5. Before dual in-line packages arrived in 1965, most integrated circuits used TO-5 variants with more than three leads.

TO-5 package dimensions from controlled drawings

Here the naming problem becomes measurable. Analog Devices publishes two legacy metal-can drawings that ought to describe different packages, plus a current package outline in the OP77 datasheet.

Dimension	3-lead “TO-39” (05-08-1330)	8-lead “TO-5” (05-08-1320)	8-pin “TO-99” (OP77, H-08)
Base flange diameter	8.890–9.398 mm	8.509–9.398 mm	8.51–9.40 mm
Cap diameter	7.747–8.509 mm	7.747–8.509 mm	7.75–8.51 mm
Cap height	4.191–4.699 mm	4.191–4.699 mm	4.19–4.70 mm
Pin circle	5.080 mm TYP	5.080 mm TYP	5.08 mm BSC
Lead diameter	0.406–0.483 mm	0.406–0.533 mm	0.40–0.53 mm
Lead length	12.700 mm min	12.700–19.050 mm	12.70 mm min

Read the cap-height row. All three run 4.19 to 4.70 mm, which is the reduced TO-78/TO-99 cap of 4.45 mm nominal — not the 6.3 mm cap that defines TO-5 proper. Analog Devices’ own OP77 outline confirms it, naming the part an 8-pin metal header, TO-99, and declaring it compliant to JEDEC MO-002-AK.

So a part described as an eight-lead TO-5 is almost certainly a TO-99. If you are modelling enclosure headroom or an optical stack, that 1.85 mm matters.

The one real difference between the two legacy drawings is the base minimum: 8.890 mm on the three-lead sheet against 8.509 mm on the eight-lead. Solder-dip lead finishes also take the lead diameter to 0.610 mm on both, which will bind in a socket sized for the bare dimension.

TO-5 pinout: the tab, and the case connection that catches people

The tab sits 45° from pin 1, and numbering runs clockwise when viewed from the bottom. National Semiconductor’s drawings, published by TI as SNOA033, register the lead counts under codes H06C, H08A, H08C and H10C — the 8-lead versions split by whether the pin circle is 5.08 mm or 5.84 mm.

The trap is what the can itself is connected to. On the OP77 in TO-99, pin 4 is V− and the datasheet marks it “V− (CASE)”. The metal body is at the negative supply, typically −15 V.

That has three consequences. The can cannot touch a grounded chassis or an adjacent component lead. Any clip-on heatsink is live. And handling a powered board by the can is a short waiting to happen.

Not every metal-can device does this — on RF transistors in four-lead variants the fourth lead is usually bonded to the case for shielding, which puts the can at whatever you tie that lead to. Check the connection diagram, not the package name.

[IMAGE 2: bottom view of an 8-lead metal can with tab, pin numbering and the case-to-pin-4 connection called out | alt: “TO-5 package bottom view pinout showing tab position and case connected to pin 4”]

What the metal can costs and what it buys

Analog Devices’ OP77 datasheet, Rev G dated October 2015, gives figures for the two hermetic options it ships.

Parameter	8-pin TO-99 (J suffix)	8-lead CERDIP (Z suffix)
θJA	150 °C/W	148 °C/W
θJC	18 °C/W	16 °C/W
Junction temperature range	−65 °C to +150 °C	−65 °C to +150 °C
Lead soldering, 60 s	300 °C	300 °C

Two things stand out. The metal can is not thermally better than the ceramic DIP — it is marginally worse on both figures. And the footnote matters: θJA is specified for worst-case mounting, meaning the device sitting in a socket. Solder it to a board and you will do better than 150 °C/W.

What the can does buy is hermeticity, a defined electromagnetic shield, and a thermal mass that damps fast ambient transients — which is why precision references and low-drift amplifiers stayed in metal long after logic left.

The price has moved against it. RS lists the OP77FJZ in TO-99 at €49.61 for a single unit and flags it as being discontinued by the manufacturer. The OP07CSZ, the plastic-package sibling that Analog Devices’ own OP07 datasheet points to for non-hermetic work, lists at €2.04 in SOIC. Roughly 24× for the same amplifier function.

The Rev G ordering guide bears this out: OP77FJZ is the only metal-can entry left, against two CERDIP options and a die.

Worked example: self-heating in a socketed metal can

Precision parts in metal cans are usually there for drift performance, so the thermal number that matters is not survival — it is offset.

Take an OP77F in TO-99, socketed, at ±15 V with no load. Power consumption is 60 mW maximum at 25 °C and 75 mW maximum over temperature.

At 75 mW into 150 °C/W:

ΔT = 0.075 W × 150 °C/W = 11.25 °C

The junction sits about 11 °C above ambient before the circuit does anything. The OP77F specifies TCVOS of 0.6 µV/°C maximum, so self-heating alone can shift input offset by:

11.25 °C × 0.6 µV/°C = 6.8 µV

Against a typical VOS of 20 µV, that is a third of the part’s untrimmed offset arriving purely from its own dissipation. It also explains the warm-up drift curve in the datasheet, which shows offset settling over the first few minutes after power-up.

Two fixes follow. Solder the part rather than socketing it, which is what the θJA footnote is telling you. And if the offset budget is tight, let the board reach thermal equilibrium before calibrating.

Four mistakes with TO-5 parts

Assuming an 8-lead TO-5 has a 6.3 mm cap. It has a 4.19 to 4.70 mm cap. That is TO-99 geometry under a TO-5 label.

Treating the can as inert. On common op-amps it carries the negative supply.

Designing to the socketed θJA. 150 °C/W is the worst case; a soldered part runs cooler and drifts less.

Sizing a socket to the bare lead diameter. Solder-dip finishes take the leads to 0.610 mm against 0.483 mm bare.

Frequently asked questions

What is the TO-5 package?

A hermetically sealed metal can registered by JEDEC as TO-205AA, with leads on a 5.08 mm pin circle and a tab 45° from pin 1. The classic three-lead version has an 8.9 mm base, 8.1 mm cap and 6.3 mm cap height. Variants exist from 2 to 12 leads, used for transistors, op-amps and photodiodes.

What is the difference between TO-5 and TO-39?

Lead length, not body size. TO-5 was registered with 38.1 mm minimum leads for point-to-point wiring; TO-39, registered as TO-205AD, shortened them to 12.7 mm minimum for through-hole boards. The cap and header are identical, which is why manufacturers use the two names almost interchangeably.

Is TO-5 the same as TO-99?

Not by registration, but often by drawing. TO-99 is the 8-lead variant with a reduced 4.45 mm cap. Analog Devices publishes one outline as “8-Lead TO-5 Metal Can” and the same geometry as “8-Pin Metal Header [TO-99],” compliant to JEDEC MO-002-AK. If a part has 8 leads, expect TO-99 dimensions.

Where is pin 1 on a TO-5 package?

Pin 1 sits 45° from the locating tab, with numbering running clockwise viewed from the bottom of the can. On many devices one pin is also bonded to the case — on the OP77 in TO-99 that is pin 4, the negative supply — so confirm the connection diagram before mounting the can against anything conductive.

What to do next

If you are laying out an existing TO-5 part, draw the footprint to a 5.08 mm pin circle, confirm the lead count and pull the outline drawing for the specific orderable number rather than a family reference. If it has eight leads, budget enclosure headroom for a 4.70 mm cap, and check whether the can is tied to a supply rail before it sits anywhere near a chassis.

If you are choosing a package for a new design, the metal can is now a deliberate choice rather than a default. It is not thermally better than a ceramic DIP, it costs around 20× a plastic equivalent, and the surviving part numbers are thinning. Specify it when you need hermeticity, a defined shield, or a qualified equivalent for a legacy board — and check the lifecycle status before the drawing is released.

External references

• Analog Devices 8-lead TO-5 metal can outline, LTC DWG 05-08-1320
• Analog Devices 3-lead metal can outline, LTC DWG 05-08-1330
• Analog Devices OP77 datasheet, Rev G
• Analog Devices OP07 datasheet, Rev G
• Texas Instruments SNOA033, Metal Can Packages

Proposed internal links

• [INTERNAL LINK: TO-39 package → TO-39 package dimensions and pinout]
• [INTERNAL LINK: TO-46 package → TO-46 package guide]
• [INTERNAL LINK: offset voltage drift → precision op-amp error budgets and thermal drift]
• [INTERNAL LINK: hermetic packages → hermetic versus plastic semiconductor packaging]
• [INTERNAL LINK: JEDEC transistor outlines → JEDEC JEP95 outline naming explained]