The TO-99 package is an 8-lead hermetic metal can, about 8.5 mm across, with leads on a 5.08 mm circle at 45° spacing. JEDEC registers the outline as MO-002-AK, not as a TO- number. Two details cost people boards: the tab marks pin 8, not pin 1, and on most op-amps the can is internally tied to the negative supply. Dimensions, thermal numbers and footprint math below.
Key takeaways
- Analog Devices publishes the TO-99 outline as drawing H-08-1, compliant to JEDEC MO-002-AK.
- The tab indicates pin 8. On the OPA627 in this package, pin 8 has no internal connection at all.
- The can is a terminal. TI states that on the TO-99 OPA627 the case is internally connected to the negative supply.
- Junction-to-ambient is 170 °C/W for TI’s LMC (TO-99) LM741. Junction-to-case (top) is 25 °C/W, so a clip-on heatsink attacks 145 of those 170 °C/W.
- Adjacent leads sit 1.94 mm apart, not 1.27 mm. Several package directories publish the wrong figure.
What is a TO-99 package?
The TO-99 package is an 8-lead hermetically sealed metal can used mainly for precision operational amplifiers, voltage references and sensors. The cap measures 7.75 mm to 8.51 mm in diameter and 3.56 mm to 4.06 mm high, with leads on a 5.08 mm pin circle spaced 45° apart. A tab on the header marks pin 8. TI calls the package LMC; Analog Devices calls it H.
[IMAGE 1: TO-99 can photographed from above on a millimeter scale, tab at 12 o’clock, with the eight leads and the 5.08 mm pin circle overlaid | alt: “TO-99 package metal can with tab marking pin 8 on a 5.08 mm pin circle”]
TO-99 dimensions
Analog Devices publishes the outline as drawing H-08-1, rev 022306-A, compliant to JEDEC standard MO-002-AK. Controlling dimensions on that drawing are inches; the millimeter values are rounded equivalents.
| Feature | Min | Max |
| Header (base) diameter | 8.51 mm | 9.40 mm |
| Cap diameter | 7.75 mm | 8.51 mm |
| Cap height | 3.56 mm | 4.06 mm |
| Lead diameter | 0.41 mm | 0.48 mm |
| Lead circle diameter | 5.08 mm BSC | 5.08 mm BSC |
| Angular lead spacing | 45° BSC | 45° BSC |
| Lead length below the seating plane | 6.35 mm | 12.70 mm |
| Tab width | 0.69 mm | 1.14 mm |
Table 1. TO-99 outline limits, per Analog Devices drawing H-08-1 rev 022306-A (JEDEC MO-002-AK).
Worth registering that this is a TO- name attached to an MO- outline. JEDEC’s TO series covers transistor outlines; the 8-lead header used for op-amps sits in the microelectronic outline series instead. If a purchasing system rejects “TO-99” as a package code, MO-002-AK is the searchable term.
Footprint: the lead spacing is 1.94 mm, not 1.27 mm
Eight leads on a 5.08 mm diameter circle at 45° gives an adjacent-lead chord of 5.08 mm × sin 22.5°, or 1.94 mm. Two package directories currently publish 0.050 in, 1.27 mm, as the TO-99 lead pitch. That is the TO-5 family’s tab-to-lead dimension, not the spacing between adjacent leads, and a footprint built on it will not accept a part.
Hole sizing follows the 0.48 mm maximum lead diameter: a 0.7 mm to 0.8 mm finished hole clears it with room for a fillet, and at 1.94 mm centers the annular rings have space.
The leads splay outward below the seating plane on many parts, which is why through-hole adapters for TO-99 are built with a circular pad ring rather than a DIP pattern.
The tab marks pin 8, not pin 1
Every other through-hole package trains you to look for pin 1. This one does not. The tab on the TO-99 header identifies pin 8, and pin numbering runs counterclockwise from it when viewed from the top.
The trap deepens on some parts. TI’s OPA627 datasheet states that pin 8 of the TO-99 package has no internal connection, so the index feature points at a lead that does nothing electrically. Get the rotation wrong by one position and the supply pins land on the inputs.
Silkscreen the tab position explicitly. A circular footprint with no orientation mark is unrecoverable at inspection.
[IMAGE 2: top-view TO-99 pad ring with the tab notch called out and leads numbered 8 through 1 counterclockwise, annotated with the 1.94 mm chord | alt: “TO-99 package footprint showing tab marking pin 8 and 1.94 mm adjacent lead spacing”]
The can is a terminal, and it usually carries the negative supply
TI’s OPA627 and OPA637 datasheet states plainly that the case, on the TO-99 metal package only, is internally connected to the negative power supply. This is the normal convention for op-amps in this can.
Three consequences follow. The can cannot touch a grounded shield or a neighboring part unless the negative supply happens to be ground. The can acts as a driven shield around the input stage, which is part of why the package survives in low-bias-current designs. And any heatsink clipped to it is electrically live at negative supply potential.
TI’s application guidance for the LMC package also covers guard-ring layout on both sides of the board for the inverting and noninverting configurations, and notes that operating the TO-99 device at lower supply voltage reduces its input bias current to roughly a quarter of the value at ±15 V.
Thermal: 170 °C/W to air, 25 °C/W to the can
TI’s LM741 datasheet gives the same die in four packages, which makes the package contribution visible.
| Package | TI designator | RθJA | RθJC(top) | Hermetic |
| TO-99 metal can | LMC | 170 °C/W | 25 °C/W | Yes |
| CDIP-8 | NAB | 100 °C/W | not specified | Yes |
| PDIP-8 | P | 100 °C/W | not specified | No |
| SO-8 | M | 195 °C/W | not specified | No |
Table 2. One die, four packages, per TI LM741 datasheet SNOSC25C / SNOSC25D.
Read the first row as two numbers, not one. In still air the TO-99 is the second-worst of the four, beating only SO-8. But 25 °C/W of the 170 is junction to case, which means 145 °C/W sits between the can and the air, and that is the part a heatsink can attack. None of the other three packages publishes a usable case path at all.
Worked example. Dissipate 200 mW in a TO-99 in still air and the junction runs 200 mW × 170 °C/W, or 34 °C above ambient. Clip a heatsink to the can and the floor becomes 200 mW × 25 °C/W, or 5 °C above the case. In a ±15 V precision amplifier that difference moves offset drift and input bias current, not just the temperature rating.
The ceramic DIP is the better free-air package at 100 °C/W. Choose the can when you want the shield, the hermetic seal or the heatsink option, not because it runs cooler.
What TO-99 still buys in 2026
Hermeticity and screening. TI’s LM741H carries the military drawing number JM38510/10101, and Analog Devices offered TO-99 grades processed to MIL-STD-883B on parts such as the AD707 and AD708 across the full −55 to +125 °C range.
Moisture handling. Because the can is hermetic, TI’s package data for LM741CH/NOPB lists MSL as Level-1-NA-UNLIM. The dry-pack and floor-life rules written for molded plastic do not apply.
Input-stage shielding. A grounded or supply-referenced metal can around a JFET input stage is hard to reproduce with copper on a two-layer board, which is why the package persists in electrometer-class and low-noise audio designs.
Availability and the EOL question
Some TO-99 parts are current and some are gone, and the split does not follow age.
Still orderable at DigiKey as of August 2026: TI’s LM741CH and LM741CH/NOPB, both listed as TO-99-8 and shipping same day, and TI’s OPA627BM in TO-99-8. TI’s package addendum shows the LMC parts shipping 500 per tube.
Already gone: the dual OPA2111AM in TO-99-8 is flagged Obsolete at DigiKey with substitutes offered, and OPA627 variants in other packages have been discontinued while the metal can survives. Do not assume the plastic version outlives the can.
Two sourcing habits apply. A plain metal can carries only a laser mark, so buy through authorized distribution and keep the tube label with the lot. And record the JEDEC outline, MO-002-AK, in the footprint library rather than the marketing name, because “TO-99” alone is ambiguous across directories that publish conflicting lead pitches.
[IMAGE 3: TO-99 op-amps in a shipping tube beside a board with a circular TO-99 pad ring and a silkscreened tab mark | alt: “TO-99 package op-amps in tube alongside a circular TO-99 footprint with tab silkscreen”]
Design mistakes that cause returns
- Building the footprint on a 1.27 mm lead pitch taken from a package directory instead of the 1.94 mm chord derived from the 5.08 mm pin circle.
- Orienting to the tab as if it marks pin 1.
- Grounding a shield or a heatsink that touches the can when the case sits at the negative supply.
- Assuming the metal can runs cooler than the ceramic DIP. In free air it does not.
- Omitting a guard ring on the inverting input and losing the low-bias-current benefit the package was bought for.
- Bending leads at the glass seal, which opens a slow leak that passes final test.
Frequently asked questions
What is a TO-99 package?
An 8-lead hermetically sealed metal can for precision op-amps, references and sensors. The cap is 7.75 mm to 8.51 mm across and 3.56 mm to 4.06 mm high, with leads on a 5.08 mm circle at 45° spacing. JEDEC registers the outline as MO-002-AK. TI calls it LMC; Analog Devices calls it H.
Which pin does the tab mark on a TO-99?
Pin 8. This is the opposite of the convention most through-hole packages follow, and it is the single most common orientation error on the package. On the TO-99 OPA627, pin 8 also has no internal connection, so the index points at an electrically inactive lead.
Is the TO-99 case connected to anything?
Usually yes. TI states that on the TO-99 version of the OPA627 the case is internally connected to the negative power supply. Treat the can as a live terminal: do not let it contact a grounded shield or a neighboring part, and insulate any heatsink clipped to it.
What is the TO-99 lead spacing?
The leads sit on a 5.08 mm diameter circle, 45° apart, which puts adjacent leads 1.94 mm apart along the chord. Several package directories quote 1.27 mm, which is wrong for adjacent-lead spacing. Derive the pattern from the pin circle, not from a quoted pitch.
Is TO-99 the same as TO-5?
They share the 5.08 mm pin circle and the general body style, but TO-99 is the 8-lead low-cap member and JEDEC lists it under the microelectronic outline MO-002-AK rather than the transistor outline series. Take dimensions from the manufacturer’s drawing, such as Analog Devices H-08-1.
What to do next
If the design needs a hermetic seal, a driven shield around a JFET input, or the option to heatsink an op-amp, specify TO-99 and build the footprint from the 5.08 mm pin circle with the tab position on the silkscreen. Confirm on the device datasheet what the case is tied to before you route anything near it.
If none of those three apply, use the ceramic DIP for the better free-air thermal path or the SOIC for cost and placement. The can is worth its price for shielding and hermeticity, not for running cool.
Primary sources
- Analog Devices 8-pin metal header [TO-99] drawing H-08-1 (JEDEC MO-002-AK) — https://www.analog.com/media/en/package-pcb-resources/package/pkg_pdf/headerh/h_08_1.pdf
- Texas Instruments LM741 datasheet (LMC / TO-99 thermal metrics) — https://www.ti.com/lit/ds/symlink/lm741.pdf
- Texas Instruments OPA627 / OPA637 datasheet (case connection, pin 8, guarding) — https://www.ti.com/lit/ds/symlink/opa627.pdf
- Texas Instruments LM741-MIL datasheet (JM38510/10101) — https://www.ti.com/lit/gpn/LM741-MIL
- Analog Devices AD707 datasheet (hermetic TO-99 grade, MIL-STD-883B) — https://www.analog.com/media/en/technical-documentation/obsolete-data-sheets/2143387AD707.pdf
Proposed internal links
- [INTERNAL LINK: “TO-72 four-lead metal can” → TO-72 package reference page]
- [INTERNAL LINK: “TO-8 metal can outline” → TO-8 package reference page]
- [INTERNAL LINK: “guard rings for low-bias-current amplifiers” → precision analog layout guide]
- [INTERNAL LINK: “reading junction-to-case thermal metrics” → thermal design guide]
- [INTERNAL LINK: “hermetic packaging and MSL” → package hermeticity explainer]
