The TO-218 package is a three-lead through-hole power outline with an exposed, electrically live metal tab — the footprint on a TIP35C, a BUV48A or a BU508A pulled off a 1990s board. It is obsolete. Every major supplier moved the die to TO-247. This guide gives the dimensions that matter, the five that change when you convert, and how to source a legacy part without buying a relabelled one.

Key takeaways

• Lead pitch is 5.40–5.55 mm; the mounting hole is 4.00–4.10 mm, per onsemi case outline 340D-02.
• SOT-93, TO-218-3 and TO-218AB all refer to the same package. None of them is a current JEDEC registration.
• Converting to TO-247 keeps your PCB footprint but changes screw size, heatsink hole position and body envelope.
• The stated 125 W applies at a 25 °C case. On a small clip-on sink the same device carries about 6 W.
• Two live distributor listings label parts “TO-218” that the manufacturer datasheets specify as TOP3 Insulated and TO-247AC.

What the TO-218 package is

TO-218 is a three-lead, through-hole plastic power package with a bare metal tab exposed at the back and one mounting hole through that tab. The tab is electrically common with the centre lead — collector on a bipolar transistor, anode or cathode on a thyristor. Body is roughly 15 mm wide by 4.8 mm thick, on 5.45 mm lead pitch.

The same outline circulates under several names. onsemi and Philips call the three-lead form SOT-93; onsemi’s ordering tables print it as “SOT-93 (TO-218)” against case outline 340D. ST used TO-218AB on BU508A-era parts. Package databases add TO-218-3 and TO-218AC as lead-count and variant suffixes.

None of that is standardised. Fairchild’s own TO-218 outline drawing states on its face that no current JEDEC outline exists for the package. In practice “TO-218” is a manufacturer house name, which is exactly why you pull the case outline from the specific datasheet rather than trusting a package encyclopedia.

[IMAGE 1: Front and rear photograph of a three-lead TO-218 power transistor, with body width, tab thickness and mounting hole called out | alt: “TO-218 package power transistor showing exposed metal tab and 4 mm mounting hole”]

TO-218 dimensions that decide your footprint

The authoritative numbers below are from onsemi Case Outline 340D-02, Issue E, dated 03 January 2002 (document 98ASB42643B), as published in the TIP35/TIP36 datasheet.

Feature	Symbol	Millimetres	Inches
Body width	B	14.70–15.20	0.579–0.598
Body thickness	C	4.70–4.90	0.185–0.193
Overall body height	A	20.35 max	0.801 max
Lead pitch	G	5.40–5.55	0.213–0.219
Mounting hole diameter	Q	4.00–4.10	0.158–0.161
Lead thickness	D	1.10–1.30	0.043–0.051
Lead width	E	1.17–1.37	0.046–0.054
Hole-centre reference	S	17.80–18.20	0.701–0.717
Overall length including leads	K	31.00 REF	1.220 REF

Two numbers do the work. Lead pitch sets the PCB footprint. Mounting hole diameter sets the hardware, and 4.00–4.10 mm is a near-zero-clearance fit on an M4 screw whose nominal major diameter is 4.00 mm. That is why commercial TO-218 mounting kits ship 4-40 or M3 fasteners with a nylon shoulder bushing rather than M4.

Be careful with secondary references here. At least one widely cited package database lists a 15 mm lead pitch for the three-lead TO-218, which is roughly the body width, not the pitch. Build to the manufacturer case outline.

TO-218 vs TO-247, TO-3P and ISOWATT218

ST’s BU208A/BU508A datasheet from June 1996 offered the same die in three packages, which makes it the cleanest package trade-off data in the discrete world.

Package	Total dissipation at TC = 25 °C	Rth(j-case) max	Tab
TO-3 (metal)	150 W	1 °C/W	Live
TO-218	125 W	1 °C/W	Live
ISOWATT218	50 W	2.5 °C/W	Isolated

Insulating the tab inside the package costs 1.5 °C/W and 60% of the dissipation budget. If you need isolation and the watts, isolate outside the package with a thin interface material instead.

TO-247 is the successor rather than a variant, and the reason is written into current datasheets. Vishay Siliconix documents the design intent in the IRFP350 datasheet (document 91225, Rev C, 24 January 2022): TO-247AC improved on the earlier TO-218 by isolating the mounting hole and by increasing creepage between pins to meet safety specifications. If your legacy TO-218 sits on a mains-referenced node, that creepage difference is a compliance argument, not only a mechanical one.

TO-3P is a separate JEITA-lineage outline that photographs almost identically and is routinely conflated with both. Verify it against the specific datasheet; do not infer it from a photo.

Why TO-218 disappeared

onsemi issued FPCN #16827 on 09 March 2012, converting every TO-218 bipolar power transistor to TO-247, first shipments 09 June 2012. Twenty-two part numbers moved, including BDV64BG, BU323ZG, MJE4343G, the MJH11017 through MJH11022 Darlingtons, MJH6284G, MJH6287G, TIP140/141/142/147, TIP2955, TIP3055, TIP33CG and the TIP35/TIP36 family. Assembly moved from PSI Manila to Nantong-Fujitsu, and onsemi stated the electrical parametric distributions were unchanged.

Central Semiconductor followed with PDN01022, revision 001 dated 07 April 2017: all TO-218 devices end-of-life, last purchase orders six months from notice, final shipments twelve. Some parts received a documented TO-247 replacement — TIP33A/B/C to TIP35C, TIP34A/B/C to TIP36C, TIP140/141 to TIP142, TIP145/146 to TIP147. Many did not. The BDV64/BDV65 family, the BDW83/BDW84 family, TIP2955, TIP3055 and the house-numbered CQ218 and CS218 series were listed with no replacement at all.

That is the real shape of the problem. TO-218 obsolescence was not a single date. The popular part numbers survived in a different package; the house-numbered ones simply stopped.

The five dimensions that change on a TO-247 conversion

onsemi published the delta itself in FPCN #16827.

Dimension	TO-218	TO-247
G — lead pitch	5.40–5.45 mm	5.45 mm BSC
Q — mounting hole	4.00–4.10 mm	3.55–3.65 mm
S — hole-centre reference	17.80–18.20 mm	19.46 mm
B — body width	14.70–15.20 mm	15.75–16.26 mm
A — overall height	20.35 mm max	20.32–21.08 mm

Lead pitch is effectively unchanged, so the PCB footprint survives. Everything else moves:

• Hardware. The hole shrinks by roughly 0.45 mm. An M4 screw that passed through a TO-218 will not enter a TO-247. Move to M3 or 4-40, then confirm your shoulder washer’s bushing outer diameter still fits the smaller hole.
• Heatsink drilling. Dimension S grows by 1.3–1.7 mm, so a sink drilled to a TO-218 pattern will strain the leads when a TO-247 is bolted down. Re-drill or slot the hole.
• Clearance envelope. The body grows about 1 mm wider and up to 0.7 mm taller. The width growth is per device, so a row of eight parts needs roughly 8 mm more span on the sink.

[IMAGE 2: Overlay drawing of TO-218 and TO-247 outlines showing identical lead pitch, shifted hole centre and reduced hole diameter | alt: “TO-218 vs TO-247 package outline overlay comparing mounting hole size and position”]

Thermal design: 125 W is a datasheet number

onsemi’s TIP35C datasheet (TIP35/D, Rev 7, June 2024) gives RθJC = 1.0 °C/W, RθJA = 35.7 °C/W and TJ max 150 °C. Those are internally consistent: (150 − 25) / 1.0 = 125 W. That figure assumes the case is held at 25 °C, which no enclosure does.

Worked example. A TIP35CG on an Aavid Thermalloy SW25-2G clip-on sink, listed by RS at 11.4 K/W. Ambient 40 °C, junction derated to 125 °C for margin. Assume 1.0 K/W for a greased mica interface — measure your own; Infineon’s mounting guidance puts the benefit of grease alone at 1.2 to 1.5 K/W:

Total θJA = 1.0 + 1.0 + 11.4 = 13.4 K/W

P = (125 − 40) / 13.4 = 6.3 W

Six watts, from a 125 W package. Move to a 1.5 K/W extruded sink and the same device carries about 24 W. The package is not the limit; the sink is. Run this calculation before you decide TO-218 versus anything else.

Torque is the other number schematics never carry. onsemi’s BUT11AF datasheet (Rev 4, March 2001) specifies 6 to 8 in·lb — 0.68 to 0.90 N·m — using a screw and compression washer. ST specifies 1.05 N·m recommended and 1.2 N·m maximum for its TOP3 Insulated outline. Over-torque deforms the package and drives RθJC back up.

[IMAGE 3: Thermal resistance stack diagram from junction through case, interface and heatsink to ambient, annotated with the worked example values | alt: “TO-218 thermal resistance stack showing junction to case, interface and heatsink to ambient values”]

Sourcing TO-218 today: three naming traps

Trap 1 — the manufacturer no longer uses the name. ST’s BTW68 SCR is stocked by Farnell and Newark as a TO-218 part. ST’s own datasheet (DS6910, Rev 4, October 2023) calls the package TOP3 Ins. and dimensions the mounting hole at 3.40–3.65 mm — a TO-247-class hole, not TO-218’s 4.00–4.10 mm. Order that “TO-218” line and your 4 mm hardware will not fit.

Trap 2 — one catalogue number, two package names. Jameco part 2056850, an ST IRFP350 rated 400 V, 16 A, 190 W, is indexed under both a TO-218 product page and a TO-247AC product page. Vishay’s IRFP350 datasheet specifies TO-247AC. The line item is not the specification.

Trap 3 — no registration to appeal to. With no current JEDEC outline, two vendors can each describe a different physical part as TO-218 and both be defensible.

Practical rule: for any TO-218 line, pull the case outline drawing from the manufacturer datasheet and check hole diameter, lead pitch and body width against your fixture before releasing the order. Buy through franchised distribution where stock still exists. Obsolete through-hole power parts are a standing counterfeit target, so where you must use an independent, use one operating to AS6081 test and inspection practice rather than an unscreened broker.

Legacy TO-218 devices you will still meet

Central Semiconductor’s TO-218 case datasheet lists the high-voltage switching bipolars built in this outline. These are CRT deflection, off-line switching and ignition parts.

Type	IC (A)	PD (W)	BVCBO (V)	BVCEO (V)	VCE(sat) max (V)
2N6931	10	150	350	300	1.0
2N6933	15	175	450	300	1.0
2N6935	15	175	650	400	1.0
BU426A	6.0	115	900	400	1.5
BU508A	8.0	125	1500	700	1.0
BUV47A	9.0	100	1000	450	1.5
BUV48A	15	125	1000	450	1.5
TIP54	3.0	100	500	400	1.5

Note the dissipation spread: 100 W to 175 W within one outline. Dissipation is set by die size and attach, not by the package name, so “TO-218 is a 125 W package” is wrong as a general statement. Check the individual datasheet.

Frequently asked questions

Is TO-218 the same as SOT-93?

Yes. SOT-93 is the Philips and onsemi name for the three-lead TO-218. onsemi’s TIP35/TIP36 ordering table prints the option as “SOT-93 (TO-218)” against case outline 340D. Treat them as one package, and still verify against the case drawing, because neither name carries a current JEDEC registration.

Can I replace a TO-218 with a TO-247?

Electrically, usually yes. onsemi converted its entire TO-218 bipolar line to TO-247 under FPCN #16827 with no change to specified electrical distributions. Mechanically it is not a drop-in. The mounting hole shrinks from 4.00–4.10 mm to 3.55–3.65 mm and the hole centre shifts, so fasteners and heatsink drilling both change. Lead pitch stays the same.

What is the TO-218 pinout?

For a bipolar transistor the leads are base, collector, emitter as pins 1, 2 and 3, with the tab electrically common to the collector. onsemi’s Style 1 for case 340D additionally designates the tab as pin 4, collector. Thyristors use a different style on the same outline, so read the style note on the case drawing rather than assuming.

Is the TO-218 tab isolated?

No. A standard TO-218 tab is live and needs an external insulator whenever the heatsink is grounded. Insulated variants exist under other names — ST’s ISOWATT218, and its TOP3 Insulated outline rated 2500 V RMS under UL file E81734 — and they trade a substantial share of the dissipation budget for that isolation.

What is the TO-218 mounting hole size?

4.00–4.10 mm, per onsemi case outline 340D-02. That is a zero-clearance fit on an M4 screw, so production hardware kits use 4-40 or M3 fasteners with an insulating shoulder washer. Do not assume the dimension carries over to a part a distributor has merely labelled TO-218.

What to do next

Sustaining an existing board? Keep the package. Source through franchised distribution while stock lasts, or through an AS6081-compliant independent, and verify the case outline against the physical part on arrival.

Respinning the board, or holding a part with no documented replacement? Move to TO-247 now. Keep the PCB footprint, change to M3 or 4-40 hardware, re-drill the sink for the new hole centre, and confirm creepage against your safety standard — that last item is the reason the industry moved.

Mains-referenced circuit? Treat TO-218 as the weaker choice regardless of stock. Reduced pin-to-pin creepage against TO-247 is the specific deficiency the newer outline was created to fix.

Just identifying a part on the bench? Measure the tab hole. Four millimetres means TO-218. Three and a half means TO-247 or TOP3, whatever the silkscreen or the invoice says.