The TO-251 package, sold almost everywhere as IPAK, is a three-lead through-hole outline with a molded body around 6.1 mm by 6.6 mm by 2.3 mm and leads on a nominal 2.29 mm pitch. It is the insertion-mount twin of the DPAK body. The dimensions are easy to find and easy to get wrong, because five manufacturer drawings publish five slightly different answers and one of them tolerances the lead pitch instead of fixing it.

Key takeaways

• Body length runs 5.97 mm to 6.35 mm and body width 6.35 mm to 6.73 mm across published limits.
• Lead pitch is quoted as 2.28 mm BSC, 2.29 mm BSC, or 2.20 mm to 2.30 mm toleranced, depending on whose drawing you open.
• Size the hole from the lead diagonal, not the lead width. Worst case across vendors is about 1.09 mm, which puts the finished hole at 1.3 mm under IPC-2222 Level B.
• The tab is drain or collector and is soldered to nothing. Heat leaves through the leads, which is why a 42 W case rating collapses to single-digit watts on a board.
• IPAK and DPAK share the same molded body. The difference is lead forming, overall height and how you attach it.

TO-251 (IPAK) dimensions at a glance

The TO-251 package is a three-lead plastic through-hole outline for power semiconductors, registered by JEDEC as TO-251AA and marketed as IPAK. The body measures roughly 6.1 mm long, 6.6 mm wide and 2.3 mm thick, leads sit on a nominal 2.29 mm pitch, and the exposed metal tab on the back carries the drain on a MOSFET or the collector on a bipolar device.

Vishay notes on its high-voltage drawing that the outline conforms to JEDEC outline TO-251AA. onsemi files the same package as CASE 369D and calls it IPAK, DPAK insertion mount. ST publishes it as IPAK (TO-251) and, across datasheets, as type A, type C or type E. Infineon indexes it as PG-TO251-3, with I2PAK reserved for TO262-3.

[IMAGE 1: Front, side and rear views of a TO-251 device with body length, width, thickness, lead pitch and tab width called out | alt: “TO-251 IPAK package dimensions showing 6.1 mm body, 2.29 mm lead pitch and exposed drain tab”]

Dimension (mm)	Vishay 71666	Vishay 91362	onsemi 369D	ST type A	ST type C
Body length D	5.97 to 6.22	5.97 to 6.22	5.97 to 6.35	6.00 to 6.20	6.00 to 6.20
Body width E	6.48 to 6.73	6.35 to 6.73	6.35 to 6.73	6.40 to 6.60	6.50 to 6.70
Body thickness A	2.21 to 2.38	2.18 to 2.39	2.19 to 2.38	2.20 to 2.40	2.20 to 2.35
Lead pitch e	2.28 BSC	2.29 BSC	2.29 BSC	2.28 typ.	2.20 to 2.30
Lead width b	0.71 to 0.89	0.64 to 0.89	0.69 to 0.88	0.64 to 0.90	0.66 to 0.79
Lead thickness c	0.46 to 0.58	0.46 to 0.61	0.46 to 0.58	0.45 to 0.60	0.46 to 0.59
Tab width	5.23 to 5.43	4.95 to 5.46	4.45 to 5.45	5.20 to 5.40	5.23 to 5.43
Lead length L	8.89 to 9.53	8.89 to 9.65	8.89 to 9.65	9.00 to 9.40	9.00 to 9.60
Overall height H	Not given	Not given	Not given	16.10 max.	16.18 to 16.78
Drawing reference	DWG 5346, Rev. 03-Jun-13	DWG 5968, Rev. 27-Dec-2021	Issue C, 15-Dec-2010	DocID022967 Rev 5	DocID022967 Rev 5

Table 1. Five published TO-251 / IPAK outlines side by side. Symbol letters differ between drawings; rows are matched by what the dimension measures, not by its label.

The lead pitch is not one number

This is the detail that costs board spins. Vishay’s IPAK drawing 71666 gives 2.28 mm BSC. Its high-voltage TO-251AA drawing 91362 gives 2.29 mm BSC, as does onsemi’s CASE 369D. ST’s type A gives a single typical 2.28 mm. ST’s type C abandons the basic dimension and publishes 2.20 mm minimum, 2.25 mm typical, 2.30 mm maximum.

A basic dimension carries no tolerance and drives the theoretically exact hole positions. A toleranced pitch does not. ST’s own outer-lead span for type C, dimension e1, confirms the consequence: 4.40 mm to 4.60 mm between the outer lead centers, against 4.58 mm if you had assumed 2.29 mm exactly.

That is a worst-case lateral offset of 0.09 mm on each outer lead. It does not sound like much until you have chosen a tight hole.

Pinout and what the tab actually does

onsemi’s CASE 369D lists the pin styles. Style 2, the MOSFET case, is pin 1 gate, pin 2 drain, pin 3 source, tab as a fourth drain node. Style 1, the bipolar case, is base, collector, emitter, tab as collector. Style 3 covers two-terminal diodes.

The tab matters for two reasons, and neither is the expected one. It is live, at drain or collector potential, so it needs clearance from anything it can touch. And it attaches to nothing: no mounting hole, no standard clip. The tab rests against the board or sits in air, and the heat path runs through the lead frame into whatever copper you gave the drain lead.

How to size the TO-251 footprint

Start from the lead cross-section, not the lead width. The lead is rectangular, so the dimension that has to pass through a round hole is the diagonal.

Take the widest published limits: 0.90 mm lead width from ST’s type A and 0.61 mm lead thickness from Vishay 91362. The diagonal is the square root of 0.90 squared plus 0.61 squared, or 1.087 mm.

IPC-2222 sets the minimum finished hole at the maximum lead diameter plus 0.25 mm for Level A, plus 0.20 mm for Level B and plus 0.15 mm for Level C. At Level B that gives 1.29 mm, so specify a 1.3 mm finished hole. Level A takes you to 1.35 mm.

Check the margin against the pitch problem. A 1.3 mm hole around a 0.90 mm lead leaves 0.20 mm of radial clearance, comfortably more than the 0.09 mm worst-case offset a toleranced-pitch part can present. Drop to a 1.1 mm hole and the clearance falls to 0.10 mm, which the pitch tolerance consumes on its own before you have accounted for drill tolerance or plating thickness.

For the land, IPC-2221 adds a fabrication allowance of 0.6 mm at Level A, 0.5 mm at Level B and 0.4 mm at Level C on top of the hole and twice the annular ring. At Level B with a 1.3 mm hole that puts the pad at 1.8 mm before annular ring, so 1.9 mm to 2.0 mm is a sensible start. Give the drain lead more copper than the other two: it is the thermal path.

[IMAGE 2: Footprint drawing with three 1.3 mm holes at 2.29 mm pitch, 2.0 mm pads, enlarged drain-lead copper pour and the tab keep-out marked | alt: “TO-251 IPAK package footprint with 1.3 mm holes on 2.29 mm pitch and enlarged drain copper”]

The thermal trap: 42 W on the datasheet, single digits on your board

Datasheet power ratings for this package are referenced to case temperature, and you cannot hold the case of a TO-251 at 25 degrees.

Vishay’s datasheet covering the IRLR024 in DPAK and the IRLU024 in IPAK quotes 42 W at a case temperature of 25 °C, with a linear derating factor of 0.33 W per °C. The same page lists 2.5 W for a PCB-mounted part at 25 °C ambient. Those numbers describe the same silicon. Vishay is candid elsewhere, describing the IPAK as suited to applications where limited heat dissipation is required.

Design to the board-referenced number. If your only figure is case-referenced, treat it as a ceiling that a heatsink you do not have would allow, and size the drain-lead copper as though the package were a small SMD.

TO-251 versus TO-252 (DPAK): same body, different lead form

ST publishes both outlines in one datasheet for the STD10P6F6 family, which puts the same die in DPAK, IPAK, TO-220FP and TO-220. Comparing its two type C tables removes the vendor variable.

Dimension (mm)	DPAK (TO-252) type C	IPAK (TO-251) type C	Comment
Body length D	6.00 to 6.20	6.00 to 6.20	Identical
Body width E	6.50 to 6.70	6.50 to 6.70	Identical
Body thickness A	2.20 to 2.38	2.20 to 2.35	Within 0.03 mm
Tab width b4	5.13 to 5.46	5.23 to 5.43	IPAK band is tighter
Lead width b	0.72 to 0.85	0.66 to 0.79	IPAK leads are narrower
Lead thickness c	0.47 to 0.60	0.46 to 0.59	Effectively the same
Lead pitch e	2.186 to 2.386 (2.286 typ.)	2.20 to 2.30 (2.25 typ.)	Same nominal, different band
Overall length	H 9.80 to 10.40	H 16.18 to 16.78	Straight leads add roughly 6 mm
Attachment	Tab soldered to a copper pad	Leads through holes, tab free	The real difference

Table 2. DPAK and IPAK type C outlines from a single ST datasheet, DocID022967 Rev 5, covering the same die in both packages.

The molded bodies are effectively identical: 6.00 mm to 6.20 mm long and 6.50 mm to 6.70 mm wide in both. What changes is lead form and height. DPAK gulls its leads and stands 9.80 mm to 10.40 mm overall; IPAK keeps them straight and reaches 16.18 mm to 16.78 mm.

The practical difference is thermal attachment. DPAK solders its tab to a copper pad. IPAK does not.

Sourcing: the one-letter decoder and lifecycle

The IPAK version of a part usually differs from the DPAK version by a single character, and the character depends on the vendor.

Vendor	DPAK marker	IPAK marker	Verified pair	Basis
Vishay / International Rectifier	IRLR	IRLU	IRLR024 / IRLU024	Both covered by Vishay document 91322, same die
Vishay / International Rectifier (HV)	IRFR	IRFU	IRFR430A / IRFU430A	Both covered by Vishay document 91276
Vishay Siliconix rebrand	SiHLR	SiHLU	SiHLR024 / SiHLU024	Listed alongside the IR numbers in document 91322
STMicroelectronics	STD	STU	STD10P6F6 / STU10P6F6	Device summary table, DocID022967 Rev 5
STMicroelectronics (suffix form)	no suffix	-1	STD12NF06L / STD12NF06L-1	ST datasheet cover and DigiKey package field

Table 3. Part-number markers that separate the through-hole IPAK from the surface-mount DPAK. Every pair listed is the same die in two packages.

A cross-reference tool matching on voltage and on-resistance will hand you the surface-mount sibling without comment, so search the base number rather than the package name.

Pricing is unremarkable, which is the point. ST’s STU6NF10, a 100 V 6 A IPAK MOSFET, was listed at DigiKey at 1.22 US dollars in ones and 0.28707 dollars at 10,050 pieces, 850 in stock. STD12NF06L-1 was 1.00 dollar in ones and 0.23359 dollars at 10,050, 968 in stock.

Lifecycle is where the attrition shows. DigiKey lists Infineon’s IRFU4105PBF, a 55 V IPAK part, as obsolete and no longer manufactured.

[IMAGE 3: Decision flowchart from assembly process and dissipation budget through height constraint to TO-251 or TO-252 | alt: “Decision path for choosing the TO-251 IPAK package over the surface-mount TO-252 DPAK”]

Frequently asked questions

What are the dimensions of a TO-251 package?

The molded body is about 6.1 mm long, 6.6 mm wide and 2.3 mm thick, with leads on a nominal 2.29 mm pitch and roughly 9 mm to 9.7 mm of overall length with the leads uncut. Drawings vary: body length runs 5.97 mm to 6.35 mm and width 6.35 mm to 6.73 mm.

Is TO-251 the same as IPAK?

Yes, in normal use. IPAK is the industry name; TO-251AA is the JEDEC outline designation. Infineon states IPAK is its designation for TO251-3, and onsemi files it as CASE 369D. Body dimensions still vary between vendors, so match a named drawing rather than a name.

What hole size should I use for a TO-251 footprint?

Size from the lead diagonal, which is about 1.09 mm at the widest published limits. IPC-2222 Level B adds 0.20 mm, giving a 1.29 mm minimum, so a 1.3 mm finished hole is the sensible specification. Anything below about 1.1 mm loses its clearance to the lead-pitch tolerance alone.

What is the difference between TO-251 and TO-252?

They share the same molded body. DPAK gulls its leads for surface mount and solders its tab to a copper pad. IPAK keeps the leads straight for insertion and leaves the tab unattached. The IPAK version runs roughly 6 mm longer overall and dissipates through its leads.

How much power can a TO-251 package actually dissipate?

Far less than the case-referenced datasheet figure suggests. One Vishay part quotes 42 W at a case temperature of 25 °C alongside 2.5 W for the same silicon PCB-mounted at 25 °C ambient. Design against the board-referenced number and give the drain lead as much copper as you can.

Specify it, or move to DPAK

Choose IPAK when the board is wave-soldered or mixed technology, when a through-hole part is cheaper to place than an isolated SMD, when the device must stand off the board for clearance or airflow, or when robustness under vibration outweighs height. Keep dissipation in the low single-digit watts unless you have measured otherwise.

Move to TO-252 when the assembly is fully surface mount, when you need the tab bonded to copper to carry heat away, or when 6 mm of extra height is a problem. The die is usually identical and the part number usually differs by one letter.

Before release: pull the specific manufacturer outline drawing and revision into your library rather than a generic footprint, size the hole from the lead diagonal at 1.3 mm rather than copying a library default, and confirm lifecycle status.

Internal links to place

• [INTERNAL LINK: TO-252 DPAK package dimensions and land pattern -> surface-mount power package reference]
• [INTERNAL LINK: IPC-2221 and IPC-2222 hole and pad sizing -> PCB fabrication rules]
• [INTERNAL LINK: thermal design for board-mounted power devices -> copper area and junction temperature]
• [INTERNAL LINK: wave soldering mixed-technology assemblies -> through-hole process design]
• [INTERNAL LINK: TO-247-3 package outline and dimensions -> larger through-hole power packages]