The TO-3PF package is a fully moulded three-lead power package on a 5.45 mm lead pitch, with the mounting tab encapsulated in epoxy rather than exposed. That built-in isolation removes the mica washer and shoulder bushing from the assembly — and costs roughly three times the junction-to-case thermal resistance.

Below is what that trade actually measures, using one die that ST ships in three different bodies.

[IMAGE 1: a TO-3PF alongside a TO-247 and a TO-3P, tabs facing the camera, on graph paper | alt: “TO-3PF package with fully moulded tab compared with TO-247 and TO-3P packages”]

Key takeaways

• The tab is inside the moulding. No insulating pad, no shoulder bushing, no hipot fixture for the interface.
• ST rates the identical die at 0.9 °C/W in TO-247 and 2.8 °C/W in TO-3PF, junction-to-case, per diode.
• Isolation voltage on that part is 2500 V DC.
• Mounting torque is 0.4 to 0.6 N·m — lower than the 0.5 to 1.0 N·m ST allows for the same die in TO-247.
• Body dimensions agree across vendors: 26.5 × 15.5 × 5.5 mm nominal, mounting hole ø3.4 to 3.8 mm.

What the full-pack body costs you

ST’s STTH60AC06C is an ultrafast dual rectifier offered in three packages from one die. The datasheet, DocID024886 Rev 1, prints separate thermal figures for each.

Parameter	TO-247 and TO3P-3L	TO-3PF
Order code	STTH60AC06CW / …CP	STTH60AC06CPF
Rth(j-c) per diode	0.9 °C/W	2.8 °C/W
Rth(j-c) total	0.55 °C/W	2.2 °C/W
Rth coupling between dice	0.2 °C/W	1.6 °C/W
Recommended torque	0.5–1.0 N·m (TO-247)	0.4–0.6 N·m
Weight	4.36 g (TO-247) / 5.26 g (TO3P-3L)	5.6 g

The junction-to-case penalty is a factor of 3.1 per diode. The coupling figure moves even harder — from 0.2 to 1.6 °C/W — because heat from each die now has to cross the moulding compound before it can spread, so the two dice interfere with each other far more.

Electrical ratings do not move at all. Both versions carry 600 V, 30 A per diode, 175 °C maximum junction temperature, and a 40 ns reverse recovery time. Only the thermal path changes.

That is the honest shape of the trade. Full-pack buys assembly simplicity and a creepage-compliant barrier; it does not buy free performance.

TO-3PF package dimensions

Two vendors publish controlled outlines for TO-3PF, and they agree on every shared dimension. Vishay’s document 95646, revision 03-Feb-16, and ST’s package tables give the same numbers under different letter schemes.

Feature	Min	Nom	Max
Body length	26.30	26.50	26.70 mm
Body width	15.30	15.50	15.70 mm
Body thickness	5.30	5.50	5.70 mm
Lead pitch	—	5.45 BSC	—
Mounting hole diameter	3.40	3.60	3.80 mm
Hole centre from top edge	9.80	10.00	10.20 mm
Overall length with leads	43.20	—	44.40 mm

ROHM’s 2024 shortform catalog lists the same package at 43.8 × 15.5 mm with a 5.5 mm thickness, which sits inside those windows — the 43.8 figure is the overall length including leads, not the body.

The mounting hole is larger than the non-isolated TO-3P’s, which runs 3.1 to 3.3 mm on ST’s TO3P-3L table. The extra diameter is there because the hole passes through moulding compound, not metal. ST notes on its TO-247 drawing that resin thickness around the mounting hole is not less than 0.9 mm; the same principle governs how hard you can clamp a full-pack part.

[IMAGE 2: dimensioned TO-3PF outline showing body envelope, 5.45 mm lead pitch and mounting hole | alt: “TO-3PF package outline dimensions with 5.45 mm lead pitch and mounting hole”]

Mounting: no insulator, but a lower torque ceiling

The assembly saving is real. A non-isolated part against a grounded sink needs a thermal pad or mica washer, a shoulder bushing to keep the screw off the tab, and an insulation test. A TO-3PF needs a screw and thermal compound.

The catch is the torque ceiling. ST recommends 0.4 to 0.6 N·m for TO-3PF and TO3P-3L, against 0.5 to 1.0 N·m for the same die in TO-247. The isolated package tolerates less clamping force because the screw bears on epoxy rather than on a metal tab.

Two more points worth designing around. The package is rated for cooling by conduction and the epoxy meets UL94 V-0, both stated on the ST datasheet — useful when a safety file asks. And because the isolation is internal, the barrier is defined by the manufacturer and tested at the factory, which removes an assembly-variable failure mode that pads and bushings introduce.

Worked example: full-pack against an insulated TO-247

Take the STTH60AC06C with one diode conducting at 15 W, junction budgeted to 125 °C rather than the rated 175 °C, in a 50 °C enclosure. Allowable rise is 75 °C, so the total path must come in under 75 / 15 = 5.0 °C/W.

TO-3PF route. Subtract Rth(j-c) of 2.8 °C/W. Direct metal-free mounting with compound leaves a small interface term, call it 0.2 °C/W. Remaining heatsink budget: 2.0 °C/W.

TO-247 route. Subtract Rth(j-c) of 0.9 °C/W, then an insulating pad. Pad performance varies widely, so check the specific pad’s datasheet — assume 0.5 °C/W for this sketch. Remaining heatsink budget: 3.6 °C/W.

The full-pack version needs a heatsink roughly 44 % better at the same dissipation. On a natural-convection extrusion that is often the difference between one part and two, or between passive and forced air.

Run the same arithmetic at lower power and the gap closes to irrelevance. Below about 5 W the 1.9 °C/W difference in Rth(j-c) costs under 10 °C of junction temperature, and the assembly saving wins outright.

Four mistakes with full-pack packages

Reusing the non-isolated thermal model. Rth(j-c) can triple. Recalculate rather than assuming the package swap is thermally neutral.

Torquing to the non-isolated spec. 1.0 N·m is fine on a TO-247 tab and is above the ceiling for TO-3PF.

Assuming isolation means no heatsink compound. The barrier is inside the package; the joint between the moulding and the sink still needs filling.

Confusing TO-3P with TO-3PF in a BOM. Aggregator sites file the isolated package under “TO-3P — Fully Isolated Plastic Package,” and Diodes Incorporated’s non-isolated TO3P drawing currently ranks for this query. The footprints differ, and so does the mounting hole diameter.

Frequently asked questions

Is the TO-3PF package isolated?

Yes. The mounting tab is fully encapsulated in the moulding compound, so no external insulator is needed against a grounded heatsink. ST rates the TO-3PF version of its STTH60AC06C at 2500 V DC insulated voltage. Isolation ratings vary by manufacturer and part, so check the specific datasheet rather than assuming a package-wide figure.

What is the difference between TO-3P and TO-3PF?

TO-3P exposes a metal tab; TO-3PF encapsulates it. That changes the thermal path substantially — ST’s figures for one die are 0.9 °C/W junction-to-case in TO3P-3L against 2.8 °C/W in TO-3PF. The mounting hole also differs, roughly 3.1 to 3.3 mm on TO-3P against 3.4 to 3.8 mm on TO-3PF.

What is the mounting torque for a TO-3PF package?

ST recommends 0.4 to 0.6 N·m for TO-3PF, using an M3 screw through the ø3.4 to 3.8 mm hole. That ceiling is below the 0.5 to 1.0 N·m the same manufacturer allows for TO-247, because the screw clamps against epoxy rather than a metal tab.

What size is a TO-3PF package?

The body measures 26.5 × 15.5 mm nominal with a 5.5 mm thickness, on a 5.45 mm lead pitch. Overall length including leads runs 43.2 to 44.4 mm. Vishay’s controlled outline 95646 and ST’s package tables agree on every shared dimension within their tolerance bands.

What to do next

Specify TO-3PF when the device sits on a grounded heatsink, the dissipation is modest enough that a tripled junction-to-case resistance still closes your thermal budget, and you want the isolation barrier factory-tested rather than assembled. Below roughly 5 W per die that is usually the right call.

Above that, run the arithmetic both ways before committing. Take the Rth(j-c) figures from the specific datasheet — not from a package-level rule of thumb — and price the pad, bushing and hipot step against the larger heatsink the full-pack version will need. Then check the lifecycle record: ROHM lists its R6030ENZ in TO-3PF as Recommended under an 8-year Product Longevity Program, while some distributor lines for the same part have already been dropped.

External references

• ST STTH60AC06C datasheet, DocID024886 Rev 1
• Vishay TO-3PF outline dimensions, document 95646
• ROHM TO-3PF dimension drawing
• ROHM 2024 shortform transistor package catalog
• Fairchild TO-3PF tube packing data

Proposed internal links

• [INTERNAL LINK: TO-3P package → TO-3P package dimensions and mounting]
• [INTERNAL LINK: TO-247 package → TO-247 package guide]
• [INTERNAL LINK: junction-to-case thermal resistance → thermal resistance fundamentals for power design]
• [INTERNAL LINK: insulating pads → thermal interface material selection for power devices]
• [INTERNAL LINK: creepage and clearance → isolation, creepage and clearance in power design]