Stud-mount diode packages bolt a rectifier directly into a heat sink or busbar through a threaded stud, with the hex base carrying heat out of the die. DO-4, DO-5 and DO-8 are the three sizes still in production, spanning roughly 25 A to 150 A average forward current. They differ in thread, torque, and thermal resistance by factors of six or more, and getting any of the three wrong cracks dice.
Key takeaways
- Thread size is the fastest identifier: DO-4 is 10-32 UNF-2A, DO-5 is 1/4-28 UNF-2A, DO-8 is 3/8-24 UNF-2A (Vishay dimension sheets 95311, 95360, 95314).
- Junction-to-case thermal resistance runs 1.5 K/W for DO-4, 0.65 °C/W for DO-5, and 0.25 K/W for DO-8, a 6:1 spread across the family.
- Torque is not a single figure. Vishay’s DO-5 datasheet lists four values depending on whether you turn the nut or the hex, and whether the threads are lubricated.
- Lubricated threads take roughly 20 % less torque than dry ones for the same clamping force.
- The R suffix puts the anode on the stud instead of the cathode, which changes the polarity of your heat sink.
[IMAGE 1: three stud diodes side by side at true relative scale, DO-4, DO-5 and DO-8, with stud threads and hex flats called out | alt: “Stud-mount diode packages DO-4, DO-5 and DO-8 compared at scale”]
What a stud-mount diode is
A stud-mount diode is a rectifier in a metal package with a threaded stud on one face and a flexible or solid terminal on the other. The stud screws into a tapped heat sink or passes through a clearance hole and is secured with a nut, making the mounting hardware both the thermal path and one electrical terminal.
That dual role is the whole design. The hex base conducts heat and current, so the joint’s mechanical quality sets the thermal performance directly.
The names, and what JEDEC actually registers
DO-4, DO-5 and DO-8 are legacy shop names. The JEDEC JEP95 diode outline index tells a more careful story: DO-5 appears only as an archived entry redirecting to DO-203AB, and DO-4 and DO-8 do not appear as index entries at all.
What JEDEC lists instead are two live families: DO-203, “Stud-hex Base, Solid Terminals” and DO-205, “Stud-Hex Base, Flex Terminals.” The mapping that matters on a datasheet is:
- DO-4 = DO-203AA
- DO-5 = DO-203AB
- DO-8 = DO-205AA
- DO-9 = DO-205AB, DO-30 = DO-205AC (per Vishay app note 95605)
Distributor filters use the paired form, “DO-203AA (DO-4).” Search either way, but specify the JEDEC number on drawings.
The parametric comparison
Three Vishay series, one per package, all datasheets revision 11 January 2018.
| Parameter | DO-4 (DO-203AA), VS-25F | DO-5 (DO-203AB), VS-1N3208 | DO-8 (DO-205AA), VS-150K |
| Stud thread | 10-32 UNF-2A (M5 × 0.8 option) | 1/4-28 UNF-2A | 3/8-24 UNF-2A (M12 × 1.75 on request) |
| Hex across flats | 10.70 to 11.50 mm | 17.15 to 17.38 mm | 27.00 mm max |
| Body diameter | Ø 6.8 mm | Ø 14.6 mm | Ø 23.30 mm max |
| IF(AV) | 25 A at TC 120 °C | 15 A at TC 150 °C | 150 A at TC 150 °C |
| IF(RMS) | 40 A | — | 235 A |
| IFSM, 60 Hz half cycle | 373 A | 250 A | 3740 A |
| I²t for fusing, 8.3 ms | 580 A²s | 260 A²s | 58 kA²s |
| VRRM range | 100 to 1200 V | 50 to 600 V | 100 to 600 V |
| TJ range | −65 to +175 °C | −65 to +175 °C | −40 to +200 °C |
| RthJC, DC | 1.5 K/W | 0.65 °C/W | 0.25 K/W |
| RthCS, greased | 0.5 K/W | 0.25 °C/W | 0.10 K/W |
| Weight | 7 g | 28.5 g | 100 g |
Note the DO-5 entry. It carries a lower average current rating than the physically smaller DO-4 because these are different device families, not a size ladder. The 1N3208 is a 15 A part in a body that also houses 60 A devices such as the 1N1183. Package size caps the dissipation; it does not set the current rating.
Polarity: which terminal is the stud
Base part numbers put the cathode on the stud. Adding R puts the anode there. Vishay states this explicitly across the families: 1N3208R, VS-25FR, VS-150KR all mean anode to stud.
The consequence is electrical, not mechanical. If the stud carries the anode and the heat sink is grounded chassis, the diode’s anode is now at chassis potential. Freewheeling and battery-charger topologies often need exactly one of the two arrangements, and swapping the suffix is cheaper than isolating the heat sink.
Vishay also offers metric threads as a separate suffix, with M denoting M5 × 0.8 on the DO-4. Two suffixes on the same part number therefore change two independent things, so read them separately.
[INTERNAL LINK: through-hole vs surface mount package trade-offs -> mounting technology comparison]
Mounting torque is four numbers, not one
The DO-5 datasheet is the clearest example. Its torque table has four rows, all with a +0 %, −10 % tolerance:
| DO-5 mounting condition | Torque |
| Dry threads, tightening on the nut | 3.4 N·m (30 lbf·in) |
| Lubricated threads, tightening on the nut | 2.3 N·m (20 lbf·in) |
| Dry threads, tightening on the hexagon | 4.2 N·m (37 lbf·in) |
| Lubricated threads, tightening on the hexagon | 3.2 N·m (28 lbf·in) |
Vishay marks the nut rows as the recommendation for pass-through holes and the hexagon rows for tapped heat sinks. The other two packages carry the same logic with different magnitudes: DO-4 takes 1.5 N·m (13 lbf·in) dry or 1.2 N·m (10 lbf·in) lubricated, and DO-8 takes 11.3 to 14.1 N·m (100 to 125 lbf·in) dry or 9.5 to 12.5 N·m (85 to 110 lbf·in) lubricated.
Two rules fall out. Lubricated threads always take less torque, because less of the applied moment is lost to friction and more becomes clamping force. And torquing on the hex instead of the nut needs a higher figure, because the thread friction path is different.
Use a torque wrench. The tolerance on these numbers is one-sided: you may go under by 10 %, never over.
The thermal chain, worked
Take the 150 A DO-8 part at full rating, 180° conduction. The datasheet gives high-level threshold voltage VF(TO)2 = 0.83 V and forward slope resistance rf2 = 0.91 mΩ at TJ maximum, with IF(AV) = 150 A and IF(RMS) = 235 A.
P = 0.83 V × 150 A + 0.00091 Ω × (235 A)² = 124.5 W + 50.2 W ≈ 175 W
Now the thermal chain. RthJC is 0.25 K/W and RthCS is 0.10 K/W with a smooth, flat, greased mounting surface:
ΔT = 175 W × 0.35 K/W ≈ 61 K
With TJ limited to 200 °C, the heat-sink surface under the diode must stay below roughly 139 °C. That is the number to design the sink to, and it assumes you achieved the 0.10 K/W interface. A dry or dirty joint can double RthCS, which costs another 18 K of junction temperature at this power, enough to move a 139 °C sink requirement down to about 121 °C.
[IMAGE 2: thermal resistance chain from junction to ambient for a stud diode, with RthJC, RthCS and RthSA labeled | alt: “Stud-mount diode thermal chain from junction through case to heat sink”]
Heat-sink preparation and the mistakes that crack dice
Vishay’s mounting application note 95556 gives numbers the datasheets do not.
Surface flatness under the device must hold to better than 0.03 mm, with surface roughness better than 0.02 mm per DIN/ISO 1302. A milled surface from sharp tooling meets this; a sawn or corroded one does not.
Particles above 0.05 mm thickness between the hex and the sink are called out specifically. On a 25 mm hex face, one steel chip that size tilts the whole interface.
Thermal compound goes on the hex side before mounting. Silicone or petroleum-based compound suits metal-to-metal joints, and it seals the joint against moisture as well as filling voids. Vishay notes that contact improves over the first hours of operation as the grease redistributes.
Never tap the stud into a threaded hole. The app note names this and excessive torque as the two failure modes that warp the hex base and crack the die. The only sanctioned fastening method is a nut and washer, with the stud entered straight and without force.
Excess compound at the edges should be cleaned off. On high-voltage parts it collects dust and creates an arc-over path across the mounting face.
Availability, lead time, and second sourcing
These are low-volume industrial parts, and the ordering experience reflects that.
| Part | Package | Position on DigiKey |
| UES704 (Microchip) | DO-203AA (DO-4), 200 V, 20 A | Not stocked; 42-week manufacturer standard lead time; $46.46 each at qty 100 |
| A170B (Powerex) | DO-205AA (DO-8), 200 V, 100 A | Listed and shipping |
| R5020210RSWA (Powerex) | DO-205AA (DO-8), 200 V, 100 A | 20-week manufacturer standard lead time |
Two consequences. First, price per amp is high compared with modern surface-mount rectifiers, so stud parts earn their place on surge capability, isolation from the board, and serviceability rather than cost. Second, lead times of 20 to 42 weeks mean the package choice is effectively frozen at design time, so plan a second source in the same outline before release.
For legacy repair work, the JEDEC 1N-series numbers help. Parts such as the 1N1183 family are made by more than one supplier to a common outline, and some carry military qualification, with MIL-S-19500/260 listed against certain DO-203AA parts.
[INTERNAL LINK: component obsolescence and second-source qualification -> lifecycle management]
Stud-mount diode FAQ
What is a stud-mount diode?
A rectifier in a metal package with a threaded stud on one face and a terminal on the other. The stud screws into a heat sink or passes through a clearance hole with a nut, so the mounting hardware carries both the heat and one of the two electrical connections.
What is the difference between DO-4 and DO-5?
Size and thread. DO-4 (DO-203AA) uses a 10-32 UNF-2A stud in a Ø 6.8 mm body; DO-5 (DO-203AB) uses 1/4-28 UNF-2A in a Ø 14.6 mm body. Thermal resistance drops from about 1.5 K/W to 0.65 °C/W junction-to-case, and mounting torque roughly doubles.
Is the stud the anode or the cathode?
Cathode, on base part numbers. Adding R to the part number, as in 1N3208R or VS-150KR, puts the anode on the stud instead. Since the stud is bolted to the heat sink, the suffix determines whether your sink sits at anode or cathode potential.
How much torque does a stud diode need?
It depends on the package and the method. Vishay specifies 1.5 N·m (13 lbf·in) dry for DO-4, 3.4 N·m (30 lbf·in) dry on the nut for DO-5, and 11.3 to 14.1 N·m (100 to 125 lbf·in) dry for DO-8. Lubricated threads take about 20 % less.
Can stud diodes be mounted on a PCB?
Only through a clearance hole with the stud secured by a nut, with the flexible or solid terminal soldered or bolted separately. The package is designed for a heat sink or busbar; a bare board cannot carry the dissipation these parts are chosen for.
Are stud diodes still available?
Yes, from Vishay, Powerex, Microchip and others, but as low-volume industrial parts. Expect manufacturer standard lead times of 20 to 42 weeks on unstocked variants and prices well above surface-mount equivalents per amp.
What to do next
Identify the package by thread first. A 10-32 stud is DO-4, 1/4-28 is DO-5, 3/8-24 is DO-8, and the JEDEC number to put on the drawing is DO-203AA, DO-203AB or DO-205AA respectively.
Size by dissipation, not current. Run the P = VF(TO) × IF(AV) + rf × IF(RMS)² calculation with your conduction angle, multiply by RthJC plus RthCS, and check the result against your sink temperature before choosing a body size.
Specify the mounting on the assembly drawing: torque value, whether it is applied to the nut or the hex, whether threads are lubricated, and the compound. Those four items decide whether you get the 0.10 K/W interface the thermal calculation assumed.
[IMAGE 3: correct stud mounting with nut and washer against an incorrect tapped-in installation showing hex base warpage | alt: “Correct and incorrect stud-mount diode installation into a heat sink”]
Primary sources
- Vishay VS-25F(R) Series, DO-4 (DO-203AA), document 93506, rev 11-Jan-18 — https://www.vishay.com/docs/93506/vs-25frseries.pdf
- Vishay VS-1N3208 Series, DO-5 (DO-203AB), document 93496, rev 11-Jan-18 — https://www.vishay.com/docs/93496/vs-1n3208series.pdf
- Vishay VS-45L(R), VS-150K(R), VS-150KS(R) Series, DO-8 (DO-205AA), document 93489, rev 11-Jan-18 — https://www.mouser.com/datasheet/2/427/VISH_S_A0004148019_1-2569277.pdf
- Vishay application note, Mounting Instructions for DO-203AA / DO-203AB Diodes, document 95556, rev 30-Mar-15 — https://www.vishay.com/docs/95556/mountingdo-203aa-do-203ab.pdf
- Vishay application note, Mounting Instructions for DO-205AA, DO-205AB, DO-205AC and B-8 Stud Diodes, document 95605 — https://www.vishay.com/docs/95605/mountingdo-205aado-205abdo-205acb-8.pdf
- JEDEC Publication 95, Diode Outlines (DO) index — https://www.jedec.org/sites/default/files/DOINDEX_0.pdf
