The TO-252, also called DPAK, is the workhorse surface-mount power package: small enough for high-volume assembly, rugged enough for automotive and industrial boards, and thermally capable when the PCB is designed properly. Its performance, however, is decided more by the copper around it than by the package itself. This article explains how a TO-252 MOSFET’s heat actually leaves the device, how the footprint and copper area set the real current rating, and where the package fits—and does not fit—in a power design.
What a TO-252 Package Is
A TO-252 is a surface-mount package with three leads and a large exposed metal tab (the drain tab in most N-channel parts) on its underside. The tab is the primary thermal and electrical path: the drain current flows through it, and most of the heat is conducted from the die through the lead frame into the tab and then into the PCB copper.
The package sits between a small SO-8 and a through-hole TO-220 in capability:
| Package | Approximate continuous capability (typical) | Thermal path | Assembly |
|---|---|---|---|
| SO-8 | 2–6 A | Pads, limited copper | Simple SMT |
| TO-252 (DPAK) | 10–40 A with proper PCB | Exposed tab to PCB copper | SMT, tab soldered |
| TO-220 | 20–60 A+ | Tab to heatsink | Through-hole or clip |
The numbers are indicative, not guarantees—the real limit is set by the die, the PCB, and the environment. The key message is that the TO-252’s advantage over SO-8 comes entirely from the exposed tab and the copper you attach to it.
The Thermal Path, Step by Step
Heat flows from the die to the lead frame, into the tab, across the solder joint, and into the PCB copper, where it spreads before reaching the ambient. Each step has a resistance:
- Junction-to-case (Rth(j-c)). The package’s internal resistance, dominated by the die and lead frame. It is fixed by the design and is the number on the datasheet.
- Case-to-solder. The solder joint between the tab and the PCB pad. A void-free joint is essential; large voids can add tens of percent to the total resistance.
- Solder-to-ambient (Rth(c-a) or Rth(b-a)). Governed by the PCB: pad area, copper thickness, vias, inner planes, and airflow. This is where the designer has the most control.
The datasheet’s thermal resistance table usually lists two values: Rth(j-c) and a board-dependent Rth(j-a) measured on a defined test board—often a 1 in² or 6 cm² pad on a 1 oz or 2 oz board. Using that number for a different board is a common error; the real Rth(j-a) can be two to three times worse on a small footprint.
Footprint Rules That Decide the Current Rating
The practical current limit of a TO-252 MOSFET is a thermal number, and four footprint choices drive it:
- Tab pad size. The pad under the tab should extend beyond the tab outline so heat can spread into the board. A pad cut exactly to the tab leaves heat nowhere to go.
- Copper thickness. 2 oz copper roughly halves the spreading resistance compared with 1 oz. For high-current boards, 2 oz on the top layer is the standard upgrade.
- Thermal vias. A grid of vias under the tab conducts heat to inner planes. The via diameter, plating, and pitch matter: many small vias with thin plating spread heat better than a few large ones. Solder-mask-defined vias that fill with solder conduct best.
- Inner planes. Connecting the pad’s vias to a solid ground or drain plane on an inner layer gives heat a large area to spread into before it reaches the ambient.
A practical budget: with a well-built 2 oz board, a large pad, and a good via array, a TO-252 part can carry significantly more current than the same die in an SO-8. With a minimal footprint, the same part overheats at a fraction of the rated current—the die was never the limit.
Solder Joint Reliability Under Thermal Cycling
The exposed tab’s large solder joint is a double-edged sword: it conducts heat well, but it also experiences mechanical stress as the package and board expand and contract at different rates. Thermal cycling in automotive and industrial applications fatigues the joint, and the failure mode is a crack that raises thermal resistance long before it opens the electrical path.
Design rules that protect the joint:
- Keep the tab solder area as large as the pad allows, for both thermal and mechanical reasons.
- Avoid long, unconstrained traces that transmit board flex into the joint; position the package away from board edges and connectors where flex concentrates.
- Match the reflow profile to the solder paste and the pad’s thermal mass; a large tab cools differently from a small pad, and tombstoning or voiding follows a poor profile.
- Verify the joint with X-ray or acoustic inspection in production—solder voids under the tab are the classic hidden defect class in power packages.
For the device families and package outlines available in this class, the Documents section of the Good-Ark site carries the outline drawings and thermal data needed for the footprint design.
When TO-252 Is the Right Package
The TO-252 earns its place in a specific window:
- Medium power with SMT assembly. Where a through-hole TO-220 breaks the assembly flow and an SO-8 cannot carry the current, the TO-252 fits.
- Automotive and industrial loads. The package has decades of field data, and its tab soldering is well understood.
- Space-constrained boards with a thermal plane available. The package’s performance depends on the board; if the board can give it copper, it performs.
It is the wrong choice when the PCB cannot provide the copper (a small, crowded board may be better served by a DFN with a smaller footprint and similar tab), or when the thermal demand exceeds what board-level spreading can remove—at which point a TO-220 with a heatsink or a module is the honest answer. The package trade-offs against leadless options are covered in the companion article on QFN and DFN packages in this series.
A Thermal Budget Worked Example
Target: 15 A continuous through a TO-252 MOSFET with RDS(on) of 6 mΩ hot, in a 70 °C ambient, with a target junction temperature below 125 °C.
- Conduction loss: 15² × 0.006 × 1.0 (continuous) = 1.35 W; add switching loss if the frequency is not zero.
- Allowable rise: 125 − 70 = 55 K.
- Required Rth(j-a): 55 / 1.35 ≈ 41 K/W.
- Check the board: Rth(j-c) might be 2–3 K/W, leaving ~38 K/W for the solder-to-ambient path—reachable with a solid 2 oz pad and via array, tight on a minimal footprint.
The calculation is an example but shows the method: define the loss, the temperature budget, and the required Rth, then design the footprint to meet it. If the required Rth is below what the board can give, the package is the wrong one.
TO-252 vs DFN vs TO-220 in Practice
The package decision is a three-way comparison, and each choice moves the thermal and assembly balance:
| Package | Best when | Cost | Thermal ceiling |
|---|---|---|---|
| SO-8 | Low power, simple assembly | Lowest | Small pads, board-limited |
| TO-252 (DPAK) | Medium power, SMT, automotive | Low | High with a good copper plane |
| DFN (leadless) | Small size, medium power, high frequency | Low, but finer assembly | Similar to TO-252 at same pad area |
| TO-220 | High power, heatsink available | Low | Very high with a heatsink |
The DFN offers a smaller body for the same pad capability and lower inductance, which matters at high switching frequency; the trade is the hidden solder joints and X-ray inspection requirement, covered in the companion article on QFN and DFN packages in this series. The TO-220 remains the answer when the board cannot absorb the heat and a heatsink is the honest solution—but it breaks the SMT assembly flow.
For automotive and industrial boards that are already SMT, the TO-252 is often the sweet spot: the exposed tab gives the thermal path, the three leads are visible for inspection, and the package has decades of field history. The decision method is the same thermal budget calculation; the packages differ in where the heat can go.
Land Pattern, Solder Paste, and the Tab Connection
The TO-252’s land pattern is where the thermal and assembly disciplines meet. Four details dominate the result:
- Pad size. The tab pad should extend beyond the tab outline so the solder spreads and the heat leaves the joint; a pad cut exactly to the tab traps the heat and concentrates the strain.
- Solder paste aperture. The stencil opening under the tab should be patterned (usually in a grid of apertures, not one solid opening) to control paste volume and avoid voids from outgassing. The aperture area ratio and the paste type come from the assembly partner’s process qualification.
- Via placement. Vias under the tab should be placed inside the pad openings or in the solder-mask-defined areas, sized and plated so they fill with solder; filled or tented vias behave differently from open ones.
- The tab is the drain. In most N-channel TO-252 parts the exposed tab is the drain connection, so the pad is part of the high-voltage or switching node. The pad capacitance adds to the drain node, the clearance to other nodes must respect the working voltage, and the EMI from the switching node can couple into the heatsink path—treat the tab connection as a circuit element, not just a heat spreader.
The migration ladder is a useful heuristic: TO-252 (DPAK) for medium-power SMT, D2PAK/TO-263 for higher current with a larger tab and pad, and TO-220 when the board cannot absorb the heat and a heatsink is the honest answer. Each step changes the pad, the assembly, and the thermal path, so the migration is a re-design of the footprint, not a drop-in swap.
The quantified effect of the board is best seen in a table of typical Rth(j-a) values; the exact numbers depend on the die and the test board, but the trend is consistent:
| Board treatment | Relative Rth(j-a) trend |
|---|---|
| Minimal 1 oz pad, no vias | Highest (baseline) |
| 1 oz pad extended, few vias | Lower |
| 2 oz pad, good via array to inner plane | Significantly lower |
| 2 oz pad, via array, airflow | Lowest |
The table is a screening heuristic for the layout review, not a specification; measure or model the actual assembly for the design’s copper, vias, and airflow.
Frequently Asked Questions
What is the difference between TO-252 and DPAK? They are the same package—TO-252 is the JEDEC designation and DPAK is the common trade name.
Can a TO-252 MOSFET handle 40 A? With a low-RDS(on) die, a large 2 oz copper pad, good vias, and airflow, a TO-252 can carry tens of amps, but the limit is thermal, not electrical. Verify the junction temperature on the real board rather than trusting a single current number.
Why does the exposed tab matter so much? The tab is both the drain connection and the main heat path. Without a large, well-soldered tab pad, most of the package’s thermal capability is wasted.
Do I need thermal vias under the tab? For any continuous current above a few amps, yes. Vias to an inner plane give the heat somewhere to spread; without them, the top-layer pad is the only heat sink.
The Tab Is the Thermal Interface
A TO-252 MOSFET is only as good as the copper attached to its tab. Design the footprint first—pad size, solder-paste apertures, copper weight, vias, and inner planes—then calculate the junction temperature at your real current and ambient. When the board can feed the tab, the TO-252 is one of the most practical power packages in production; when it cannot, no datasheet number will save the design. To download the TO-252 package outline and recommended land pattern, check the Documents section, and contact Good-Ark for footprint-specific support.
FAE note before publication: add the TO-252 package outline and recommended land-pattern drawing, plus a measured Rth(j-a) or temperature-rise example for a defined copper area and via configuration.