Heatsink compound before the thermal runaway

A linear regulator without proper thermal coupling to its heatsink is just a slow fuse. You sized the part, you checked the dropout, you even did the dissipation math—but if the thermal interface between die and metal is an air gap, none of that matters when your board pulls current.

The thermal path you cannot see

Power dissipation is voltage drop times current. That heat has to leave the silicon, cross the package, traverse the interface material, and spread into the heatsink before convection or conduction can carry it away. The weakest link in that chain is almost always the interface between the component and the metal.

Most TO-220 and TO-263 packages have a metal tab that looks flat. It is not flat. Neither is your heatsink. Those microscopic air pockets are excellent insulators. Thermal compound fills the voids so conducted heat doesn't stall at the boundary.

Application that actually works

A rice-grain-sized blob in the center of the component tab is enough for most small regulators. Spread it thin with a plastic card edge or the applicator tip—you want coverage, not thickness. Excess compound squeezes out when you torque the fastener, which is fine, but a half-millimeter layer adds thermal resistance instead of removing it.

Clean both surfaces with isopropyl alcohol before application. Oil from your fingers, oxidation, and old compound residue all raise the thermal resistance. If you are reusing a heatsink, scrape off the old material completely.

Torque the fastener to the datasheet spec if one exists. Under-torqued mounting leaves gaps; over-torqued hardware can crack the package.

When to skip it

Some regulators have isolation requirements and use a mica or silicone insulating washer between the tab and heatsink. You still need compound—one thin layer on each side of the washer. The insulator is not a thermal barrier by design, but it becomes one if you skip the interface material.

If your regulator dissipates under half a watt and the ambient is controlled, you might survive without a heatsink at all. That is a different decision. Once you have committed to a heatsink, commit to the compound.

What to keep on the bench

Non-conductive thermal compound in a small syringe. You do not need exotic materials for regulators under 10 W. Zinc oxide or silicone-based pastes are sufficient and stable.

Avoid compounds marketed for overclocked gaming PCs unless you enjoy paying for performance you will not measure. Avoid pads unless your production volume justifies the convenience—they add thermal resistance compared to paste, and you are not building thousands of boards.

Store the syringe tip-up so the compound does not separate. Most pastes have a shelf life of a few years; mark the purchase date on the tube.

The failure mode you will not enjoy

A regulator in thermal shutdown does not always fail permanently, but it will drop out of regulation or current-limit in ways that look like circuit bugs. You will chase phantom voltage sags and assume your bulk capacitors are undersized, when the real problem is a 5 °C/W interface that should be 0.5 °C/W.

If your heatsink is warm to the touch but your regulator is scorching, your thermal path is broken. That heat should be spreading into the metal, not staying in the silicon.

Check your work after assembly. A properly mounted regulator with thermal compound will run cooler than the heatsink under sustained load—heat flows from component to metal. If the component is hotter than the sink, your interface has failed.

Measure what you built

Run your board at full load for ten minutes, then check the regulator temperature with an IR thermometer or thermocouple. Compare it to the junction temperature limit in the datasheet minus your calculated thermal resistance. If you are within 20 °C of the absolute maximum, you have a problem.

Thermal compound is not dramatic. It does not make sparks or smoke when you forget it. It just lets your regulator do the job you designed it for without throttling or dying early.

Apply it once, apply it correctly, and your voltage will stay honest under load. Skip it and you will troubleshoot a thermal problem that looks like every other kind of failure until you finally touch the part.

See also: LDO headroom when your wall wart drifts for keeping your regulator in the linear region, and Reverse polarity protection you'll thank yourself for for upstream safeguards.