How does a vapor chamber cool a phone?

Apple says its new iPhone Duo has a custom-designed vapor chamber, and the iPhone 17 Pro has one too. Inside each is a little liquid, sealed in a thin copper pocket. How can a few drops of liquid move a chip’s heat better than solid metal?

▶ Try it yourself in Vapor Chamber Lab: turn a phone chip from idle up to a 13 W burst and watch a vapor chamber and a solid copper plate side by side, seen like a thermal camera.

The heat starts in one small chip

A phone’s processor turns nearly all the power it uses into heat, in a patch about the size of a fingernail. That heat has to spread across the whole phone before it can leave through the glass and the frame. Spread it badly and the chip runs hot, and the phone slows it down to protect itself.

A sheet of copper spreads heat by conduction, metal atom to metal atom. It helps, but a thin sheet can only carry so much. In the tool’s model, a 6 W chip under a 0.5 mm copper plate the size of a phone makes a hot spot of 59.7 °C while the far corners sit near 42 °C.

Water that boils at 48 °C

A vapor chamber is a flat, sealed copper pocket. Before it is sealed, a little liquid goes in and nearly all the air is pumped out. With no air, the only gas inside is the liquid’s own vapor, so the pressure inside is just its vapor pressure. For water at 48 °C that is about 11 kPa, 11% of the air outside, and at that pressure water boils at 48 °C. The chamber does not need to reach 100 °C; it boils at the temperature of a warm phone.

Boiling soaks up a lot of heat without making the water any hotter: about 2,390 kJ for every kilogram near 48 °C, more than five times what it takes to warm that water from freezing to boiling. So very little water has to move. At 6 W the chip boils off only about 2.5 µL a second, one drop every 20 seconds.

Apple has said the iPhone 17 Pro’s chamber holds deionized water. It has not published what fills the Duo’s, how much or at what pressure, so the tool models a generic water-filled chamber, not Apple’s.

The wick closes the loop

The vapor rushes from the hot spot toward the cooler parts of the plate. There it condenses back into liquid and gives up the heat it soaked up. A wick lining the chamber, a thin layer of porous metal, then pulls the liquid back toward the chip by capillary action, the way a paper towel drinks up a spill. Then the loop starts again.

Because moving vapor carries heat far faster than copper conducts it, the whole chamber stays within half a degree of one temperature. One 0.5 mm test chamber measured an effective conductivity of about 20,900 W/m·K, about 52 times copper. In the tool, with the same 6 W chip, the chamber’s hot spot is 48.4 °C: solid copper runs 11.3 °C hotter.

The catch is the wick

A wick can only pull so hard. That test chamber could carry at most 7.58 W lying flat. Push past what the wick can return and the liquid front retreats from the chip, leaving a dry patch where nothing boils. Under it the chamber is just its two thin copper walls, thinner than the copper plate. In the model at 13 W a dry patch about 33 mm across opens, and the chamber’s hot spot (108 °C) ends up hotter than solid copper’s (100 °C).

Real phones never get there: they slow their chips long before. Testers measured an iPhone 17 Pro shedding about 6.1 W while gaming, with peaks near 13 W, and phone chambers are built and filled for the loads they will see.

Predict first, then try

Before you touch anything, predict: at 1 W, a phone idling, is the gap between copper and the chamber bigger or smaller than at 6 W? Then try it in Vapor Chamber Lab, and turn the chip up past 7.6 W to watch the wick give out.

What does the model leave out?

The model’s tag reads “bare plate in still air.” It leaves out:

• The rest of the phone. It is a bare plate, not a phone: no battery, screen, case or other heat paths, and no throttling.
• Any one phone’s chamber. The conductivity and wick limit are one lab chamber’s, lying flat. Real chambers vary with size, wick, fill and how the phone is held, and very thin chambers can do worse than solid metal at low power.
• The inside, to scale. The cross-section is not to scale, and its bubbles, vapor and droplets are drawn to show the loop, not to measure it.

The film uses thermal-camera tests and real iPhone vapor chamber plates by @iphonedo on TikTok, credited on screen; the plates, the chamber and every number are drawn from the model. The same boil-and-condense loop moves heat uphill in a heat pump: see Heat Pump Lab. More on the new phones: how a folding screen bends without breaking.

Sources: Apple, Apple unveils iPhone Duo (9 September 2026), as reported by iThinkDiff; 9to5Mac, Here’s how the iPhone 17 Pro vapor chamber actually works; Notebookcheck, iPhone 17 Pro thermal results; A novel ultra-thin vapor chamber for heat dissipation in ultra-thin portable electronic devices (Applied Thermal Engineering, 2020); Yadavalli, Weibel and Garimella, Performance-governing transport mechanisms for heat pipes at ultra-thin form factors (2015); Wikipedia, Heat pipe, Vapor pressure and Enthalpy of vaporization.