How does a vapor chamber cool a phone?

A loop that runs on its own. A vapor chamber is a flat, sealed metal pocket. Before it is sealed, a little liquid goes in, usually water, and nearly all the air is pumped out. A wick lines the inside: a thin layer of porous metal or fine mesh.

Heat from the chip boils the water right above it. The vapor rushes to wherever the plate is cooler and condenses there, giving its heat to the metal. The wick pulls the liquid back toward the chip, the way a paper towel drinks up a spill, and the water boils again. NASA Spinoff, describing a titanium vapor chamber, puts it the same way: the liquid evaporates at the heat source, the vapor gives up its energy somewhere cooler, and “a porous metal structure” brings the liquid back. Nothing moves but the water.

Why does water boil in a phone nowhere near 100 °C? Water boils when its vapor pressure matches the pressure around it. With the air pumped out, the only gas left inside is the water’s own vapor. At 48 °C that is about 11 kPa, 11% of the air outside, Vapor Chamber Lab’s notes show, so the water boils at 48 °C. The chamber starts working as soon as the phone warms up.

Because boiling soaks up a lot of heat without getting any hotter. Turning water into vapor takes about 2,390 kJ for every kilogram at a warm phone’s 45–50 °C. Even at 100 °C, where it is a little lower, it is more than five times the energy it takes to heat the same water from 0 to 100 °C. The vapor gives all of it back when it condenses.

So very little water has to move. At 6 W the chip boils off about 2.5 µL a second: one drop every 20 seconds, going round and round.

Copper has no such trick. Heat flows through metal only from hotter to cooler, and the thinner the sheet, the steeper the slope it needs. In the model, a 0.5 mm copper plate with a 6 W chip is 59.7 °C at the hot spot and about 42 °C at the far corners. A vapor chamber of the same size and thickness stays within half a degree of 48 °C everywhere. One 0.5 mm test chamber measured an effective conductivity of about 20,900 W/m·K, Applied Thermal Engineering reports, about 52 times copper’s 400. Long heat pipes can approach 100,000 W/m·K.

The patch over the chip dries out. A wick can only pull so hard. The 0.5 mm test chamber could carry at most 7.58 W lying flat, at its best fill. Past that, the chip boils water faster than the wick brings it back. The liquid front retreats, and a dry patch opens over the chip where nothing boils. Under it, the chamber is just its two thin metal walls.

In the model, at 13 W, about the A19 Pro chip’s reported peak, the dry patch is about 33 mm across. The chamber’s hot spot reaches 108.3 °C, hotter than solid copper’s 100.2 °C, because the dry walls are thinner than the copper plate. The model dries the whole patch at once; real chambers dry out over a narrow range of power.

Which way the chamber faces matters too. The same chamber carried 9.06 W when gravity helped the liquid back to the hot spot and 6.33 W when gravity worked against it.

Apple’s iPhone 17 Pro has one, and Apple has said what is inside it. “Deionized water is sealed inside the vapor chamber, which is laser-welded into the aluminum chassis,” Apple’s Greg Joswiak said at the phone’s launch event, as 9to5Mac reported. The foldable iPhone Duo has one too. Its engineers told PCMag that heat goes from the A20 Pro chip into a vapor chamber, 9to5Mac relays. A flexible graphite pipe through the hinge then carries heat to the other half, which gives the phone more surface to shed it from.

It seems to help. Testers at Geekerwan measured an iPhone 17 Pro shedding about 6.1 W while gaming, against at best 5.3 W for the A18 Pro in the iPhone 16 Pro Max. Notebookcheck reads that as cooling that handles about 15% more heat. The A19 Pro chip can also draw up to 13 W in short bursts.

How thin is a phone’s chamber? Apple has not published the thickness, fill or wick of either one, so the tool does not claim them. The lab chamber it uses is 0.5 mm thick. Thinner gets harder: Yadavalli, Weibel and Garimella found that at ultra-thin sizes and low power, the vapor itself starts to resist the flow, and a heat pipe can do no better than a solid spreader.

1. 1 See

   Open Vapor Chamber Lab. It starts at Gaming · 6 W: Hot spot, solid copper reads 59.7 °C and Hot spot, vapor chamber 48.4 °C. Boiling off the chip reads 2.5 µL/s, a drop every 20 s.

2. 2 Change

   Tap Idle · 1 W. Copper reads 30.8 °C and the chamber 28.9 °C: with little heat to move, the gap almost closes. Then tap Near the limit · 7.5 W. Copper reads 68.4 °C, the chamber 54.2 °C, and Load on the wick 99%.

3. 3 Understand

   Now tap Peak burst · 13 W. The load on the wick reads 172%. A dry patch about 33 mm across opens over the chip, and the chamber’s hot spot, 108.3 °C, is 8.0 °C hotter than solid copper.

Vapor Chamber Lab is a bare plate in still air, not a phone. A real phone also has a battery, a screen, a case and other paths for heat, and it slows its chip down long before the model’s hottest numbers. Air takes heat from both faces at an assumed 12 W/m²·K, for natural convection plus radiation.

The chamber is one lab chamber’s measured conductivity and wick limit, lying flat. Real chambers vary with size, wick, fill and orientation, and very thin ones can do worse than solid metal at low power. The color scale runs from 30 to 80 °C, like a thermal camera’s range, so anything hotter reads white. The cross-section is not to scale: its bubbles, vapor and droplets are drawn to show the loop, not to measure it.

The film’s thermal-camera footage and real iPhone vapor chambers are by @iphonedo on TikTok, credited on screen. The plates, the chamber and every number come from the model, not from the clips.