iPhone 18 Pro boils over – how Apple cools the A20 Pro Chip
Inside the iPhone 18 Pro there is water, and under load it turns into steam. This sounds like a risky way to cool a smartphone for $1199, but in reality it is a sealed evaporative chamber where liquid can circulate for years without moving parts.
In the iPhone 18 Pro and iPhone 18 Pro Max, Apple significantly redesigned this system. The vapor chamber area has tripled compared to the iPhone 17 Pro, and the A20 Pro processor is now positioned closer to it. As a result, the company claims up to a 40% increase in sustained performance relative to the previous generation.
Water is indeed inside the iPhone
The evaporator chamber is a very thin, sealed metal structure. Inside it are a small amount of deionized water and a porous structure that acts as a wick.
When the A20 Pro heats up, the liquid in the adjacent area evaporates. The vapor spreads through the internal volume of the chamber toward cooler regions, where it condenses back into water and releases the transferred heat. Then, thanks to capillary action along the wick, the liquid returns to the hot area, and the cycle repeats.
No pump, fan, or other mechanism is required for this. In fact, heat is transferred not only through metal heating but also along with steam across a significantly larger surface area of the enclosure.
Why water should not be heated to 100 °C
Here arises an obvious question: if water is boiling inside the iPhone, why doesn’t the smartphone need to be held with a kitchen mitt?
The boiling point of water depends on pressure. 100 °C applies to standard atmospheric pressure, whereas air is removed from evaporation chambers during manufacturing and the chambers are sealed airtight. As pressure decreases, the temperature of water’s phase transition also drops.
Therefore, the working fluid can evaporate intensely at temperatures normal for an electronic cooling system. Apple does not disclose the internal pressure of the evaporative chamber in the iPhone 18 Pro, but water inside it can boil already at 40 °C.
Deionized water with very low dissolved salt content is used specifically. Water is well-suited for such systems due to its high latent heat of vaporization, and removing impurities is also critical for the long-term operation of thin internal channels and the wick.
Apple had to redesign the A20 Pro chip
Increasing the evaporative chamber alone was not enough. In the A20 Pro, Apple reconfigured the processor package layout using a principle the company compares to its M-series chips.
Previously, RAM was located above the processor die and effectively sat between the main heat source and the cooling system. In the A20 Pro, the die and memory are placed side by side.
This allows the processor to directly contact the thermal interface of the vapor chamber. Heat no longer needs to pass through a memory layer before reaching the cooling system.
This is especially important during prolonged workloads. A modern processor can briefly operate at high frequencies even with mediocre cooling, but as temperatures rise, the system is forced to reduce power consumption and clock speeds. That is why peak performance after 10-20 minutes of gaming or video processing can drop significantly.
The A20 Pro is manufactured using a 2-nm process. It features a 6-core CPU, a 7-core GPU, and two 16-core Neural Engines. Memory bandwidth relative to the A19 Pro has increased by 50%, while graphics performance, according to Apple, has risen by up to 40%. The longer the processor operates under high load, the more the result depends not only on the chip’s capabilities but also on how quickly the smartphone can dissipate the heat it generates.
Apple was not the first to come up with this
The funniest thing about “iPhone water cooling” is that this technology is far from new for smartphones.
Apple for the first time used its own vapor chamber in the iPhone 17 Pro and 17 Pro Max in 2025. Android smartphone manufacturers began using similar solutions much earlier.
For example, Samsung already used a thin water-based vapor chamber in the 2016 Galaxy S7: liquid evaporated near the processor, carried heat away, and then condensed again after cooling. Later, such systems became common in flagship and gaming Android smartphones.
