To make the iPhone Duo as thin as it is, Apple needed to make a lot of design choices to fend off heat. Here's what they did, how they did it, and where in the device the innovations live.

Folding devices like the iPhone Duo often raise engineering challenges due to their unique design. However, solving problems like making as thin a smartphone as possible can lead to the creation of others, such as keeping it cool.

In an interview with PCMag, Apple's VP of silicon engineering Dan Murray and senior manager of thermal product design Banafsheh Barabadi discussed how Apple fixed heat issues in the iPhone Duo.

The discussion first covered the "M-Class-inspired packaging design" of the A20 Pro, referring to a change from the package-on-package (POP) process Apple previously used. Rather than a vertical stack for the system-on-chip (SOC) and the memory, Apple moved the DRAM module next to the processor.

The "M-Class" stems from the idea that a notebook like a MacBook Pro has a lot more internal space to play with than a smartphone. The memory can easily be placed separately from the processor, rather than requiring it to be stacked to save space.

The use of a wafer-level multichip module (WMCM) packaging technique helped spread out the heat instead of confining it to one place. The location also meant that the memory still had fast communications with the chip itself.

A thermal redesign

While Apple has used vapor chambers in the iPhone 17 Pro and Pro Max, Apple redesigned the idea for the A20 Pro used in the iPhone 18 Pro and iPhone Duo. The chassis were also redesigned to work better with the thermal technique.

There are four pillars to heat management, with the first being direct silicon access. The shifting of the DRAM from the vertical stack helped this, since heat from the silicon has a more direct path to the vapor chamber.

"Metal over foam" is the second pillar, creating a direct thermal and mechanically compliant coupling between the vapor chamber and chip. "It took us months of development to get the material properties exactly right," the engineers explained.

The technique was first spotted during a teardown of the iPhone 18 Pro, described at the time as "thermal chainmail" inside a pad.

The third is the vapor chamber, which now as three times the surface area as the previous version, along with more de-ionized water. The chamber also serves as the "central structural plane for the device," said Barabadi, with the battery and logic board mounted directly to it.

Lastly, the fourth is a new layer on the back of the foldable display to keep it sturdy. It's described as a "nanotube copper display metal plate material" that provides mechanical strength for the display.

The copper plate also has a high level of thermal conductivity, combined with a high-conductivity graphite layer. The result is that heat is spread more evenly across the display, reducing hotspots.

"When we combine these four pillars together, it meaningfully improves the thermal performance across every metric," said Barabadi.

Unhinged thermal performance

The thermal antics aren't just limited to the main body of the iPhone Duo. The all-important hinge does its part too.

Since the iPhone Duo has two halves, connected by a hinge, the two thin halves become one much thicker device when closed. For heat management, this is a problem, because the vapor chamber in the iPhone Duo floats in the middle.

To thermally connect the two halves of the chamber, there's a flexible graphite pipe going through the hinge. This transfers heat between the two sides, balancing the temperature throughout the devices.

It also considerably increased the surface area used for heat dissipation.

To Apple, the part needed to be worked on by both the thermal and mechanical teams. All due to being a flexible material that needed to survive thousands of hinge activations throughout the iPhone Duo's lifespan.