Apple routes a graphite pipe through the iPhone Duo hinge to spread heat across both halvesApple routes a graphite pipe through the iPhone Duo hinge to spread heat across both halves

Apple cools the iPhone Duo with a flexible graphite pipe that runs through the phone's hinge, two of its engineers have told PCMag.

The interview was with Dan Murray, Apple's vice president of silicon engineering, and Banafsheh Barabadi, its senior manager of thermal product design. PCMag's article could not be read for this report; the details below come from 9to5Mac's and Digital Trends' accounts of it, which agree with each other. They are one interview relayed twice, not two sources. Apple's own iPhone Duo page says only that the phone has an advanced thermal management system with a vapour chamber.

Why is a folding phone harder to cool?

Both reports give the same reason. The A20 Pro, Apple's first 2nm iPhone chip, is shared by the iPhone 18 Pro, iPhone 18 Pro Max and iPhone Duo, and more performance means more heat. Digital Trends adds that a folded phone is twice as thick as either half, which makes it much harder for heat to escape. Barabadi called the problem "one of the most technically challenging aspects of our design", according to both outlets.

Apple's answer is to move heat across the fold. The graphite pipe carries it from one half to the other, so the whole phone's surface can shed it rather than only the side with the chip. Because a hinge moves thousands of times, Digital Trends says the thermal and mechanical teams designed the pipe together so that its cooling does not degrade with folding.

What else changes inside the A20 Pro phones?

• Memory placement: earlier iPhone chips had memory stacked on top of the processor. The A20 Pro puts it beside the chip, giving heat a more direct path out, Digital Trends reports.
• Metal over foam: a new material that couples the chip to the vapour chamber while giving a little mechanically. 9to5Mac quotes Apple saying it took months to get right.
• Display backing: copper and graphite layers behind the display spread heat evenly, per Digital Trends.
• A second display engine: Murray says the Duo's version of the A20 Pro adds one so the inner and outer screens can run at the same time, and supports a FaceTime camera behind the inner screen.

The vapour chamber is where the Duo differs. Digital Trends says the iPhone 18 Pro's chamber has three times the area of the iPhone 17 Pro's and acts as the phone's backbone, while the Duo uses a slimmer chamber in the middle of the phone. A Chinese-language account of the interview attributes the larger, structural chamber to the Duo; we follow Digital Trends, and Apple has not published the Duo's chamber size.

Our take

Much of this confirms what teardowns had already shown. The first iPhone 18 Pro teardown found the A20 Pro moved so it touches the vapour chamber through a new thermal pad, and the pad now has a name. The hinge pipe is the genuinely new part, and it is a sensible design: a foldable's biggest surface is useless for cooling if heat cannot cross the fold.

What the interview does not contain is any number a buyer can check. There is no sustained performance figure for the Duo against the iPhone 18 Pro, no surface temperature and no word on whether the Duo throttles sooner in long games or video recording. Apple's iPhone Duo page gives CPU and memory bandwidth gains over the A19 Pro, which are peak figures, not sustained ones. A slimmer chamber suggests the Duo has less thermal headroom than the 18 Pro, and the graphite pipe is how Apple tries to make up the difference.

The pipe also adds one more part to a hinge that has to last. Apple has published no fold-cycle rating for the iPhone Duo hinge, and a cooling path that flexes with every fold is another thing that has to survive years of use. APPDOOK's reading is that this is careful engineering, but the claim is untested until reviewers run the Duo hard. The first sustained gaming and video recording tests, once the phone reaches reviewers later this month, will tell us whether the hinge pipe works as described.