Thin is a hardware problem: batteries, boards and the 5 mm limit

A thin phone looks like a styling decision. It is mostly a hardware problem. Every millimetre you take out of the thickness, the z-height in engineering terms, comes out of the battery, the camera, the cooling or the frame, and each of those pushes back in a different way. When we drew the Humanly Phone concept at 5.4 mm, most of our time went into working out where that 5.4 mm would actually go.

We are a software studio, not a hardware lab, and the concept is not a product. The figures in this post are rounded estimates based on public specifications and teardowns, labelled as such. The physics behind them is the interesting part, and it does not change much from one phone to the next.

The z-height budget

Start with the layers that cannot shrink much. From the front of a typical slab phone:

• Cover glass, around 0.5 to 0.7 mm. Thinner glass breaks more easily, so there is a floor.
• OLED panel with touch layer, around 0.3 to 0.5 mm. Flexible OLED is already remarkably thin.
• Mid-frame, the metal plate that holds everything, around 0.5 to 0.8 mm where it sits under the display.
• Internals: battery, logic board, cameras, speakers and connectors.
• Back cover, glass or ceramic, around 0.5 to 0.8 mm.

Add the fixed layers and you have spent roughly 2 mm before the phone has a battery or a processor. In a 5.4 mm phone that leaves around 3.2 mm for the internals, and every part inside has to fit in that one slice. In an 8 mm phone the same slice is over 5 mm. That difference of 2 mm is most of the story.

There are also gaps you cannot see: adhesive layers, foam, clearances so parts do not rub, and room for the battery to swell slightly as it ages. They are fractions of a millimetre each, and on a thin phone they add up to a noticeable share of the budget.

Battery: energy lives in volume

A battery stores energy per unit of volume. Modern lithium ion phone cells land somewhere around 700 to 900 watt hours per litre, and the newer silicon carbon anode cells that several manufacturers adopted in 2024 and 2025 sit at the higher end of that range. You cannot make the chemistry denser by wishing, so the only lever left is the size of the box.

Here is a rough worked example. A cell measuring 60 by 70 by 3.0 mm has a volume of about 12.6 cubic centimetres. At 750 Wh per litre that is roughly 9.5 Wh, which at a nominal 3.9 V is around 2,400 mAh before packaging overhead. Make the same cell 5 mm thick and it holds about two thirds more. Thickness is capacity, almost linearly.

There is a second problem hiding in thin cells. A pouch cell has fixed overheads: the pouch material, the tabs, the protection circuit and the edge seals. Those do not get thinner when the cell does, so a 3 mm cell carries proportionally more dead weight than a 5 mm one. Thin cells lose a little energy density on top of losing volume.

This is why thin phones lean so hard on software efficiency, and why our concept only works on paper with a true black interface and very few background tasks.

Boards: stacked or side by side

The logic board holds the processor, memory, storage, power management and radios. In most flagships since around 2017 it has been built as a stack: two boards sandwiched together with an interposer between them. That roughly halves the board's footprint and frees area for a bigger battery. The cost is thickness, because a sandwich is thicker than a single board.

A thin phone usually has to go the other way. The board lies flat beside the battery, not on top of it, so both parts can use the full internal height. Now the board takes up area that the battery wanted. The designer is trading area for thickness, and there is no free option.

Recent thin phones show different answers. Some keep a flat slab and accept a smaller battery. Apple's iPhone Air, released in 2025, moved the logic board and cameras into a raised plateau across the top of the back, so the rest of the body could be almost entirely battery. The plateau is honest about where the thickness went. It is the same trade, placed where your hand notices it least.

Camera modules and the bump

The camera is where thin phones hurt most. A lens needs a certain distance between its front element and the sensor, often called total track length. For a given field of view, that distance grows with the size of the sensor. A large main sensor, the kind that gives good low light photos, needs a lens stack of several millimetres plus the sensor and its mount. In most flagships the main camera module is taller than the body, which is why nearly every phone has a bump.

Periscope telephoto lenses get around the problem by folding the light path sideways with a prism, so the long part lies flat inside the phone. That works for zoom lenses with narrow fields of view. It does not really help the wide main camera.

So a thin phone has three options: a smaller sensor, a bump, or both. Our concept chose a smaller sensor and no bump, and we said plainly that it would be a good everyday camera rather than a great one. Most real thin phones keep a bump and accept it.

Heat, antennas and stiffness

Heat. A phone has no fan. Heat from the processor spreads through graphite sheets or a thin vapour chamber into the frame and glass, and from there into your hand and the air. Manufacturers limit the surface temperature to stay comfortable to hold, which in practice is somewhere in the low to mid 40s Celsius. A thinner phone has less material to spread heat into, so it reaches that limit sooner and has to slow the processor down earlier during a long game, a video export or a hot day in a car. Short bursts feel the same. Sustained performance is where thin phones lose.

Antennas. Metal frames make good structures and awkward antennas. Phones get around this by splitting the frame into segments with plastic breaks, the thin lines you can see on the edges, and using those segments as antennas. Antennas need clearance from other metal parts to work well, and a thin phone has less room to give it. Adding every modern band (several cellular bands, Wi-Fi, Bluetooth, GPS, ultra wideband, sometimes mmWave 5G) into a slimmer edge is careful, fiddly engineering.

Stiffness. The bending stiffness of a flat plate grows with the cube of its thickness. Halve the thickness and you get one eighth of the stiffness, all else equal. That is why thin phones use stiffer materials such as titanium or stainless steel in the frame, and why the 2014 bending complaints about a large aluminium phone got so much attention. A thin phone can be strong. It just has to be designed around bending from the start rather than fixed later.

The repair cost of thin

Thin devices tend to be glued. Screws, brackets and connectors take height, and adhesive takes almost none, so a thin phone is often held together with adhesive around the display, the back and the battery. That makes it harder and riskier to open, and it makes battery replacement, the most common repair, a job for a workshop.

Regulation is pushing against this. EU ecodesign rules for phones that started applying in 2025 set requirements for spare parts and repair information, and the EU battery regulation pushes toward batteries that users can replace, with its main requirements due from 2027. Manufacturers have responded with things like stretch release adhesive tabs and electrically released adhesive for batteries, which keep a thin design without making a repair so painful. How much further the rules go for very thin designs is still unclear, and we would not bet on the details.

Our own view, after drawing a 5.4 mm phone, is that thin only makes sense if the phone does less. A full flagship squeezed into 5.4 mm runs hot, dies early and costs a lot to fix. A small, focused device with a black screen and a short feature list can live inside that budget. The 3.2 mm slice decides what the phone can be, and it is a good idea to read that slice first, before drawing the outside.