Whether you’re in the camp that declares Moore’s Law is dead, or you’re in the camp that believes it still has a chance, one thing continues to ring true—if you want to remain competitive in the smartphone marketplace, you’re going to continue packing as much performance and power as you can into a device that fits in the palm of your hand.
Unfortunately, while Moore’s Law is up for theoretical debate, there’s no getting around the Second Law of Thermodynamics. The more power (both physical and digital) you pack into a smartphone, the more those internals are going to heat up. Laptop and desktop manufacturers have the luxury of using fans, liquid cooling systems, and large surface area heat sinks to keep their devices cool. But what does the smartphone designer have at their disposal for keeping handhelds cool? In this article we’ll take a look at common techniques designers use to keep smartphones from overheating.
1. Dynamic Thermal Management (DTM) with Processor Throttling
First rule in the zen of smartphone thermal management design—keeping cool starts from within. The processor on an ARM chip is one of the main sources of heat within a smartphone. Hot spots can lead to power leakages, performance loss, and eventual degradation of the chip. While you can’t fit a fan inside a smartphone, you can still employ processor throttling to mitigate heat generation during high performance loads.
Temperature-Aware Scheduling: A software technique that involves slowing down hot processes (or selecting threads based on access to int and fp register files in multithreaded processors) as identified based on CPU activity and feedback from temperature sensors positioned within your device.
Thermal Herding: Also known as distributed traffic throttling, this technique involves steering network traffic throughout the chipset to avoid hotspots. This reduces 3D power density and can be used to direct heat to the die closest to the heat sink in the chipset.
Clock Gating: Global clock gating involves stopping the bulk of processor logic for a few microseconds at a time. This has a drawback of higher performance impact than the software techniques discussed earlier, but the cooling impact is higher. Fetch gating involves stopping only part of the processor, utilizing ILP to mask the performance impact. While not as effective at cooling as global clock gating, it’s still decent.
Dynamic Voltage and Frequency Scaling (DVFS): DVFS involves balancing performance and energy consumption by adjusting the GPU and CPU’s frequencies. While processes are slowed, it can quickly reduce temperature while the processor continues running.
Activity Migration: This technique involves running computations on spare units in colder areas of the chip to reduce overall power density. In the simplest case, this is achieved through activity ping-ponging between one unit and a redundant unit. Alternatively, a thermal-aware superscalar microprocessor (TAM) may be employed, where the primary pipeline is clock gated and the system resorts to a simpler, ultra-low power secondary pipeline.
2. Material Selection
Choice of materials can have a big impact on keeping smartphone temperatures down. That’s why it’s important to perform a thermal analysis on your design using tools like Cadence Celsius Thermal Solver which includes a Finite Elements Analysis (FEA) solver to develop a better thermal picture of your device’s internals. High thermal conductivity and structural integrity are key in choosing the right materials for your smartphone’s body.
Aluminum/Copper: These are pretty standard materials used in smartphone design due to their ubiquity, integrity, and high thermal conductivity. Often used as the backing for the PCB in the phone. Aluminum may also be used to form the chassis and case itself both for aesthetic appeal and to facilitate thermal dissipation to the environment.
Graphite/Graphene: The crystalline allotrope of carbon has been used in everything from pencil lead to battery electrodes. With its excellent thermal and electrical properties you’ll find it in the smartphone’s battery and the heat spreader plate where it can serve as a passive cooling system. Graphene is a monolayer (one atom thick) of graphite—this gives it even better thermal, electrical, and structural properties (40 times stronger than diamond).
3. Heat Dissipation
Now that we’ve introduced the materials, let’s take a closer look at the structures used to dissipate heat within a smartphone. Be sure to also factor in the thermal resistance of PCB itself when designing features around your device. A PCB thermal conductivity calculator can help.
Heat Sinks: Passive heat exchangers that use fans, pins, and other facets of geometry to increase the surface area available for convective cooling. Smartphone architectures rarely use true heat sinks because of the lack of fans (though you’ll often hear people use the term heat sink in reference to passive cooling features in general). However, novel techniques have been able to work surface area increasing geometries into smaller form factors.
Heat Spreaders: A flat plate made from a highly thermally conductive material like graphite is used to draw heat from the PCB and pass it to the external chassis of the smartphone (usually aluminum) which in turn conducts it through the case to atmosphere. This is the heat dissipating feature of choice for most modern smartphone designs.
Cold Plates: Active cooling in smartphones is rare, but they do exist. Graphene heat spreaders already provide excellent passive cooling. Run a current through graphene and it’s possible to turn that heat spreader into an active cold plate via thermoelectric cooling.
Heat Pipes: The other way to transform a heat spreader into a cold plate is to employ heat pipes. While expensive, this design mirrors industrial scale heat exchangers where a fluid is used to enhance heat exchange between surfaces. While you might be wary using a fluid like water inside a smartphone, Samsung already succeeded using water filled copper pipes in their S7 devices, as well as their S8 and S9. Just be sure to do your due diligence and perform a thermal expansion simulation on any fluids you intend to use within your device.