With the Spring here in the northern hemisphere, and the threat of another long hot summer drawing ever closer, now is the time to think about how to ensure your Mac and its peripherals keep cool. In this article I clear up some of the more common misunderstandings about how you should go about that, not just for your Mac but for other vital systems including external and networked storage. This looks first at the mysteries of thermal paste, heat sinks and fans.

The problem

Computers and peripherals are powered by electricity, and almost all the power they use is generated as heat. If that heat can’t be dissipated as fast as it’s produced, then the temperature of critical components will rise, and ultimately cause them to cease functioning. Before that, some materials such as plastics inside the computer or device may melt, resulting in damage to the logic board, requiring expensive replacement. Unchecked temperature rise can even cause fires, although those are thankfully exceedingly rare.

Heat transfer

Heat generated by electronic components is normally dissipated by three means:

• conduction from the component direct to another solid in immediate contact with it;
• convection of a fluid, almost always air, which moves when it’s heated by the component; this can be free convection, or forced by a fan;
• radiation, invisible exchange between two surfaces that can ‘see’ one another, even when there’s a vacuum separating them, by infra-red radiation.

Conduction

Hot electronic components surrounded by still air don’t dissipate heat well, as still air is a good insulator. An obvious illustration of that is in the duvet jacket you put on in the cold, which traps layers of still air in its filling (down or synthetic fibre), and reduces heat loss from your body.

In many circumstances, electronic components make physical contact with a metal heat sink, with the aim of heat being conducted away from the component into the heat sink, and dissipated from there. But even with precision machining, it’s common for small air gaps to impair conduction, so a paste is applied between the component and the heat sink. Provided that thermal paste is more conductive than air, it’s likely to increase the rate of heat conduction away from the component.

CPUs and other chips inside Macs are the most likely to be equipped with heat sinks, and to have thermal paste applied between them to improve heat transfer. This is often seen with more powerful GPUs as well.

Thermal paste

In some cases, thermal paste may not have been well applied to begin with; in others, it migrates or is otherwise lost over time. In those cases, careful application of thermal paste between the chip and its heat sink can significantly improve cooling by conduction.

In some older models of Mac, an experienced and careful user may be able to disassemble the Mac to the point where its heat sink(s) can be removed for application of thermal paste. That may be delicate surgery: before even contemplating trying it, watch iFixit’s account of how to disassemble your Mac down to its logic board. Bear in mind that any damage you do won’t be covered by any warranty or AppleCare, so you’d have to bear the cost of any replacement logic board yourself.

Most recent Macs require special tools to perform this, and some need special replacement components too. The best way to obtain these, together with full instructions, is through the Apple Self Service Repair programme, if that’s available in your location.

For the great majority, the only practical way to check and replace thermal paste in a Mac is to have this performed by an Apple Authorised Service Provider, who will be delighted to quote a cost that might convince you that, without good evidence that your Mac needs it, it may be worth leaving for the moment.

External SSDs

Some external SSDs also rely on conduction to a heat sink for their cooling. That isn’t normally accomplished using thermal paste, though, but a thermal pad made of specially conductive silicon rubber. If your SSD can benefit from better thermal contact with the aid of a thermal pad, they’re cheap and readily available. The common failing here is not ensuring good contact between the upper surface of the pad and its heat sink, which requires a pad of just the right thickness.

Free convection

Unless the air around a heat sink or bare component is blown by a fan, its heat is then dissipated by free convection, occurring when the air in immediate contact with the hot surface is heated and rises because warmer air is less dense. This isn’t an efficient way to lose heat, and is normally aided by increasing the surface area of the hot surface using cooling fins. The larger the fins, the greater their surface area, and the more heat they’re able to dissipate, provided that the air surrounding them is able to move away to let cooler air come into contact with the surface of the heat sink.

Here, heat sink design becomes most important, together with ensuring that air can flow freely. Blocked cooling slots on the case must be cleaned out, and the warm case of a notebook like a MacBook Air must have good clearance to allow the air to move around it. Some notebook stands encourage this, while putting a notebook onto a soft surface can prevent it.

Forced convection

Even a small low-powered fan is far better at moving cooling air than relying on free convection. This is easy to experience on your own skin when it’s hot.

Cooling fans inside a computer case are carefully designed to draw or blow cooler ambient air over hot components and heat sinks. They rely on being kept clean at all times, and their intakes and outlets remaining free of all obstructions. Removing dust and dirt from them is thus a vital step in ensuring that they work efficiently.

Some small external SSD enclosures now offer cooling fans, although here they’re not certain to prove superior to a good heat sink. Designs that try to incorporate both a fan and heat sink may not be optimal either, as the fan works with forced air movement, while the heat sink relies on good surface contact without air.

Cooling fans should be provided inside all cases of sufficient size, including those in a NAS system. Provided that ambient air temperature remains within the comfort zone of the user, cooling fans should provide sufficient heat dissipation for SSDs and similar components, as has been demonstrated by vendors such as Asustor. The only remaining question is how those fans are controlled.

Radiation

This is a catch for those vendors who don’t know what they’re talking about: any company making claims to cool your Mac or devices using radiated heat is almost certainly trying to fool you. While changing material and colour of a case may alter its infra-red reflectance, almost all materials lose heat by infra-red emission at similar rates, as they behave close to a theoretical ‘black body’, and are near-perfect emitters. Thus there’s little that you can do to enhance any radiant heat loss.

When radiant heat does become important is with anything left in sunlight, even if that light has been transmitted through glass. Always ensure that the whole of your Mac, its display, and all peripherals, are kept out of sunlight, to minimise any radiant heating.

Evaporation

Some Macs of the past were liquid cooled, but none is anything like a human and cools by evaporation. But if you’re warm enough to even start sweating, that’s a good indication that your Mac may well be hotter than you, and you should ensure that it’s kept cool.

Summary

• Thermal paste is normally applied to ensure good conduction of heat from electronic components to a heat sink.
• Checking or replacing thermal paste is best performed by an Apple Authorised Service Provider.
• Thermal mats can be valuable to ensure good conduction between electronic components such as SSDs and their heat sink or enclosure.
• Keep all cooling slots in the case clean, free of dust and dirt, and unobstructed.
• Keep any fans clean and free of dust and dirt.
• Keep all electronics out of sunlight, even if that has passed through glass.
• If you’re warm or hot, your Mac may well be even hotter.