[MacBook Air Teardown] Air Plays Key Role in Thermal Design [Part 6]

Mar 6, 2008
Mayuko Uno, Nikkei Electronics

What measures were taken against the heat problem in the MacBook Air? After all, heat is what confronts engineers who design digital consumer electronics including personal computers.

With the help of engineers specializing in thermal design, we explored Apple's stance on this issue. We gradually found that the notebook was designed so that users hardly feel the heat. The company employed simple heat dissipation methods such as exhausting heat by using air and preventing the unnecessary heat transfer by providing a gap (air) between the heat source and the adjacent parts.

The distinctive features of the thermal design in the MacBook Air include the followings: (1) The adoption of a slim fan and the layout of the ventilation ports, (2) a heat dissipation mechanism to prevent heat generated by the microprocessor and other parts from being directly transferred to the chassis and (3) treatment of the heat from the battery.

Consideration for making the heat unnoticeable

Among the parts used in a notebook computer, microprocessor is the component most likely to be affected by heat. The MacBook Air seems to be mounted with Intel Corp's 1.6GHz Core 2 Duo with a 4Mbyte built-in cache memory designed for laptops. The thermal design power, which corresponds to the maximum power consumption, of this processor is 17W. Considering the dimensions of this notebook, heat dissipation does not seem to be a big problem.

However, there is a problem that the entire chassis is made of aluminum alloy, which easily conducts heat to the entire body. When heat generated from one component is transferred to a certain part of the chassis, it is highly possible that the palm rest or other portions, where users often feel heat, also become hot. The thermal design in the MacBook Air showed Apple's efforts to reduce the heat the users feel.

When the Nikkei Electronics Teardown Squad used the MacBook Air for a short period of time, the temperature rise was negligible. We did not measure the heat generation and power consumption because the excited Squad members started to break down the main unit right away.

Hiding the fan

One of the key features of the MacBook Air's thermal design is the ventilation ports located at the back of the body and under the hinge block. A slim fan is used to draw in external air from the back side and exhaust air heated by a heat source again from the back side.

The layout is well designed to prevent the heat of warm air from being conducted to the chassis as much as possible.

"The ventilation ports located on the back of the body prevent exhaust air from blowing on the user and make the fan noise lower," one of the engineers who attended the teardown operation said.

This is the advantage of using the proprietary thin Li polymer secondary battery. Many Japanese slim laptops mount the secondary battery on the back side of the body to reduce the thickness of its front portion and to improve the usability. And they usually use a general-purpose secondary battery.

In this case, ventilation ports are located at the lateral side of the body. This often brings an undesirable thermal condition due to the lack of ventilation port area. And users may be annoyed by hot exhaust air blowing on their hands.

Preventing the heat transfer to the chassis

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When we opened the back lid, we could see the microprocessor, a heatsink and so forth. An engineer said that arranging heat generating parts under the keyboard is an established technique of designing laptops. It is because a keyboard is usually made of plastic, which has a poor heat conductivity, and is touched by the hands for a relatively short time. Thus, the users are less likely to feel heat on the keyboard.

The MacBook Air dissipates heat generated by the microprocessor and the image processing LSI by transferring it to the heatsink via grease. Then, the heatsink is cooled by air from the fan, and hot air is exhausted to the outside.

The heatsink was probably made of aluminum alloy painted black. A high-conductive graphite sheet was attached on the microprocessor side. A similar technique is used in Sony Corp's "type G." The MacBook Air did not employ a combination of a heatpipe and a comb-shaped heatsink (RHE, or remote heat exchanger), which is widely used in recent laptops.

"Apple probably gave top priority to slimness," an engineer said. "It could design a simple heat dissipation mechanism by utilizing the heatsink because the power consumption of the MacBook Air is relatively low."

When we examined the heatsink in more detail, we found that a rubber "bank" is surrounding it. On the back side, the heatsink is surrounded by a sponge bank. The rubber and sponge banks were cut only at the ventilation ports. This probably indicates that the heatsink serves as an air duct as well as a heat dissipation mechanism.

"The MacBook Air was designed so that air from the fan gathers around the components that need to be cooled, such as, the microprocessor, image processing LSI, their power circuits and the memory," an engineer said.

There are two air flow paths. The first flow path is as follows.

The ventilation port near the fan → The power circuit of the microprocessor (the cutout portion of the heatsink) → Through the cutout portion to the upper surface of the heatsink → The air duct surrounded by the heatsink, the bottom surface of the chassis and the rubber bank → Another ventilation port

The second flow path is as follows.

The ventilation port near the fan → The power circuit of the microprocessor → The lower surface of the heatsink → The air duct surrounded by the heatsink, the circuit board and the sponge → The ventilation port

In other words, the air flow branches into two directions by means of the cutout portion on the heatsink, which is used to cool the power circuit. An engineer speculated that the heat dissipation from the heatsink to air is improved by blowing air onto the both sides of the heatsink.

Also, an engineer estimated that the rubber and sponge banks work not only to block the gap between the heatsink and the chassis and the gap between the heatsink and the circuit board, but also to prevent the heatsink from directly conducting heat to the chassis. In addition, they might be used for insulation and EMI prevention, he said.

The fan is 5.25mm thick and has the company name "DELTA ELECTRONICS, INC." and the model number "KSB05105HB" marked on the back side. An L-shaped piece of hardware "fastens the heatsink to the mounting board with a screw in the center and presses the heatsink against the microprocessor and the image processing LSI," an engineer said. We checked the Website of Delta Electronics Inc of Taiwan but found no product with this model number.

A graphite sheet covered with a black insulation sticker is attached to the portion opposing the heatsink on the bottom face of the chassis. The sheet was probably used to prevent the partial temperature rise in the bottom face of the chassis due to the heat generated from the microprocessor and other parts.

"Covered with the black sticker, the sheet may also enhance the heat transfer from the heatsink to the chassis bottom by increasing the radiation," an engineer said.

Innovative battery

Apple also improved the battery design to make heat less noticeable to the users. The battery face on the side of the palm rest is covered with a graphite sheet and some sort of cushioning material with holes.

An engineer estimated that the graphite sheet was used to "dissipate heat generated in certain areas of the battery into its entire surface." On the other hand, the cushioning material was probably used to block the heat transfer to the chassis.

"Users tend to be annoyed by heat on the palm rest even if it isn't overly hot because they are constantly touching the palm rest," an engineer said.

It seems that Apple took these measures because the battery is placed under the palm rest in the MacBook Air.

Another major heat generating component in laptops is an HDD. But in the MacBook Air, there seemed to be no special heat prevention measure taken against the HDD, considering that the outlet of the cooling fan faces opposite to the HDD.

When we used the notebook, we hardly felt heat on the front surface on which the keyboard is located. However, its bottom was slightly hot.

The temperature increase on the bottom of the chassis due to the HDD is probably tolerable in terms of both the parts' specifications and user's perception. But users tend to be annoyed is there is a temperature gap in the areas they touch, feeling that "this part is hotter." This seems to be an issue for the future because it is a universal problem for all laptop thermal designers.