Thermal Design Series 3: The Nightmare of Heat Dissipation

For many product manufacturers, failures caused by heat are not unrelated to them. Even companies that did not pay much attention to heat dissipation in the past have found it increasingly difficult to dissipate heat as their products have become smaller and more powerful. For example, in notebook computers, the power consumption of the CPU related to heat dissipation has been "around 30 to 35W in recent years" (Nobuhiro Nohara, Director of the Mobile Support Center of Intel's Mobile Platform Group in Japan). However, in order to further pursue thinner, smaller, and lighter products and to ensure space for adding new functions such as optical drives, the area of ​​the motherboard has been shrinking. Note 2-3). As a result, the packaging density of the motherboard has continued to increase, and heat dissipation has become increasingly difficult.

Note 2) Don't forget that as the power consumption of the CPU increases, the power consumption of the chipset and memory also increases. Currently, the power consumption of the memory controller LSI (north bridge chip) is generally 10W, the input/output controller LSI (south bridge chip) is 2~3W, and the memory is 3~5W. In particular, regarding the memory, "Since the OS has changed to 'Windows Vista', the heat generation has become noticeable, and we have to start considering cooling" (NEC Personal Products PC Business Headquarters Development and Production Business Department Mobile Product Development Department Hiromitsu Ohku).

Note 3) For example, the motherboard of Sony's "VAIO type T" ("VGN-TZ50B", etc.) notebook computer is about 30% smaller than the previous model, with an area of ​​10,000 mm2. The motherboard of Toshiba's "dynabook SSRX" notebook computer is about 35% smaller than the previous model, with an area of ​​11,600 mm2.

For portable products, the temperature limit of the housing also makes heat dissipation more difficult. Currently, many manufacturers have "internal regulations to set the upper limit of the body temperature to 14°C above the ambient temperature" (Nashi Kunimine, Director of the Information Planning Department of OKI Industrial Co., Ltd. Note 4). The housing of portable products often acts as a radiator to dissipate heat to the outside, but with the trend of miniaturization and high performance of products, it is not easy to achieve this.

Note 4) The basis is that it is generally believed that low-temperature burns will occur if the device is in contact with an object at 44°C for 6 hours. For example, when the device is used in an environment with a room temperature of 27°C, the upper limit of the device body temperature is 41°C.

Challenging the limits of heat dissipation

Kunimine of OKI Electric pointed out that naturally cooled electronic products such as mobile phones "have already seen the limit of heat dissipation." This is because the amount of heat that can be dissipated from the surface of the product is limited. Generally speaking, if the power consumption of a product exceeds 10W per 1L of volume, it will be difficult to achieve natural cooling.

For example, a mobile phone that can watch TV programs for 4 hours continuously using a battery with an output voltage of 3.7V and a capacity of 800mAh consumes 0.74W of power when watching TV. Assuming that the volume of the mobile phone is about 0.1L, the power consumption of 1L is 7.4W. This value itself has not reached the heat dissipation limit, but there is no room for improvement.

Despite this, various portable products, such as mobile phones, are becoming increasingly small and high-performance. Thermal design is the key to supporting these products. In the early stages of design, the heat generated by components, the heat dissipated by the product surface and fans, and the heat dissipation path of the finished product are considered, and the temperature information of each part is obtained through simulation and local measurement. The goal of product manufacturers is to make all parts of the product dissipate heat "evenly without recklessness or waste".

No thermal design, no product

Thermal design, which product manufacturers are beginning to focus on, is not a new technology. It has been used in large computers and power supplies in the past. However, for a long time in the past, thermal design has always been an ideal for consumer products and could not be used in actual development due to cost accounting.

Now, this situation is changing fundamentally. Now you can hear many product manufacturers' developers emphasize that "without thermal design, there is no product."

Matsushita Electric Industrial Co., Ltd. has put into production a notebook computer optical drive that uses thermal design to add Blu-ray disc function to the original CD/DVD and keep the size comparable to the original product (Figure 3). The laser source used for Blu-ray discs generates more heat than the laser source used for CD/DVD and has poor heat resistance. "To ensure the life of the laser head, the temperature conditions must be fully guaranteed, and thermal design based on simulation plays a big role" (Mitsuro Moriya, Counselor of the BD Business Development Working Group of Matsushita Electric Industrial Co., Ltd.) Note 5). (To be continued)

Figure 3: Thermal design is essential in Blu-ray Disc devices. To achieve a 12.7mm high Blu-ray Disc device, Panasonic adopted simulation in the design of the laser head and optical platform. The optical drive is a general product suitable for notebook computers, so the external dimensions cannot be changed. The key is to package it in the same size as the original CD/DVD two-wavelength product after adding the Blu-ray Disc optical system.

Note 5) The power consumption of the laser source and laser driver IC for Blu-ray Disc devices is about 1.3W, which is 1.5 to 2 times that of conventional CD/DVD devices. This is packaged together with the laser source for CD/DVD devices in a laser head with a volume of only about 5.5cc.