Mobile computing is poised to shape the way we live our lives, allowing people to generate, retrieve, and process data anywhere and at any time. The infrastructure for mobile computing is rapidly developing, as evidenced by the recent growth of cellular phone networks and a constant stream of innovations in portable electronic equipment.
Mobile computing is also presenting a variety of engineering challenges as demand grows for the faster processing of ever increasing volumes of data between mobile terminals. One particular challenge facing engineers is the design and manufacture of portable computers. For commercial success, it is important to consider not only the processing power required, but also the needs of the user – how they will carry, operate and use the computer, its weight, size, shape, and overall feel.
For certain applications the computer will become a part of a person’s everyday attire, a part of the human body, and the possibility of a wearable computer is now being studied in some universities and corporate laboratories. Portable computers were first introduced in the 1970s. Few attempts have been made, however, to develop generic guidelines for the design and manufacture of miniaturized systems, similar to those that exist for the design and fabrication of electronic circuits on chips.
The establishment of guidelines for achieving competitive mobile computer designs inevitably involves thermal management – the subject of the present article.
Planning the power-resource allocation to the CPU chip and other devices that operate on a given battery capacity is an important part of thermal management. The proximity of the components within an extremely confined space make them thermally interdependent. Hence, component location and thermal design are inseparable.
The thermal design criteria for portable computers include issues which need not be considered in the design of desktop PCs and other larger systems. One such issue is the temperature of the enclosure or case, which must be kept low for the comfort of the user. Diverse thermal environments must also be considered – portable computers are used not only in the office environment, but also outdoors, in crowded trains, on factory floors, and other unfavorable environments.
This article focuses on a range of portable computers recently launched by Japanese manufacturers and illustrates the complex nature of thermal management. The examples also illustrate the art of miniaturizing systems and components as pursued by Japanese engineers.
Competition in the Japanese portable computer market is now heating up, with each manufacturer now introducing approximately 3 new models each year. Portable computers are selling well, unlike their desktop counterparts.
Portable computers: recent japanese models
Major Japanese companies in the electrical machinery and the consumer electronics sectors are competing in the portable computer market. This article will focus on 3 models – the “Libretto” from Toshiba (Figure 1(a)), the “Vaio” from Sony (Figure 1(b)), and the “Pedion” from Mitsubishi Electric (Figure 1(c)). These models are currently sold only in the Japanese domestic market. The “Libretto”, a computer designed to fit in a jacket pocket achieved considerable media coverage on account of its extremely small size. The “Vaio” – aesthetically appealing to young customers – leapt to the top of the league in the latter part of 1997. The “Pedion” caused a sensation amongst customers and its competitors on account of its thinness, made possible by using a lithium-ion polymer film battery.
The dimensions and the weight of the three models are given in Table 1.
Table 1. Exterior dimensions of portable computers
All the models incorporate Intel’s Pentium for the CPU, although the clock frequency is varied according to the performance specifications. The Pentium processor is supplied in a Tape Carrier Package (TCP). The Pentium TCP has 320 pins with a 0.25 mm lead pitch and measures 26 mm x 26 mm x 0.85 mm. Other chips that perform functions such as input/output control, graphics management, PC card control, and memory, vary among the models. All of the models are passively cooled. The literature incorporates limited information on thermal design. The following descriptions about thermal design are therefore rather sketchy. The references used in writing this section are cited in Future Circuits, Issue 3, 1998, pp.81-88, of which this article is essentially a shortened version.
One immediate concern for the packaging designer is how to spread heat in the immediate vicinity of the CPU and other power-intensive chips.
In Toshiba’s earlier design, the Pentium TCP is sandwiched between two heat spreaders, as shown in Figure 2(a). The lower spreader (called a ‘cold plate’) has three tiers. The central tier is made to make direct contact with the die through a 14 mm x 14 mm opening in the printed circuit board (PCB). The upper spreader is called a ‘cover’. The cover and the cold plate are clamped to the TCP using screws on four corners. Adhesives are used for thermal contacts between the die and the upper and lower heat spreaders. With fan-assisted forced convection cooling, this design allows heat dissipation from the TCP in a range 5-7 W, with the junction temperature being held below 90°C.
(a) Cold plate assisted heat spreading. (b) Heat spreading to ground plane of PCB.
Figure 2. Heat spreader design for Pentium TCP (Toshiba Corporation).
For the mini-notebook Libretto, space was at a premium. The structure around the TCP is much simplified as shown in Figure 2(b). The heat spreaders are eliminated; instead, the heat spreading function is incorporated in the PCB. To enhance heat spreading in the PCB, the ground and the power layers are made thicker than those in conventional PCBs. The thickness of each layer is increased to 105 micrometers. The ground layer is particularly important, because its ends are attached to the enclosure with screws, thus forming both an electrical and a thermal path from the ground layer to the enclosure. In order to reduce the thermal resistance between the die and the ground layer, a part of the PCB below the die is removed, exposing the ground layer. A metal spacer is set in the cavity thus created. The spacer and die are then bonded to the ground layer using conductive thermal hardening adhesive.
Other sources of high-power are the circuits used to perform control functions such as I/O, graphics, and PCMCIA control. Normally, computer assemblers purchase those chips from vendors who assemble the chips in multi-chip modules called ‘chip sets’. Toshiba stopped relying on chip-set vendors for the Libretto and used its in-house research and development to reduce the chip-set volume. Toshiba replaced a chip set originally supplied by a vendor with a single ASIC chip. The newly developed ASIC chip has 230,000 gates, operates on 3.3V and is encapsulated in a package called T-BGA, shown in Figure 3. The T-BGA is composed of a TCP having a ball grid array and two metal plates – one being a stainless steel plate called a stiffener, while the other is a copper plate called a ‘cover plate’. The stiffener has an opening in it, providing a cavity for the die and giving the die access to the cover plate. The cover plate serves also as a heat spreader. The package has 576 balls spaced with a 1.27 mm pitch.
Figure 3. Heat spreaders on ASIC chip (Toshiba Corporation).
Its external size is 40 mm x 40 mm x 1.4 mm. The thermal resistance in the natural convection environment is reported as 15 K/W.
In the aforementioned examples conventional metals are employed as heat spreaders.
Nonetheless, research into alternative materials for heat spreaders continues. Matsushita Electric recently developed a graphite sheet with an in-plane thermal conductivity of 800 – 1000 W/mK.
The arrangement of packages and heat spreaders in the system enclosure is central to creating a favorable temperature distribution on the packages and the PCB. Figure 4 shows a Hitachi design, where the CPU chip, the memory chips and the control chip (noted as ‘LSI’) are squeezed into a three-dimensional structure. Figure 4 shows the structure schematically; for the implementation of this structural scheme advanced packaging techniques and bonding materials are required.
Figure 4. Stack of chips and heat spreaders (length in mm; Hitachi, Ltd.).
In order to increase the heat conduction paths from the chips to the PCB, thermal bumps and thermal vias are implanted across the PCB. Heat from the CPU chip is conducted to the keyboard plane through an aluminum plate (plate 2) and bonding agent (elastomer). It is also transported to the bottom plane through an aluminum plate (plate 1), an aluminum block and another aluminum plate (plate 3), all bonded together by an elastomer.
A thermal analysis indicates that a heat dissipation of 13 W is feasible where the temperature rise of the CPU chip is allowed to reach 60 K and that of the keyboard can reach 15 K.
The location of PCBs and other devices within the system enclosure must be designed to create a favorable temperature distribution throughout the system. Figure 5 shows the arrangement of major components and devices in the Mitsubishi Pedion. The heat spreader next to the top chassis and the heat transfer plate next to the bottom chassis are configured to produce a desired temperature distribution throughout the system. This temperature distribution reflects the different maximum allowable temperatures for the chips and for the system PCB – 100°C and 50°C, respectively.
Figure 5. Exploded view of Mitsubishi Pentium.
For the system-level thermal design, thermal simulation codes are indispensable tools. Japanese manufacturers are using both commercially available numerical analysis codes and codes developed in-house.
Commercial codes are favored by designers and trouble shooters in the factory, while laboratory researchers tend to develop and work with their own codes. The in-house code can be modified more readily and can be used to evaluate the effects of various parameters involved in the numerical solution.
Common to all three of the portable computers detailed in Figure 1 is the use of magnesium alloy for the system enclosure. In earlier models, the system housing was made from ABS (acrylnitrite-butadiene-styrene) resin. However, the molding process dictated that the thickness of ABS enclosure must be in the range 1.2 – 1.4 mm. Using die-casting techniques however it was not possible to create enclosure frames of less than 1mm thick. Furthermore, the resin had a low thermal conductivity.
In contrast, magnesium alloy offers high thermal conductance, is light weight and can be injection molded to reduce the thickness of the enclosure to just 0.7 mm. Toshiba found that by manufacturing the case for its Libretto in magnesium rather than in resin, they were able to effectively equalize the temperature distribution on the enclosure surface and reduce the maximum temperature of the enclosure by 2-3 K. The Pedion has an interesting feature on the outermost surface of the enclosure. It is coated with a 0.2 mm-thick layer of paint. The paint has a bubbly finish, created by foaming the paint using a hydrocarbon agent prior to application. The foamed paint softens the touch of the metallic enclosure. However, this leads to an optimization problem, where the enhancement of heat transfer by the metal enclosure has to be weighed against the increase of thermal resistance by surface coating.
One drawback of magnesium-alloy enclosures is the relatively high cost when compared to conventional resin. Fujitsu has adopted a composite design where an aluminum plate (about 1 mm thick) is embedded in the resin enclosure. Heat transfer paths from the heat sources to the aluminum plate appear to include air gaps between the PCB and the plate.
Nonetheless, the plate is reported to have some effect on the temperatures of the key components. According to the analysis, the temperatures of the CPU chip and other devices decrease by about 2 K, compared to those in the all-resin enclosure. In order to mold the aluminum plate to the resin, the plate must first be coated with a 20 – 30 µm (mm) thick layer of adhesive, then placed in the molding die. During molding, heat from the resin melts the adhesive and so bonds the plate to the resin.
Portable computers will become an indispensable part of the mobile computing information network. The packaging of portable computers presents engineers with a challenge that must be met by research and innovation, if computers are to become more powerful, more compact and even lighter. Thermal management is one of the most challenging issues to be addressed, where consumer demands for reliable, easy-to-carry equipment, which feels good to touch, often conflict with the need for more processing power.
Ultimately, portable computers must be cooled by combined natural convection/radiation heat transfer in what is called passive cooling.
Passive cooling sets the upper bound on the system-level power consumption. It is obvious that the processing performance of the portable computer has to be upgraded under the tight constraints imposed by passive cooling. The issue of prime importance is not the development of high-performance cooling, which has occupied the interest of heat transfer engineers for decades, but the need to improve the processing performance through various power saving measures and to achieve the ideal thermal situation – namely, an isothermal state over the system’s outer enclosure.
In the future, portable computers may have to possess certain features analogous to biological bodies – a heat transport medium that is circulated in microscopic channels running throughout the computer which removes heat from the heat sources and spreads it over the system enclosure.
The author would like to thank the following individuals for their kind assistance regarding material for this article- Mr. Haruo Kozono, Sony Corporation; Dr. Masaru Ishizuka, Toshiba Corporation; Mr. Takashi Kobayashi, Mitsubishi Electric Corporation; Mr. Kuichi Kimura, Fujitsu Laboratories Ltd.; Mr. Shigeo Ohashi, Hitachi, Ltd, and Mr. Clemens Lasance, Philips Research.