Microsoft Takes Heat from Xbox 360

A week ago, Microsoft adjustedits warranty policy on the Xbox 360 to cover general hardware failure signaledby the E74 error message.

The Xbox 360 has been a huge success since its launch in2005, notching sales of more than 28 million worldwide, but there have beenthermal management problems with the high-tech gaming device. In the course of developingnew technology to dissipate heat from microprocessors, Microsoft was awardedtwo U.S.patents.

The power in the Xbox 360 was more than doubled from theoriginal Xbox, going from under 100 to more than 200W, and several otherimportant features were added, including an online capability. The box issmaller, and had to be cost competitive. The Xbox mechanical engineering teamtook steps to deal with thermal management, but overheating and reliabilityproblems dogged some of the models.

The original engineering of the Xbox 360 was coordinated byJeff Reents, the lead mechanical engineer. In an interview called "Xbox 360:The Guts," posted on the xbox.com Web site, Reents provides insight into howthermal issues were attacked.

"Weoriginally wanted to put in a single 80-mm fan. But we couldn't fit it in thefinal form factor chassis," said Reents. "We couldn't just go with one 60-mmfan because it would not provide the required cooling - as the diameter of thefan increases the air flow performance per revolution increases exponentially.So we ended up putting in two 60-mm fans, and we came up with a water-cooled heatsink, as well, for the CPU."

Theheat sink includes a copper base, aluminum-stamped fins and a copper heat pipe,which contains water. A vacuum is pulled on the pipe prior to sealing. Water boilsfaster under a vacuum. The steam rises and moves the heat higher into the finswhere the air flow from the fans can extract the heat more efficiently. Thesteam condenses and flows back to the copper base of the heat sink, which isattached to the processor.

Copperand other conductive materials have replaced aluminum as the material of choicefor heat sinks.

Therequired cooling capacity of the Xbox 360 is determined by system electronics. Fanscool the unit, but can be noisy. "We use thermal sensing diodes in the main ICs(GPU and CPU) and thermal algorithm firmware that reads the diode temperaturesand adjust the fan RPMs by altering the fan voltage to a predetermined minimumrequirement that minimizes the fan speed (minimizes the acoustics) whileproviding the required cooling capacity for the system," says Reents.

Therehave been significant revisions to the Xbox 360 since its release. They includethe mundane,such as surrounding the CPU and GPU with epoxy to prevent board movement duringheat expansion. More importantly, a second heat sink was added.

Two Patents issued

Microsoft and Reents have been issued two patents on newthermal management concepts for electronics devices since engineering workbegan on the Xbox 360.

A patentwas awarded last December for an integrated heat sink that provides efficienttransfer of heat from a non-planar surface. In an example, one of the heatsinks is compressively coupled to an integrated circuit via an integral springassembly. The spring assembly pushes one of the heat sinks against one of theintegrated circuits while allowing the other heat sink to remain positionedagainst the other integrated circuit. The integrated heat sink compensates forvariations in circuit height, which occur due to manufacturing toleranceissues.

In 2006, Microsoft was awarded a patentfor Reents' invention of a system in which a heat sink is mounted to a processorusing a spring fastener. It's an interesting way to boost heat release from theprocessor.

A plastic clip had typically been used to provide a tightconnection of the heat sink to the processor. As a result, a poor conductingmaterial (plastic) is placed over the hottest section of the heat sink. Use ofa metal fastener would prevent the use of fins across a portion of the surfaceof the heat sink.

Reents' design goal was to develop a heat sink mountingsystem that could avoid placing excessive forces on the PCB, while stillallowing the heat sink to tightly press against the CPU.

In Reents' invention, the spring element includes a thrustplate made of an insulating material located between the spring element and thePCB and presses on the bottom of the PCB to create a force that keeps the CPUand the heat sink together.

It's not known to what extent the invention was used in the Xbox360 because Microsoft turned down an interview request from Design News. "It is our policy not todiscuss specific details of internal components or manufacturing processes,"said Jeremy Bartram, a company spokesman.

The Xbox 360 is available in three models: the "Arcade," the "Premium" and the"Elite" console, each with its own accessories. The centralprocessing unit (CPU) is a 3.2-GHz PowerPC Tri-Core Xenon.

The majority of the heat problem in electronics, such as theXbox 360, is caused by the electrical leakage of the transistors that make up aCPU. Efforts to reduce the heat output of CPUs have been mostly unsuccessfulbecause the number of transistors on a CPU increased from about 6,000transistors on an Intel.RTM.8080 in 1974 to more than 50 million resistors onCPUs in recent years.

The maximum die temperature for a silicon-based CPU is around90C. Temperatures that high, however, will cause a short operating life.

Reents was one of the original dozen Microsoft employeesrecruited by ToddHolmdahl  to work on the first Xbox.Holmdahl is corporate vice president of the Gaming and Xbox Product Group atMicrosoft Corp., and is responsible for the design, engineering, testing andmanufacturing of all Xbox consoles worldwide.

Other Design News stories on the Xbox 360:

"Learningfrom Failure" by Dean Takahashi

"Software-DefinedInstrumentation" by Kevin Bisking

Microsoft Takes Heat from Xbox 360 A