Documentation Archive Developer

Thermal considerations for Mac Pro FB-DIMMs

This document outlines design considerations and specifications for the AMB and Heat spreader for the Mac Pro


Creating "Fully Buffered Dual In Line Memory Modules" [FB-DIMMs] for the Mac Pro requires the developer to provide for thermal conditions other than those proposed in Intel's original reference design.

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Mac Pro Air Flow for FB-DIMMs

The Mac Pro controls the air flow for FB-DIMMs and regulates the maximum permitted memory bandwidth to maintain a safe operating temperature for the FB-DIMMs.

To minimize acoustic noise and maximize memory bandwidth, FB-DIMMs are recommended to be equipped with sufficient thermal dissipation capabilities, to maintain a safe operating temperature on the FB-DIMM components, while with an air flow of 0.5 meters per second and at an ambient temperature of 25° C, during maximum memory bandwidth utilization.

The Mac Pro will ensure FB-DIMMs maintain a safe operating temperature by raising fan speeds to increase air flow, up to a maximum of 2 meters per second. If needed, the Mac Pro will also lower the maximum permitted memory bandwidth.

Note: To maintain a safe operating temperature at an ambient temperature of 35°C on FB-DIMMs using a JEDEC standard heat spreader requires an air-flow of approximately 3 to 6 meters per second. Therefore, designing adequate thermal dissipation capabilities on FB-DIMMs for the Mac Pro is strongly recommended to avoid raised fan speeds and lowered bandwidth during user operation.

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Advanced Memory Buffer [AMB]

In order for the Mac Pro to maintain a safe operating temperature of the FB-DIMMs, the AMBs for each FB-DIMM must provide a functional temperature sensor.

Note: The Mac Pro will run the fans at full speed when detecting FB-DIMM's without a functional temperature sensor, which will maximize cooling on the FB-DIMM's. While the full speed of the fans maximizes the preservation of thermal integrity, FB-DIMMs with inadequate heat sinking may still overheat depending on ambient temperature and system load. In this case, the Mac Pro may shut down the system to protect the FB-DIMM from possible thermal damage.

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Serial Presence Detect (SPD) personality byte

The personality bytes and other information not directly produced by the FB-DIMM developer should be provided by the AMB manufacturer.

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Heat Spreader

There are several companies that have produced heat spreaders that meet the thermal conditions for the Mac Pro.

Netlist :

For more information contact .

Apple Designed Heat spreader :

Apple has proposed the MO-256 FBDIMM Heatsink Option to the JEDEC FB-DIMM specification.

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Document Revision History

2006-11-10Design considerations and specifications for the AMB and Heat spreader for the Mac Pro