SPCR's 2010 CPU Heatsink Test Platform [Updates: 10 April & 31 May]

UPDATED CPU HEATSINK TEST PLATFORM

Key Components

• Intel Core i7-965 Extreme Nehalem core, LGA1366, 3.2GHz, 45nm, 130W TDP.
• Asus P6T SE ATX motherboard. A LGA1366 X58 chipset board with short solid-state capacitors around the CPU socket, low profile northbridge and VRM heatsinks, and mounting holes for both LGA1366 and LGA775 coolers.
• Asus EAH3450 Silent graphics card.
• Intel X25-M 80GB 2.5" solid-state drive. Chosen for silence.
• 2GB QiMonda DDR3 memory. 2 x 1GB DDR3-1066.
• Seasonic X-650 SS-650KM 650W ATX power supply. This PSU is semi-passively cooled. At the power levels of our test platform, its fan does not spin.
• Arctic Silver Lumière: Special fast-curing thermal interface material, designed specifically for test labs.
• Nexus 120 fan (part of our standard testing methodology; used when possible with heatsinks that fit 120x25mm fans)

The system is silent under the test conditions, except for the fan on the heatsink, which is a controlled variable.

The Asus P6T SE has short but effective northbridge and VRM heatsinks, allowing for plenty of clearance around the CPU.

Measurement and Analysis Tools

• Extech 380803 AC power analyzer/datalogger for measuring AC power at the wall to ensure that the heat output remains consistent.
• Custom-built, four-channel variable DC power supply, used to regulate the fan speed during the test.
• PC-based spectrum analyzer: SpectraPlus with ACO Pacific mic and M-Audio digital audio interfaces.
• Anechoic chamber with ambient level of 11 dBA or lower
• Various other tools for testing fans, as documented in our standard fan testing methodology.
• SpeedFan, used to monitor the on-chip thermal sensors. The sensors are not calibrated, so results are not universally applicable. The hottest core reading is used.
• Prime95, used to stress the CPU heavily, generating more heat than most real applications. 8 instances are used to ensure that all 4 cores (with Hyper-threading) are stressed.
• CPU-Z, used to monitor the CPU speed to determine when overheating occurs; throttling has been observed to occur at between 95~100°C.

Testing Methodology

As in the past, the main question we ask in our review is, What is the cooling power of this heatsink with this quiet fan whose characteristics are well known? By asking this question, we put all the heatsinks on the same playing field — no screaming 100 CFM fans. All have only the aid of the same quiet, low airflow fan. The heatsink, then, is the only unknown variable. This approach guarantees that all heatsinks are tested under the same acoustic and airflow conditions.

When a standard fan is included, we run two sets of tests, one with our reference fan, and one with supplied fan. Stock fans are profiled according to our standard fan testing methodology, which uses a similar noise-centric approach. With heatsinks that have a fully integrated fan not easily replaced, it is tested as delivered.

Load testing is accomplished using Prime95 to stress the processor, and the graph function in SpeedFan is used to make sure that the load temperature is stable for at least ten minutes. Larger, high performance heatsinks will undergo an additional test with the CPU overclocked to 3.6GHz and overvolted to 1.40V. The stock fan is tested at various voltages to represent a good cross-section of its airflow and noise performance.

We assess the heatsink and mounting mechanism together as a unit. A heatsink's intrinsic cooling power is determined mainly by:

• its radiating surface area
• the heat transfer coefficient of its materials
• the spacing and number of fins
• its geometry
• the smoothness and flatness of the CPU contact surface
• overall mass
• ease and efficacy of the mounting mechanism

The mounting mechanism is mentioned because it maintains the all-important contact between CPU and heatsink. The amount of pressure brought to bear on the interface also affects cooling. It is also the only real interface between HS and user. We may say we use a HS, but it's not the same way that we use a car, for example. We interact constantly with a car while using it. User interaction with a HS really happens only when the HS is installed or uninstalled. If this design aspect is poor and results in the user having difficulty with installation, or failing to mount the HS correctly, then poor cooling of the CPU can result. Some mounting mechanisms are poor, both difficult to install and lacking in precision or security; others are integrated wonderfully into the heatsink and easy to use. The mounting mechanism is a critical part of the HS design.

Test platform.

Power consumption is also monitored, both from the wall and from the ATX12V connector. An 0.01 ohm shunt resistor is placed in-line and the voltage drop (in mV) is measured with a high precision digital multimeter. Knowing the resistance and the voltage across the resistor gives us enough information to calculate the current using Ohm's Law. A second multimeter measures the voltage going into the connector which is around 12V. Multiplying the current by the voltage gives us the power draw of the CPU and VRMs in watts. By monitoring power consumption at this point, we can possibly spot increases in VRM efficiency caused by airflow generated by top-down coolers.

Measuring the voltage across the shunt resistor.

CPU Power Consumption

Now you may be wondering exactly how much more power the i7-965 uses compared to the Pentium D 950. Unfortunately, that is a rather difficult question to answer. With socket 775 processors, all the power for the CPU channels through the AUX12V connector. This is not true for socket 1366 processors.

We measured the power consumption at the AUX12V plug to be about the same on both platforms, 86W including losses in the VRMs. The i7's system power was 37W higher and it definitely put out considerably more heat as some of the heatsinks we re-tested failed at lower fan speeds, causing the CPU to throttle down. We had calculated the HD 3450 graphics card power consumption at idle to be 11W. The boards themselves use the about same amount, so there is some 26W unaccounted for.

Lost Circuits has a detailed article about the power configuration of the i7. As it turns out, the i7 CPU also draws power from the +5V and +3.3V rails through the motherboard's main 24-pin power connector for the integrated memory controller. The author states that Intel's own specifications indicate the total non-AUX12V power to the i7 "can weigh in with as much as 44.58 Watts." This accounts for the extra power we measured for the i7 system, and leaves us without an accurate power consumption figure for the i7-965 CPU.

We can't say with certainty that the unaccounted 26W is the extra power going into the i7-965 via the +5V and +3.3V lines, but if it is, then the CPU power would be 112W, which is a bit closer to the rated 130W TDP than the 86W we measured at the AUX12V socket. In any case, since we cannot determine the i7-965's power draw accurately, our traditional degrees Celsius rise per watt of CPU power (°C/W) will have to be shelved; instead we will rely solely on temperature rise over ambient.

Some readers may note the PSU change from the old platform: A new Seasonic X-650 80 PLUS Gold rated power supply instead of the previous Silentmaxx Fanless 400W MX460-PFL01. There were two main reasons for the change:

1. The X-650 PSU is far more efficient, drawing only 168W AC under load with the new test platform with default BIOS settings, compared to the Silentmaxx's 180W AC. We like to minimize our energy consumption whenever possible.
2. The X-650 actually runs much quieter; it is essentially silent under the test load. In contrast, under the load of the new i7 platform, the Silentmaxx made enough electronic noise (buzzing, humming — well over 20 dBA@1m) that its fanless operation became moot.

For those who are interested, here's a snapshot of the power profile of our new platform, cooled with a Prolimatech Megahalems and the reference fan at 12V. The power consumption can vary substantially with different heatsinks, especially with low airflow, due to the VRMs becoming less efficient when they get really hot. Just before throttling occurs, AC power can be some 20W higher and the AUX12V power can be 16~18W higher.

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