Heatsink Mounting Pressure vs Performance

Heatsink clamping force directly affects thermal compound performance – Joe

SUMMARY: Heatsink clamping force directly affects thermal compound performance.

One of the critical determinants of thermal grease performance is the pressure loading of the heatsink. As this graph shows,

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there is a strong correlation between thermal performance and the amount of pressure holding the heatsink to the CPU. The most efficient thermal grease available will perform poorly if the bond line interface (the thickness of the thermal grease between the heatsink and CPU) is too thick.

One way to minimize bond line thickness is to craft a thermal grease that flows easily – low viscosity. The ubiquitous white grease is a good example. The problem with low viscosity grease is “pump-out” – the tendency for the grease to ooze out of the interface between the heatsink and CPU, in this case due to thermal cycling.

Another approach is to increase heatsink clamping pressure. The ancient ones among us remember in the early overclocking days of increasing heatsink pressure with thicker springs – some to the point of flexing the motherboard! However, coupled with a low viscosity grease, performance degraded over time due to pump-out and grease re-application every 2-3 months was required.

A bulk-loaded higher viscosity grease avoids the pump-out problem but does require careful attention to heatsink pressure for optimum results, as the graph above shows.

How Much Pressure?

A quick look at the “Intel ® Core ™2 Extreme Processor QX9775Ä Thermal and Mechanical Design Guidelines Addendum, February 2008″ gives the following:

The attach mechanism for the heatsink developed to support the processor should
create a static preload on the package between 18 lbf and 70 lbf throughout the life
of the product.

Not trivial! If you’ve encountered some stiff mounting pressure on that high performance heatsink, there’s a reason – you get best performance at the top end of this spec – 70 lbf. I should mention that if the heatsink is even slightly off balance, you will see an adverse impact.

Large, heavy tower-type heatsinks which mount horizontally in a desktop can, through its long lever arm, exert enough force to impact performance. (This is why larger heatsinks such as the Tuniq Tower use high mounting force.)

CONCLUSIONS

Optimal heatsink performance requires careful attention to heatsink mounting pressure and ensuring that the heatsink base is absolutely parallel to the CPU’s IHS.
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Summary: Lapping for a flat, uniform contact patch can significantly improve heatsink performance.

Heatsink lapping has been used as one means to improve heatsink performance. As part of evaluations IC Diamond is doing to better understand user thermal grease performance data, a special pressure sensitive film from Sensor Products was given to a number of users in different forums who agreed to participate in this test using their own PCs as a test bed.

The test involved first inserting the pressure film between the heatsink and CPU and bolting the heatsink on. The film was sent to Sensor Products for data analysis – it records pressures on the IHS from 30 to 90 pounds which is translated into data and images for analysis by Sensor Products. After lapping, a new film was used again to record the difference in contact area and pressure. For each test, IC Diamond was applied and temperatures were recorded.

The results are presented below for two approaches:

  • Lapping for Overall Flatness
  • Lapping to a High Center Spot

Lapping for Overall Flatness

Progressive lapping on test #23B: Before lapping, high load and high spots were found on the two edges of the IHS. After the first lapping, the high edges were eliminated. The last lapping smoothed out the high spots resulting in a relatively flat IHS (note: the numbers of the left are pounds of pressure).

Starting Point

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Two Dimensional Image

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Three Dimensional Image

First lapping -3ºC

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Two Dimensional Image

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Three Dimensional Image

Second lapping -5ºC

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Two Dimensional Image

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Three Dimensional Image

The performance result was an impressive 5ºC improvement in CPU temps. As the last image shows, the pressure pattern over the IHS is fairly uniform.

Lapping to a High Center Spot

Progressive lapping on test #20B: This was a test to remove the high spots on the edges of the IHS and leave the IHS’ center as the main contact area, testing the idea that most of CPU heat is concentrated on center and improved contact there would be beneficial.

Results showed that it was not a viable idea – it limited the amount of area available for heat transfer. Mounting the heatsink on the bump was like balancing a sheet of plywood on a basketball – the pressure on center, while high, tended to transmit the load to a bias on one side.

Starting Point

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Two Dimensional Image

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Three Dimensional Image

First lapping -0.7ºC

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Two Dimensional Image

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Three Dimensional Image

Second lapping -1.4ºC

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Two Dimensional Image

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Three Dimensional Image

While performance did improve by 1.4ºC, it was nowhere near the degree of improvement for a uniformly flat IHS as shown in the first series – quite a dramatic difference even considering the high center contact pressure.

Conclusions

The best lapping objective is a uniformly flat contact patch between the heatsink and IHS. To some this is a “DUH!”, but visual verification is nice to see.
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Disclosure: Joe Citarella has a financial interest in a company developing products for electronic chip cooling.

Summary: Thermal grease responds best with higher mounting force and parallel mating surfaces.

As a continuation of work to better understand the dynamics impacting thermal grease performance, tests were conducted by users in four different forums using contact pressure film from Sensor Products to evaluate why some users saw marked improvements in heatsink performance after changing to IC Diamond and some not.

Test Results

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The test involved inserting pressure sensitive film to record mounting force between the heatsink and CPU after installing the heatsink on the CPU. The film records pressures on the IHS from 28 to 98 pounds, which is translated into data, two and three dimensional images by Sensor Products for analysis.

The results below are selected samples; samples not included were multiples of the same
heatsink and CPU mount or those that appear to have been roughly handled in shipping and look
to be over exposed; others that were lacking complete thermal data were not included.

In most cases, improved performance is indicated by total force loads above 45 lbs,
averaging 53.6 lbs for 16 users. The contact images show broad, even contact with few high spots:

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Three users experienced no change even though they had adequate force with an average of 50.8 lbs. Closer examination of the images show uneven contact, with most of the force applied on the edges:

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Four users who experienced an increase in temperatures had the worst combination of both low
average force (37 lbs) and uneven edge contact, as seen below:

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While almost all results fell into the pressure analysis comfortably, samples 14
and 24 did not. A possible explanation is that under marginal conditions, the accuracy of the Intel internal diode or hardware monitoring program are at their limit, being only accurate to within a degree or so. Alternative speculation is
welcomed.

Mounting force over 60 lbs indicates diminishing returns.

Conclusions

The total span of temperatures in the sample group ranged from -5ºC to
+3.9ºC, an almost 9ºC spread. Results show that careful attention to contact and pressure can yield significant
benefits – as much as the difference between a stock heatsink and an expensive, high performance
heatsink. Forces over 60 lbs will not yield significant performance gains.

Many thanks to those that assisted us in this effort.

Appendix

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Email Joe

Disclosure: Joe Citarella has a financial interest in a company developing products for electronic chip cooling.

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