User:Kandi111777/Thermal interface material

A Thermal Interface Material or Mastic (aka TIM) is used to fill the gaps between thermal transfer surfaces, such as between microprocessors and heatsinks, in order to increase thermal transfer efficiency. These gaps are normally filled with air which is a very poor conductor.

They take many forms. The most common is the white-colored paste or thermal grease, typically silicone oil filled with aluminum oxide, zinc oxide, or boron nitride. Some brands of thermal interfaces use micronized or pulverized silver.

Another type of TIM are the phase-change materials. These are solid at room temperature but liquefy and behave like grease at operating temperatures. They are easy to handle and are not messy.

Compressed Interface Application
A soft metal alloy-thermal interface material (SMA-TIM) made of indium offers uniform thermal resistance at lower applied stresses in compressed interfaces. The malleability of indium minimizes surface resistance and increases heat flow. Also, heat-spring technology will further reduce the thermal resistance.

Reliability
SMA-TIM delivers superior performance over time. As SMA-TIM products are made of metal, they do not experience pump out problems even under power cycling. The heat-spring material, which does not contain silicone, will conform to surface disparities over time, thereby reducing thermal resistance through the life of the TIM. Due to its solid state, the SMA-TIM also resists bake out as shown in the diagram below.

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Measuring performance
When measuring the performance of thermal interface materials, characterize them based on their thermal resistance. This value is typically more valuable than bulk thermal conductivity. For a compressible TIM, the thermal resistance assumes the actual contact which will be made between the interface material and it’s mating surfaces. This provides a measurement of thermal performance which is as close to real-world per Watt or per cm2 as can be provided without being application-specific. The thermal resistance value is incredibly valuable, but even that is not the deciding factor for whether to implement a specific thermal interface material. In addition to an application’s unique requirements, there are other material properties which are commonly investigated to justify the selection of a thermal interface material. For instance, there is the compatibility between the TIM and the device in which it will reside. Some interface materials are affected by the specific surface finish of the device. Other interface materials require a cure cycle, which may exceed the device’s specs. Also, it is important to consider the reworkability of an interface material. TIMs such as compressible metal heat-spring are very simple to rework. Others, such as conductive epoxies, can be quite difficult.

Variables
Thermal resistance data is presented in a chart of thermal resistance vs. pressure. In general, higher applied pressure produces a drop in resistance. This is because, under higher loads, thermal interface materials fill any surface imperfections, effectively lowering the contact resistance and thinning the interface bondline. Thermal resistance is also affected by the thermal conductivity of the TIM material and its bond-line thickness. Logically, the higher the thermal conductivity the less sensitive the resistance is to bond-line thickness.

Optimizing performance
For compressible metal thermal interface materials, the key to optimizing their performance is maximizing applied pressure. These are metal foils intended to be compressed at room temperature. The more pressure the better. For instance, if you place a 0.004-inch-thick pure indium compressible TIM and apply 50PSI to it, you will achieve a thermal resistance of approximately 0.12 cm2-ºC/W. If the same TIM receives a load of 100PSI, the thermal resistance drops to approximately 0.06 cm2-ºC/W.

Compression
The rule of compression applies to most metal thermal interface materials primarily because pressure thins the interface bondline. Other metal thermal interface materials are less affected by pressure. Solder TIMs, for example, provide significantly lower thermal resistance then a compressible metal thermal interface, and only light pressure is ever applied to these. Solder TIMs have the benefit of being reflowed and melted for attachment. In its molten stage, solder TIMs flow as a liquid, filling in surface irregularities without a notable applied pressure. Pure indium, used as a solder TIM delivers a thermal resistance to 0.03 cm2-ºC/W.

Performance
When extreme thermal performance is required, liquid metals or phase change metals outperform solder TIMs. One issue with solder TIMs is that, when they re-solidify after they are reflowed, they may trap air or flux in the bond. These voids degrade thermal resistance. Liquid metal TIMs never re-solidify, so voiding isn’t an issue. The thermal resistance of a liquid metal TIM, with a 0.002" bondline, can be as low as 0.015 cm2-ºC/W. Another benefit is that they are not reflowed; therefore reflow equipment is not necessary. When liquid metals are used, the assembly must be designed to seal in the liquid metal.

Storage and packaging
SMA-TIM preforms come in a variety of packaging options, including tape and reel. To minimize excessive handling and oxidation due to exposure to air, SMA-TIM Preforms should be packed according to the quantity used during a typical work shift. Store in the original container, closed securely, in 55%RH or less and at a temperatures less than 22°C. SMA-TIM preforms can also be stored in an inert atmosphere such as a nitrogen air box. The SMA-TIM can also be packed in custom made individual adhesive carriers for direct attach to heat sinks and substrates.