Thermal greases

Figure 6. Scanning electron micrographs of hard-baked PMMA films before and after a 1.0-min oxygen-RIE treatment at the following conditions a, unetched and b, etched, 0.125 W/cm, 35 mTorr, -230 VDC, no thermal grease. Continued on next page.

Figure 6. Continued, c, Etched, 0.75 W/cm, 35 mTorr, -500 VDC, no thermal grease and d, same as in c except with thermal grease.

For design purposes the contact conductance values given in Table 2-2 may be used in the absence of more specific information. Thermal contact resistance can be reduced markedly, perhaps as much as 75 percent, by the use of a thermal grease like Dow 340. 

Note If we want to know how much thermal resistance is typically attributable to thermal grease, we must remember that without this grease we would have air in the spaces between the device and heatsink, and that is a very poor thermal conductor. Thermal grease lowers this interface resistance significantly by filling the... 

Surface-mount adhesive, solder paste, solder mask, and RTV dispensing Thermal grease, solder paste, and surface-mount adhesives... 

Thermal grease uses a hydrocarbon oil or silicone as a base and is filled with a thermally conductive material such as aluminum oxide or zinc oxide. The typical thermal grease used in production applications has a thermal conductivity of approximately 1.0 W/m K. The typical thickness is 0.001-0.003 in. Newer thermal greases have thermal conductivities as high as 16 W/m K [15,24]. 

Thermal grease does not provide adhesion. Therefore, some form of mechanical attachment is necessary to apply sufficient pressure and minimize the thickness. 

Mica insulators, having a thickness of 0.002-0.003 in., have been used for many years in mounting power devices to heat sinks. Used in conjunction with thermal grease, they provide a low-cost method of reducing the thermal resistance caused by air gaps. Mica has a thermal conductivity of 0.75 W/m K [9]. 

FIGURE 17.11 A stacked package configuration on a PCS with a perforated RF shield that is being used to spread heat from the top package.Hiemial gap filler material such as thermal grease is used to conduct the heat from the top package into the RF shield. 

Power modules are integrated in electric and hybrid vehicles, for example, in order to connect the transistors of the power electronics to the heat sinks. The use of power modules is typical for power levels above one kilowatt (Hensler 2012). Power modules have electrical, mechanical, and thermal functions. The silicon chip, the bonds, and the copper layer form the electrical circuit. This electrical circuit is insulated from the rest of the module by means of an electrical insulator (typically ceramic). The insulation is applied to a heat spreader that ensures that heat is distributed uniformly within the module and that the heat is thus dissipated away from the chip. The heat spreader is typically made of copper and also provides mechanical stabilization. It is connected to the heat sink by a so-called thermal grease, and the heat sink in turn is cooled by means of a cooling medium (air or liquid). 

The improvement in thermal conductivity of SR nanocomposites could lead to its many promising industrial applications, e.g., circuit boards in power electronics, thermal greases, elastomeric thermal pads, and phase change materials [185,186]. Therefore, many attempts at SR nanocomposites with higher thermal conductivity have been developed [39,124,130,179]. 

Conductance between dry, rough surfaces is much worse in a vacuum than in air. Such a contact can be improved by the use of thermal grease - generally filled with a metal oxide. On the other hand, use of a thermal grease degrades the conductance if the surfaces are very flat, smooth, and/or of malleable materials (copper to indium, for example). 

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