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Designated use of thermal interface material (TIM)
When in operation, power modules produce losses which increase the module temperature and impair module efficiency and/or functionality. To dissipate the heat that builds up in power modules, the power modules are mounted onto heat sinks. The heat is then dissipated from the power module via the heat sink. Heat transfer between the heat-dissipating surface of the power module and the heat sink surface depends on the surface quality of the different surfaces. Both the heat sink surface and the heat-dissipating surface of the power module are uneven. As a result, air is trapped between the two surfaces, preventing direct heat transfer. As air is a poor thermal conductor (the specific thermal conductivity of air is λair ≈ 0.03 W/m•K), only very little heat can be conducted to the heat sink (see Figure 1).
Figure 1. Heat transfer from a power module to a heat sink without TIM
A suitable way of improving heat transfer is to fill up the air pockets with a TIM (see Figure 2).
Figure 2. Heat transfer from a power mo dule to a heat sink using TIM
TIM normally consist of a plastic carrier material (e.g. silicon oil) and thermal conductive filler substances such as zinc oxide, graphite or silver. They are available in the form of pastes, adhesives, phase-change materials and foils. TIMs conduct heat better than air and typically have a specific thermal conductivity of λ≈ 0.5 - 6 W/m•K. The thermal conductivity of TIM is thus approx. 20 - 200 times better than that of air. To enable the thermal conductivity properties of TIMs to be categorized, table 1 shows the specific thermal conductivity of materials commonly used in power modules. The thermal paste P12 from the company Wacker has been taken by way of example. The R(th) values shown are based on the module-specific thermal spreading.
Table 1. Specific thermal conductivity of materials commonly used in a power semiconductor modules
If the thermal conductivity of thermal paste is compared with the thermal conductivity of other components in a power module (see Tab. 1), the thermal paste does not rate particularly well. The extent to which thermal paste contributes to the overall thermal resistance Rth(j-s) of the module amounts to around 20-65%, depending on the module with the combination of the heat sink. The thermal paste layer therefore has to be as thin as possible but as thick as necessary (see Figure 3).
Figure 3. Dependence of thermal resistance on thermal paste layer thickness
Too thin a thermal paste layer results in air pockets between the underside of the module and the top of the heat sink, causing a high thermal resistance Rth(c-s). Once the optimum has been reached, the thermal resistance Rth(c-s) increases quickly again in line with the increase in thermal paste layer thickness; this happens because the specific thermal conductivity λ of thermal conductive media is very low compared with other materials in a power semiconductor module. The minimum value as shown in the diagram above is different in each system (module on heat sink) and has to be defined in tests.
The thickness of the thermal paste layer is different for different module types. This is why the mounting instructions of power modules specify the given thermal paste layer thickness and describe the quality of the surface of the heat sink.
The thermal paste used and recommended by SEMIKRON is P12 thermal paste from the company Wacker and is at the lower end of the specific thermal conductivity range. The following factors are the key arguments in favor of this thermal paste:
- R(th) tests have shown that the thermal conductivity of a thermal paste in actual application does not only depend on its specific thermal conductivity λ, but also on it its structure. The larger the filler particles in a thermal paste are, the higher the specific thermal conductivity. The particle size of the filler determines the minimum layer thickness. In other words, the thermal paste layer applied cannot be thinner than the largest particles in the paste. After several temperature cycles, a paste with small particles (e.g. P12: particle size 0.04μm - 4μm) allows almost for metal-to-metal contact at points where the pressure is particularly high, resulting in a substantial reduction in Rth(c-s).
- The paste is highly resistant to “bleeding” and “drying out”.
Procedures for thermal paste application
Thermal paste can be applied either to the module or to the heat sink. This is done using a roller or in printing processes. In roller application, a rubber roller is normally used (see Figure 4), while the printing process is normally silk screen printing or stencil printing.
Figure 4. Paste application using a rubber roller
Applying thermal paste with a rubber roller can lead to sufficient results provided this assembly step is performed by experienced professional staff that are properly trained in this critical process. This process also has disadvantages, however, for instance inhomogeneity, poor reproducibility and the risk of contamination.
In stencil printing, a stainless steel stencil and stainless steel scraper are normally used. The “effective” thermal paste layer thickness, however, is determined by the ratio of filled area to non-filled area, as well as by the height of the dots applied, which in turn are determined by the thickness of the stencil itself.
In screen printing, Monolen-PET meshes and a polyurethane scraper with a shore hardness of 75 are used. The thickness of the yarn and the number of yarns per unit of length determine the thickness of the thermal paste layer.
In stencil and screen printing, far better results can be achieved than in the roller process, provided the printing is done automatically. Performing this process manually can lead to considerable process fluctuations. The development of a process with an automatic stencil printer that features continuous process monitoring, as is the case at SEMIKON, requires substantial investments, however, which in economic terms only makes sense for large production quantities.
The stencil and screen printing process exists in all automated stages. An example of a manual screen printing process used to print onto a heat sink is explained below (see Figure 5):
a) Clean the surface with a cleansing agent to remove all grease. Position the heat sink in the device. Here, it is important that the screen does not touch the surface of the heat sink. To ensure this happens, the clearance between the screen/mesh and heat sink has to be 4 - 7 mm.
b) The screen is “flooded” with thermal paste (low-pressure application).
c) The thermal paste is spread using the polyurethane scraper; here, sufficient pressure must be applied to ensure that the mesh is pressed onto the print surface.
d) Visual inspection.
Figure 5. Thermal paste application using manual screen printing
In addition to complying with recommended layer thickness, care should be taken when applying the thermal paste to ensure that the thermal paste layer is evenly and homogenously spread on the underside of the module or the heat sink surface. An inhomogeneous thermal paste layer (extreme case: application of one or more thermal paste blobs) (Figure 6) can result in fractures or breakage in the DBC ceramic substrate. This applies to modules with and without a base plate alike. In addition to this, thermal paste inhomogeneity can also lead to local overheating resulting from the air pockets between the underside of the module and the upper side of the heat sink surface.
Figure 6. Module underside showing problematic thermal paste layer application
Quality control check on thermal paste layer
The thickness of a thermal paste layer can be measured directly or indirectly. An indirect way of measuring the thickness is, for example, to weigh the thermal paste by performing a Tara weight measurement using suitable scales. An example of a direct contact-free measurement of the thermal paste layer is a measurement using an optical profilometer such as the μSCAN from Nano Focus. Other measurement equipment that can be used to measure the thermal paste layer directly includes, for example, thickness gauges such as wet film combs or wet film wheels; the downside of these, however, is that they may destroy the layer in places.
The edges of the wet film comb (e.g. from Zehntner (ZND 2051) or Elcometer Instruments or BYK Gardner (PG- 3504)) have support teeth and measurement teeth which have a defined distance to the surface. The comb is held perpendicular to the surface and run across the surface horizontally (see Figure 7); when this is done, some paste residue will be left on the teeth of the comb that are beneath the surface of the paste layer. As shown in Figure 7, the thickness of the layer measured in this example lies in the range of between 25μm and 30μm. This measurement can be very easily distorted, however, through improper handling, surface unevenness or even by raised points in the thermal paste surface.
Figure 7. Measuring the thermal paste layer with wet film comb (here PG-3504 from BYK Gardner)
A wet film wheel (e.g. from Zehntner (ZWW 2100-2102), see Figure 8 or BYK Gardner) produces more accurate results in thermal paste layer thickness tests than a wet film comb. The wet film wheel consists of two support discs which are positioned at the outer edges, and one measurement disc located between the support discs. The measurement wheel is rolled across the surface that has been coated with thermal paste (see Figure 8).
Figure 8. Measuring the thermal paste layer with a wet film wheel (here ZWW 2102 from Zehntner)
The thermal paste layer thickness can be read from the scale, taken from the end of the wet segment of the middle measurement disc (see Figure 9).
Figure 9. Reading the wet film wheel
Determining the optimum thermal paste layer thickness
Thermal paste layer thickness is different for different module types. This is why the mounting instructions of power modules specify the given thermal paste layer thickness and describe the quality of the surface of the heat sink. In most cases, however, the specified layer thicknesses apply to the thermal paste Wacker P12. If other thermal pastes are being used, we recommend observing the following procedures:
Varying, pre-defined thermal paste layer thicknesses are applied to the modules or the heat sink. A module can be screwed onto a standard module or onto an aluminium plate in accordance with the given mounting instructions. When tightening the mounting screws, the tightening torques specified in the given mounting instructions must be observed. To achieve a relaxed system state, i.e . with no mechanical load, the mounted and secured module should undergo three thermal cycles (20°C/100°C/1h) (see Figure 10).
Figure 10. Thermal cycling performed to determine optimum thermal paste layer thickness
As the module is pressed onto the heat sink/aluminum plate and sticky thermal paste distributed in the space between, once the screws have been undone, a module without base plate cannot be easily removed without causing destruction. To ensure non-destructive removal, the module should be left untouched at room temperature for 12 hours after the screw has been loosened or should undergo 1-2 thermal cycles. The remaining thermal paste can then normally be easily removed using a clean, solvent-free, lint-free cloth (where applicable, thermal paste manufacturer specifications are to be complied with).
Each module should be used just once, as repeated loosening and tightening of the screws can alter the pressure properties. For each thermal paste layer thickness to be tested, a minimum of two modules should be used.
When module underside surface is fully covered with paste, the thermal paste layer application is optimum (see Figure 11).
Figure 11. Optimum thermal paste layer thickness
If the visual inspection reveals areas on the contact surface of the module which have not been filled, then the thermal paste layer is too thin (see Figure 12).
Figure 12. Too little thermal paste
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