An - Thermal Paste

Application Note

Revision:

Issue Date:

Prepared by:

0

2010-03-30

Dieter Esau

Key Words: Thermal paste

THERMAL PASTE APPLICATION

AN-10-001

1 / 6 2010-03-30 – Rev0 © by SEMIKRON

Designated use of thermal interface material (TIM) .................................................................................................. 1 Procedures for thermal paste application .................................................................................................................. 3 Quality control check on thermal paste layer............................................................................................................. 4 Determining the optimum thermal paste layer thickness.................................................................................................................................................................... 5 References................................................................................................................................................................. 6 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 Fig. 1).

air gaps Fig. 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 Fig. 2).

thermal interface material Fig. 2: Heat transfer from a power module to a heat sink using TIM

Heat sink

Heat sink

baseplate or DCB of power module

baseplate or DCB of power module

Application Note AN-10-001

2 / 6 2010-03-30 – Rev0 © by SEMIKRON

TIM normally consist of a plastic carrier material (e.g. silicon oil) and thermal conductive filler substances such as zinc oxide, graphite1 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 categorised, 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.

Tab. 1: Specific thermal conductivity of materials commonly used in a power semiconductor module 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 R(thjs) of the module amounts to around 20-65%3, 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 Fig. 3).

1 These materials are electrically conductive. 2 The filler particles (ZnO) of Wacker P12 thermal paste are between 0.04µm and 4µm in diameter, which is why this thermal paste is very good for achieving very thin thermal paste layers. 3 A thermal paste with a higher specific thermal conductivity will not normally result in much improvement here, since this would normally mean a considerable increase in thermal paste layer thickness, too.

Fig. 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 Rthcs. Once the optimum has been reached, the thermal resistance Rthcs 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 favour 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 (see Tab. 2). 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 Rthcs.

• The paste is highly resistant to “bleeding” and “drying out”.

The table below shows an excerpt from a series of thermal conductive media which have undergone testing at SEMIKRON.

Material Specific Thermal

Conductivity λ

[W/(m*K)]

Thickness

[µm]

Portion of R(th) for SKiM modules

Chip 106 120 2.92%

Chip solder 57 70 3.65%

DBC (copper) 394 300 1.94%

DBC (Al2O3) 24 380 32.91%

DBC (copper) 394 300 1.31%

Thermal paste (P12 from WACKER2)

0.81 30 57.26%

Thickness of TIM Minimum Maximum

Rthcs


3 / 6 2010-03-30 – Rev0 © by SEMIKRON

Tab. 2: Selection of tested TIMs

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 Fig. 4), while the printing process is normally silk screen printing or stencil printing. 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.

Fig. 4: Paste application using a rubber roller

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: Thermal paste application using manual screen printing (Fig. 5):

Name; Producer Description Filled with silicon

Conduc-tive

Possible Application process

Applicable thickness of TIM in µm

Thermal Resis-tance

Thermal resistance λ W/(m*K)

(Data sheet)

P12, Wacker Paste, Filler: Al2O3

Yes No Roller, stencil-/ screen-printing

10-100 + 0,81

HTC, Electrolube Paste, Filler: AL2O3

No No Roller, stencil-/ screen-printing

10-100 + 0,9

PSX-P8, Hala Contec GmbH

Phase-Changer, Filler: aluminium powder

No No Roller, stencil-/ screen-printing

10-100 + 3,4

TIC 1000A, Bergquist

Paste, Filler: Al2O3

Yes Yes Roller, stencil-/ screen-printing

15-100 O 1,5

TIC 4000, Bergquist

Paste, Filler: fluid metal

Yes Yes Roller, stencil-/ screen-printing

ca. 100 + 4,0

KU ALC-5, Kunze Phase-Changer;

Aluminium foil with wax coating

No Yes Manually ca. 76 O 220

KU ALF, Kunze Phase-Changer;

Aluminium foil with wax and graphite coating

No Yes

Manually ca. 76 + 220

Keratherm 86/50, Kerafoil

Foil, Filler: boron nitride

Yes No Manually 120 - 2,9

Q2-Pad, Bergquist Aluminium foil with graphite coating

Yes Yes Manually 152 - 2,5


4 / 6 2010-03-30 – Rev0 © by SEMIKRON

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.

Fig. 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) (Fig. 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.

Fig. 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 Fig. 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 Fig. 8, the thickness of the layer measured in this example lies in the range of between 25µm and 30 µm. This measurement can be very easily distorted, however, through improper handling, surface unevenness or even by raised points in the thermal paste surface.

Fig. 7: Measuring the thermal paste layer with wet film comb

Fig. 8: 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 Fig. 9 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 Fig. 9). The thermal paste layer thickness can be read from the scale,

Thermal paste

Edges

Measurement teeth

2 25 30 µm

Heat sink

90°

Heat sink

Scrape direction

Measurement comb

Thermal paste

Screw holes

thermal paste blobs

Screw holes

a b

c d

Thermal paste


5 / 6 2010-03-30 – Rev0 © by SEMIKRON

taken from the end of the wet segment of the middle measurement disc (see Fig. 10).

Fig. 9: Measuring the thermal paste layer with a wet film wheel (here ZWW 2102 from Zehntner)

Fig. 10: Measuring the thermal paste layer with a wet film wheel (here ZWW 2102 from Zehntner)

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 Fig. 11).

Fig. 11: Thermal cycling performed to determine optimum thermal paste layer thickness As the module is pressed onto the heat sink/aluminium plate and sticky thermal paste distributed in the space between, once the screws have been undone, a module without base plate can not 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 Fig. 12).

Fig. 12: Optimum thermal paste layer thickness

If the visual inspection reveals areas on the contact surface of the module which haven’t been filled, then the thermal paste layer is too thin (see Fig. 13).

Fig. 13: Too little thermal paste

Temperaturzyklus

0 60 120 180

t [min]

Tem

per

atu

r

100°C

20°C

Temperaturzyklus

0 60 120 180

t [min]

Tem

per

atu

r

Temperaturzyklus

0 60 120 180

t [min]

Tem

per

atu

r

Temperaturzyklus

0 60 120 180

t [min]

Tem

per

atu

r

100°C

20°C

Area with less contact to the measurement disk

The layer thickness can be read here

Temperature Cycle

One Cycle

Tem

pera

ture


6 / 6 2010-03-30 – Rev0 © by SEMIKRON

References

1. Freyberg, M.: Application of thermal paste for power modules without base plate. SEMIKRON International, 1999.

2. Kolpakov, A. I.: „SKiiP – intellektual’nye silovye IGBT moduli SEMIKRON“ in „Komponenty i technologii“ Nr. 1, 2003

3. Kolpakov, A. I.: „SKiM – novoe pokolenie intellektual’nych silovych modulej SEMIKRON“. In ölektronnye komponenty, Nr. 1, 2003

4. Kolpakov, A. I.: „SEMITOP kak al’ternativa TO“. In: Silovaja ÷lektronika, Nr. 2, 2004

5. Goldman, W. E.: An Introduction to the Art of Heat Sinking. In: Electronic Packaging and Production, 1966

6. Strube M (2007) Wärmeleitpastenauftrag als Dienstleistung. Elektronik Praxis Nr.10, S. 24

7. Strube M (2007) Thermal Paste Spread and Ready for Use. Bodo's Power System May 2007, S. 24

DISCLAIMER SEMIKRON reserves the right to make changes without further notice herein to improve reliability, function or design. Information furnished in this document is believed to be accurate and reliable. However, no representation or warranty is given and no liability is assumed with respect to the accuracy or use of such information. SEMIKRON does not assume any liability arising out of the application or use of any product or circuit described herein. Furthermore, this technical information may not be considered as an assurance of component characteristics. No warranty or guarantee expressed or implied is made regarding delivery, performance or suitability. This document supersedes and replaces all information previously supplied and may be superseded by updates without further notice.

SEMIKRON products are not authorized for use in life support appliances and systems without express written approval by SEMIKRON.

SEMIKRON INTERNATIONAL GmbH P.O. Box 820251 • 90253 Nürnberg • Deutschland • Tel: +49 911-65 59-234 • Fax: +49 911-65 59-262

[email protected] • www.semikron.com

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