Technical Explanation Thermal Interface Materials

Transcription

1 Technical Explanation Thermal Interface Materials Revision: 02 Issue date: Prepared by: Stefan Hopfe Approved by: A.Wintrich Keyword: Thermal Interface Material, Thermal Paste, Grease, TIM, Phase Change Material 1. Introduction... 1 Advantages Technical Details TIM Materials Print pattern tolerances Surface specification for heat sink Qualification Tests Typical qualification tests for shelf life of pre-printed modules inside blister Typical qualification tests for reliability (module mounted on heat sink) Conclusion Production Handling Packaging units Surface cleaning Opening blister packaging Removing modules from blister packaging Mounting Evaluation of imprint pattern First time of operation Labelling Labelling on delivery packaging Labelling on blister packaging Catalogue acceptable / non-acceptable parts Introduction SEMIKRON offers a broad range of pre-applied Thermal Interface Materials (TIM) for a wide product range. The TIM materials are applied to the power modules by SEMIKRON s automated screen and stencil printing process prior to delivery to the customer. Advantages Silicone and silicone free as well as phase changing Thermal Interface Materials are available. TIM materials with improved thermal performance are Phase Change Material HALA P8 for baseplate modules (silicone-free) and the High Performance Thermal Paste for baseplate-less modules (siliconebased). Following advantages can be utilized by using modules with pre-applied TIM: - Optimum heat dissipation, i.e. low thermal resistance thanks to the optimum conditions provided by the evenly distributed TIM layer - Easy and fast assembly of the module - Increased productivity due to reduced handling costs and logistics - Outsourcing of a dirty manufacturing process - Optimized thickness of thermal paste layer, which drastically reduces the risk of damage to the DBC while providing the lowest thermal resistance - Excellent long-term stability thanks to the use of well proven TIM materials Page 1/19

2 2. Technical Details The modules with pre-applied Thermal Interface Material are printed in a clean environment on an automated and SPC controlled silk screen or stencil printing line. The amount of TIM printed on the module surface can be found in the datasheets entitled typical thickness or typical weight. The value for the thickness is valid after module is mounted and the thermal paste has been distributed evenly across the entire bottom surface of the module. If the weight is stated in the datasheet, an inhomogeneous pattern is applied to the module. For these patterns it is not applicable to define the amount of TIM material by a thickness as the calculated resulting thickness is not constant over the printed surface. The thermal resistance values R th(j-s) for baseplate-less and R th(c-s) for baseplate modules specified in the module data sheets are valid in combination with the standard thermal paste material Wacker P12 (thermal conductivity λ=0.81 K/(W*m). If a module is applied with Phase Change Material (thermal conductivity λ=3.4 K/(W*m) or the High Performance Thermal Paste (thermal conductivity λ=2.5 K/(W*m), the thermal resistance R th(c-s) /R th(j-s) will be lowered. Therefore a second R th value with the new TIM material is specified in the datasheet. Moreover for baseplate-less modules like MiniSKiiP, SKiM or SEMITOP the output current I c is calculated by the thermal resistance R th(j-s). In consequence also for the output current I c two values are specified, one for the standard material Wacker P12 and one for the High Performance Thermal Paste. It must be noted, that the thermal conductivity λ is used here only to differentiate between the TIMs. It is not the only reason for the lower R th. A TIM material with higher thermal conductivity could also lead to a higher R th if the minimum layer thickness cannot be achieved or the interconnection between TIM and metal surface is poor (high contact resistance). 2.1 TIM Materials Phase Change Material Thermal Grease Available TIM type: HALA P8 (TPC-Z-PC-P8/Henkel Loctite PSX-Pe) Characteristic: aluminium particles as the thermal conductive filling material wax as a carrier matrix resolvent for the processability Phase Change Material will be offered for baseplate modules only. HALA P8 is a compound consisting of substances mentioned above which is pasty before application. The material is applied via screen or stencil printing process. The application is followed by an additional heating process which ensures that all resolvents are evaporated. After this the appearance is stiff like candle wax at room temperature and low viscous (pasty) at temperatures above 45 C. At this temperature the material starts to flow under pressure and to wet the contact surface between heat sink and baseplate. Available TIM types: Wacker P12: silicone and zinc oxide Electrolube HTC: paraffine and zinc oxide High Performance Thermal Paste: silicone and zinc oxide Characteristics: metal oxide particles as a filling material silicone or paraffine as a carrier matrix no resolvents Thermal greases will be offered for baseplate-free modules only. The materials are applied via screen or stencil printing process. An additional heating process is not required. Page 2/19

3 2.2 Print pattern tolerances The screen printing process is subject to a process tolerance; i.e. that the amount or thickness of thermal interface material can vary. These tolerances can be found in the datasheet titled min./max. thickness or min./max. weight. Due to the automated screen-printing process, only slight variations in print positioning occur but have no influence on the mounting process or the thermal properties. Additionally, slight defects in the print structure may occur during the printing process and are acceptable. The maximum permissible deviation in size of the honeycomb structure is ~10%. There might be a slight sub-surface migration of the stencil with thermal interface material. This optical change in the screen printing image does not affect the module s thermal properties or the mounting process. For more information about good and bad parts, please refer to chapter Surface specification for the heat sink To achieve a good interconnection between module and heat sink and to obtain an optimum heat transfer, the heat sink must comply with the following specification (see Figure 1). It is recommended that the cooler is milled by a carbide indexable insert. This tooling method delivers the best result with a typical surface roughness of ~1-3µm. - Heat sink must be free from residues, particles or dust - Evenness 50 µm on 100mm (DIN EN ISO 1101) - Roughness R z : 6.3 µm (DIN EN ISO 4287) - No steps > 10 µm Figure 1: Surface specification Heat sink 50 µm > 10 µm 6.3 µm Page 3/19

4 3. Qualification Tests Thermal Interface Materials are applied to different qualification tests to quantify the max. shelf life during storage and transport of modules inside the blister packaging and to prove the reliability of assembled modules in application. The TIMs are tested according to IEC xx for environmental conditions (like high and low temperature storage, according to IEC for minimum 12 Month in a shelf life test and according to IEC in power cycle tests. 3.1 Typical qualification tests for shelf life of pre-printed modules inside blister Tests P12 and HTC Tests P8 and HPTP Standard High-Temperature Storage IEC Bb Low-Temperature Storage IEC Ab High Humidity, High Temperature Storage Not applied IEC Climatic Change Not applied Close to IEC Pass Criteria R th measurement after test within spec, mounting according to mounting instruction ok none Shelf Life 18 month inside blister conditions: -25 C +60 C, 10 95% r.h. 12 month inside blister 1K2 conditions: 5 C +40 C, 10 85% r.h. IEC Typical qualification tests for reliability (module mounted on heat sink) Tests P12 and HTC Tests P8 and HPTP Standard High-Temperature Storage Low Temperature Storage High Humidity, High Temperature Storage IEC Bb IEC Ab IEC Climatic Change Close to IEC Power Cycling Test IEC Temperature Cycling Test IEC Na Pass Criteria R th measurement before and after storage test/thermal cycling criteria: variation of thermal resistance within spec, For Power Cycling EOL reached at R th+20% none Page 4/19

5 3.3 Conclusion TIM P12 HTC HPTP P8 Shelf life conditions 18 month inside blister conditions: -25 C +60 C, 10 95% r.h. 18 month inside blister conditions: -25 C +60 C, 10 95% r.h. 12 month inside blister 1K2 conditions: 5 C +40 C, 10 85% r.h. 12 month inside blister 1K2 conditions: 5 C +40 C, 10 85% r.h. Max. storage or transport temperature* +55 C +55 C +55 C +55 C Max. operation temperature T op** +125 C +125 C +125 C +105 C *limited by the blister packaging **In an assembled status. For baseplate-less modules: T op to be measured on heatsink, for baseplate modules: T op to be measured on baseplate In principal it is recommended that modules with pre-applied Thermal Interface Material are stored at room temperatures (if necessary air conditioned). Page 5/19

6 4. Production Handling Phase Change Material Thermal grease The HALA P8 phase change material applied on a baseplate of a power module is solid when delivered to customers and is changing to a paste-like consistency only when the assembled module is heated up (e.g. during burn-in, or first operation) Contamination with dust or particles can be easily removed from the solid phase change material layer. Please use a brush tool to remove any contaminations. Damage to the printed phase change material pattern by e.g. accidental touching or moving over the heatsink is not likely to occur, unless a strong force is applied. Baseplate-less power modules are applied with thermal grease and therefore have to be handled carefully due to the pasty TIM layer. The main advantage of a thermal grease instead of a phase change material on a baseplate-less module is the increased assembly process robustness. The low viscous TIM layer can easily distribute during the mounting and therefore reduces the mechanical stress on the DBC. But the product has to be handled with care, accidental touch will smudge the printed pattern and may influence the performance. A contamination with particles can be removed by using a tweezer tool. To avoid contamination of the TIM printed surface area with particles and dust, the blister packaging should be opened not earlier than the time of module assembly. To open the blister packaging please follow the instructions given in chapter Packaging units Following an overview about the packaging unit size for each module family and case size: Module Quantity per blister packaging MiniSKiiP0 66 MiniSKiiP1 40 MiniSKiiP2 24 MiniSKiiP3 16 MiniSKiiP8AxB 16 MiniSKiiP8AC 12 SKiM 63 4 SKiM 93 4 SkiM 4 4 SEMITOP 2 56 SEMITOP 3 35 SEMITOP 4 20 SEMITOP E1 28 SEMITOP E2 20 SEMiX 3p 6 SEMiX 1 8 SEMiX 3 6 SEMiX 13 4 SEMiX 5 4 SEMITRANS 3 6 SEMITRANS 10 2 Page 6/19

7 4.2 Surface cleaning Before module mounting it has to be ensured that the heat sink is free of contamination with any foreign particles or dust. A lint free wipe with isopropanol can be used to clean the surface. In case of module replacement, any residues of thermal grease or phase change material on contact surfaces can also be removed by using isopropanol and lint free wipes. 4.3 Opening blister packaging The surrounding environment must be clean and free of dust. The blister should remain closed during production stops to avoid contamination. The TIM pattern should be free of contamination and the screen-printing image should not be damaged. Larger sized particles which are visible to the naked eye can be removed by using tweezers. All parts are subjected to a final optical inspection. In the case of contamination with particles or any damage of the printing pattern, please refer to the catalogue which parts should be rejected. (see chapter 9) 4.4 Removing modules from blister packaging All ESD regulations for electronic components (IEC ) must be observed when removing the modules from the blister packaging as well as for further handling steps. ESD gloves must be worn when handling the modules. Contamination of the modules with thermal paste or hand perspiration must be avoided at all times. For easier removal of the modules from the blister packaging it is recommended for MiniSKiiP that the package is turned before removal so that the spring pins (upper side of modules) are visible (see Fig. 2 and 3). For SKiM 63/93 it is recommended to turn the package so that the printed DBC is visible. Than the blister package can be opened and the module can be taken out. Figure 2: MiniSKiiP and SKiM63/93 Figure 3: Removal of MiniSKiiP and SKiM63/93 Page 7/19

8 5. Mounting Phase Change Material Thermal grease Phase change material will be applied on baseplate modules only. For each module baseplate a certain print pattern layout is developed according to the baseplate bending (see Fig.4). This layout ensures a direct metal-to-metal contact of the module baseplate and heat sink. Only the air gaps coming from module bending and roughness are filled out with thermal interface material. This approach leads to two main advantages: Thermal greases will be applied on baseplate-less modules only. For standard thermal greases like Wacker P12 and Electrolube HTC a homogeneous honeycomb pattern is applied underneath the DBC (see Fig.5). Target is to gain a well distributed TIM layer between DBC and cooler without any air voids. - Thermal resistance R th(j-s) is optimized - No screw retightening required after phase change material has melted Fig.5: MiniSKiiP3 homogeneous print layout STANDARD TIM Fig.4: Print pattern according to baseplate bending For each module family (e.g. SEMITRANS, SEMiX) mounting instructions are given. These mounting instructions with prescribed values for mounting torque and speed will not change, when the modules has been applied with phase change material. After the module has been mounted on the heatsink and heated up for the first time, the phase change material melts above a module baseplate temperature of 45 C and fills out the bending and the roughness. The module has to be mounted according to the given module specific mounting instructions. After the module assembly the thermal grease distributes and fills out the roughness and the bending of the module. The High Performance Thermal Paste has a higher viscosity compared to standard pastes like Wacker P12 or Electrolube HTC. To reduce mechanical stress during the assembly the print pattern has been adapted by applying less material on the pressure areas of the module (see Fig.6) Fig.6: Optimized print layout High Performance Thermal paste Result: -Mounting according to existing Mounting Instructions -Even though the material is melted and distributed, the module mounting screws do not have to be re-tightened at any time. Result: -Mounting according to existing Mounting Instructions -The optimized print pattern layout allow to apply always the same mounting procedure independent from the thermal grease type. Page 8/19

9 6. Evaluation of imprint pattern Phase Change Material Thermal grease The pictures below show the typical appearance of baseplate modules applied with phase change material HALA P8: The pictures below show the typical appearance of baseplate-free modules applied with thermal grease: Fig.7 before mounting Fig.8,9,10 after mounting + thermal cycling Fig.11 before mounting Fig.12,13,14 after mounting and thermal cycling Fig.7: Typical baseplate module+tim before mounting Mounting and heat up procedure: Mount the module according to mounting instructions on the heat sink Perform 3 thermal cycles 20/90 C: Option 1: Put the heat sink with the mounted module e.g. on a heating plate and increase the temperature as fast as possible up to 90 C. As soon as the upper temperature has been reached it can be removed from the heating plate and should be cooled down as fast as possible to ~20 C. Repeat this procedure at minimum 2 times. Option 2: Apply approx % of the nominal module DC current that the baseplate can heat up to a temperature of 90 C. Then cool down after down to a baseplate temperature of ~20 C. Both options give the material the possibility to melt, spread out and evenly distribute Evaluation of print pattern: - Dismount the module and take it carefully off the heat sink - For baseplate modules it is not mandatory that the TIM material is completely spread out over the whole baseplate area. Only in the central area of the module it should be ensured that the honey combs have disappeared and the material is well distributed. (Fig.8) - Especially the area around the mounting holes and the module edges are free of TIM material that metal-metal contacts can be established. This approach leads to an optimum thermal resistance and an additional retightening Fig.11: Typical baseplate-less module+tim before mounting Mounting and heat up procedure: Mount the module according to mounting instructions on a heat sink Perform 3 thermal cycles 20/90 C: Option 1: Put the heat sink with the mounted module e.g. on a heating plate and increase the temperature as fast as possible up to 90 C. As soon as the upper temperature has been reached it can be removed from the heating plate and should be cooled down as fast as possible to ~20 C. Repeat this procedure at minimum 2 times. Option 2: Apply approx % of the nominal module DC current that the heat sink can heat up to a temperature of 90 C. Then cool down to a heat sink temperature of ~20 C. Both options give the material the possibility to spread out and evenly distribute Evaluation of print pattern: - Dismount the module (unscrew, wait 24 hours till the module can be taken off easily without separation of the DBC and the module housing - it is absolutely mandatory that the predominant part of the DBC surface (except the DBC edges with the dimples) and the cooler area is wetted with TIM material. All honeycombs should have touched the heat sink surface or better completely disappeared. - An optimum imprint pattern is when the honeycombs are disappeared and the thermal paste has spread out evenly without any air voids (Fig.12). Any significant air gaps will Page 9/19

10 process is not required. - Figure 9 shows a suboptimal imprint pattern after disassembly, where the TIM material has not completely spread out after 3 thermal cycles. But the largest baseplate bending underneath the chips is filled out with TIM material and all honey combs have touched the heat sink surface. It can be supposed that with further thermal or power cycles the distribution of the TIM material further improves. - Figure 10 show a unacceptable imprint pattern after disassembly. Even though 3 thermal cycles were performed the TIM material has filled out the baseplate bending insufficiently. The majority of honey combs is still visible. Imprint pattern: Good: increase the thermal resistance and lead to a chip temperature increase during operation. Figure 13 show an still acceptable but not optimal imprint pattern. Some areas with honey combs are still visible but all of them have already touched the heat sink. It can be supposed that during the next 10 power cycles these air voids will fully disappear. - Picture 14 shows an unacceptable imprint pattern. Large areas of honey combs have even not touched the cooler. It is not expected that any load cycles will close these air voids. Imprint pattern: Good : Fig.8: After 3 thermal cycles 20 C/85 C and disassembly Fig.12: After 3 thermal cycles 20 C/85 C and disassembly Not optimum but still acceptable: Not optimum but still acceptable: Fig.9: After 3 thermal cycles 20 C/85 C and disassembly Fig.13: After 3 thermal cycles 20 C/85 C and disassembly Page 10/19

11 Not acceptable: Not acceptable: Fig.10: After 3 thermal cycles 20 C/85 C and disassembly Fig.14: After 3 thermal cycles 20 C/85 C and disassembly When assembly trials have been performed and the imprint pattern after disassembly shows a bad interconnection between module and heat sink as shown in Fig.10 and 14, it is mostly related to a deviation to the cooler surface specification. (refer to chapter 2.3) Anyway, to verify a proper thermal interconnection it is recommended to evaluate the thermal resistance junction to heat sink R th(j-s). Page 11/19

12 7. First time of operation Phase Change Material Thermal grease After the power module has been mounted to the heat sink according to the Mounting Instructions it can be put directly into operation with nominal current. The R th(j-s) of the module directly after mounting will be higher than after the phase change material has melted and distributed. The melting process starts at a temperature as low as T c =45 C so that R th(j-s) will drop very quickly and will not cause any junction temperature overshoot exceed the desired steady state value under nominal load conditions. Nevertheless thanks to relaxation and distribution effects, the R th(j-s) will slightly improve during the first heat up/cool down phases. The final R th(j-s) is typically reached after the first power cycles. In consequence a certain load pattern or burn in process is not required as long as the power module is not exposed to a high overload condition during the first time of operation. After the module has been mounted on the heat sink according to the standard Mounting Instructions it can be put into operation. After the assembly of a baseplate-less module the thermal grease starts to spread out and distributes evenly. In this stage the initial thermal resistance R th(j-s) is approx % above the final R th(j-s). Thanks to a settling process this R th improves significantly with an exponential curve progression over time until the final R th(j-s) is obtained after several days (see Fig.15). Thermal pumping effects induced e.g. by thermal cycles or active power cycles will accelerate this settling process. In consequence following the module mounting is recommend to either: a) wait 1 to 2 days before applying a load to the power module with full load or b) do 3 thermal cycles with a heat sink temperature swing of 20 to 90 C or c) do 3 load cycles with a heat sink temperature swing of 20 C to 90 C and a max. current limited to 70% to 80% of the nominal load current, to achieve full power capability For reference: The datasheet values for R th(j-s) were measured after 3 thermal cycles (20 C to 90 C) had been performed. This corresponds to approx. 2 to 5 days relaxation time. Fig.15: Typical thermal resistance over relaxation time Page 12/19

13 8. Labelling 8.1 Labelling on delivery packaging From the outside of a delivery packaging it is not visible if the modules come with or without pre-applied Thermal Interface Material. The label gives only information about the module type, quantity and order number. (see Fig. 16 and 17) Figure 16: Position of type label on delivery boxes The label contains the following items: Figure 17: Type label on paper box 1. SEMIKRON Logo 2. Product type designation 3. Dat. Cd.: Date code - 6 digits: YYWWL (YY=Year, WW=Week, L=Lot of same type per week) 4. Au.-Nr : Order Confirmation number / Item on Order Confirmation 5. Menge or QTY: Quantity on modules inside the box also as bar code 6. Id.-Nr : SEMIKRON part number also as bar code Bar Code in compliance with: - Standard: EEC Format: 19/9 Page 13/19

14 Each delivery box contains a note with the following information: Figure 18: Delivery paper box Always observe the thermal paste manufacturer s specifications as set down in the safety data sheet. The latest versions can be found on the following websites: Wacker P12: asten/siliconpasten.jsp Electrolube HTC: HALA TPC-Z-PC-D_P: High Performance Thermal Paste, (HPTP): No guarantee is given, either expressly or implied, for the accuracy, completeness, timeliness, efficacy or reliability of the information given in the safety data sheets. Further, we accept no liability for any direct or indirect damages arising from the use of the safety datasheets. Page 14/19

15 8.2 Labelling on blister packaging If the service pre-applied thermal interface material is requested, one label gives information about the date of application and the date of expiry (Fig. 19). A second label warns of opening the blister too early (Fig.20). Both are fixed to the blister packaging (Fig. 21). Figure 19: Label with date of expiry Figure 20: Advice label on blister packaging Figure 21: Position of labels on blister packaging Further, our General Terms and Conditions (GTC) in the currently applicable version apply exclusively. These can be viewed or downloaded on our website Page 15/19

16 9. Catalogue acceptable / non-acceptable parts The TIM application process is done in a separated, controlled environment and the printing process is subjected to an automated process control which ensures a consistent and reproducible printing quality. Despite all precautions a damage or contamination of the printing pattern cannot completely avoided and therefore cannot constitute a cause for complaints. The modules should be visually checked and any contamination with particles, fibers which are visible to the naked eye needs to be removed prior to the assembly by using a tweezer! In this case the pattern has to be evaluated as shown in the table below. Deviation due to the stencil or screen printing process Defect description Mechanical defect picture Quality acceptance Slight defects in printing pattern (small voids) OK Deformed honeycomb edges OK Slight sub-surface migration OK Few dots contain a plateau OK Page 16/19

17 Defects caused by handling or assembly process Defect description Mechanical defect picture Quality acceptance Only a few dots are blurred OK Contamination with particles e.g. hairs, fibers OK only when removed prior to the assembly (removable with tweezers) OK only when removed prior to the assembly (removable with tweezers) OK only when removed prior to the assembly (removable with tweezers) Page 17/19

18 Surface discolouration ok Many dots or whole printed sub-areas are blurred or smeared NOT OK (The application of a new TIM layer is recommended) Page 18/19

19 IMPORTANT INFORMATION AND WARNINGS The information in this document may not be considered as guarantee or assurance of product characteristics ("Beschaffenheitsgarantie"). This document describes only the usual characteristics of products to be expected in typical applications, which may still vary depending on the specific application. Therefore, products must be tested for the respective application in advance. Application adjustments may be necessary. The user of SEMIKRON products is responsible for the safety of their applications embedding SEMIKRON products and must take adequate safety measures to prevent the applications from causing a physical injury, fire or other problem if any of SEMIKRON products become faulty. The user is responsible to make sure that the application design is compliant with all applicable laws, regulations, norms and standards. Except as otherwise explicitly approved by SEMIKRON in a written document signed by authorized representatives of SEMIKRON, SEMIKRON products may not be used in any applications where a failure of the product or any consequences of the use thereof can reasonably be expected to result in personal injury. No representation or warranty is given and no liability is assumed with respect to the accuracy, completeness and/or use of any information herein, including without limitation, warranties of non-infringement of intellectual property rights of any third party. SEMIKRON does not assume any liability arising out of the applications or use of any product; neither does it convey any license under its patent rights, copyrights, trade secrets or other intellectual property rights, nor the rights of others. SEMIKRON makes no representation or warranty of non-infringement or alleged non-infringement of intellectual property rights of any third party which may arise from applications. This document supersedes and replaces all information previously supplied and may be superseded by updates. SEMIKRON reserves the right to make changes. SEMIKRON INTERNATIONAL GmbH Sigmundstrasse 200, Nuremberg, Germany Tel: , Fax: sales@semikron.com, Page 19/19