Thermal Interface Materials (TIM) for a Wide Product Range
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Technical Explanation Revision: 03 Thermal Interface Issue date: 2019-09-25 Prepared by: Stefan Hopfe Materials Approved by: A.Wintrich Keyword: Thermal Interface Material, Thermal Paste, Grease, TIM, Phase Change Material 1. Introduction ............................................................................................................................... 1 Advantages .................................................................................................................................. 1 2. Technical Details ........................................................................................................................ 2 2.1 TIM Materials ........................................................................................................................ 2 2.2 Print pattern tolerances .......................................................................................................... 3 2.3 Surface specification for the heat sink ...................................................................................... 3 3. Qualification Tests ...................................................................................................................... 4 3.1 Typical qualification tests for shelf life of pre-printed modules inside blister .................................. 4 3.2 Typical qualification tests for reliability (module mounted on heat sink) ....................................... 4 3.3 Conclusion ............................................................................................................................ 5 4. Production Handling .................................................................................................................... 6 4.1 Packaging units ..................................................................................................................... 6 4.2 Surface cleaning ................................................................................................................... 7 4.3 Opening blister packaging ...................................................................................................... 7 4.4 Removing modules from blister packaging ................................................................................ 7 5. Mounting ................................................................................................................................... 8 6. Evaluation of imprint pattern ....................................................................................................... 9 7. First time of operation ............................................................................................................... 12 8. Labelling ................................................................................................................................. 13 8.1 Labelling on delivery packaging ............................................................................................. 13 8.2 Labelling on blister packaging ............................................................................................... 15 9. Catalogue acceptable / non-acceptable parts ............................................................................... 16 1. 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 (silicone-based). 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 © by SEMIKRON / 2019-09-25 / Technical Explanation / Thermal Interface Materials Page 1/19 PROMGT.1026/ Rev.6/ Template Technical Explanation 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 Rth(j-s) for baseplate-less and Rth(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 Rth(c-s)/Rth(j-s) will be lowered. Therefore a second Rth value with the new TIM material is specified in the datasheet. Moreover for baseplate-less modules like MiniSKiiP, SKiM or SEMITOP the output current Ic is calculated by the thermal resistance Rth(j-s). In consequence also for the output current Ic 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 Rth. A TIM material with higher thermal conductivity could also lead to a higher Rth 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: Available TIM types: − HALA P8 (TPC-Z-PC-P8/Henkel Loctite PSX-Pe) − Wacker P12: silicone and zinc oxide Characteristic: − Electrolube HTC: paraffine and zinc oxide − aluminium particles as the thermal conductive − High Performance Thermal Paste: silicone and filling material zinc oxide − wax as a carrier matrix Characteristics: − resolvent for the processability − metal oxide particles as a filling material − silicone or paraffine as a carrier matrix Phase Change Material will be offered for baseplate − no resolvents modules only. HALA P8 is a compound consisting of substances Thermal greases will be offered for baseplate-free mentioned above which is pasty before application. modules only. The material is applied via screen or stencil printing The materials are applied via screen or stencil process. printing process. The application is followed by an additional heating An additional heating process is not required. 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. © by SEMIKRON / 2019-09-25 / Technical Explanation / Thermal Interface Materials Page 2/19 PROMGT.1026/ Rev.6/ Template Technical Explanation 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 9. 2.3 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 Rz: 6.3 µm (DIN EN ISO 4287) - No steps > 10 µm Figure 1: Surface specification 50 µm Heat sink > 10 6.3 µm µm © by SEMIKRON / 2019-09-25 / Technical Explanation / Thermal Interface Materials Page 3/19 PROMGT.1026/