Application Note Power Modules with Phase-Change Material Handling of Power Modules with Pre-Applied Phase-Change Material Application Note no.: AN_2012-07_001-v01 Table of Contents Re v is i on h is to r y: ................................................................................................. 3 1 A bs tr ac t ................................................................................................... 4 2 In tr o d uc t i o n ............................................................................................. 4 3 T he P has e - c h an g e M a ter i a l ....................................................................... 4 4 V inc o tec h M o du l es wi t h Pr e - ap p l ie d P C M ................................................... 5 5 Mo u nt i n g / As s em bl y ................................................................................. 6 6 O p er a t io n ................................................................................................ 7 7 Dis as s em bl y / R eus e ................................................................................ 8 8 La b e li n g a nd Mar k i n g ................................................................................ 9 9 Co nc lus i o n ............................................................................................... 9 Disclaimer: The information in this document is given as an indication for the purpose of implementation only and shall not be regarded as any description or warranty of a certain functionality, condition or quality. The statements contained herein, including any recommendation, suggestion or methodology, are to be verified by the user before implementation, as operating conditions and environmental factors may vary. It shall be the sole responsibility of the recipient of this document to verify any function described herein in the given practical application. Vincotech GmbH hereby disclaims any and all warranties and liabilities of any kind (including without limitation warranties of noninfringement of intellectual property rights of any third party) with respect to any and all information given in this document. Revision history: Date July 2012 Revision Level 1 Description First release Handling of Power Modules with Pre-Applied Phase-Change Material Page number(s) 9 pages Page 3 of 9 1 Abstract There are several advantages to using phase-change material (PCM) rather than conventional thermal grease as the thermal interface material (TIM) between the power module and heatsink. Vincotech offers modules with a layer of pre-applied PCM. The thermal interface material is applied in a layer with uniform thickness by a screen-printing process. This document describes the benefits of this phase-change material and provides tips on handling modules. 2 Introduction Heat has to be transferred from the module to the heatsink and thermal interface material is a necessary evil that gets the job done. However, thermal resistance increases if the layer is too thin or thick. Vincotech has addressed this issue by supporting modules with a pre-applied layer of phase-change material. The module’s size and technology determines the layer’s thickness. The phase-change material is solid at room temperature, so it requires no special care during transportation, handling and application. Because of its thixotropic consistency, the material softens but does not flow when heated during soldering. The surface needs protection only if the power module’s phase-change material comes into contact with other objects during or directly after soldering, for example, the soldering oven’s carpet. The screen-printing process is precise, thereby maximizing heattransfer capability. The customer is spared the task of applying thermal interface material, thereby saving time and reducing the failure risk. 3 The Phase-change Material Vincotech uses Loctite PSX-Pm phase-change material. It has the advantage that it can be applied by screen or stencil printing. It is fluid during the application and dries out over time and temperature. Once the phase-change material solidifies, the module may be handled like any conventional module. Key benefits include: Faster, easier module mounting Optimum thickness of the thermal interface material Improved Rth and reduced risk of DBC cracking Streamlined production; no need for screen-printing facilities Automated screen printing for utmost reliability No risk of smearing thermal paste; material is solid at room temperature Standard solder profile applicable (e.g. J-STD-001, J-STD-003) Compatible with Press-fit pins Handling of Power Modules with Pre-Applied Phase-Change Material Page 4 of 9 The table below lists the phase-change material’s physical and thermal properties. Please refer to the manufacturer’s datasheet for more details. Parameter Value Unit Specific gravity 2 g/cm² Thermal conductivity 3.4 W/m*K Phase-change temperature 45 °C Viscosity above phase-change temperature Thixotropic Color Grey Table 1: Physical and thermal properties of the phase-change material 4 Vincotech Modules with Pre-applied PCM Vincotech offers modules with a pre-applied layer of phase-change material as indicated in Table 3. All modules are UL-listed; therefore modules with phase-change material are also UL-approved. They come in a standard blister box with a protective lid. Figure 1: Modules with applied phase-change material in a blister Modules must be stored in blister boxes under the following conditions: Temperature: -25°C < T < 60°C Relative humidity: 10% < rH < 95% No aging effect is known. This compound was subjected to a battery of tests to verify its reliability. Test Conditions High temperature storage 2 x 500h @ 85°C High humidity and high temperature 2 x 500h @ 85% rH and 85°C Thermal shock test 100 x (30 min. @ 125°C / 30 min. @ -40°C) Table 2: Phase-change material reliability tests All modules had phase-change material applied and were mounted vertically to an aluminum heatsink for these tests. Handling of Power Modules with Pre-Applied Phase-Change Material Page 5 of 9 The following table and figures show the pattern, thickness and dimensions of the applied phase-change material at the back of the module after screen printing. Parameter Min. Typ. Thickness flow 0 Al2O3 DBC 30 55 Thickness flow 1 Al2O3 DBC 40 60 Thickness flow 1B 70 90 Diameter of pattern (D1, see Figure 1) 3.00 Web (D2, see Figure 1) 0.5 Table 3: Dimensions after screen-printing Max. 80 80 110 Unit μm μm µm mm mm Figure 2: Dimensions and pattern after screen printing The small triangle at the top right of the printed phase-change pattern is one of four corner markers used to align the press-in tool for modules with Press-fit pins. 5 Mounting / Assembly The module can be mounted to the heatsink after it has been soldered or pressed in. The procedure is the same as the standard mounting process described in the housing specifications or handling instructions, apart from one major difference: Screws to the heatsink can be fastened and tightened in a single step. The phase-change material is solid at room temperature, so screws can be tightened immediately without having to give the material any relaxation time. Handling of Power Modules with Pre-Applied Phase-Change Material Page 6 of 9 Pictured below is a module with melted phase-change material assembled to an inspection block. Figure 3: Module assembled to an inspection block Please refer to each module’s handling instructions to learn more about the heatsink properties, mounting and handling. 6 Operation Upon initial start-up, the Rth between the junction and the heatsink of a system without soft material will be 10% to 15% higher than that of an operating system; that is, a system where the module is operating a temperature higher than 45°C and the phase-change material has attained its ultimate thickness. The higher Rth is not a problem because the heatsink temperature is below 45 to 50°C, a state at which chips cannot overheat. Time-to-melt is a function of temperature and the speed of temperature change. The material will not flow unless heat and force are applied. Figure 4: Thermal resistance versus heatsink temperature Once the module has been mounted, the system should be heated up (e.g. during the system’s burn-in test) while leaving enough time for the phase-change material to melt. If the temperature of the PCM exceeds 45°C, the material will melt, fill gaps and after a short time provide an optimal thermal connection between the module and heatsink. Screws do not have to be tightened again. Handling of Power Modules with Pre-Applied Phase-Change Material Page 7 of 9 Figure 5: Decrease in thickness over time The above curve shows how thickness decreases as the heatsink temperature increases at a rate of 15 K/min. with the temperature starting at 35°C. The force applied to the phase-change material was around 100 kPa, which is below the flow modules’ pressure handling capability and therefore leaves a margin for safety. The phase-change material returns to its solid state when the temperature drops below 45°C. This means the material’s phase changes every time it reaches 45 °C. 7 Disassembly / Reuse No special care has to be taken when removing the module from the heatsink. Simply loosen the screws and use a knife to lift the module from the heatsink. Standard isopropyl or other alcohol with similar properties may be used to clean the back of the module and the heatsink. A non-woven antistatic microfiber cloth is best for this purpose. It is not advisable to reuse the module without cleaning it first. The picture below shows a disassembled module that had been in operation and its heatsink print. Figure 6: Disassembled module and its heatsink print Standard thermal grease or the Loctite phase-change material described above can be applied before the module is remounted. Handling of Power Modules with Pre-Applied Phase-Change Material Page 8 of 9 8 Labeling and Marking All Vincotech modules come in a carton. The suffixes -/3/ indicate that it contains modules with preapplied phase-change material. Figure 7: Label on carton The part number printed on the module does not indicate that these modules come with phase-change material pre-applied. Figure 8: Part number printed on module The “03” prefix to the RoHS indicates that this module is in revision status 03, and has no bearing on whether or not it comes with pre-applied phase-change material. 9 Conclusion Vincotech offers modules with pre-applied phase-change material. This PCM is applied in a screen printing process that leaves an optimum and always consistent layer thickness. This phase-change material is thixotropic and therefore will not flow without the application of pressure. Standard soldering profiles may be used. A lid or a foil can prevent the phase-change material’s surface from coming into contact with the soldering oven’s carpet. Power modules with Press-fit pins and phase-change material are compatible. Please refer to the module’s handling instructions to learn more about this. Handling of Power Modules with Pre-Applied Phase-Change Material Page 9 of 9