AN827 Vishay Siliconix Torque Recommendations for TO-220 Devices Kandarp Pandya INTRODUCTION When the TO-220 was first introduced, most applications required something less than the full power handling capabilities of this package. Hence, the TO-220 is almost taken for granted in terms of its excellent power handling capacity and ruggedness. Today, however, advances in semiconductor technologies are bringing application demands closer to the TO-220’s capabilities, so an understanding of these is more relevant than ever. A reliable power electronics design requires close attention to both thermal management and mechanical mounting of devices. To ensure a successful implementation, designers must be aware of and understand the thermal resistance of the interface between the device and the heat sink, issues in mechanical fastening, the thermal properties of the interface medium, and the flatness (or roughness) of the interface surfaces of the device and the heat sink. For the MOSFET/heat sink assembly, a specially designed heat sink assembly of a copper block (4 in. x 4 in. x 0.75 in.) was used to simulate an infinite heat sink attached to the case of the TO-220 device. The design of the heat sink also ensured the best possible flatness of the device-mounting surface could be achieved through appropriate machining techniques. The cooling system maintained the ambient at the desired temperature of 25 _C. The fastening method employed a standard M3 screw-washer-nut. A calibrated torque wrench was used to assemble the part with known torque values. The device under test (DUT) was an engineering sample of the Vishay Silicionix SUP50N06-16L power MOSFET in the TO-220 package. The DUT was mounted to heat sinks with the following assembly variations in the heat transferring interface between part tab and the heat sink: Vishay Siliconix has conducted a laboratory experiment to help designers understand the torque spec for the TO-220 and its impact on thermal resistance. The experiment likewise addresses the difference between various interface mediums and the applicable torque for the assembly fastener, which is typically an M3 screw. Set-up for the experiments and their consolidated results are reported below. (a) Part mounted directly onto the heat sink (no use of thermally conductive grease or heat sink compound) THE EXPERIMENT SET-UP (d) Part mounted on the heat sink with “Bergquist” BOND PLY 100 Our experimental set-up was similar to that used for thermal characterization of power MOSFETs, and consisted of a MOSFET/heat sink assembly and a semiconductor thermal test system. Document Number: 72674 01-Dec-03 (b) Part mounted on the heat sink with grease (c) Part Mounted on the heat sink with “Bergquist” SIL-PAD A1500 (e) Part mounted on the heat sink with Mylar without grease (f) Part mounted on the heat sink with Mylar with grease www.vishay.com 1 AN827 Vishay Siliconix FIGURE 1. “Analysis Tech” Semiconductor Thermal Test System Figure 1 shows the semiconductor thermal test system used for the experiment. The power stimulus generation and junction temperature derivation was managed through built-in computerized equipment especially designed for this function. The electrical schematic is shown in Figure 2. This arrangement facilitated temperature calibration for the DUT and then the actual testing. V_PORT SENSE V_PORT D I_PORT SENSE Sink R4 Sense Channel G I_PORT Source S C3 0.3 mF R5 D1 500 W POWER_GND SENSE_GND FIGURE 2. Electrical Schematic Diagram www.vishay.com 2 Document Number: 72674 01-Dec-03 AN827 Vishay Siliconix ÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁ TABLE 1: Rth(j-c) VS. TORQUE Rth(j-c) Bergquist Rth(j-c) Torque (In lbs) H Directly Onto Heat-Sink H Mylar Without Grease H With Grease H Sil Pad A1500 H Bond Ply 100 H Mylar With Grease 1 2.00 1.32 3.36 4.45 4.21 3 1.71 1.10 3.25 4.17 4.20 4 1.68 1.06 3.22 4.00 5 1.64 1.01 3.20 3.91 5.02 4.11 7 1.62 0.97 3.15 3.71 4.92 4.09 10 1.60 0.92 3.08 3.58 4.88 4.05 12 1.59 0.91 3.04 3.47 4.86 4.00 15 1.56 0.90 2.95 3.46 4.84 3.95 4.15 FIGURE 3. Rth vs. Torque (TO-220) 5.5 Mylar Without Grease 5.0 Impedance (C/W) 4.5 Mylar With Grease 4.0 Bergquist Bond Ply 100 3.5 Bergquist Sil_Pad A1500 3.0 2.5 2.0 Directly Onto Heat-Sink 1.5 With Grease 1.0 0.5 0 3 6 9 12 15 Torque (In lbs) For each of the assembly variations, a steady-state value of thermal resistance was obtained against known torque values from 1 in-lb to 15 in-lb. Comparing Interface Mediums The results are tabulated in Table 1. The corresponding chart in Figure 3 facilitates visual comparison. As Figure 3 shows, the part directly mounted on the heat sink with grease (heat sink compound) performs the best. The use of any medium to electrically isolate the part from the heat sink results in higher thermal resistance, however. Mylar without grease is the worst-case scenario with the thermal resistance value increasing to 4.8 °C/W. OBSERVATIONS Breaking Torque For The Assembly The Torque Spec For TO-220 – 15 in-lb The negative slopes of each curve indicate that an increase in the torque value does improve (decrease) the thermal resistance value. However, there is a point of diminishing return beyond 10 in-lb. The curve almost flattens around 15 in-lb. The Impact Of Torque On Thermal Resistance The increase in the mounting torque beyond 15 in-lb does not improve the thermal resistance value. Document Number: 72674 01-Dec-03 This value was obtained last with a destructive test. The screw was tightened with gradually increasing torque. The M3 screw broke at around 26 in-lb to 27 in-lb. CONCLUSION Typically 15 in-lb torque is adequate for fastening a TO-220 device on the heat sink and obtains the best (lowest) possible thermal resistance value. Use of a heat sink compound improves the thermal resistance by almost 0.6 _C/W, but electrical isolation between part tab and heat sink increases the thermal resistance of the interface by a factor of 3 to 6. www.vishay.com 3