VISHAY SILICONIX www.vishay.com MOSFETs Application Note AN808 Mounting LITTLE FOOT®‚ SO-8 Power MOSFETs By Wharton McDaniel Surface-mounted LITTLE FOOT® power MOSFETs use integrated circuit and small-signal packages which have been modified to provide the heat transfer capabilities required by power devices. Leadframe materials and design, molding compounds, and die attach materials have been changed, while the footprint of the packages remains the same. See Application Note 826, Recommended Minimum Pad Patterns with Outline Drawing Access for Vishay Siliconix MOSFETs, (www.vishay.com/doc?72286), for the basis of the pad design for a LITTLE FOOT SO-8 power MOSFET. In converting this recommended minimum pad to the pad set for a power MOSFET, designers must make two connections: an electrical connection and a thermal connection, to draw heat away from the package. In the case of the SO-8 package, the thermal connections are very simple. Pins 5, 6, 7, and 8 are the drain of the MOSFET for a single MOSFET package and are connected together. In a dual package, pins 5 and 6 are one drain, and pins 7 and 8 are the other drain. For a small-signal device or integrated circuit, typical connections would be made with traces that are 0.020 inches wide. Since the drain pins serve the additional function of providing the thermal connection to the package, this level of connection is inadequate. The total cross section of the copper may be adequate to carry the current required for the application, but it presents a large thermal impedance. Also, heat spreads in a circular fashion from the heat source. In this case the drain pins are the heat sources when looking at heat spread on the PC board. 0.050 1.27 0.196 5.0 0.027 0.69 0.078 1.98 0.050 1.27 0.088 2.25 0.088 2.25 0.027 0.69 0.078 1.98 0.2 5.07 Fig. 2 - Dual MOSFET SO-8 Pad Pattern with Copper Spreading The minimum recommended pad patterns for the single-MOSFET SO-8 with copper spreading (Fig. 1) and dual-MOSFET SO-8 with copper spreading (Fig. 2) show the starting point for utilizing the board area available for the heat-spreading copper. To create this pattern, a plane of copper overlies the drain pins. The copper plane connects the drain pins electrically, but more importantly provides planar copper to draw heat from the drain leads and start the process of spreading the heat so it can be dissipated into the ambient air. These patterns use all the available area underneath the body for this purpose. Since surface-mounted packages are small, and reflow soldering is the most common way in which these are affixed to the PC board, “thermal” connections from the planar copper to the pads have not been used. Even if additional planar copper area is used, there should be no problems in the soldering process. The actual solder connections are defined by the solder mask openings. By combining the basic footprint with the copper plane on the drain pins, the solder mask generation occurs automatically. A final item to keep in mind is the width of the power traces. The absolute minimum power trace width must be determined by the amount of current it has to carry. For thermal reasons, this minimum width should be at least 0.020 inches. The use of wide traces connected to the drain plane provides a low impedance path for heat to move away from the device. 0.2 5.07 Fig. 1 - Single MOSFET SO-8 Pad Pattern with Copper Spreading Revision: 02-May-16 Document Number: 70740 1 For technical questions, contact: [email protected] THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE 0.288 7.3 0.288 7.3