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STEVAL-TDR029V1 Datasheet(PDF) 6 Page - STMicroelectronics |
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STEVAL-TDR029V1 Datasheet(HTML) 6 Page - STMicroelectronics |
6 / 10 page STAC2942B mounting recommendations STEVAL-TDR029V1 6/10 Doc ID 17470 Rev 1 4.3 Surface conditions of the mounting base The mounting base, or surface of the heatsink to which the backside of the package is mounted, must be flat, clean, and free of tool marks. ● Flatness: 400 µin/in (0.4 µm/mm) ● Roughness: Ra<30 µin (0.8 µm) Flatness and roughness values can be achieved on the above-mentioned materials with standard machining processes, such as milling. Holes for screws, solder wells, etc. should be slightly countersunk and/or de-burred. 4.4 Thermal interface material Careful consideration must be paid to the actual interface layer between the backside of the package and the mounting base. Cleanliness is key: particulates, grease, and oxidation should be removed using standard practices. A thermal interface material (TIM) of good thermal conductivity should be used in order to fill any discontinuities between the package and mounting base surface. The decision of which TIM material to use is based on many factors. Many types of TIMs are available today with an incredible range of thermal/electrical/mechanical properties. Several are listed here: ● WPSII (a silicone-free compound, Austerlitz Electronics), ● Type 340 (Dow Corning) ● Type 120 (a silicone compound, Wakefield Thermal Solutions) ● Indium foil (Indium Corp.) ● Sn63Pb37 or Pb-free solder In all cases, the chosen TIM should be no thicker than necessary and applied as evenly as possible. For liquid compounds, it is easiest to apply to the backside of the package, keeping in mind that excessive TIM thickness results in a higher thermal resistance than using no TIM at all. For foils and soldering it is best to specify an appropriately-sized “preform” and apply it to the thermal base. 4.5 Seating plane The seating plane refers to the distance between the thermal base and the bottom of the transistor leads. This is shown on the package outline drawing to indicate the total thickness of the PCB, or PCB+channel depth of the heatsink. Some points to consider are: ● The leads of the transistor should launch perpendicular to the package body onto the traces of the PCB to reduce mechanical stress to the package and solder joints. In some cases, the PCB fabrication tolerances are such that the leads extend above the plane defined by the top of the PCB; this situation is best handled with a judicious selection of solder pad preform or paste thickness, or by a slight downward bending of the transistor leads (no more than 0.01 in (0.25 mm). In no case should the leads be bent higher than the topside of the PCB, to avoid shear stresses which could cause damage to the interface between the lead and the package body. ● Care should be taken to factor in the thickness of the TIM used, which is equivalent to increasing the seating plane dimension. |
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