Design News, March 2013

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Mechatronics Reliability in a Clutch ...a clutch, brake or power transmission part for that matter. Since 1903 Carlyle Johnson has solved some of history's toughest motion control challenges – it's what we love to do. Our precision electrical, mechanical, air and hydraulic power transmission products consistently prove reliable and dependable in every application. Underwater, on the ground and in the air, CJM is everywhere. Figure 2: 2 oz features connect control circuits while 20 oz features carry high current loads. trace can safely carry, we must find a way to estimate the heat rise associated with the applied current. An ideal situation would be to reach a stable operating temperature where the rate of heating equals the rate of cooling. Fortunately, we have an IPC formula we can use to model this event. IPC-2221A, calculation for current capacity of an external track [1]: I = .048 * DT(.44) * (W * Th)(.725) Where I is current (amps), DT is temperature rise (°C), W is width of the trace (mil), and Th is thickness of the trace (mil). Internal traces should be derated by 50 percent (estimate) for the same degree of heating. Using the IPC formula we generated a chart, showing the current carrying capacity of several traces of differing cross-sectional areas with a 30C temperature rise (see it online at designnews. com). What constitutes an acceptable amount of heat rise will differ from project to project. Most circuit board dielectric materials can withstand temperatures of 100C above ambient, although this amount of temperature change would be unacceptable in most situations. Standard and Custom Clutch, Brake & Power Transmission Solutions 291 Boston Turnpike • Bolton, CT 06043 Phone: 860-643-1531 Circuit Board Strength and Survivability Circuit board manufacturers and designers can choose from a variety of dielectric materials, from standard FR4 (operating temperature 130C) to high-temperature polyimide (operating temperature 250C). A high-temperature or extreme environment situation may call for an exotic material, but if the circuit traces and plated vias are standard 1 oz/ft2, will they survive the extreme conditions? The circuit board industry has developed a test method for determining the thermal integrity of a finished circuit product. Thermal strains come from various board fabrication, assembly, and repair processes, where the differences between the coefficient of thermal expansion (CTE) of Cu and the PWB laminate provide the driving force for crack nucleation and growth to failure of the circuit. Thermal cycle testing (TCT) checks for an increase in resistance of a circuit as it undergoes air-to-air thermal cycling from 25C to 260C. Design News | march 2013 | www.d esign n –M6–

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