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An Engineer's Guide to High Reliability Components 10 Thermal expansion—the physical expansion of certain materials, often metals, at high temperatures—is one way heat decreases device lifetime. In our stack-up, high temperatures make the different layers in the device want to expand in size. Each layer offers a different thermal coefficient of expansion, causing uneven device expansion and generating stresses in the material. The higher the temperatures, the larger the stresses; the larger the stresses, the sooner the device will break. Further, the bond wires that interconnect components are susceptible to failures resulting from a combination of high power and high temperatures. Increased power demand can lead to cyclic power stresses detrimental to device lifetime. Consider driving a high-power motor constantly speeding up and slowing down. This process causes power pulses inside analog power electronic devices, and these pulses make the wire bonds inside the devices stretch and shrink continuously. Already mechanically weakened by the high temperatures, wire bonds can start to experience breaks and liftoffs where they connect to a die or substrate. So, the higher the temperature, the shorter the mean time to failure is for a device. A device failure can lead to a product failure, decreasing the reliability of the system as a whole. Apex's Thermal Mitigation Techniques To ensure the highest reliability possible in its analog power electronics, Apex Microtechnology has adopted several thermal management design techniques. One technique Apex leverages to mitigate thermal challenges is building its device stack-ups to allow for higher thermal conductivity. Apex uses a very rigid packaging style, which consists of thick base plates made from either cold-rolled steel or copper in some of its highest-power devices. These thick plates allow for higher thermal conductivity and a larger thermal mass, which effectively sinks heat away from the device components. The device dies are solder-attached to the substrate in a way that optimizes thermal contact, and the substrate is solder-attached to the base plate in the