Features

The avionics systems of military aircraft support maneuverability and situation awareness for the pilot, process essential images, operate sensors, record data, and provide for broadcast and communication between home base and wingman. The newest generation of military aircraft cannot be flown without avionics, and modern combat is making the leap to electronic warfare.

Figure 1. Die-attach voids shown in the derivative of the structure function<sup>1</sup>.

New processors and other electronic components are smaller, faster and more powerful. But with these advances, power density increases, making thermal management essential to ensure the reliability of these components. An electronic failure in mid-air is simply not an option.

As more electronic components are incorporated into aircraft designs, they become heavier. More weight can mean less time in the target zone. Thus, the tradeoff of size, weight, and power (SWaP) is a crucial system design consideration.

Lifetime Testing and Failure Characterization

High reliability of safety-critical components is important to ensure perfect functioning of a system over its lifetime. A lifetime may be several thousand hours under constantly changing conditions such as temperature variations and shocks, pressure variations, humidity, etc., which increase the aging process resulting in component or material failures. Methods such as highly accelerated life testing (HALT), in which components are exposed to harsh environments worse than the actual environments to accelerate the aging process and pinpoint degradation, shortens the original testing time.

Figure 2. Test set with two failures show in the derivative of the structure function, a) die-attach delamination in the package, b) imperfect package soldering.

Thermal characterization is a nondestructive measurement and can reveal failures inside the component. If the die attach is degrading and the die is delaminating, for example, thermal resistance will rise, which increases the junction temperature of the component because the heat isn’t dissipating efficiently anymore. As a result, the component is likely to fail sooner than a healthy component because excessive temperature over a long time accelerates the aging process.

The following examples illustrate different failures in a component in general and during the aging of the component. The structure function clearly shows increased thermal resistance of the component for these failures (Figures 1 and 2).

Failure in the package structure and in the heat path outside the structure can be measured up to the environment. The rest of the structure function, depending on the magnitude of the failure, is often just shifted horizontally or vertically, which indicates that the rest of the heat path has kept its properties and the failure only exists in the responsible layer of the package or assembly.

When different LEDs were compared over a lifetime of up to 6,000 hours, a set of LEDs failed after around 3,000 hours compared to some LEDs that reached 6,000 hours without substantial degrading of the components2.