the Engineers' Guide to VME, VPX & VXS 2013 - Subscribe

Tackling the Challenge of Building Smaller,
Lighter, and More Efficient Componentbased
Avionics with CFD
Component thermal issues have to be managed in order
to maintain high reliability in military aircraft and
avionics environments. The ratio and tradeoff of size,
weight, and power (SWaP) is a crucial system design
consideration in modern defense systems. As aircraft
are fitted with an increasing amount of electronics and
with more components, they become heavier—and
more weight can mean less time in the target zone. But
they also become hotter as size density increases. The
thermal behavior can also vary from component vendor
to vendor but also over the component aging process.
This is due to the degradation of the materials used in
the component and the influence of thermal management
over the lifetime of the component.
A good understanding of semiconductor components’
thermal behavior is important because it is crucial to
the optimum thermal design for a low SWaP ratio. Insufficient
understanding of a component can lead not only
to an oversized cooling system but also to a bad choice
of the selected components intended for the lifetime system use.
Thermal Characterization of Semiconductor Components
A good way to begin formal component characterization is to create
a “smart” implementation of the static test version of the Joint Electron
Devices Engineering Council (JEDEC) JESD51-1 electrical test
method [1] that allows for continuous measurement during a heating
or cooling transient.
The characterization method uses the temperature sensitivity of
the semiconductor component. This sensitivity has to be measured
before the actual characterization can begin, and it should be done
according to the JESD51-14 standard to record the cooling curve of
the component.
Once the measured temperature sensitivity parameters (TSP) are
obtained, the component can be characterized by powering up the
device (heating it) with PH [Watt] until a steady state is reached.
Once the junction temperature TJ is constant, the heating current
is switched off to a lower measuring current that creates a low
SPECIAL FEATURE
Computational Fluid Dynamics software helps defense systems designers doubly:
it aids in optimizing component SWaP, and models device thermal behavior across
time and vendors.
By Boris Marovic, Product Marketing Manager,
Mentor Graphics
Figure 1: Cooling curve of a sample component.
measuring power PM [Watt]. The measuring current is negligible
compared to the heating current. This sharp power step introduces
the cooling process and is recorded until a steady state is reached.
From the temperature sensitivity of the component and the lower
steady state temperature, ideally realized with a cold plate, the transient
cooling curve is created as shown in Figure 1. The temperature
difference ΔT (Kelvin) is derived by the temperature sensitivity of
the component; and the thermal resistance of the component can be
calculated as shown in the equation: Rth = ΔT/(PH ‒ PM).
From the recorded cooling curve, a structure function can be
derived as shown in Figure 2. This structure function shows thermal
resistance and capacitance of the single component layers from
junction to environment. The vertical sections of the curve show
[K/W] materials such as metallic layers in the component structure;
whereas horizontal lines show higher thermal resistance layers such
as die attach, glue, grease, and other thermal interface materials
(TIM), PCB layers, and so on.
www.eecatalog.com/vme 21