Meeting stringent requirements for ruggedness is a top
concern for designers of military and critical system applications. Developers
need the ability to scale solutions and add features within embedded form
factors without dramatically affecting the energy variables, such as thermal
output. This is especially true in cases where space is at a premium, such as
unmanned vehicles and rugged mobile systems. These requirements must be
balanced with the availability of suitable embedded computing technologies. In
an ideal world, there would be a technology that could elevate nearly any
commercial design to MIL-spec compliance without the need for extensive base
product modification. However from a realistic perspective, this is virtually
an insurmountable and impractical feat. Fortunately, a new ruggedized coupling
agent is now available to thwart thermal challenges while increasing MTBF, and
overcome ruggedization hurdles to transform commercial-grade products into
products suitable for harsh-environment use.

Overcoming thermal challenges

The ability to cool rugged, high-performance systems in
smaller footprints has become increasingly difficult. This is especially true
in military and aerospace applications where extreme environments and long
program support and service life cycles, perhaps as long as 20 years, are a
must.

Accordingly, a new
ruggedized coupling agent methodology has emerged to help address some of these
thermal challenges. This concept is essentially an add-on milled heat drain (or
heat sink) that is added as an option for a standard commercial board at the
end of the manufacturing process. The boardís topside topology is
captured in 3D in the CAD/CAM environment by the manufacturer, to fit with the
components of the board. The resulting surface pattern is then reproduced, in
negative form, as a milled aluminum plate that becomes the board-level heat
sink. It is then machined from
aluminum 6082 in accordance with standard QQ-A-250/11 TEMP T6 and subjected to
surface treatment in accordance with standard MIL-C-5541. Next, the ruggedized
coupler agent is secured to the circuit board by screws using the existing VME
connector and front-panel attachment points. The plate/circuit board
assembly then fits into a standard 0.8. VME slot (Figure 1).

The most significant
breakthrough is that it provides heat conduction by facilitating heat
dissipation between all board
components and the board-level heat sink. To provide adequate heat
transfer, a thermal coupling agent (a type of thermal film) is applied to the
commercial-grade circuit board. The coupling agent is critical in ensuring the
maximum heat transfer with complete electrical isolation. In addition to
thermal and electrical resistance characteristics (see Table 1), the coupling
agent also offers resistance to aging, pliability, and elasticity along with
removal/reapplication capability.

Since the ruggedizer add-on also acts as a thermal equalizer,
it lowers operating temperatures of the hottest points on the board by routing
heat to cooler areas. In many cases, components that limit high-temperature
operation of the board are not equipped with individual heat sinks in the
original commercial-grade board design. As a consequence, the average
temperature of the board, and all its components, is lowered by 10 ∞C to 20 ∞C
(50 ∞F to 68 ∞F), depending on the components. Since, these components can now operate
at a cooler temperature, the long-term reliability of the board increases significantly.
For instance, the benefit of a 10 ∞C (50 ∞F) temperature reduction can increase
the MTBF by 20 to 30 percent on average.

Within the system itself, there are a number of cooling
methods – convection (moving air), conduction (using solid materials),
and liquid (circulating, spray, and immersion) – each with its own set of
pros and cons. The choice of method really depends on a number of specific
design factors such as altitude, G-forces, noise, accessibility, size, and
weight. Conduction-cooled boards have traditionally been employed in
applications where heat evacuation with an airflow is impractical. These
include situations where there is no air available such as space applications,
where the air is not efficient enough to carry off all the calories at the
right speed. This also includes environments where creating an airflow with
moving parts would affect system reliability. As a result, there is not a
single method of cooling that can meet the needs of all critical applications.
Some flexibility on the part of the overall system design is required.

In its standard convection-cooled environment, the ruggedized
coupler adapts to all standard VME racks with no modification and complies with
all the dimensional constraints of VME boards. The surface is optimized to
provide maximum heat dissipation in the circulating air. It can also adapt to
conduction cooling. However, because each conduction-cooled application has a
specific set of space and other constraints related to confined environments, a
more customized approach is required for those environments.

Overcoming ruggedization hurdles

The other major hurdle is ensuring the system can withstand
extremes in shock and vibration as defined in VITA 47 Class V2 and V3, which
provides very detailed specifications for shock and vibration parameters.  Historically, there have been a number
of ruggedization methods to make SBCs and their components more resistant to
shock and vibration in harsh environments: Some vendors have introduced
families of products to cover increased levels of ruggedization requirements. These
products are usually derived from a commercial design by screening the parts
used in manufacturing, or by doing a ìre-layoutî of the original product. In
both cases, improvements in ruggedness incur higher costs, along with lower
levels of performance and functionality. Typically this could mean that
operating frequencies could by derated by 30 to 50 percent, and some
functionalities such as add-in option cards might not be available.

The previously mentioned ruggedizer add-on board assembly
provides mechanical stiffening, without modifications to the board. The special
coupling agent that provides heat transfer also provides mechanical buffering
and prevents contact between the aluminum plate and the components. A minimum
clearance from the components is maintained, while ensuring a zero contact
risk, even in harsh operational conditions. By using this technique, the
ruggedized product can fit in the same VME slot count as the original
commercial-grade product. Adding a conformal coating to the circuit board
before the thermal coupling agent is applied can also provide
moisture/corrosion resistance. These conformal coatings are applied in
accordance with standard MIL-46058-72, and are UL approved.

Because the milled aluminum plate covers the entire top face
of the board, it protects all board components, acting as a mechanical shield
when manipulating the board during maintenance operations. The sandwich
structure of the board reduces the amplitude of vibration and its subsequent
impact, decreasing the risks of fatigue-induced physical failure. In
particular, the characteristics of the aluminum mass help eliminate or dampen
low-frequency resonances that can be frequently associated with failure. Shock
testing has revealed that, in most cases, the shock resistance of a board
equipped with this technology is double that of a nonruggedized, commercial-grade
version of the same product.

Overall, the performance benefits of the rugged add-on
coupling agent include improved thermal characteristics, resistance to shock
and vibration (VITA 47 Class V2 and V3), and improved overall Mean Time Between
Failure (MTBF). Depending on specific board characteristics, the gain in
operational ambient temperature limits (compared with the nonruggedized
commercial-grade board) can be as much as 20 ∞C (68 ∞F).

With the availability of the rugged add-on coupling agent,
commercial-grade, board-level products can be elevated to a variety of
ruggedization levels, as shown on Table 2.

A bright future for rugged designs

Today, innovative methods of ruggedizing components are
helping bring more cost-effective and military and rugged applications to
market faster. Even more promising, cost-effective ruggedization via add-on
options is enabling commercial-grade VME board products to be used in various
harsh environments without compromising performance or reliability.

Joe Eicher, director
of sales for military-aerospace- government, Eastern North American Region for
Kontron America, holds a Bachelor of Science in Electrical Engineering (cum
laude) from the University of Texas at Austin and a Masterís degree in Electrical
and Computer Engineering from the U.S. Air Force Institute of Technology (AFIT)
in Dayton, Ohio. Before working at Kontron, he held the positions of executive
director, VME products, and director, global field application engineering, for
Thalesí Computers division. He can be contacted at [email protected].

Kontron America

888-294-4558

www.kontron.com