What does it take for a plug-in module to be rugged enough
for harsh military environments? The answer is not simple since there is a wide
range of environmental levels and durations to meet depending on application
and program requirements. Consequently, a product designed for one environment
(such as shipboard) might not be rugged enough for another (such as combat
aircraft). In addition, military programs typically require high availability
over long program lifetimes, so high reliability is an integral element of
plug-in module ruggedness. To answer the original question, three VITA
standards factor into the equation: ANSI/VITA 46 (VPX), ANSI/VITA 47 (Environments, Design and Construction, Safety, and Quality
for Plug-In Units), and VITA 48 (VPX-REDI). However, we will focus primarily on VITA
47, and a specific example of temperature cycling will serve to demonstrate
what is required.
Standards for rugged applications
Several VITA standards and specifications are useful guides
for rugged plug-in module design, among them ANSI/VITA 46 and VITA 48 (see
sidebar), along with ANSI/VITA 47. The main purpose of ANSI/VITA 47 is to
specify environmental requirements for various classes of COTS plug-in modules.
The environmental classes range from benign to very harsh, and are intended to
represent commercial and military mobile applications. According to the
foreword in VITA 47, ìcertification of COTS plug-in units to this standard will
facilitate the cost-effective integration of these items in larger systems.î
Click here to read more about ANSI/VITA 46 and VITA 48
VITA 46 and 48 recommend VITA 47 implementation
Both the VITA 46 and 48 specifications recommend that
implementers consider meeting one or more of the environmental classes in
ANSI/VITA 47 (see again Table 1). These classes are segregated by cooling type,
for example, forced air cooling over components (EAC in VITA 47); forced air
cooled heat exchanger (EFC); conduction cooled (ECC); and liquid cooled (ELC).
Each cooling type has a range of levels for the following environments:
operating temperature, non-operating temperature, temperature cycling,
vibration, and mechanical shock. For example, the ECC4 class requires complying
with the following environments:
n -40
∞C to 85 ∞C card edge operating temperature
n -55
∞C to 105 ∞C nonoperating temperature
n 500
cycles of temperature cycling between -55 ∞C and 105 ∞C
n 0.1
g2/Hz random vibration
n 40
g, 11 ms mechanical shock
Meeting the VITA 47 standard
There is little guidance on how to actually meet the
ruggedization levels in ANSI/VITA 47, with the exceptions of testing criteria
and a section on construction requirements and recommendations. These, however,
are not sufficient to produce rugged plug-in modules. Module vendors need to
understand how to design their products to pass the ANSI/VITA 47 tests, or they
risk facing a long and arduous test-fix-test process, which, even if completed,
might not result in acceptable reliability due to limited test samples.
The example of temperature cycling
To understand the implications of ANSI/VITA 47 testing,
consider the example of temperature cycling. The ANSI/VITA 47 standard requires
a module to be exposed to 500 cycles of one of four temperature ranges (from
-40/85 ∞C to -55/105 ∞C), with no performance degradation
afterwards. Assuming a 10 ∞C/minute ramp rate and 25 minute total dwell times
(10 minute temperature stabilization plus 15 minute dwell), the test duration
can be as long as 28 days. It behooves the module vendor to maximize the
chances of a successful test the first time around; otherwise, retests might be
required, causing significant schedule delays.
For a rugged module design to be capable of successfully
passing the ANSI/VITA 47 temperature cycling test, the designer must evaluate
and mitigate the risks posed by the various associated failure modes (such as
cracked solder joints, PWB barrel cracks, and so forth). Some of this
preparation can be achieved through analyses like solder joint reliability
analysis or finite element modeling/analysis. Specialized testing is also
highly recommended to validate the analyses. For example, Curtiss-Wright
Controls Embedded Computing performs extensive reliability testing on numerous
samples to ensure that risky interconnects such as BGA/CSP solder joints and
blind or buried PWB vias can survive harsh temperature cycling.
This kind of testing will become increasingly critical as
lead-free parts become the norm and rugged module suppliers are induced to use
them, and eventually to solder them with lead-free solder, either due to supply
market forces or program requirements. Figure 1 shows how solder joint
reliability is degraded when soldering a lead-free part with lead-free solder
(ìCBGA All Pb-freeî data), compared to the reliability of the same part with
tin-lead balls soldered with tin-lead (ìCBGA All SnPbî data).
]
Other ANSI/VITA 47 environments
Other environmental tests in ANSI/VITA 47 include operating
and non-operating temperature, mechanical shock, random vibration, humidity,
corrosion resistance, and Electrostatic Discharge (ESD) for LRMs. Similar to
temperature cycling, analysis can be used to discover and mitigate risks posed
by some of these environments (for example, vibration); however, others will
require the benefit of previous testing experience to design in sufficient
ruggedness. Testing is also used to validate the analysis results. This testing
experience will produce the valuable, hard-won knowledge of what works and what
does not for surviving harsh environments.
Figure 2 shows some examples of what has not worked. From top
left, clockwise, the failure analysis photos show:
n Pad
cratering under BGA after vibration due to excessive local strains
n Salt
bridge after 500 hour salt fog exposure due to direct exposure of module
n Connector
contact fretting corrosion due to excessive micromotion during vibration
n ìFriedî
processor due to insufficient cooling during high-temperature testing
The discovery and resolution of failures such as these
invariably leads to improved ruggedness. As more experience is gained, fewer
and fewer failures are likely to occur, and customers will gain increased
confidence in the module supplierís ruggedization capabilities. Of course, the
previously mentioned analyses are also critical in gaining this confidence.
VITA specifications and beyond
The process of designing a plug-in module to survive harsh
military environments, also known as ruggedization, requires substantial
knowledge of and experience with the particular environments of concern and how
they might cause electronics to fail. A small portion of this know-how is
contained in key standards and specifications such as ANSI/VITA 47, and also
ANSI VITA 46 and VITA 48; however, the ìrest of the icebergî resides within
companies that have focused on ruggedization and reliability for a long time.
Sadly, as the saying goes, there is no free lunch.
ANSI/VITA 46 and VITA 48 pave the way
The VITA 48 (VPX-REDI) family of specifications is a close
mechanical complement to the increasingly popular VITA 46 (VPX) specifications.
VPX and VPX-REDI both contain many features applicable to rugged plug-in module
design, with conduction-cooled packaging typically ranking among the most
important. Conduction-cooled modules are frequently used in rugged military
applications because of their high shock and vibration resistance, good cooling
performance, and ability to be housed in a sealed chassis for protection
against harmful contaminants.
Additionally, the VITA 46 standard provides guidance on
conduction features for 3U and 6U cards that stem from the IEEE 1101.2 standard
(Mechanical Core Specifications for Conduction Cooled Eurocards). This makes
VPX modules compatible with chassis designed for either conduction VME or
CompactPCI cards, for example, by changing the backplane to one populated with
VPX connectors. VITA 48 takes the conduction concept further by providing
increased functional density (via 0.85" and 1" pitches), improved
cooling, and two-level maintenance compatibility (with Line Replaceable Modules or LRMs).
Note that both VITA 46 and 48 also standardize air-cooled module features, and
VITA 48 is standardizing Air Flow Through (AFT) and Liquid Flow Through (LFT)
for very high-power cooling.
Another critical rugged feature of the VITA 46 and 48
specifications is the high-speed connector they specify. This connector was
originally chosen for its high-speed performance, but needed to be tested to
determine its ruggedness. An ensuing extensive testing program proved that it
was more than up to the task. Table 1 summarizes the test results with details
available at www.vita.com/VITA46ContechTestReportrev1.4.pdf. It is worthwhile
to note that the levels and durations of the various environmental tests met or
exceeded the requirements of the highest levels in ANSI/VITA 47, ensuring that
the connector was capable of operating in harsh environments.
Ivan Straznicky
is a principal mechanical engineer for Curtiss-Wright Controls Embedded
Computing, where his responsibilities include advanced thermal and packaging
technologies. Ivan is currently the vice chair of the VITA Standards
Organization and a key contributor to the following standards/specifications:
ANSI/VITA 46, ANSI/VITA 47, VITA 48, and VITA 42. He has a degree in Mechanical
Engineering from McGill University in Montreal, Canada. He can be contacted at
[email protected]
Curtiss-Wright
Controls Embedded Computing
613-599-9199
www.cwcembedded.com