A Primer on VITA 47
February 18, 2009 | BY: Ivan StraznickyDownload PDF
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, certification of COTS plug-in units to this standard will facilitate the cost-effective integration of these items in larger systems.
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:
- -40°C to 85°C card edge operating temperature
- -55°C to 105°C non-operating temperature
- 500 cycles of temperature cycling between -55°C and 105°C
- 0.1 g2/Hz random vibration
- 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 (CBGA All Pb-free data), compared to the reliability of the same part with tin-lead balls soldered with tin-lead (ìCBGA All SnPb 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:
- Pad cratering under BGA after vibration due to excessive local strains
- Salt bridge after 500 hour salt fog exposure due to direct exposure of module
- Connector contact fretting corrosion due to excessive micromotion during vibration
- Fried 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 'rest of the iceberg' resides within companies that have focused on ruggedization and reliability for a long time. Sadly, as the saying goes, there is no free lunch.
Read the full story from VME and Critical Systems, February 2009 here