Empfasis - Naval use of Environmental Testing to ensure COTS Reliability

Military programs are using commercial off the shelf components (COTS) to reduce costs and take advantage of the latest innovations in commercial technology. The use of COTS by many defense programs has been occurring more frequently as the defense industry is forced to look for resources other than the dwindling number of defense-based component suppliers. COTS offer significant advantages in functionality, cost, and availability. The use of technologically advanced COTS components is only one part of a Navy strategy to reduce costs and increase functionality. The EMPF has also been utilizing COTS materials and processes to improve the performance of digital, power, and radio frequency (RF) Navy systems, bringing these systems to the same technology level as many advanced commercial devices. While COTS offer substantial benefits, many of these commercial systems were not designed for high-reliability Navy applications. Because of this, adequate reliability and durability must be assured when using COTS to meet the long lifetime and harsh environment requirements of Navy systems.

Test Planning
The good news for military programs using COTS systems is that the number of today’s commercial technologies that are able to perform well in harsh environments is increasing. This has produced many components for high-reliability system designs. However, these components must be thoroughly tested to the system’s specifications prior to final integration in the military system. These system specifications often include mechanical stress, thermal stress, corrosion resistance, and moisture permeability requirements.

The system specifications vary from program to program. The requirements for a high power radar system in a ship board application such as DDG 1000 are significantly different than that of a communications antenna on a multi-mission aircraft, although both systems share some of the same componentry. Many Navy programs perform reliability testing based on the operating environment and performance requirement of the application. This means that COTS technology that is to be integrated into Navy systems must undergo testing designed specifically for that system. To accomplish this, careful test plans must be developed.

The EMPF has been successfully developing and utilizing test plans for specific new technology insertion and sustainment projects. Test plan development requires reliability modeling that is based upon the actual operating conditions under which the COTS technology is required to perform. In the absence of specific operating specifications, generic assumptions can be made about the product lifetime, performance parameters, and operating environment. Military and select commercial industry testing guidelines can be used as a basis for planning environmental testing. Some of these specifications include:

MIL-STD 883
MIL-STD 202
MIL-STD 810
MIL-STD 167
IPC TM-650
IPC 6012
JEDEC specifications

Good qualification plans require the prediction of potential failure mechanisms. Once the potential failure mechanisms have been defined, the selection of the reliability tests will follow, based on the accelerating factors that trigger those failure mechanisms.

Corrosion Resistance
In implementing technologically advanced near-hermetic coatings for RF application in the DDG 1000 ship platform, a series of tests will be developed to ensure corrosion resistance and moisture permeability. The project utilizes COTS technology for wafer-level coating of electronic die. The wafer level coating solution that is being developed is based on commercially available coating materials and processes. This COTS coating technology offers a significant cost and weight advantage over the ceramic packaging of die. Although the solution has shown a high-level of reliability in many commercial applications, implementation into the DDG platform requires that the technology meet the specific requirements of the program. The test plan for the near hermetic coating includes temperature-humidity-bias (THB) and highly accelerated stress testing (HAST).

THB is designed to accelerate corrosion of the metallization on the die surface. When contaminants are present, the corrosion of the metallization occurs under high temperature and humid conditions. Bias is applied to the device during testing to accelerate the corrosion and provide a driving force corrosion migration. The bias applied will be similar to the bias placed on the device while operating onboard the ship. Testing will be paused at intermediate read points (typically after 8, 24, 48, 96, 168, and 500 hours of testing). The read points are necessary for making predictions about the reliability performance of the coatings.

HAST also tests the ability of the coating to protect against corrosion of the die. Like THB, HAST is performed under bias. HAST exposes the coated die to temperature, humidity, and bias while accelerating the penetration of moisture through the use of pressure. The pressurized HAST chamber also allows for increased temperature and humidity which further accelerates the corrosion failure mechanism.

Both THB and HAST are excellent tests for evaluating COTS technologies such as components, circuit boards, and coatings that must perform in high-humidity and corrosive environments. Rain, leak testing, and salt fog/spray exposure are other environmental tests used to evaluate the corrosion resistance of technologies used in high-reliability applications.

Thermal Requirements
Almost all military programs have a thermal requirement that the components of the system must meet. The most common thermal test is thermal cycling. Thermal cycling is designed to exploit the effects of coefficient of thermal expansion (CTE) mismatches between adjoining materials. The range of temperature changes to which the product will be exposed are accelerated in a thermal chamber. Thermal cycle test samples are exposed to alternating periods of high and low temperatures. The temperature extremes may vary but -55 to 125°C is a good starting point for high-reliability military applications. The ramp rate of the thermal profile may also vary but it is very common to use ramp rates between 4°C to 10°C per minute. Thermal shock is similar to thermal cycling but the ramp rate is increased. Generally hot and cold transitions that occur faster than 20°C per minute can be considered thermal shock. Thermal shock testing is much more strenuous than thermal cycling. Because of this, the total duration of the testing tends to be much shorter (typically, less than 500 cycles). The failure mechanism that occurs in thermal shock can be quite different from failures observed in thermal cycling. Because of this, thermal shock cannot be used as an accelerated thermal cycling test. They should be considered a separate test.

Thermal cycling is currently being used to qualify commercial technologies for packaging flip chips that will be used in DDG 1000 antenna systems. In this case, the commercial technology is not a component, but instead includes the attachment methods for connecting the chip to the substrate. The commercially available materials and attachment processes must meet the thermal requirements of greater than 30 years of active service. Thermal shock was used to measure the robustness of innovative solder attachments for the Monolithic Microwave Integrated Circuit (MMIC) program. The MMICs are to be used in F/A-18 radar systems. Thermally shocking the boards created greater stress than what was available through thermal cycling and provided much needed data about the robustness of the connections under extremely harsh conditions.

The High Temperature Operating Life (HTOL) and Low Temperature Operating Life (LTOL) can be performed to determine the reliability of devices under operation at high and low temperature conditions over an extended period of time. These tests consist of subjecting the parts to a specified bias or electrical stressing, for a specified amount of time, and at specified temperatures

Mechanical Stress
The mechanical stresses that a COTS component can incur during operation can include vibration, static load, and shock. Mechanical stress tests have a wide range of testing parameters and conditions. Components used in airborne Navy applications such as the Microwave Integrated Circuit (MMIC) flip chips developed by the EMPF and its industry partners for the F/A-18 have a strict vibration requirement. Components designated for internal shipboard applications such as the System on Chip (SOC) components developed for DDG 1000 may be exposed to long duration high static loads or mechanical shock. Technologies designated for munitions (Figure 5-1) require high-G load testing to verify reliability under the strenuous loads created during launch or in some cases survival upon impact. High-G load testers are well suited for simulating high-G loads for short durations.

The goals of high-G survivability (15,000gs) and reduced size packaging were accomplished in the Micro-electromechanical System (MEMS) based Inertial Measurement Units (IMU) project using COTS technology that had undergone high-G load mechanical testing.

There are many other environmental tests used throughout the Department of Defense to ensure that the emerging and existing commercial technologies are robust enough to survive the harsh environments and life expectancy of military applications, including tests that combine mechanical, chemical, and thermal test methods. Regardless of the environmental test selected, it is imperative that the test performed has a direct correlation to the operating environment in which the COTS technology is selected to perform. COTS components, materials, and processes allow for new technology to be implemented much faster. However, implementing this new technology should not result in significantly compromised reliability and performance. Properly designed reliability testing and qualification of COTS devices ensures that the new technology meets the requirements of high-reliability applications. More information regarding COTS implementation into various DoD programs is available on the EMPF website.