Environmental Stress Screening (ESS) is often considered an "add on" requirement introduced during production. As a result, design engineering often considers the temperature and vibration techniques of ESS to be environments that will "break" the hardware. Consideration of ESS during the development phase precludes any concerns that ESS techniques will overstress the design.
ESS limits may exceed the limits of the environmental envelope required for product performance. Qualification testing assesses design maturity and the ability of the hardware to function in its environmental envelope. In contrast, ESS stresses the hardware in a nondestructive manner to stimulate parts and workmanship defects in electronic assemblies. Therefore, the ESS temperature and vibration limits may be significantly different than the qualification test environments. Power should be applied to the hardware to the maximum extent practical. Power should not be applied when the hardware is exposed to temperature outside the hardware's operating limits.
A common misconception is that ESS is a rigid program with standardized technical requirements that must be executed only at the highest practical assembly level. Practice dictates a dynamic program that allows for intelligent tailoring of temperature limits, number of temperature cycles, temperature rate-of-change, etc., and the level of assembly where best applied in the manufacturing process.
Burn in testing and environmental stress screening are sometimes considered identical. Burn in tests require operating the hardware at elevated temperatures for an extended period of time (typically, over 100 hours). ESS uses a combination of temperature cycling and random vibration, and a reduced test duration. Burn in tests primarily precipitate parts of semiconductor defects; ESS techniques primarily precipitate assembly and workmanship defects such as poor soldering or weak wire bonds.
Temperature cycling and random vibration are the most efficient environmental stress screens. ESS requirements should be tailored during the design process, and any necessary tradeoffs between the most effective screening limits and ESS compatibility with the design should be completed during FSD. It is key to remember that manufacturing screening is not intended to damage the hardware; it is intended to stimulate parts and workmanship defects.
Effective use of ESS requires flexibility. ESS techniques at the highest practical level of assembly, using a combination of temperature cycling and random vibration, is an excellent starting point. The number of cycles depends on the complexity of the hardware. The temperature rate of change is another key parameter. A 10 to 15 C/ min temperature rate of change is most effective in precipitating assembly and workmanship defects. A random vibration regime that includes 6g rms at frequencies between 100 to 1000 Hz for a 10-minute total duration on three axes is recommended. Power should be applied to the hardware for maximum effectiveness. The combination of parameters, including their levels, chosen for ESS in a particular application should be proofed during development and adjusted as appropriate during production. Further technical guidance on using ESS techniques can be found in DoD 4245.7-M.
Depending on the nature of defects found, it may be cost-effective to introduce ESS techniques at other points in the manufacturing process (e.g., after the circuit card soldering is completed). One purpose of ESS at this point in the manufacturing process is to reduce expensive rework at the higher assembly levels.