Public Lessons Learned Entry: 0643

Environmental stresses affect parts in different ways. Table 3 illustrates the principal effects of typical environments on system parts and materials.

High temperatures impose a severe stress on most electronic items, since it can cause catastrophic failure (such as melting of solder joints and burnout of solid-state devices). High temperature also causes progressive deterioration of reliability due primarily to chemical degradation effects. It is often stated that excessive temperature is the primary cause of poor reliability in electronic equipment.

In electronic systems design, great emphasis is placed on small size and high part densities. This generally requires a cooling system to provide a path of low thermal resistance from heat-producing elements to an ultimate heat sink of reasonably low temperature.

Solid-state parts are rated in terms of maximum junction temperatures. The thermal resistance is usually specified from this point to either case or to free air. Specification of the maximum ambient temperature for which a part is suitable generally is not a sufficient method for part selection, since the surface temperature of a particular part can be greatly influenced by heat radiation or heat conduction effects from nearby parts. These effects can lead to overheating, even though an ambient temperature rating appears not to be exceeded. It is preferable to specify thermal environment ratings such as equipment surface temperatures, thermal resistance paths associated with conduction, convection, and radiation effects, and cooling provisions such as air temperature, pressure, and velocity. In this manner, the true thermal state of the internal components of temperature-sensitive components can be determined. Reliability improvement techniques for high temperature stress include the use of heat dissipation devices, cooling systems, thermal insulation, and heat-withstanding materials.

Low temperatures experienced by electronic equipment can cause reliability problems. These problems usually are associated with mechanical system elements. They include mechanical stresses produced by differences in the coefficients of expansion (contraction) of metallic and nonmetallic materials, embrittlement of nonmetallic components, mechanical forces caused by freezing of entrapped moisture, stiffening of liquid constituents, etc. Typical examples include cracking of seams, binding of mechanical linkages, and excessive viscosity of lubricants. Reliability improvement techniques for low temperature stress include the use of heating devices, thermal insulation, and cold-withstanding materials.

Additional stresses are produced when electronic equipment is exposed to sudden changes of temperature or rapidly changing thermal cycling conditions. These conditions generate large internal mechanical stresses in structural elements, particularly when dissimilar materials are involved. Effects of thermal shock-induced stresses include cracking of seams, delamination, loss of hermeticity, leakage of fill gases, separation of encapsulating materials from components and enclosure surface leading to the creation of voids, and distortion of support members.

A thermal shock test may be specified to determine the integrity of solder joints since such a test creates large internal forces due to differential expansion effects. Such a test also has been found to be instrumental in creating segregation effects in solder alloys leading to the formation of lead-rich zones, which are susceptible to cracking effects.

Electronic equipment often is subjected to environmental shock and vibration during both normal use and testing. Such environments can cause physical damage to parts and structural members when deflections produced cause mechanical stresses which exceed the allowable working stress of the constituent parts.

Natural frequencies of items comprising the equipment are important parameters that must be considered in the design process since a resonant condition can be produced if a natural frequency is within the vibration frequency range. The resonance condition will greatly amplify subsystem deflection and may increase stresses beyond the safe limit.

The vibration environment can be particularly severe for electrical connectors, since it may cause relative motion between members of the connector. In combination with other environmental stresses, this motion can produce fret corrosion. This generates wear debris and causes large variation in contact resistance. Reliability improvement techniques for vibrational stress include the use of stiffening, control of resonance, and reduced freedom of movement.

Humidity and salt air environments can cause degradation of equipment performance since they promote corrosion effects in metallic components. They also can foster the creation of galvanic cells, particularly when dissimilar metals are in contact. Another deleterious effect of humidity and salt air atmosphere is the formation of surface films on nonmetallic parts. These films cause leakage paths and degrade the insulation and dielectric properties of these materials. Moisture absorption by insulating materials also can cause a significant increase in volume conductivity and the dissipation factor of these materials. Reliability improvement techniques for humidity and salt environments include use of hermetic sealing, moisture-resistant material, dehumidifiers, protective coatings/covers, and reduced use of dissimilar metals.

Electromagnetic and nuclear radiation can disrupt performance levels and, in some cases, cause permanent damage to exposed equipment. Therefore, it is important that such effects be considered in determining the environmental strength for electronic equipment that must achieve a specified reliability goal.

Electromagnetic radiation often produces interference and noise effects within electronic circuity, which can impair system performance. Sources of these effects include corona or lightning discharges, sparking, and arcing phenomena. These may be associated with high voltage transmission lines, ignition systems, brush type motors, and even the equipment itself. Generally, the reduction of interference effects requires incorporating filtering and shielding features or specifying less susceptible components and circuity.

Nuclear radiation can cause permanent damage by alteration of the atomic or molecular structure of dielectric and semiconductor materials. High energy radiation also can cause ionization effects that degrade the insulation levels of dielectric materials. The migration of nuclear radiation effects typically involves materials and parts possessing a higher degree of radiation resistance, and the incorporation of shielding and hardening techniques.

Each environmental factor experienced by an item during its life cycle requires consideration in the design process. This ensures that adequate environmental strength is incorporated into the design for reliability.

References:

• Government
  1. MIL-HDBK-217E Notice 1, "Reliability Prediction of Electronic Equipment," January 1990.
  2. MIL-HDBK-251, "Reliability/Design Thermal Applications," January 1978.
  3. MIL-HDBK-338-1A, "Electronic Reliability Design Handbook," October 1988.
  4. MIL-STD-810E, "Environmental Test Methods and Engineering Guidelines," July 1989.
• 
  
• Industry
  5. EID-00866, Rocketdyne Division, Rockwell International, "Space Station Freedom Electric Power System Reliability and Maintainability Guidelines Document," 1990.
  6. SAE G-11, Society of Automotive Engineers, Reliability, Maintainability, and Supportability Guidebook, 1990.

Lesson(s) Learned:

Failure to perform a detailed life cycle environment profile can lead to overlooking environmental factors whose effect is critical to equipment reliability. If these factors are not included in the environmental design criteria and test program, environment-induced failures may occur during space flight operations.

Recommendation(s):

At the onset of the design process, identify the operating conditions that will be encountered during the life of the equipment.

Evidence of Recurrence Control Effectiveness:

Programs That Certified Usage:

Space Electronic Rocket Test (SERT) I and II, Communication Technology Satellite (CTS), ACTS, Space Experiments, Launch Vehicles, Space Power Systems, and Space Station Freedom.

Documents Related to Lesson:

Mission Directorate(s):

• Exploration Systems
• Science
• Space Operations
• Aeronautics Research

Additional Key Phrase(s):

• Computers
• Environment
• Flight Equipment
• Ground Equipment
• Hardware
• Launch Process
• Launch Vehicle
• Spacecraft

Additional Info: