ENVIRONMENTAL CONSIDERATIONS

1-42 ENVIRONMENTAL CONSIDERATIONS The environmental requirements of each system design are defined in the PROCUREMENT SPECIFICATION. Typical environmental requirements for an IC, for example, are shown in table 1-1. After these system requirements have been established, components, applications, and packaging forms are considered. This then leads to the most effective system form. Table 1-1.—Environmental Requirements Temperature Operating Nonoperating - 28º C to +65º C - 62º C to +75º C (MIL-E-16400E) Humidity 95 percent plus condensation (MIL-E-16400E) Shock 250 to 600 g (MIL-S-901C) Vibration 5 to 15 Hz, 0.060 DA 16 to 25 Hz, 0.040 DA 26 to 33 Hz, 0.020 DA Resonance test in three mutual perpendicular planes. (MIL-STD-167) RF Interference 30 Hz to 40 GHz In the example in table 1-1, the environmental requirements are set forth as MILITARY STANDARDS for performance. The actual standard for a particular factor is in parentheses. To meet each of these standards, the equipment or component must perform adequately within the test guidelines. For example, to pass the shock test, the component must withstand a shock of 250 to 600 _Gs (force of gravity). During vibration testing, the component must withstand vibrations of 5 to 15 cycles per second for 0.06 day, or about 1 1/2 hours; 16 to 25 cycles for 1 hour; and 26 to 33 cycles for 1/2 hour. Rf interference between 30 hertz and 40 gigahertz must not affect the performance of the component. Temperature and humidity factors are self-explanatory. When selecting the most useful packaging technique, the system designer must consider not only the environmental and electrical performance requirements of the system, but the maintainability aspects as well. The system design will, therefore, reflect performance requirements of maintenance and repair personnel. ELECTRICAL CONSIDERATIONS The electrical characteristics of a component can sometimes be adversely affected when it is placed in a given system. This effect can show up as signal distortion, an improper timing sequence, a frequency shift, or numerous other types of unwanted interactions. Techniques designed to minimize the effects of system packaging on component performance are incorporated into system design by planners. These techniques should not be altered during your maintenance. Several of the techniques used by planners are discussed in the following sections. Ground Planes and Shielding. At packaging levels I and II, COPPER PLANES with voids, where feed-through is required, can be placed anywhere within the multilayer board. These planes tend to minimize interference between circuits and from external sources. At other system levels, CROSS TALK (one signal interfering with another), rf generation within the system, and external interference are suppressed through the use of various techniques. These techniques