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Impact of Space Environmental Factors on Microtechnologies 73

mechanisms can include metal corrosion or the generation of subtle electrical
leakage currents, which disrupt the function of the device. The following factors
are responsible for internal moisture-related failures: moisture, a path for the
moisture to reach the active area, a contaminant, and for dendritic growth voltage.
Space grade microcircuits, in contrast to MEMS devices, are protected by glassivat-
ing the die and controlling the sealing environment to preclude moisture and other
contaminants. To be space qualified, a hermetic package requires a moisture
content of no greater than 5000 ppm (by volume). This must be verified by
performing an internal water vapor content check using residual gas analysis
(RGA) in accordance with 1018.2 of MIL-STD-883. All space-qualified hermetic
packages containing cavities receive a seal test to assure the integrity of the seal.
Some space flight components, such as the computer of the Delta launch vehicle,
are hermetically sealed assemblies. External to the parts, all assembled boards are
conformally coated to reduce the chance for moisture or impurities to gain access to
the leads, case, etc. Polymerics used in the conformal coating of assembled boards
for NASA projects must comply with NASA-STD-8739.1 (formerly NHB 5300.4
(3J)). NASA has found the need to restrict certain materials in parts used for space
flight. For instance, MIL-STD-975 prohibits the use of cadmium, zinc, and bright
tin plating.
For outgassing requirements, an informal, but accepted, test specification used
4
by all NASA centers is ASTM-E-595. This specification considers the effects of a
thermal vacuum environment on the materials. ASTM-E-595 does not set pass or
fail criteria but simply lists the test results in terms of total mass loss (TML) and
collected volatile condensable material (CVCM). The results are accumulated in the
materials listings: NASA Reference Publication 1124 and MSFC-HDBK-527. The
maximum acceptable TML and CVCM for general use are 1.0 and 0.10%, respect-
ively. Materials used in near proximity or enclosed hermetically with optical
components or surface sensors may require more stringent TML and CVCM
percentages (such as TML < 0.50% and CVCM < 0.05%). Outgassing is of
particular concern to EEE parts such as wire, cable, and connectors. Materials for
space electronics must be able to meet a unique set of requirements. These are:

. Stability under high vacuum and thermal vacuum conditions
. Stability to the radiation of space (stability in high AO and UV conditions
may also be required)
. Stability to sterilization conditions such as thermal radiation of outer space
and ethylene oxide exposure
. Low outgassing under thermal vacuum conditions, nontoxicity of out gassed
materials

4.2.4 ELECTRICAL STRESSES
Electrical stresses run the gamut from on-Earth damage as a result of electrostatic
discharges through on-orbit damage due to degradation through radiation effects.
Concerns for the prelaunch environment, launch, and postlaunch are addressed later

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