Vacuum-deposited film protects Space Station electronics

KANATA, Ontario - Neptec Design Group Ltd., has developed the Space Vision System (SVS) to help build the International Space Station. A thin, transparent layer of Parylene, a vacuum-deposited polymer film, protects critical electronic components of this new vision system.

By Mike Gauthier

Neptec manufacturing manager

KANATA, Ontario - Neptec Design Group Ltd., has developed the Space Vision System (SVS) to help build the International Space Station. A thin, transparent layer of Parylene, a vacuum-deposited polymer film, protects critical electronic components of this new vision system.

The SVS uses special electronics, software, and graphic displays to improve guidance information to operators, thereby enhancing control of the huge shuttle robot arm. The vision system tracks optical targets. This optical tracker will give operators a precise, moving graphic image of load position in all three dimensions.

A series of shuttle missions beginning this month will carry the components of the huge International Space Station into orbit, with completion anticipated by 2004, after 100 shuttle flights.

The new vision system will be part of each shuttle mission as the station is assembled, enabling astronauts to control the spacecraft`s robotic arm for docking, moving cargo, and assembling space station components. Neptec, with offices in Kanata, Ontario, and Houston, is supplying NASA with vision systems for ground training of astronauts at Houston`s Johnson Space Flight Center, and for use during construction of the space station.

Before engineers developed the SVS concept, astronauts relied on two-dimensional video images, and on limited views through small spacecraft windows to guide them in controlling robot functions.

Protective coating

An effective conformal coating was required to protect SVS circuit assemblies from moisture and other environmental threats. Coating protection is critical for ground-based SVS training systems as well as the shuttle-mounted systems. Parylene is Neptec`s preferred coating for aerospace electronics because of its superior gas and moisture barrier performance, very low outgassing, pinhole-free coverage, very low physical mass, and other important properties.

Parylene is a thin transparent polymer film that is applied to substrates in a vacuum deposition chamber by means of gas phase polymerization. The coating process is non-solvent, with no catalysts or solvents, and no liquid phase.

The resulting conformal film has useful dielectric and barrier properties per unit thickness, as well as extreme chemical inertness. Non-liquid Parylene neither pools, bridges, nor exhibits meniscus properties during application. The deposited film has minimal physical mass, so it does not produce mechanical forces on the underlying substrate during deposition or with subsequent thermal cycling, unlike conventional liquid coating materials with greater mass.

The Parylene raw material, di-para- xylylene dimer, a white crystalline powder, is first vaporized at approximately 150 degrees Celsius, then molecularly cleaved or pyrolyzed in a second process phase at 680 C to form the diradical, para-xylylene, and finally introduced as a monomeric gas that polymerizes on substrates in the vacuum chamber at room temperature.

Substrate temperatures remain near ambient during this sequence, and the coating grows as a conformal film (poly-para-xylylene) on all exposed surfaces, edges, and in crevices. During deposition, the active monomeric gas polymerizes spontaneously on the surface of coated objects at ambient temperature with no stresses induced initially or subsequently.

The Parylenes form at a vacuum of approximately 0.1 torr, and under these conditions the mean free path of the gas molecules in the deposition chamber is in the order of 0.1 centimeter. As a result the gaseous monomer uniformly impinges all sides of an object being coated, resulting in a high degree of conformity. Polymerization occurs in crevices, under devices, and on exposed planar and edges surfaces at the same rate.

The first event in the International Space Station project will be the delivery of the self-supporting control module by a Russian Proton rocket launched from the Baikonur Cosomodrome in Kazakstan. This orbiting vehicle will provide control capability and power for early stages of space station construction.

Mike Gauthier is vice president of manufacturing and product development of Neptec Design Group Ltd. in Kanata, Ontario. He has been with Neptec for 20 years, where his responsibilities have included packaging and hardware design. He has an associate degree in electrical engineering from Marion Technical College in Marion, Ohio.

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