Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1999-12-07
2001-07-17
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S330000, C428S423100
Reexamination Certificate
active
06261680
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electronic assemblies, and, more particularly, to the dissipation of accumulated charges and the protection of such electronic assemblies.
In one common architecture, electronic devices are assembled onto substrates, forming electronic subsystems. The subsystems are assembled into a chassis or other housing which, with appropriate interconnections and external connections, forms the electronic system. A well-known version of this architecture is the printed wiring board (PWB) or printed circuit board (PCB) widely used in consumer electronics.
PWBs are sometimes used in applications which may be subjected to adverse environments, such as in military or space applications. It is known to protect the PWB, including the circuitry and electronic components thereon, with a conformal coating. This conformal coating seals and protects the PWB circuit against adverse external influences such as foreign objects, moisture, corrosion, and solvents. The conformal coating aids in the mechanical support of the electronic components and helps protect the PWB circuitry against thermal shock.
The conformal coating also electrically insulates the PWB. In many applications, such electrical insulation of the PWB is desirable in that it reduces the risk of electrical shorting by foreign objects which may contact the PWB. In others, it may be undesirable because it results in charge accumulation which, in turn, can cause arcing or other electrical damage to the components on the PWB. In space applications, for example, the PWB is subjected to a flux of electrons during service. The electrons remain generally stationary on the PWB, resulting in charge accumulation. The PWB is therefore typically electrically grounded in such applications, although the grounding may not be fully effective because there still may be harmful charge accumulations in some regions of the PWB.
Two types of conformal coatings are in widespread use today. One is the standard MIL-I-46058 insulating coating, which provides for no dissipation of the accumulated charge. The other is a two-layer coating in which one layer is sufficiently electrically conductive to dissipate accumulated static charge. The two-layer coating, while operable to some degree, suffers from the shortcomings that it must be applied very carefully to avoid inhomogeneities that interrupt the charge dissipation, and it may also act as a capacitor to interfere with the operation of the electronic components.
There is a need for an improved conformal coating for electronic devices and substrates. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
This invention provides a single-layer conformal coating for use on electronic assemblies such as printed wiring boards. The single-layer conformal coating protects the electronic assemblies from damage by foreign objects, moisture, corrosion, and solvents, aids in the mechanical support of the electronic components, and helps protect the PWB circuitry against thermal shock. The single-layer conformal coating has sufficient electrical conductivity to dissipate electrical charges. The single-layer conformal coating of the invention has also been found to protect the coated electronic assembly from radiation damage, while existing coatings provide little such protection. The conformal coating may be applied by straightforward techniques such as spraying, wiping, dipping, or painting. Because there is only a single layer, there is no concern with the relative thickness inhomogeneities that may affect the performance of two-layer coatings. Another important advantage is that the conformal coating is sufficiently transparent that the underlying circuitry and components of the electronic assembly may be visually inspected through the conformal coating, due to the low hiding power of the conformal coating. By comparison, the coating according to MIL-I-46058 is transparent, but the conventional two-layer coating is opaque.
In accordance with the invention, a coated electronic assembly comprises an electronic assembly, and a single-layer conformal coating overlying at least a portion of the electronic assembly. The electronic assembly may be of any type, such as, for example a PWB, a chip-on-board structure, or a ceramic-substrate device. The coating has a volume resistivity of from about 10
8
to about 10
14
ohm-centimeter, more preferably from about 10
10
to about 10
12
ohm-centimeter, and comprises an organic polymeric binder and a plurality of electrically semiconductive filler particles dispersed in the binder. The filler particles are preferably metal oxide particles. The single-layer conformal coating has substantially the same mixture composition of binder and particles throughout its volume. In one embodiment, the binder and the filler particles have similar visible-light refractive indices, so that the conformal coating is sufficiently transparent to permit inspection of the underlying electronic assembly.
In a preferred version of the conformal coating, the organic polymeric binder is a cured urethane (polyurethane). Such urethanes may be selected for curing by heat or by ultraviolet light. The filler particles, which desirably have a band gap of about 50 meV, are preferably (Al,Mg,Zn)
3
O
4
or ZnO, doped with a cationic or anionic dopant. The filler particles and binder are preferably, but not necessarily, present in a weight ratio of from about 1:1 to about 5:1. For the case where the UV-curable polymeric binder is used, the particles are selected to be transparent to UV energy of the curing wavelength.
By selecting the binder and the particles so that their indices of refraction to visible light are about the same, the conformal coating may be made sufficiently transparent so that the underlying structure may be inspected through the conformal coating. In the preferred case, the index of refraction of the binder is from about 1.48 to about 1.50, and the index of refraction of the particles is from about 1.8 to about 2.0. This coating is translucent and light yellow to tan in color, but is still sufficiently transparent to allow inspection of the underlying structure. To achieve sufficient transparency, the indices of refraction for the particles and the binder would be selected such that they are different by no more than about 0.5. To achieve full transparency, the indices of refraction for the particles and the binder would be selected such that they are different by no more than about 0.05.
The conformal coating may be applied by any convenient technique such as painting, spraying, wiping or dipping. The conformal coating, after application and curing, typically has a thickness of from about 0.002 inch to about 0.020 inch. A carrier fluid such as a ketone, an acetate, or a naphtha may be provided in the uncured conformal coating to modify its viscosity and thus aid in its application. The conformal coating may be applied in one or several coats. The application of multiple coats of the single-layer conformal coating composition is distinct from prior conformal coatings in which two different layers of different compositions are applied.
The conformal coating of the invention protects the electronic assembly against damage by foreign objects, moisture, corrosion, and solvents. It also shields the electronic assembly against radiation damage. The conformal coating dissipates electrical charge, protecting the electronic assembly against arcing. However, the electrical resistance of the conformal coating remains sufficiently high that it protects against shorting and other electrical damage. The conformal coating is sufficiently transparent to permit inspection of the underlying circuitry after curing of the coating. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the inv
Gudmestad T.
Hughes Electronics Corporation
Nakarani D. S.
Paulraj Christopher
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