Patent Description:
<CIT> relates to a quick-drying, transparent, conductive coating composition comprising a conductive polymer, a non-aqueous acrylic binder polymer, and an ether- or alcohol-based solvent.

<CIT> relates to a conformally coated structure comprising a microelectronic device and a conformal coating comprising a base polymer and an electrically conductive polymer.

Described herein are static dissipating coatings and thermally-stable static-controlled (TSSC) electronic circuits, comprising such coatings. Also described herein are methods of forming such coatings and circuits. The static dissipating coating comprises a conductive polymer and a thermally-stable base polymer. The conductive polymer comprises polyaniline and, in some examples, a conductive agent, such as dinonylnaphthalene sulfonic acid (DNNSA), dodecyl benzene sulfonic acid (DBSA), and/or camphor sulfonic acid (CSA). The thermally-stable base polymer comprises one or more copolymers of butyl-methacrylate, such as poly-butylmethacrylate-co-methyl methacrylate (PBM). The amount of the conductive polymer is specifically controlled to ensure the coating's overall conductivity and thermal stability. In some examples, the conductive polymer concentration is at or less than <NUM>% by weight. The conductivity of the coating is preferably between <NUM>-<NUM> S/cm and <NUM>-<NUM> S/cm, even after being at a temperature of <NUM> for up to <NUM> hours.

Provided is a thermally-stable static-controlled electronic circuit comprising a base structure, electronic components, disposed on and supported by the base structure, and a static dissipating coating, conformally covering the base structure and each of the electronic components. The static dissipating coating comprises a conductive polymer and a thermally-stable base polymer. The conductive polymer comprises polyaniline. The thermally-stable base polymer comprises one or more copolymers of butyl-methacrylate.

Also provided is a static dissipating coating suitable for conformal coating over electronic components. The static dissipating coating comprises a conductive polymer, comprising polyaniline, and a thermally-stable base polymer, comprising one or more copolymers of butyl-methacrylate.

Also provided is a method of forming a thermally-stable static-controlled electronic circuit comprises providing a conductive polymer and a thermally-stable base polymer. The conductive polymer comprises one or more polyanilines. The thermally-stable base polymer comprises one or more copolymers of butyl-methacrylate. The method further comprises forming a static dissipating ink using a solvent, the conductive polymer, and the thermally-stable base polymer. The method then proceeds with forming a static dissipating coating using the static dissipating ink. The static dissipating coating is formed over a base structure with electronic components disposed on and supported by the base structure, such that the static dissipating coating conformally covers the base structure and each of the electronic components and provides static dissipation to the electronic components while preventing electrical shorts among the electronic components.

In the following description, numerous specific details are outlined to provide a thorough understanding of the presented concepts. In some examples, the presented concepts are practiced without some or all of these specific details. In other examples, well-known process operations have not been described in detail to unnecessarily obscure the described concepts. While some concepts will be described in conjunction with the specific examples, it will be understood that these examples are not intended to be limiting.

As noted above, static charge dissipation is desired for many circuit types and applications, such as aircraft and spacecraft applications. While a conductive coating can provide this static control function, many conventional coatings lack uniform conductivity (e.g., filler based coatings) or are not thermally stable (e.g., polyurethane-based coating). Both characteristics are desired. For example, the uniform conductivity ensures that the static charge dissipation happens in substantially all portions of the coating (e.g., all portions have a sufficiently high conductivity) without causing any electrical shorts among electronic components (e.g., by coating portions with excessive conductivities). For purposes of this disclosure, the uniform conductivity is defined as a conductivity that varies less than <NUM> times or less than <NUM> times through the entire volume of the coating. Furthermore, the thermal stability of static dissipating coatings is particularly desired in dense electronic circuits (e.g., with significant thermal output) and/or applications with temperature fluctuations (e.g., aircraft and spacecraft). For purposes of this disclosure, the thermal stability is defined as a conductivity change of less than <NUM> times or less than <NUM> times when at <NUM> for at least <NUM> hours.

Described herein are static dissipating coatings and thermally-stable static-controlled (TSSC) electronic circuits, comprising such coatings. These static dissipating coatings are configured to be thermally stable and maintain their resistance within an operating range while being exposed to elevated temperatures. This performance is not possible with conventional polyurethane-based coatings. The thermal stability of the described coatings is achieved by a specific combination of a conductive polymer and a thermally-stable base polymer. The thermally-stable base polymer comprises one or more copolymers of butyl-methacrylate, such as poly-butylmethacrylate-co-methyl methacrylate (PBM). PBM has high thermal stability (up to <NUM>) and is soluble in a range of aromatic solvents (e.g., up to <NUM>% by weight in cymene). In some examples, the thermally-stable base polymer has a solubility of between about <NUM>% and <NUM>% by weight, between about <NUM>% and <NUM>% by weight, and between about <NUM>% and <NUM>% by weight in one or more solvents used to form a static dissipating ink. For purposes of this disclosure, solubility is defined as a lack of particle precipitation in a solution at a stated concentration. PBM's thermal stability allows increasing the thermal stability of the static dissipating coating. At the same time, PBM's solubility allows integrating PBM with other polymers, such as polyaniline, to achieve the desired conductivity of the static dissipating coating. A combination of the desired conductivity and thermal stability is achieved by a specific formulation of the static dissipating coating, e.g., specific weight ratios of the conductive polymer and the thermally-stable base polymer. Furthermore, the performance uniformity of the static dissipating coating is achieved by specific methods of forming this coating, e.g., first forming a solution of the conductive polymer and the thermally-stable base polymer and then casting or otherwise depositing this solution onto the base structure and electronic components of the TSSC electronic circuit.

<FIG> is a schematic illustration of TSSC electronic circuit <NUM>, in accordance with some examples. TSSC electronic circuit <NUM> can be used for various applications, such as aircraft and spacecraft. For example, spacecraft applications present unique challenges due to the lack of atmosphere around TSSC electronic circuit <NUM>. Without proper static measures, a static charge can accumulate and cause static charge dissipations, which are potentially damaging to various circuit components. On aircraft, precipitation static (P-stat) can cause charge build-up on the aircraft surfaces during flight, which can also lead to static build-up on electronic components.

TSSC electronic circuit <NUM> comprises base structure <NUM>, electronic components <NUM>, and static dissipating coating <NUM>. Static dissipating coating <NUM> conformally covers base structure <NUM> and each of electronic components <NUM>. Furthermore, static dissipating coating <NUM> provides static dissipation to electronic components <NUM> over a wide temperature range, for example, temperatures of from about -<NUM> to about +<NUM> or, more specifically, from about -<NUM> to about +<NUM>. At the same time, static dissipating coating <NUM> prevents electrical shorts between electronic components <NUM>. These characteristics are achieved by maintaining the resistance within a set operating range and the resistance being uniform throughout static dissipating coating <NUM> as further described below.

Some examples of base structure <NUM> include, but are not limited to, a printed circuit board and a flexible circuit. Electronic components <NUM> are disposed on and supported by base structure <NUM>. For example, electronic components <NUM> are surface mounted to base structure <NUM> using, e.g., an adhesive. Electronic components <NUM> are interconnected using, e.g., conductive leads of base structure <NUM>. Some examples of electronic components <NUM> include, but are not limited to, resistors, capacitors, power sources, memory components, and the like.

In some examples, TSSC electronic circuit <NUM> further comprises ground contact <NUM>, disposed on and supported by base structure <NUM>. Ground contact <NUM> is electrically insulated from electronic components <NUM>. Static dissipating coating <NUM> conformally covers ground contact <NUM> and provides static dissipation from each of electronic components <NUM> to ground contact <NUM>. During the operation of TSSC electronic circuit <NUM>, ground contact <NUM> is connected to a common ground, such as an aircraft fuselage.

Referring to <FIG>, static dissipating coating <NUM> comprises conductive polymer <NUM> and thermally-stable base polymer <NUM>. The concentration of conductive polymer <NUM> in static dissipating coating <NUM> is preferably a non-zero percentage of at or less than <NUM>% by weight or, more preferably, less than <NUM>% by weight or even less than <NUM>% by weight. The rest of the weight represents thermally-stable base polymer <NUM>. A higher concentration of conductive polymer <NUM> increases the conductivity of static dissipating coating <NUM>. If this conductivity exceeds the operating range, static dissipating coating <NUM> can cause electrical shorts. In some examples, the concentration of conductive polymer <NUM> in static dissipating coating <NUM> is between <NUM>% by weight and <NUM>% by weight, such as between <NUM>% by weight and <NUM>% by weight, or even between <NUM>% by weight and <NUM>% by weight.

Conductive polymer <NUM> comprises polyaniline <NUM>. Other examples of conductive polymers include polyethylene dioxythiophene (PEDOT). However, other solvents (other than cymene), such as esters, ketones, or glycol ethers, are needed to facilitate dissolution of both PEDOT and PBM in the same solvent. In more specific examples, conductive polymer <NUM> further comprises conductive agent <NUM>, such as dinonylnaphthalene sulfonic acid (DNNSA), dodecyl benzene sulfonic acid (DBSA), camphor sulfonic acid (CSA), and combinations thereof. For example, conductive agent <NUM> is DNNSA.

Thermally-stable base polymer <NUM> comprises one or more copolymers of butyl-methacrylate. In some examples, one or more co-polymers comprise poly-butylmethacrylate-co-methyl methacrylate (PBM), e.g., a PBM with a molecular weight (MW) of between about <NUM>,<NUM> and <NUM>,<NUM> or, more specifically, between about <NUM>,<NUM> and <NUM>,<NUM> such as about <NUM>,<NUM>. The molecular structure of PBM is presented in <FIG>. Other examples of thermally-stable base polymer <NUM> include, but are not limited to, polyvinylpyrrolidone (PVP), polyvinylpyrrolidone-vinyl acetate (PVP-VA), polyvinyl caprolactamcovinylacetate-ethylene glycol, hydroxypropyl cellulose (HPC), hydroxypropylmethyl cellulose (HPMC), hydroxypropyl methyl cellulose acetate succinate (HPMCAS), polyethylene oxide (PEO), polybutyl methacrylate-co-<NUM>-demethylamino ethyl-methacrylate-co-methyl methacrylate, polyethyl acrylate-co-methyl methacrylate-co-trimethylammonio ethyl methacrylate chloride, polymethacrylic acid-co-methyl methacrylate, polyethyl acrylate-co-methyl methacrylate-co-trimethylammonioethylmethacrylate chloride, poly(e-caprolactone) (PCLa), and ethylcellulose polyvinyl alcohol-polyethylene glycol graft copolymer (PVA-PEG).

In some examples, static dissipating coating <NUM> is substantially free from any metal-based structures and any carbon-based structures. For example, the concentration of any non-polymer materials (e.g., metals, carbon structures) in static dissipating coating <NUM> is less than <NUM>% by weight or less than <NUM>% by weight. This feature distinguishes static dissipating coating <NUM> from the conventional conductive coating, which relies on conductive particles for electrical conductivity. In some examples, static dissipating coating <NUM> further comprises antioxidant agent <NUM>.

In some examples, conductive polymer <NUM> and thermally-stable base polymer <NUM> are both soluble in one or more aromatic solvents. Some examples of such solvents include, but are not limited to, toluene, xylene, cymene, and mixtures thereof. This feature of being able to dissolve in the same solvents allows preparing an ink comprising conductive polymer <NUM> and thermally-stable base polymer <NUM> and coat this ink over base structure <NUM> as further described below.

Static dissipating coating <NUM> conformally covers base structure <NUM> and each of electronic components <NUM> as, e.g., is schematically shown in <FIG>. In some examples, the average thickness of static dissipating coating <NUM> is between <NUM> micrometers and <NUM> micrometers or, more specifically, between <NUM> micrometers and <NUM> micrometers, such as between <NUM> micrometers and <NUM> micrometers. Smaller thicknesses provide insufficient encapsulation, static dissipation, and overall coverage. On the other hand, larger thicknesses increase the overall circuit weight and can even cause electrical shorts due to excessive conductivity. In some examples, the thickness variation of static dissipating coating <NUM> is a non-zero number of less than about <NUM>% or, more specifically, less than about <NUM>%, such as from about <NUM>% to about <NUM>% or, more specifically, from about <NUM>% to about <NUM>%.

In some examples, the conductivity of static dissipating coating <NUM> is between <NUM>-<NUM> S/cm and <NUM>-<NUM> S/cm or, more specifically, between <NUM>-<NUM> S/cm and <NUM>-<NUM> S/cm such as between <NUM>-<NUM> S/cm and <NUM>-<NUM> S/cm. These conductivity ranges ensure the static dissipation of static dissipating coating <NUM> while preventing electrical shorts between electronic components <NUM>. In some examples, the conductivity of static dissipating coating <NUM> is substantially uniform. For example, the conductivity of static dissipating coating <NUM> varies less than <NUM> times through the entire volume of static dissipating coating <NUM> or even varies less than <NUM> times. Similar to the conductivity ranges described above, this conductivity uniformity ensures static dissipation throughout the entire TSSC electronic circuit <NUM> while preventing local electrical shorts.

In some examples, the resistance of static dissipating coating <NUM> changes between about <NUM> times and <NUM> times when at <NUM> for <NUM> hours or, more specifically, between about <NUM> times and <NUM> times such as between about <NUM> times and <NUM> times. In some examples, the resistance of static dissipating coating <NUM> changes less than about <NUM> times when at <NUM> for <NUM> hours or, more specifically, less than about <NUM> times or even less than about <NUM> times. This thermal stability provides that the static dissipating coating <NUM> can be operated up to <NUM>.

In some examples, static dissipating coating <NUM> is transparent. For purposes of this disclosure, the transparency of static dissipating coating <NUM> is defined as the ability to see electronic components <NUM> and other features on the surface of base structure <NUM> through static dissipating coating <NUM>. For example, the transmittance of static dissipating coating <NUM> (within the visible light spectrum) is at least about <NUM>% or, more specifically, at least about <NUM>% or even at least about <NUM>%. The transparency of static dissipating coating <NUM> allows inspection of TSSC electronic circuit <NUM> after placing static dissipating coating <NUM>, such as light emitting diodes (LEDs), optical sensors, and other like devices.

<FIG> is a process flowchart corresponding to method <NUM> of forming TSSC electronic circuit <NUM>, in accordance with some examples. Various examples and features of TSSC electronic circuit <NUM> are described above with reference to <FIG> and <FIG>.

In some examples, method <NUM> comprises (block <NUM>) providing conductive polymer <NUM> and thermally-stable base polymer <NUM>. Conductive polymer <NUM> comprises one or more polyanilines <NUM>. Thermally-stable base polymer <NUM> comprises one or more copolymers of butyl-methacrylate.

In some examples, at least one of conductive polymer <NUM> and thermally-stable base polymer <NUM> is provided as a polymer solution in a base solvent. For example, conductive polymer <NUM> is provided as a solution in toluene. The base solvent forms a portion of static dissipating ink <NUM>, described below. In some examples, the base solvent has the same composition as solvent <NUM> used to form static dissipating ink <NUM>. Alternatively, the base solvent and solvent <NUM> have different compositions.

In some examples, method <NUM> proceeds with (block <NUM>) forming static dissipating ink <NUM> using solvent <NUM>, conductive polymer <NUM>, and thermally-stable base polymer <NUM>. The composition of static dissipating ink <NUM> is schematically shown in <FIG>. In general, static dissipating ink <NUM> has the same composition as static dissipating coating <NUM> but also comprises one or more solvents. In some examples, solvent <NUM> is one or more aromatic solvents. Some examples of suitable solvents include but are not limited to toluene, xylene, cymene, and mixtures thereof. For example, <FIG> illustrates the molecular structure of cymene. In some examples, the viscosity of static dissipating ink <NUM> is between about <NUM> cPs and <NUM>,<NUM> cPs, such as between about <NUM> cPs and <NUM> cPs, which is suitable for spraying, brushing, or roll coating.

In some examples, method <NUM> proceeds with (block <NUM>) forming static dissipating coating <NUM> using static dissipating ink <NUM>. Specifically, static dissipating coating <NUM> is formed over base structure <NUM> with electronic components <NUM> disposed on and supported by base structure <NUM>. Static dissipating coating <NUM> conformally covers base structure <NUM> and each of electronic components <NUM> as described above with reference to <FIG>.

In some examples, forming static dissipating coating <NUM> comprises one or more of dip-coating, spray-coating, or <NUM>-D printing. Other coating techniques are also within the scope.

In some examples, forming static dissipating coating <NUM> comprises removing solvent <NUM> from a layer formed using static dissipating ink <NUM>. In other words, static dissipating ink <NUM>, deposited onto base structure <NUM>, is dried to form static dissipating coating <NUM>. For example, drying is performed at room temperature (e.g., <NUM>-<NUM>) for <NUM> hours followed by drying at <NUM> for <NUM> hours.

In some examples, method <NUM> further comprises (block <NUM>) applying isopropanol to static dissipating coating <NUM> to modify the resistance of static dissipating coating <NUM>.

Various samples of static dissipating coatings were tested for thermal stability, e.g., resistance changes while being heated at <NUM>. All samples used a polyaniline-based conductive polymer, which was combined with different base polymers. The polyaniline-based conductive polymer was provided as a <NUM>% solution in toluene, such as BM1720 available from Boron Molecular in Raleigh, NC. Control samples used polyurethane, in combination with the polyaniline-based conductive polymer. Test samples used different amounts of PBM, also in combination with the polyaniline-based conductive polymer. Before combining with the polyaniline-based conductive polymer, PBM was dissolved in cymene at room temperature.

Both control and test inks were drop-cast onto a surface insulation resistance (SIR) test board, having a set of interdigitated contacts. These SIR test boards are specially configured to measure the resistance of coated layers. Each coated layer was dried at room temperature (e.g., <NUM>-<NUM>) for <NUM> hours followed by drying at <NUM> for <NUM> hours. The coated boards were then placed in a convection oven maintained at <NUM> and the electrical resistance of each board was periodically measured. Specifically, the electrical resistance of the circuit was measured using a Kiethley <NUM> Semiconductor Characterization System (available from Tektronix, Inc. in Beaverton, OR) performing voltage (V) scans from +<NUM> V to -10V and measuring the current (I) during each scan. The resistance was then calculated from the slope of the I-V profile. The results of this test are presented in <FIG> and <FIG>.

Specifically, <FIG> illustrates line <NUM> corresponding to a control sample formed with <NUM>% by weight of polyurethane, line <NUM> corresponding to a test sample formed with <NUM>% by weight of PBM, and line <NUM> corresponding to another test sample formed with <NUM>% by weight of PBM. The <NUM>%-PBM test sample showed the best thermal stability.

<FIG> illustrates the results for different amounts of the polyaniline-based conductive polymer. Resistance values were obtained after <NUM> hours, <NUM> hours, <NUM> hours, and <NUM> hours of thermal exposure at <NUM> in the convection oven. Specifically, line <NUM> corresponds to a test sample formed with <NUM>% by weight of the polyaniline-based conductive polymer, line <NUM> corresponds to a test sample formed with <NUM>% by weight of the polyaniline-based conductive polymer, line <NUM> corresponds to a test sample formed with <NUM>% by weight of the polyaniline-based conductive polymer, and finally line <NUM> corresponds to a test sample formed with <NUM>% by weight of the polyaniline-based conductive polymer. Only samples with <NUM>% by weight and <NUM>% by weight of the polyaniline-based conductive polymer are within the resistance operating range, which is between 1e+<NUM> and 1e+<NUM>. The sample with a smaller amount of the polyaniline-based conductive polymer (i.e., line <NUM>) has a higher resistance than the operating range, while the sample with a higher amount of the polyaniline-based conductive polymer (i.e., line <NUM>) has a lower resistance than the operating range.

In some examples, methods, and systems described above are used on aircraft and, more generally, by the aerospace industry. Specifically, these methods and systems can be used during the fabrication of aircraft as well as during aircraft service and maintenance.

Accordingly, the apparatus and methods described above are applicable for aircraft manufacturing and service method <NUM> as shown in <FIG> and for aircraft <NUM> as shown in <FIG>. During pre-production, method <NUM> includes specification and design <NUM> of aircraft <NUM> and material procurement <NUM>. During production, component, and subassembly manufacturing <NUM> and system integration <NUM> of aircraft <NUM> takes place. Thereafter, aircraft <NUM> goes through certification and delivery <NUM> in order to be placed in service <NUM>. While in service by a customer, aircraft <NUM> is scheduled for routine maintenance and service <NUM>, which also includes modification, reconfiguration, refurbishment, and so on.

In some examples, each of the processes of method <NUM> is performed or carried out by a system integrator, a third party, and/or an operator, e.g., a customer. For the purposes of this description, a system integrator includes without limitation any number of aircraft manufacturers and major-system subcontractors; a third party includes without limitation any number of vendors, subcontractors, and suppliers; and an operator can be an airline, leasing company, military entity, service organization, and so on.

As shown in <FIG>, aircraft <NUM> produced by method <NUM> includes airframe <NUM> with plurality of systems <NUM> and interior <NUM>. The airframe <NUM> includes the wings of the aircraft <NUM>. Examples of systems <NUM> include one or more of propulsion system <NUM>, electrical system <NUM>, hydraulic system <NUM>, and environmental system <NUM>. Any number of other systems can be included.

Claim 1:
A thermally-stable static-controlled electronic circuit comprising:
a base structure;
electronic components, disposed on and supported by the base structure; and
a static dissipating coating, conformally covering the base structure and each of the electronic components, wherein:
the static dissipating coating comprises a conductive polymer and a thermally-stable base polymer,
the conductive polymer comprises polyaniline, and
the thermally-stable base polymer comprises one or more copolymers of butyl-methacrylate.