Thin-film coatings polyimides

Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4>5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history. 

Wholly aromatic polyimides are highly thermally stable engineering plastics, and have been widely used as the reliable insulating materials in microelectronics. Recent developments in this field toward higher integration of devices required ultra thin films of polyimides. Minimum thickness of polyimide films cast by spin coating was about 0.1 Urn. 

Until now, the macroscopic tribological properties of the nanotubes had not been studied. The only studies carried out used the nanotubes as reinforcement in various matrices diamond thin film [87], polyimide [88], Ni [89], Ni-P [90], carbon/carbon composites [91] and alumina [92, 93]. These composite coatings present a better wear resistance and a lower friction. In nanotribology, Ohmae et al. studied friction of a gold tip on a nanotube forest. The friction coefficient obtained was high (1.2 to 1.5) and independent of humidity [94]. A very weak adhesion of the tip on the nanotubes was observed. During friction, no distortion of the nanotubes intervenes and there is no transfer of nanotubes on the tip. Recently, Miyoshi et al. 

Condensation polyimides or SP-polyimides from the solvent-phase reaction of an aromatic tetra-acid with a diamine. Despite the thermoplastic form they are infusible and generally insoluble. Consequently, the producer can only mould this type. It is particularly convenient for the manufacture of thin parts and films, coatings... 

Another common location for creep failures of encapsulated assemblies is at sharp corners or edges. Many encapsulants such as polyimides must be applied in thin coats, and coverage of points, edges, or corners is difficult or impossible. Sharp corners, characteristic of most thin-film devices provide ideal conditions for the initiation of creep failures due to the resulting irregularity of the encapsulant coverage. 

Fabrics coated with FEP and PFA can be laminated and heat-sealed into protective garments, canopies, etc. FEP-coated polyimide films are used in electronics and as a wire tape. FEP-based anticorrosion coatings are used in the chemical industry and as chemical barriers.18 A thin coating of FEP or PFA can be used as a hot melt adhesive for a variety of substrates, including PTFE-coated fabrics and laminates. 

Attaching thin adherent titania films to polyimide polymers demonstrates the promise of such coatings as barrier layers. Potential applications must examine the effect of the deposition process on the mechanical and thermal properties of the polymer and the requisite thickness of the ceramic layer. Nevertheless, the low-cost and convenience of LPD coatings, their mild conditions and their lack of line-of-site limitations, recommend their use. 

The polyimide thin film samples wre prepared by spin coating an approximately 2 pm film of electronic grade PMDA-ODA polyamic acid precursor (Dupont PI-2545) onto a 3n silicon wafer. This film was dried under vacuum at 85 °C for several hours and cured at 350 °C in the preparation chamber for 30 minutes. 

In general, polyimides exhibit a high refractive index typically between 1.6 and 1.7 and hence can be of definite practical interest particularly as antireflection coatings. Also, similar to parylene thin films, polyimides exhibit anisotropic optical properties. The reported values of refractive index of polyimides were n = 1.653 and n = 1.624 before curing and... 

Vapour Deposition Polymerisation. This is a little studied approach but one that offers significant potential for the fabrication of very thin films and for elaborate multilayer structures. A commercial process has been developed by the Ulvac Corporation in Japan to coat magnetic relay switches with an insulating polyimide layer. A polyamic acid is sythesised by co-deposition of two reactive monomers and is then thermally imidised. The same approach can be used for the condensation polymerisation of poly(azomethine)s, ° poly(ox-adiazoles) and poly(quinoxalines) all of which have been used in LED structures. This approach to polymer synthesis is ripe for further development. 

The high refractive indices of polyimides, compared to many other polymers, combined with excellent optical clarity, have been exploited in the development of coatings in optoelectronic applications117. Soluble thin-film polyetherimides (OptiNDEX ), such as 188, with controlled refractive indices for use as optical coatings, have been prepared (Scheme 35). The refractive index is controlled through variation in the dianhydride and diamine composition. The polyimides exhibit good thermal stability at 400 °C, and Tg... 

Screen-Printable Polyimide. Bending beam results for a screen-printable polyimide used for chip protection are presented in Figure 8. The EPO-TEK 600 polyimide paste was applied to a quartz beam, then the beam was spun at 5000 rpm to achieve high uniformity. Results are presented as inverse radius of curvature, 1/R, because the 50 m coating thickness on the 84 im substrate violates the thin film criterion of Equation 2. We have no knowledge of the polyimide s viscoelastic modulus, which is needed in order to convert 1/R into interfacial stress using Equation 1. 

One of the limitations with packed columns is eddy diffusion. This is a remixing of the separated compounds as they go around the particles in the column packing, much like you get whirlpools when water flows around a rock in a stream. To completely eliminate this problem requires the elimination of the column packing If you make the column diameter very small (< 1.0 mm), the inside wall of the column can act as the inert phase. A thin film (0.1 pm) of stationary phase is coated onto the inside of the tubing wall or even bonded to it. These capillary columns are made of fused silica and then coated with a polyimide polymer to reduce breakage. They are quite flexible and can be tied in a knot, but that is seldom done. Their natural tendency is to form a straight column, and they are hard to coil and keep... 

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