Meniscus coating is yet another method for applying uniform coatings to large flat surfaces. The process involves generating a meniscus or thin layer of the coating on top of an applicator and transferring the meniscus to a substrate.
The pores or slot are located along the length of the tube perpendicular to the direction of coating. For a coating that contains no fillers, a porous applicator is used, while coatings that have filler particles must use a slotted applicator. The coating is continuously pumped into one end of the applicator tube and recirculated as it drips from the bottom of the tube into a reservoir.[41] The substrate is secured to a vacuum chuck then inverted so that the substrate is suspended 8–10 mils above the applicator. The substrate is then moved past the applicator, or optionally, the applicator may be moved past the substrate. As the substrate passes beyond the applicator, the meniscus splits and recoils towards both the applicator and substrate.
Meniscus coating of photoresists, photocurable coatings, and dielectrics is applicable to the fabrication of large flat panel displays and the large area processing of thin-film multilayer substrates for multichip mod-ules.[42]
Curtain Coating
Like roller coating and meniscus coating, curtain coating is yet another process for applying a coating to large flat surfaces. In curtain coating, a bath with a slot in the base allows a continuous flow (curtain) of coating to fall into a gap between two conveyor belts. The parts to be coated are moved along the conveyor at a controlled speed and receive the coating on their upper surfaces (Fig. 3.20). Curtain coating is useful in depositing photoresists, solder maskants, and dielectric insulation in the processing of printed circuit boards, multichip module substrates, and flat panel displays.
Brush Coating
Brushing does not lend itself to production and results in non-uniform and non-reproducible thicknesses even for small engineering quantities. Brushing is used mainly to touch up or repair a coating in a damaged area, to cover spots where components have been removed and replaced, and to cover pinholes left from electrical test probing. Applying the coating with a small camel’s hair brush is ideal for these small touch-up jobs.