METHOD UTILIZING A MAGNETIC ASSEMBLY DURING ETCHING THIN SHADOW MASKS

Abstract: In an improved method of etching apertures (44) in a thin metal sheet (32) to form a shadow mask for a color picture tube, the metal sheet has a first acid-resistant stencil (38) on one major surface thereof and a second acid-resistant stencil (40) on the other major surface thereof. At least one of the stencils has openings (42) therein at locations of intended apertures. The improvement comprises the steps of magnetically holding the metal sheet with a flat magnetic assembly (30), and moving the magnetic assembly magnetically holding the metal sheet thereon through an etching chamber (12). The magnetic assembly includes a magnetic layer (36) that is supported on an acid-resistant board (34).

Description: METHOD UTILIZING A MAGNETIC ASSEMBLY DURING ETCHING THIN SHADOW MASKS This invention relates to a method of etching apertures in a thin metal sheet to form a shadow mask for a color picture tube, and particularly to such a method that utilizes a magnetic assembly during etching of a thin tension shadow mask, to magnetically hold the mask material. Background of the Invention A color picture tube includes an electron gun for generating and directing three electron beams to the screen of the tube. The screen is located on the inner surface of a faceplate of the tube and is made up of an array of elements of three different color emitting phosphors. A color selection electrode, or shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel or Invar, that is usually contoured to somewhat parallel the inner surface of the tube faceplate. One type of color picture tube has a tension shadow mask mounted within a faceplate panel thereof. The tension shadow mask includes an active apertured portion that contains a plurality of parallel vertically extending strands. A multiplicity of elongated apertures are located between the strands. The electron beams pass through the elongated apertures in the active portion during tube operation. Handling of tension shadow masks during their manufacture can be very difficult, especially if there are no tie bars or other connections between the strands of the mask. For example, if a tension shadow mask were etched by the process used to make conventional domed masks, there would be excessive movement of the strands during etching, and therefore great difficulty in obtaining repeatable results. The present invention provides a method that utilizes a magnetic assembly to overcome the difficulties that may arise during etching of tension shadow masks. Summary of the Invention The present invention provides an improvement in a method of etching apertures in a thin metal sheet to form a shadow mask for a color picture tube. The metal sheet has a first acid-resistant stencil on one major surface thereof and a second acid-resistant stencil on the other major surface thereof. At least one of the stencils has openings therein at locations of intended apertures. The improvement comprises the steps in the etch method of magnetically holding the metal sheet with a flat magnetic assembly, and moving the magnetic assembly magnetically holding the metal sheet thereon through an etching chamber. The magnetic assembly includes a magnetic layer that is supported on an acid-resistant board. Brief Description of the Drawings In the drawings: FIGURE 1 is a schematic representation of an apparatus that may be used for practicing a first embodiment of the novel method. 5 FIGURES 2 and 3 are cross-sections of a magnetic assembly and metal sheet at different stages of practicing the first embodiment of the novel method. FIGURE 4 is a schematic representation of an apparatus that may be used for practicing a second embodiment of the novel method. FIGURES 5 through 9 are cross-sections of two magnetic assemblies and a 10 metal sheet at different stages of practicing the second embodiment of the novel method. Detailed Description of the Preferred Embodiments FIGURE 1 shows a horizontally oriented insulative strip 10, while it is moving left-to-right through an etching chamber 12. The etching chamber 12 has an entrance 15 port 14 and an exit port 16. A sump 18 is located at the bottom of the chamber 12 to collect a liquid etchant emitted from spray nozzles 20 positioned at the top of the chamber. The etchant in the sump 18 is pumped by a pump 22 through piping 24, which includes a control valve 26, to a header 28 to which the nozzles 20 are attached. A flat magnetic assembly 30 is shown on the top of the strip 10 within the 20 chamber 12. On top of the magnetic assembly 30 is a metal sheet 32 used to produce a shadow mask. As shown in FIGURE 2, the magnetic assembly 30 is an insulative circuit board material 34 that includes a thin magnetic layer 36 adhered thereto. The metal sheet 32 includes an upper first acid-resistant stencil 38 on one major surface thereof and a 25 lower second acid-resistant stencil 40 on the other major surface thereof. The upper stencil 38 has openings 42 therein at locations of intended apertures in the completed shadow mask. The magnetic assembly 30 and metal sheet 32 are kept in the etching chamber 12 a sufficient time to ensure that apertures are completely etched through the sheet. FIGURE 3 shows the magnetic assembly 30 and the metal sheet 32 after 30 they have left the etching chamber 12, with complete apertures 44 formed in the metal sheet 32. In another embodiment, shown in FIGURE 4, an insulative strip 46 passes through two etching chambers 48 and 50. The construction of the second chamber 50 is similar to the etching chamber 12 of the previous embodiment. The first chamber 35 48 differs from the second chamber 50 in that the former includes spray nozzles 52 that spray from below the strip 46 instead of from above. Preferably, the insulative strip 46, as well as the strip 10 of the first embodiment, is moved continuously during etching. FIGURE 5 shows a metal sheet 54 including a lower first acid-resistant stencil 56 on one major sur ace thereof and an upper second acid-resistant stencil 58 on the other major surface thereof. Both stencils 56 and 58 have openings 60 and 62, respectively, at locations of intended apertures in the completed shadow mask. At the beginning of the etch process, the metal sheet 54 is magnetically held against the bottom of the strip 46 by magnetic assembly 64 that includes an insulative circuit board material 66 with a thin magnetic layer 68 attached thereto. Partial apertures 70 are etched in the metal sheet 54 in the first chamber 48 to depth of about 40% of the thickness of the sheet, as shown in FIGURE 6. After the magnetic assembly 64 and the metal sheet 54 leave the first chamber 48, a second magnetic assembly 72, including an insulative circuit board material 74 with a thin magnetic layer 76 attached thereto, is placed against the lower side of the metal sheet 54, as shown in FIGURE 7. Next, the first magnetic assembly 64 is removed from the top of the strip 46 and the second magnetic assembly 72, with the metal sheet 54 magnetically attached, is placed on top of the strip 46, as shown in FIGURE 8. The metal sheet 54 then enters the second etching chamber 50 with the second acid-resistant stencil 58 facing upward. The metal sheet 54 is etched through to the partial apertures 70, thus forming final apertures 78, as shown in FIGURE 9. Following etching, the acid- resistant stencils are removed, and the remaining metal sheet 54 is a shadow mask. The magnetic layers 36, 68 and 76 preferably are continuous rectangles that are at least as large in area as the metal sheets 32 and 54. Alternatively, the magnetic layers could be magnetic strips aligned parallel with the direction of movement through the chambers. For example, a magnetic assembly was constructed with magnetic strips running parallel to the direction of movement through the etching chamber that were attached to a G-10 stripped circuit board sheet. In this case the circuit board material was chosen because of its small thermal expansion coefficient and because of its resistance to etching solutions used to make the mask. Preferably, the magnetic strips are positioned outside the active area of the masks to allow for solution exchange during etching to avoid staining. The magnets hold the mask material to prevent excessive movement of the material during etching, and to permit handling of the mask without the chance of any tangling of mask strands or other mask damage occurring. Also, additional magnet members can be used on top of the metal sheet to further hold the mask in place during etching and to prevent any contact with the mask by the etching equipment.

Claims: CLAIMS 1. A method of etching apertures (44, 78) in a thin metal sheet (32, 54) to form a shadow mask for a color picture tube, said metal sheet having a first acid-resistant stencil (38, 56) on one major surface thereof and a second acid-resistant stencil (40, 58) on the other major surface thereof, at least one of said stencils having openings (42, 60, 62) therein at locations of intended apertures in said sheet, comprising said method including the steps of magnetically holding said metal sheet (32, 54) with a flat magnetic assembly (30, 64, 72), said magnetic assembly including a magnetic layer (36, 68, 76) that is supported on an acid-resistant board (34, 66, 74), and moving said magnetic assembly through an etching chamber (12, 48, 50), while said assembly is magnetically holding said metal sheet thereto. 2. The method as defined in claim 1 , wherein only one of said stencils (38) has openings (42) therein at said locations of intended apertures (44) in said metal sheet (32), and said metal sheet is oriented with said stencil with said openings facing upward. 3. The method as defined in claim 2, wherein said metal sheet (32) is etched in said chamber (12) from the upper side of said sheet only. 4. The method as defined in claim 1 , wherein said magnetic layer (36, 68, 76) is a continuous rectangle at least as large in area as said metal sheet (32, 54). 5. The method as defined in claim 1 , wherein said magnetic layer (36, 68, 76) comprises magnetic strips aligned parallel with the direction of movement through said chamber (12, 48, 50). 6. The method as defined in claim 1 , wherein both said first (56) and second (58) acid-resistant stencils have openings (60, 62) therein at said locations of intended apertures (78) in said sheet (54). 7. The method as defined in claim 6, wherein said etching chamber (48) has an insulative strip (46) passing therethrough; said metal sheet (54) is placed under said insulative strip with said first acid-resistant stencil (56) facing downward; said magnetic assembly (64) is placed on top of said insulative strip with said magnetic layer (68) facing downward to magnetically hold said metal sheet against said insulative strip; said insulative strip, with magnetic assembly and metal sheet thereon, is moved through said etching chamber; and said metal sheet is partially etched through from the bottom side thereof. 8. The method as defined in claim 7, wherein said partially etched through metal sheet (54) is transferred to and magnetically held by a second magnetic assembly (72); said second magnetic assembly is transferred to the top of said insulative strip (46) with the second acid-resistant stencil (58) on said metal sheet facing upward; and said second assembly and metal sheet are passed through a second etching chamber (50), wherein the etch-through of said apertures (78) in said metal sheet is completed.

Inventors: ESHLEMAN CRAIG CLAY, MCCOY RANDALL EUGENE, WETZEL CHARLES MICHAEL, ESHLEMAN, CRAIG, CLAY, MCCOY, RANDALL, EUGENE, WETZEL, CHARLES, MICHAEL
Applicants: ESHLEMAN CRAIG CLAY, MCCOY RANDALL EUGENE, THOMSON LICENSING SA, WETZEL CHARLES MICHAEL, ESHLEMAN, CRAIG, CLAY, MCCOY, RANDALL, EUGENE, THOMSON LICENSING S.A., WETZEL, CHARLES, MICHAEL
Application year: 2000
Main classification: H01J9
IPC codes: C23F1, H01J9