Method for making shadow mask for color picture tube

Abstract: Disclosed is a method for making a shadow mask for a color picture tube, including the steps of: forming a pattern, corresponding to apertures of the shadow mask, on a screen mesh (31) fixed to a frame; applying metal paste (27') on the screen mesh having the pattern; disposing a flat AK steel shadow mask (25) under the screen mesh; printing a metal layer on a face of the flat AK steel shadow mask by squeezing the metal paste (27') on the screen mesh (31) with a constant pressure along a direction; and pressing the flat shadow mask to form a skirt portion and a bead portion of the shadow mask.

Description: BACKGROUND The present invention relates generally to a method of forming an anti-doming material of a shadow mask. A color picture tube of a shadow mask type has electron beams emitted from an electron gun which pass through apertures of the shadow mask to land on R, G and B pixels, respectively, on a phosphor layer. However, part of the electron beams pass through the apertures of the shadow mask and the rest strike the inner face of the shadow mask to heat it. As a result, the shadow mask is thermally expanded and domes out, such that the position of the apertures is changed against the electron beam. Thus, a demand for compensating the change is proposed. Referring to FIG. 5, there is illustrated a conventional shadow mask 1 which is secured to a frame 3 which is mounted at a panel by a spring 5. On the inner surface of the panel 7, there is deposited a phosphor layer containing phosphor pixels that respectively emit light of red R, green G and blue B. The shadow mask 1 is spaced in a predetermined distance from the phosphor layer. In addition, the shadow mask 1 is generally made of pure iron, for example aluminium killed (AK) steel. This AK steel has a thermal expansion coefficient of about 11.7 x 10 -6 /K. When the tube operates, electron beams emitted from an electron gun pass through corresponding apertures of the shadow mask 1 and correctly lands on the aimed phosphor pixels to display a picture. However, about 80% of the electron beams strike the inner surface of the shadow mask to thereby increase the temperature of the shadow mask to about 80-90°C. As a result, the shadow mask 1 is thermally expanded and domes out as shown in a broken line of FIG. 5 such that the paths of the electron beams which pass through the shadow mask are shifted from the phosphor pixels to thereby deteriorate the white uniformity. That is, path of the electron beam is displaced from a position B1 to a position B2 and thereby the corresponding phosphor pixel is also displaced from a position P1 to a position P2. To solve the above described problem, shadow masks made of invar alloy having an extremely low thermal expansion coefficient are disclosed in Japanese Laid-Open Patent NO. S59-15861 and U.S. Patent Nos. 647,924 and 4,528,246. However, invar alloy is difficult to form and the cost thereof is high which increases manufacturing costs. Therefore, the Korean Patent No. 85-1589 discloses a method for forming an electron radiation layer on the shadow mask to solve the doming problem. The European Patent No. 139,379 discloses a method for forming a low expansion layer on the shadow mask. However, since all the methods described above is technically complicated, it is difficult to apply the methods to actual production. SUMMARY It is an object of the present invention to provide a method for fabricating a shadow mask for a color picture tube with a much simpler fabrication process while providing low thermal expansion, and high electron reflecting and a thermal radiation effect. The above and additional objects are realized in accordance with the present invention which provides a method for making a shadow mask for a color picture tube, comprising the steps of: forming a pattern, corresponding to apertures of the shadow mask, on a screen mesh fixed to a frame; disposing a flat AK steel shadow mask under the screen mesh; printing a low thermal expansion material layer on a face of the flat AK steel shadow mask by squeezing pastes of a low thermal expansion material through the screen mesh onto said AK steel shadow mask with constant pressure along a direction; and pressing the flat shadow mask to form a skirt portion and a bead portion of the shadow mask. Preferably, the pastes comprises one or more metals or an oxide selected from the group consisting of tungsten, carbonated tungsten and bismuth. The step of printing the layer may be performed two or more times to increase the thickness of the layer. If required, to further printing of the layer on the other face of the flat AK steel shadow mask, the process from the step of forming the pattern to the step of printing the layer is further applied to the other face of the flat AK steel shadow mask. The method further comprises the step of heating the shadow mask, which is obtained after pressing the flat AK steel shadow mask, in a reduction heating furnace to induce substance diffusion of both the material layer and the flat AK steel shadow mask so as to obtain an alloy steel between the layer and the flat AK steel shadow mask. According to preferred embodiment, the temperature of the reduction heating furnace is set at about 850-1,200°C. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects of the present invention will become apparent from the detailed description below when taken in conjunction with the following drawings in which: Fig. 1 is a partial sectional view for showing a shadow mask made by a method in accordance with a first embodiment of the present invention; Figs. 2A to 2E are views for showing a method for fabricating a shadow mask in accordance with a first embodiment of the present invention; Fig. 3 is a partially sectional view for showing a shadow mask made by a method in accordance with a second embodiment of the present invention; Fig. 4 is a partial sectional view for showing a shadow mask made by a method in accordance with a third embodiment of the present invention; and Fig. 5 is a sectional view showing a conventional color picture tube. DESCRIPTION While the invention will be described and illustrated in connection with certain preferred embodiments and examples, it should be understood that it is not intended to limit the invention to those particular embodiments and examples. To the contrary, it is intended to cover all alternatives, modifications and equivalents falling within the scope of the invention as defined by the appended claims. Reference will now be made in detail to the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. Referring first to FIG. 1, there is partially illustrated a shadow mask 21 made by a method according to a first embodiment of the present invention. The shadow mask 21 comprises an AK steel shadow mask 25 having a thermal expansion coefficient of about 11.7 x 10-6/K and is provided with a plurality of apertures through which electron beams pass. The AK steel shadow mask 21 is coated on a face facing an electron gun (not shown) with a low thermal expansion material layer 27. The layer 27 comprises one or more materials selected from the group consisting of tungsten(W), carbonated tungsten(WC) and bismuth(Bi). The layer 27 is formed to be less than 50µm in its thickness such that when the beams pass through the apertures, passing characteristic of the beams is not to be deteriorated. Referring now to the method for making such shadow mask 21 according to a first embodiment of the present invention in conjunction with Figs. 2A to 2E, as the first step, a screen mesh 31 made of material selected from the group consisting of stainless steel, polyester and nylon is mounted on a frame 35. And then, photo resist material 33 is covered over the complete surface of the screen mesh 31 in a constant thickness and is then dried (see Fig 2A). Thereafter, as the second step, the photo resist material 33 covered on the screen mesh 31 is exposed to light from a light source 20 through the AK steel shadow mask 25 (see Fig. 2B) and the unexposed portion of the photo resist material 33 is etched, thereby, as shown in Fig 2C, forming photo resist pattern 33' corresponding to the apertures 23 of the AK steel shadow mask 25 as shown in Fig. 2C. As the third step, the screen mesh 31 which goes through the above described steps is mounted on a screen printer which is well known in the art. And then, metal paste 27' is is applied on the upper surface of the screen mesh 31 in a constant thickness (see Fig. 2C). The paste comprises one or more materials selected from W, WC and Bi. Thereafter, as the fourth step, the shadow mask 25 is disposed under the screen mesh 31 having the photo resist pattern 33' and the metal paste 27' is then squeezed by a squeeze 39 to thereby be moved in a direction so as to print the metal layer 27 on the AK steel shadow mask 25 (see Figs. 2D), thereby obtaining the shadow mask 21 as shown in Fig. 2E. This step may be repeatedly performed two or more times, if required, to increase the thickness of the layer 27 or to print another metal material. It is also possible to regulate the thickness of the layer 27 in accordance with the types of screen mesh and paste, and pressure and speed of the printing. And, as the fifth step, the shadow mask 25 is pressed to thereby to form a skirt portion and a bead thereof, thereby obtaining a finished shadow mask. The layer 27 made by the above-described steps performs as a low thermal expansion layer as well as an electron reflecting layer and thermal radiation layer to suppress doming of the shadow mask 25. More in detail, the material used for the layer 27 has a thermal expansion coefficient of less than 4.5 x 10 -6 /K. This shows that thermal expansion of the shadow can be considerably reduced when considering that the AK steel has a thermal expansion of approximately 11.7 x 10 -6 /K. In addition, since each of the materials W, WC and Bi have a relatively high electron-reflecting efficiency of about 0.45-0.50, the extinction amount of the electron beams incident to the shadow mask is reduced to thereby suppress doming of the shadow mask. Further, each of the materials W, WC and Bi has a relatively high thermal radiation efficiency of about 0.8-0.9, this also helps to suppress doming of the shadow mask. Fig. 3 shows a shadow mask 210 manufactured by a method according to a second embodiment of the present invention. The AK steel shadow mask 25 is covered on its opposite faces with the layer 27. To achieve this, before the fifth step of the first embodiment, the first to fourth steps are performed to print the layer on the other face. Referring to Fig. 4 showing a shadow mask 211 made through a method according to a fourth embodiment of the present invention, an alloy layer 29 is formed between the AK steel shadow mask 25 and the layer 27. The alloy layer is formed by substance diffusion of both the layer 27 and the Ak steel shadow mask 25. To form the alloy layer 29 between the AK steel shadow mask 25 and the layer 27, in this fourth embodiment, a cementation process is additionally performed by heating the shadow mask 21 or 210, which is obtained through the first or second embodiment, in a neutral or reduction heating furnace. The temperature of the heating furnace is set at about 850-1,200°C in consideration that the temperature of transformation point of the AK steel is approximately 800°C. However, the temperature of the heating furnace may be set at a relatively higher temperature in accordance with the kind of material. By this cementation process, between the layer 27 and the AK steel shadow mask 25, substance diffusion occurs resulting in a changing of the inherent characteristic thereof to thereby form the alloy layer 29. More in detail, the alloy layer 29 comprises alloy steel selected from the group consisting of Fe-W, Fe-WC and Fe-Bi. The alloy layer 29 has a thermal expansion coefficient of about 4.5-11.7 x 10 -6 /K. This shows that the shadow mask obtained by this fourth embodiment has a lower thermal expansion amount than that obtained by the first or second embodiment. Further, each of the alloy steels Fe-W, Fe-WC and Fe-Bi has a relatively high thermal radiation efficiency.

Claims: A method for making a shadow mask (21, 210, 211) for a color picture tube, comprising the steps of: a) forming a pattern (33'), corresponding to apertures (23) of the shadow mask, on a screen mesh (31) fixed to a frame (35); b) disposing a flat aluminium killed (AK) steel shadow mask (25) under the screen mesh; c) printing a low thermal expansion material layer (27) on a face of the flat AK steel shadow mask by squeezing pastes (27') of a low thermal expansion material through the screen mesh onto said AK steel shadow mask with a constant pressure along a direction; and d) pressing the flat shadow mask to form a skirt portion and a bead portion of the shadow mask. The method according to claim 1 wherein the pastel (27') comprises one or more materials selected from the group consisting of tungsten, carbonated tungsten and bismuth. The method according to claim 1 wherein the step of printing the layer is performed two or more times to increase the thickness of the layer. The method according to claim 1 further comprising the step of forming a second layer on an opposite face of the flat AK steel shadow mask by repeating steps a) - e) on said opposite face of the AK steel shadow mask. The method according to claim 1 further comprising the step of cementation of the shadow mask, which is obtained after pressing the flat AK steel shadow mask, in a neutral or reduction heating furnace to induce substance diffusion of both the layer and the flat AK steel shadow mask so as to obtain an alloy steel (29) between the layer (27) and the flat AK steel shadow mask (25). The method according to claim 5 wherein temperature of the reduction heating furnace is set at about 850-1,200°C.

Inventors: HAN DONGHEE, KIM JEAMYUNG, RHO HWANCHUL, HAN, DONGHEE, KIM, JEAMYUNG, RHO, HWANCHUL
Applicants: SAMSUNG DISPLAY DEVICES CO LTD, SAMSUNG DISPLAY DEVICES CO., LTD.
Application year: 1996
Main classification: H01J9
IPC codes: H01J29, H01J9