Aptitude à la découpe chimique de l'INVAR® pour masques de tubes cathodiques couleurs à haute performance

Ability of invar in etching for high performance color picture tube masks

Imphy S.A.

Abstract

In a cathode ray tube for color TV, the shadow-mask is a key factor for the quality of the picture. This essential component has two main requirements :

• –

  perfection of the geometry (shape, position of apertures, surface and regularity of apertures...)

• –

  stability of the shape when the tube is operating.

For this second requirement, the use of Invar (Fe-36% Ni alloy) instead of steel avoids local deformations due to the expansion (Invar has a coefficient of thermal expansion ten times smaller than steel).

The process of the shadow-mask manufacturing includes a step of etching to form apertures and we intend to illustrate the control of four parameters which show the ability of Invar strip for this process:

CONTROL OF PITTING ON INCLUSIONS - In iron-nickel alloys, sulphide inclusions can lead to pits ten times greater than their own size. The elaboration of the alloy and the control of sulphur content below 10 ppm gives a material which is not affected by inclusions during etching.

CONTROL OFTHE TEXTURE - It is known that etching is affected by crystal orientation.

The process of strip manufacturing (successive cold rolling and annealing operations) can control the intensity of the cubic texture.

CONTROL OF THE ROUGHNESS - To achieve a good adherence of the photosensitive coating and a good contact of the negative an isotropic roughness with an Ra of 0.2pm to 0.6pm is required.

CONTROL OF SURFACE CHEMISTRY - Regularity of aperture requires uniformity of the speed of the etching. Free potential graphs in nitric acid show the importance of the surface analysis which can be analysed by glow discharge spectrography.