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AUTOMATIC LEVELER FOR SUBSTRATE TOP SURFACE WHEN SCREEN PRINTING

IP.com Disclosure Number: IPCOM000005869D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2001-Nov-13
Document File: 3 page(s) / 149K

Publishing Venue

Motorola

Related People

William L. Oates: AUTHOR

Abstract

When screen printing or stencil printing substrates, the best recommended tooling is fully nested (See figure 2). But it's effectiveness is limited when, because of the necessity for manufacturing tolerances, there are substrate thickness variations.

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m MOmR0L.A Technical Developments March 1990

AUTOMATIC LEVELER FOR SUBSTRATE TOP SURFACE WHEN SCREEN PRINTING

by William L. Oates

   When screen printing or stencil printing substrates, the best recommended tooling is fully nested (See figure 2). But it's effectiveness is limited when, because of the necessity for manufacturing tolerances, there are substrate thickness variations.

Description of Invention:

   Printing ink patterns by means of silk screening is a technique that has been used for generations and the thick film electronics industry makes extensive use of the same basic process, called Screen Printing, by replacing the frame of silk with a woven metal mesh screen. More recently, as the demands of surface mount technology require heavy deposits of solder paste, mesh screens have been replaced by templates made from metal foil.

   Figure 1 shows the usual arrangement of a framed template or stencil, with a moving squeegee that applies downward pressure to force the paste through the template openings, coating these areas with paste on the surface of the flat substrate below. This is simple low cost tooling consisting of a flat plate, usually with vacuum hold down plumbing, (not shown) but the squeegee adjustments are quite critical in order to minimize damage to the template at the pinch point that occurs along the substrate edge. This tendency to crease or blend the stencil material is called "coining" which as a result of distortion, no longer remains flat against the substrate surface, permitting the paste to spread under the template, resulting in poorly defined images,

   Figure 2 shows that by recessing the substrate in a cavity formed in the tool plate so that it is exactly flush with the top surface, the template can be supported along the whole stroke area of the squeegee and the squeegee pressure cannot harm or distort the template. This gives the optimum sat-up and the best quality printing but only if all the substrates are exactly the same thickness. Unfortunately in a production environment it is not economically feasible to manufacture PC. boards so that they are the same identical size and plus or minus 077 inches would be considered a normal tolerance for a typical thickness dimension.

   Figure 3 shows the best state-of-the-art compromise set-up. The substrate is fully nested but the depth of the cavity is only correct for substrates that are at the minimum thickness tolerance. (Typically minus ,007 inches). Because it is not practical for the template to contact the substrate surface if it is below the surrounding tool plate, all the other substrates, although good parts, made to the correct tolerance range, will extend above the tooling nest cavity and will present an uneven step against which the template will be forced by the squeegee pressure.

   It can readily be seen and understood that a typical metal template of .OlO inches material thickness cannot be repeatability forced down by the squeegee blade against a s...