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Making Printed Circuits

IP.com Disclosure Number: IPCOM000092728D
Original Publication Date: 1967-Jan-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 3 page(s) / 41K

Publishing Venue

IBM

Related People

Sponder, R: AUTHOR

Abstract

Initially, printed circuit board 10 has two or more spaced continuous conductor sheets 11, 12, and 13 interspersed with insulating sheet material 14 and 15 and compressed in laminated form. Through-holes 16 in a matrix pattern are drilled in the board 10. Then a circuit pattern is formed and through-holes 16 are plated to form electrical connections between layers. The use of photoresists is common to form the conductor patterns. To avoid defects in the through-hole conductors, photoresist and other foreign particles must not be present in through-holes 16 when the plating operation is performed.

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Making Printed Circuits

Initially, printed circuit board 10 has two or more spaced continuous conductor sheets 11, 12, and 13 interspersed with insulating sheet material 14 and 15 and compressed in laminated form. Through-holes 16 in a matrix pattern are drilled in the board 10.

Then a circuit pattern is formed and through-holes 16 are plated to form electrical connections between layers. The use of photoresists is common to form the conductor patterns. To avoid defects in the through-hole conductors, photoresist and other foreign particles must not be present in through-holes 16 when the plating operation is performed.

The problem is avoided by applying photoresist layer 17 to drilled board 10 in all areas except the through-holes 16 and an enlarged circular area 18 concentric with such holes. For this purpose, a stencil, such as a silk screen or the like, is used which has a blocking pattern corresponding with the through-hole matrix and land area pattern. Such a stencil is prepared using a stainless steel screen. To the latter a photoresist is uniformly applied to completely fill the interstices of the screen. The screen is then photographically exposed using a photographic master having a pattern corresponding to the desired hole and land arrangement. The hole pattern on the screen has a larger diameter than the drilled holes in the board. In those areas exposed to light, the photoresist is hardened to block the interstices of the screen. The unexposed portions are removed by washing, or the like, and the screen stencil is then applied to the surface of the board 10 with the stencil pattern coincident with the through-hole pattern. Photoresist is screened onto board 10 leaving through-holes 16 and circular areas 18 therearound and the lands uncoated.

In the preferred form, a stainless steel screen is used having a 200 mesh screen with 0.0031" mesh openings. The photores...