Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Patent
1996-02-27
1998-12-01
Young, Christopher G.
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
430296, 430312, 430313, 430315, 430319, 430330, 118 72, 118239, 118641, G03F 738, B05B 500, B05C 100, B05C 1100
Patent
active
058436219
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a process for coating printed circuit boards with a coating that is crosslinkable by electromagnetic radiation, especially UV radiation,. The invention relates also to an apparatus suitable for that purpose and to the use of the process for various purposes.
The function of the printed circuit board is to provide the conductive connection to the components. As miniaturisation progresses, the number of connections is becoming ever greater, with the result not only that printed circuit board technology has produced multilevel circuitry, but also that conductive tracks are becoming ever narrower, drill hole diameters are becoming ever smaller and the number of conductive tracks between two holes is becoming ever greater.
With the development of surface-mounted devices it has been possible to achieve a further reduction in the surface area required. This has led to conductor widths of less than 100 .mu.m, to drill hole diameters of from 0.3 to 0.2 mm and to solder point diameters of only up to 0.4 mm with up to seven conductive tracks between a drill hole grid of 2.54 mm. At the same time, more and more connections must be made per integrated circuit, which results in connection pad grids of from 0.3 to 0.4 mm. The problems resulting from the increasingly high integration density are very complex and require a comprehensive solution. They begin first with the production of the conductive pattern.
For the production of a conductive pattern, the drilled copper-clad base material is coated with a resist. While such resists were in the past screen-printable etching or electro resists, nowadays photoresists are predominantly used, which are solid or liquid resists. They are either laminated on to the surface (solid resists) or applied by means of a pouring machine or using rolls (liquid resists). After the application of a mask, the conductive pattern is fixed, for example, by exposure to UV light, with polymerisation of the resist, and exposed by development. This leads to the so-called pattern plating process. In pattern plating, first of all a mask is applied and only the conductor-free areas are exposed and developed. The conductive tracks are then built up by electroplating and the drilled contact holes are clad with copper. After the conductors have been deposited by electroplating, they are, for example, provided with a tin coating, the electro resist is removed and the conductive pattern is etched. Since the etching speed is the same in all directions, the undercutting corresponds approximately to the thickness of the copper film used. The undercutting that occurs in the etching stage of the pattern plating process represents the limit for large-scale application of that process. Moreover, the production of conductors of equal height is frequently not possible as a result of the geometry of the bath or of the printed circuit board.
Accordingly, for microconductor technology, the so-called panel plating process was developed. In that process, starting with the drilled copper-clad base material, first of all the surface of the printed circuit board and the drilled holes are clad with copper by electroplating in order to achieve a uniform thickness of the copper layer. A dry film resist is then laminated on, exposure is effected using a mask, and development is carried out.
In the case of microconductors, however, constrictions frequently occur because the line pressure of the laminating roller is unable to compensate for uneven areas of the base material, so that the dry film resist does not adhere to the same extent in all places. It is especially important that the already copper-clad drill holes should also be protected from the effects of the etching. That is achieved by covering the holes with resist, so-called "tenting".
Further miniaturisation and the technology of surface-mounted components has led to so-called "rest-ring"-free through-holes. Dry film lamination technology cannot be used in this case, because without the so-called rest-rings the re
REFERENCES:
patent: 5049478 (1991-09-01), Koch et al.
"Advances in Liquid Photoimageable Solder Mask Technology", Susan Crum, Electronic Packaging & Production, vol. 33, No. 9, Sep. 1993, pp. 78-81.
Ciba-Geigy AG
Young Christopher G.
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