GB2304061A

GB2304061A - Curtain coating printed circuit boards using movable curtain guides

Info

Publication number: GB2304061A
Application number: GB9516376A
Authority: GB
Inventors: Adrian Richard Edmundson; Jeremy Pal White; Alexander James Brooks
Original assignee: Coates Brothers and Co Ltd
Current assignee: Coates Brothers and Co Ltd
Priority date: 1995-08-10
Filing date: 1995-08-10
Publication date: 1997-03-12
Anticipated expiration: 2015-08-10
Also published as: GB9516376D0; GB2304061B

Abstract

A process for curtain coating printed circuit boards 4 with liquid resist compositions by passage through a curtain 2, the width of the lower edge of the curtain being controlled by curtain guides 6 at each end of the curtain, wherein the curtain guides are movable and are so controlled that the width of the curtain is greater when a circuit board is passing through the curtain than when no circuit board is passing through the curtain.

Description

COATING This invention is concerned with improvements in and relating to curtain coating processes and apparatus for use therein.

Curtain coating processes, for coating an object with a liquid coating composition, are well known and well-established and, basically, comprise passing the object to be coated through a falling curtain of the liquid coating composition. The process is particularly suitable for coating relatively flat objects, which are passed horizontally through the curtain and has found especial application in the coating of printed circuit boards and like flat rectangular substrates used in the electronics industry. Thus liquid resist compositions, especially liquid solder resist compositions, may be applied to printed circuit boards or the like by a curtain coating process.

When coating some substrates, such as printed circuit boards, it is often desirable to leave the outer edges of the substrate uncoated to facilitate further processing of the substrate. To this end curtain splitters (e.g. as described in GB-A-2202467 and EP-A-0145648) may be employed so that only a generally central position of the curtain impinges on the substrate, the side margins of the curtain being deflected away from the edges of the substrates by the curtain splitters. Such an arrangement has the disadvantage that the edge beads can form on the substrate, i.e. longitudinally extending strips of greater coating height. The edge bead is formed because the surface tension pulls the sides of the curtain inwards so that, on impingement, the edges are thicker than the centre.

It has been proposed to overcome this problem by inserting in the lateral margins of the curtain severing devices or curtain guides which serve to "stretch" the curtain at its lower edge. (Such severing devices and/or curtain guides are also described in GB-A-2202467 and EP-A-0145648). Such edge guides do achieve the desired objective of reducing the edge bead height but, in their turn, suffer from disadvantages. Thus we have found that, in practice, a "stretched" curtain is more unstable than an "unstretched" curtain so that, in the course of operation, the curtain can break up and, as a result, does not give the desired uniform coating.

Reforming the curtain after such a break is generally a time-consuming operation and necessarily involves discontinuing the curtain coating operation.

In accordance with the present invention there is a provided a process for coating discrete articles with a liquid coating composition by passing the articles through a curtain of liquid coating composition from a pouring head, the width of the lower edge of the curtain coming into contact with the articles to be coated being controlled by curtain guides at each end of the curtain, in which process the curtain guides are movable and are so controlled that the width of the curtain is greater when an article to be coated is passing through the curtain than when no article is passing through the curtain.In accordance with the process of the invention, the curtain is stretched only when a substrate to be coated is passing therethrough and if, for any reason, the curtain should break at this stage, it will reform when the curtain guides are moved, to decrease the width of the curtain, after the object to be coated has passed through the curtain.

The invention also provides apparatus for carrying out the process of the invention and generally comprising a pouring head for forming a vertical curtain of a liquid coating composition; conveyor means for conveying articles to be coated through said curtain and generally at right angles to the line of the curtain; curtain guide means arranged to control the width of the lower edge of the curtain which will come into contact with articles to be coated; and control means for moving the curtain guide means so that the width of the curtain is greater when an article to be coated is passing through the curtain than when no article is passing through the curtain.Such control means may, for example, take the form of detectors for sensing the approach of an article to the curtain and the departure of an article from the curtain, appropriately linked to means moving the curtain guides so as to increase or decrease the width of the lower edge of the curtain.

The invention also provides curtain splitting means, suitable for use in accordance with the invention comprising: - (i) an upwardly extending curtain severing plate; (ii) a downwardly extending curtain guide plate connected to the lower edge of the severing plate with an obtuse angle (e.g. of 130-175 , preferably about 1600) between the planes of the plates; and (iii) a downwardly extending curtain deflecting plate connected to the severing plate along a line intermediate its upper and lower edges and extending from the severing plate in the opposite sense to the curtain guide plate.

In order that the invention may be well understood reference will now be made to the accompanying schematic drawings in which: Fig. 1 is a side view, partly in section, of coating apparatus for use in accordance with the invention in the position when no article to be coated is passing through the curtain; Fig. 2 is a similar view showing the curtain in the "stretched" position with an article passing through the curtain; Fig. 3 is a section, on an enlarged scale, of a curtain guide for use in accordance with the invention; and Fig. 4 is section on the line A-A' through Fig. 1.

As shown in Figs. 1, 2 and 4 of the drawings, curtain coating apparatus for use in accordance with the invention comprises a pouring head 1 from which is poured, through an elongate slit in the bottom thereof, a curtain 2 of coating liquid. An article 4 (see Fig.

2) to be coated is passed through the curtain by conveying means 3 (see Fig. 4). The outer margins of curtain 2 are defined by fixed lateral guides 5 and the falling curtain is divided into three portions, namely a pair of outer portions and a centre portion by means of symmetrically disposed pivotably mounted curtain guides 6 extending upwardly into the curtain.

When no article 4 is passing through curtain 2, (see Fig. 1) all of the portions of the curtain fall into drainage trough 10 whence the coating material is recycled (by means not shown) to pouring head 1. When an article 4 is passing through curtain 2 (see Fig. 2) only the outer portions of the curtain fall into trough 10.

Curtain guides 6 have the structure shown in Fig. 3 of the drawings and comprise an upwardly extending elongate flat severing member 7 having, at about the mid point thereof, a downwardly and outwardly (with respect to the curtain) extending flange portion 8 and at its lower end an inwardly and downwardly directed (with respect to the curtain) lower flange 9. Plate 7 and flange 8, in combination, serve to separate the outer portions of the curtain which are fed into trough 10.

The form of construction of curtain guides 6 makes possible the use of a single continual curtain splitter and curtain lower edge guide, in contrast to known arrangements (see e.g. EP-A-0145648) in which separate splitter and guides are employed.

The width of the curtain at its point of impingement on an article 4, is defined by the distance between the lower edges of lower flanges 9 of curtain guides 6 which are rotatable from the position shown in Fig. 1 of the drawings to that shown in Fig. 2 of the drawings. In the position shown in Fig. 1, the curtain is generally stable but would, if allowed to fall upon an article 4, form edge beads thereon. With the curtain guides 6 in the position shown in Fig. 2, the lower edges of lower flanges 9 of curtain guides 6 are further apart than they are in the position shown in Fig. 1 and further apart than the distance between the upper portions of plates 7 of curtain guides 6.As a result the lower edge of the curtain is "stretched" so that any edge bead formed upon an article 4 passing through the curtain will be of reduced height, as compared with that which it would have been had the substrate passed through the curtain with curtain guides 6 in the position shown in Fig. 1.

In the condition shown in Fig. 2, the curtain 2 can be less stable and may break up whilst an article is passing therethrough. In this event the article 4 will not be properly coated but once it has passed through the curtain 2, curtain guides 6 will, operating in accordance with the invention, revert to the position shown in Fig. 1 of the drawings and, under these conditions, the curtain will automatically reform; to be stretched again when another article 4 is to be passed therethrough (as shown in Fig. 2).

The process and apparatus of the invention may be employed for the curtain coating of a variety of substrates, especially flat substrates such as printed circuit boards or the like of the application of liquid resist coatings thereto. Such resist coatings, after drying, may be imagewise exposed to radiation (such as W-light) to partially cure them (where exposed to radiation) and then developed by removing unused portions of the coating. Suitable compositions for the formation of resists are described, for example, in EP-A-467945, EP-A-536272, EP-A-575426 and EP-A-650657.

Claims

CLAIMS:

1. A process for coating discrete articles with a liquid coating composition by passing the articles through a curtain of liquid coating composition from a pouring head, the width of the lower edge of the curtain coming into contact with the articles to be coated being controlled by curtain guides at each end of the curtain, in which process the curtain guides are movable and are so controlled that the width of the curtain is greater when an article to be coated is passing through the curtain than when no article is passing through the curtain.

2. Apparatus for carrying out the process claimed in claim 1 and comprising a pouring head for forming a vertical curtain of a liquid coating composition; conveyor means for conveying articles to be coated through said curtain and generally at right angles to the line of the curtain; curtain guide means arranged to control the width of the lower edge of the curtain which will come into contact with the articles to be coated; and control means for moving the curtain guide means so that the width of the curtain is greater when an article to be coated is passing through the curtain than when no article is passing through the curtain.

3. A process or apparatus as claimed in claim 1 or claim 2 in which the curtain is split by curtain splitting means comprising: (i) an upwardly extending curtain severing plate; (ii) a downwardly extending curtain guide plate connected to the lower edge of the severing plate with an obtuse angle between the planes of the plates; and (iii) a downwardly extending curtain deflecting plate connected to the severing plate along a line intermediate its upper and lower edges and extending from the severing plate in the opposite sense to the curtain guide plate.

3. A process as claimed in claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

4. Apparatus as claimed in claim 2 substantially as hereinbefore described with reference to the accompanying drawings.