Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Controlling current distribution within bath
Reexamination Certificate
2000-03-27
2001-11-20
Dawson, Robert (Department: 1712)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Controlling current distribution within bath
C205S137000, C205S136000, C204S205000, C204S206000
Reexamination Certificate
active
06319383
ABSTRACT:
DESCRIPTION
The invention relates to a device and a method for evening out the thickness of metal layers on electrical contact points on flat items to be treated, such as conductor foil and printed circuit boards, during electrolytic treatment, especially electrolytic metallisation, of the items guided in a horizontal plane of conveyance in a continuous electroplating plant. The device finds particular application in plants in which the items to be treated are guided in a horizontal position.
In electroplating plants for metal deposition, flat items to be treated are usually gripped at one side or at both sides at the edge by clips or clamps. The clips or clamps serve to convey the items to be treated through the plant and simultaneously to provide electrical contact, i.e. to supply the electroplating current to the items. The items to be treated are, for example, printed circuit boards, especially so-called multilayer circuits, with board thicknesses of up to 8 mm. It can happen that in a continuous electroplating plant both printed circuit boards with a thickness of that kind and toils with a thickness of only 0.1 mm or less are produced.
A further requirement of an electroplating plant consists in the required evenness and precision of the distribution of the thickness of the metal layer. The electrolytically deposited metal layers must, for technical and economic reasons, be very evenly thick right into the edge regions of the items to be treated. The edge regions, in which clear deviations from the pre-determined standard layer thickness occur, should be as narrow as possible. In precision conductor technology (conductor track widths and spacings of approximately 120 &mgr;m and less), for example, a relative layer thickness tolerance in the usable range of the items to be treated (outside the non-utilisable edge region) is required which is below 10%. Even in the proximity of the points on the items to be treated to which the clips or clamps are applied during electroplating, the required even distribution of the layer thickness cannot be achieved. Therefore, the regions in the vicinity of these contact points count as edge regions.
In DE 36 24 481 C2 is described a clamp which is used in a horizontal continuous electroplating plant. Many clamps of this kind are secured at a spacing from one another to a revolving conveyor belt. As they run into the electroplating plant, the items to be treated are grasped at their side edge by the clamps. For this purpose, two straps are swung towards one another and the edge of the items to be treated is held tight by the clamp with the aid of a pressure spring in the clamp. In another embodiment, displacing the straps perpendicular to one another is suggested in order to open the clamps. The contact pressure is in this case applied by a tension spring. At the exit from the electroplating plant the clamps are opened again by means of an oblique stop face. Thus the printed circuit boards are released again and are generally conveyed further by conveying rollers.
During the electroplating of items to be treated, the metallic straps of the clamps are also simultaneously metallised. These thus act as pirate cathodes in relation to the surface of the items to be treated which lies in the vicinity of the clamps. Because of the small layer thickness which occurs in this region, a correspondingly broad edge strip cannot be used. Experience has shown that this non-utilisable width is approximately 60 mm. In order to avoid the formation of fluctuations in layer thickness, in the publication mentioned it is recommended that the straps be provided with a plastics material cover. Only the ends which provide the contact to the items to be treated should, according to this document, remain metallically bare. In the publication is further stated that these points are deplated again electrolytically in a deplating chamber during the return travel of the clamps in the plant.
The plastics material cover makes it possible to operate the clamps within the electrolytic bath. Additional sealing walls to keep the electrolyte away from the clamps are therefore not necessary. This type of operation will be referred to below as wet contacting. The plastics material cover of the clamps consists, for example, of ECTFE (poly(ethylene trichlorofluoroethylene)). Manufacturing a plastics material coating with this chemically resistant plastics material is very involved and therefore expensive.
Mechanical damage to the plastics coating, caused by sharp-edged tools or printed circuit boards, can, however, also not be avoided when this or some other suitable plastics material is used. The service life of the clamps is therefore on average only twelve months.
During electroplating, the clamps are cathodically polarised. In practice it has been shown that the plastics material cover of many clamps is metallised after lengthy use. The undesired metallisation even takes place if the clamps are regularly electrolytically deplated during their return travel. However, the causes for this are not known. It is thought that the ageing of the plastics material cover in conjunction with the organic and/or inorganic electrolyte additives plays a role: the securely adhering metallic layer on the plastics cover begins, in the electrolyte, to grow from the metallically bare points of the clamps outwards, for example the bare contact points at the ends of the straps. In particular, however, these are damaged areas of the plastics material covers of the clamps which are caused by cleaning work during heavy duty, for example by incorrectly positioned and very sharp-edged printed circuit boards or by inappropriate treatment of the covers.
The following processes take place: through the partially very small open damaged areas, such as for example cracks in the plastics material covers, an electroplating process begins which produces a thin electrically conductive connection between the metal strap of the clamp and the metallic layer slowly growing on the outer side of the plastics material cover. During the return of the clamp in the continuous electroplating plant, the electroplated layer is intended to be electrolytically removed. The electrically conductive connection in a damaged area is removed first by preference and thus in a very short time. Thus the electrical connection of the metallic layer, which has grown on the plastics material cover, to the metal strap is lost even before this layer could be completely removed electrolytically. If the clamp is located again in the metallising region of the electroplating plant, the remaining metallisation becomes electrically conductive again through the damaged area, by new metal being electrolytically deposited inside the damaged area over the electrical connection to the metal strap. The metallisation therefore continues to grow. This repeated slow process leads, in a plant which is operated constantly, after weeks or months, to the clamps being unusable, since the undesired metallisation on the plastics material cover acts as a pirate cathode in relation to the metallisation on the items to be treated. The plastics material cover must therefore be renewed. This is connected with high costs. In addition, loss of production occurs.
In DE 32 36 545 C3 is described a device for electroplating individual plate-shaped work pieces which are guided in succession in a horizontally aligned direction by means of rotatable conveying devices through an electrolytic bath provided at its entrance and exit with seals, a plurality of cathodically switched contact wheels, located opposite one another in pairs and able to be pressed against one another, being provided as a conveying device in the bath and in particular also to provide electrical contact for the work pieces at one side of the conveyance path. Furthermore, in order to completely shield the contact wheels from the bath, they are provided with screens which have openings appropriately slit for the passage of the workpieces, wiping devices being provided along the slit openings of the screens an
Kopp Lorenz
Langheinrich Peter
Schneider Reinhard
Atotech Deutschland GmbH
Dawson Robert
Feely Michael J
Paul & Paul
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