Electrolysis: processes – compositions used therein – and methods – Electrolytic coating
Reexamination Certificate
2002-02-25
2004-02-24
King, Roy (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
C204S297010, C204S297060, C204SDIG007, C205S096000, C205S145000
Reexamination Certificate
active
06695961
ABSTRACT:
The invention relates to carriers serving to supply current to workpieces to be treated electrolytically or counter-electrodes and to a method for the electrolytic treatment of workpieces.
Workpieces to be treated electrolytically, for example printed circuit boards, are brought together with suitable counter-electrodes into contact with a treatment fluid. For electrolytic metallisation (electroplating) anodes are used as counter-electrodes. In conventional electrolytic treatment, the workpieces and counter-electrodes are dipped into a bath of the treatment fluid and a flow of electric current is generated by the workpieces and the counter-electrodes. In most cases, one counter-electrode is placed on each side opposite each workpiece, in order to be able to treat both sides of the workpiece.
The workpieces and the counter-electrodes are, for this purpose, secured to suitable carriers. Whilst the counter-electrodes are disposed stationary in a bath, the workpieces are secured detachable to elongate goods carriers and conveyed by means of conveying devices from bath to bath into the individual treatment stations. During electroplating, the workpieces are polarised cathodically during electrolysis and the counter-electrodes are polarised anodically. During electrolytic etching, cleaning, roughening and during other anodic processes (for example during electrophoretic processes), on the other hand, the workpieces are polarised anodically and the counter-electrodes cathodically. Solely the case of electroplating is described below in a representative manner. However the invention also relates to the cases in which the workpieces are polarised anodically and the counter-electrodes cathodically, or in reverse pulse plating are both polarised alternately anodically and cathodically.
Generally electrolytically deposited metal layers on workpieces have to be applied with a very uniform thickness. Mostly a large number of individual workpieces are secured to a goods carrier. In order to achieve uniform coating, all the workpieces must be exposed to substantially the same physical and chemical conditions during treatment. Here a very essential factor is the locally effective current density since this reacts proportionally to the amount of deposited metal. This means that substantially the same current density must be set at all the workpieces secured to a goods carrier. To this end, the same cell voltage must be applied to all the workpieces, i.e. the voltage measured between the surfaces of the individual workpieces and of the anodes facing the workpieces in the treatment bath must be the same for each workpiece.
In production plants, for example for treating printed circuit boards, the goods carriers and anode carriers bridge the relatively long treatment cells and are thus generally several meters long. The width of the so-called electroplating window (projection of the printed circuit boards, which are attached beside one another and in addition partially above one another to frames secured to the goods carriers) is thus very large. Through simultaneous electroplating of a large number of printed circuit boards it is possible to achieve high productivity of a plant. The whole electroplating current at a goods carrier must be set to very high values in plants of this kind in order to achieve a high current density at the printed circuit boards and thus short electroplating times. In particular, in very large plants in which also long goods carriers are used with a very large number of printed circuit boards attached thereto, the current required is very great.
Electroplating windows have a width of up to 8 m and a height of up to 1.5 m. The printed circuit boards are arranged in these plants very close together. The necessary current density at the printed circuit boards is for example 5 A/dm
2
. If printed circuit boards are electrolytically copper-plated over their whole surface with direct current by means of such a system, on each side of a printed circuit board flows a current of 80 dm·15 dm·5 A/dm
2
=6000 A. Since generally both sides of the printed circuit board are treated simultaneously, in this case a current of approximately 12,000 A flows. (This calculation does not take into account the fact that the through-bores present in the printed circuit boards have an additional surface.) In a high current of this kind, within the goods carriers a voltage drop occurs which cannot be disregarded, and which for the printed circuit boards secured in the vicinity of the location for current to be led into the goods carrier is low, and can assume very high values away from this location (for example several 100 mV) such that very different cell voltages occur for the individual printed circuit boards. Also at the anodes arranged opposite the printed circuit boards a non-negligible voltage drop can be noticed which occurs through the high current through the anode carrier. Therefore the mentioned objective or very even electroplating can only be achieved imperfectly.
When the reverse pulse technique (bipolar current pulse form) is used, even greater currents flow than quoted above. In this case the current is switched alternately cathodically and anodically during the treatment. In order to achieve a pre-determined electroplating result, the effective cathodic current intensity must be further increased in relation to the rated current intensity. Typically, during approximately 85% of the time electroplating is carried out and during roughly 15% of the time deplating takes place again. If the same amount of metal is to be deposited as during a direct-current treatment, the current intensity during the electroplating pulse phase must be increased by the amount which flows during the deplating phase. Instead of a current intensity in the previously described example of 6000 A or 12,000 A (using direct current) in this case a current intensity of 6900 A or 13,800 A must be set.
In EP 0 619 846 B1 is described that through the voltage drop at the goods carrier individual partial electrolytic cells are formed in addition between the respective edges of adjacent printed circuit boards in that a voltage drop occurs between the securing points of the printed circuit boards. From the edge regions, lying opposite one another and separated by a spacing, of two adjacent printed circuit boards, some act as a local anode and the others as a local cathode with a cell voltage. This leads to the partially cathodically polarised edge regions being more strongly metallised than the adjacent partially anodically polarised edge regions. When measured, as a result of this effect layer thickness differences of 20% and more were found.
In the past there has been an attempt to avoid these disadvantages by the following measures:
In EP 0 110 846 B1 has been described that the voltage drop at the cathode bars (goods carriers) and supporting rods of the frames holding the printed circuit hoards and at the anode carriers can be reduced by a reduction in the overall current, by increasing the electrical conductivity of the metal of the bars and the frame supporting rods and/or by increasing the cross-sections of the bars and frame supporting rods. However reference was also made to the fact that with a reduction of the current density, the electroplating times have to be lengthened, such that this measure works against the economic efficiency of an electroplating plant. An improvement in the electrical conductivity of the materials could be achieved by the use of copper instead of the stainless steel usually used. However in electroplating plants copper tends to severe corrosion, such that at both the anode and printed circuit board contacts additional and uncontrolled voltage drops occur. Stainless steel, which admittedly has a roughly 40 times lower electrical conductivity, is therefore preferred. Increasing the cross-section of the current-supplying structural elements is generally not possible to any extent and in particular with stainless steel on account of its high specific weight. Moreover very high
Atotech Deutschland GmbH
King Roy
Leader William T.
Paul & Paul
LandOfFree
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