Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal or alloy coating on...
Patent
1998-06-04
2000-10-17
Gorgos, Kathryn
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal or alloy coating on...
2042306, 2042308, 205104, C25D 518
Patent
active
061325848
DESCRIPTION:
BRIEF SUMMARY
SPECIFICATION
The invention relates to a method for generating short, cyclically repeating, current pulses with great current intensity and with great edge steepness. In addition, it relates to a circuit arrangement for electrolytic metal deposition, especially for carrying out this method. The method finds application in electrolytic metal deposition, preferably in the vertical or horizontal electroplating of printed circuit boards. This type of electroplating is referred to as pulse-plating.
It is known that the electrolytic deposition of metals can be influenced with the aid of pulse-like currents. This affects the chemical and physical properties of the layers deposited. It also affects, however, the even deposition of the layer thickness of the metals on the surface of the workpiece to be treated, the so-called dispersion. The following parameters of the pulsating electroplating current influence these qualities:
In publication DE 27 39 427 A1, electroplating with a pulsating bath current is described. The unipolar pulses here have a width of 0.1 millisecond maximum. The pulse time, the pause time and the pulse amplitude are all variable. Semiconductor switches, here in the form of transistors, serve to generate these pulses. What is disadvantageous about this is that, through the use of switching transistors, the maximum applicable pulsating bath current is technically and economically limited. The upper limit lies at approximately 100 amperes.
The process described in the publication DE 40 05 346 A1 avoids this disadvantage. Here thyristors which can be switched off are used as quick switching elements (GTO: Gate turn-off thyristor) to generate the current pulses. Technically available GTOs are suitable for currents of up to 1,000 amperes and more.
In both cases, the technical outlay has to be reflected, i.e. to be doubled, if bipolar pulses are used. In publication GB-A 2 214 520, which is likewise concerned with pulse plating, a second bath current source is avoided in one form of embodiment by using mechanical, electromechanical or semi-conductor switches to reverse the polarity of the direct current voltage fed in. The necessary high current switches are disadvantageous however. Moreover this system is inflexible since the method must proceed in both polarities with the same current amplitude, for, with short high current pulses, the amplitude cannot be readjusted quickly enough in the bath current sources which are available in practice. Thus, in a further form of embodiment in this publication, two bath current sources are also used which can be adjusted independently of one another. These bath current sources are connected via a change-over switch with the work-piece located in the electrolytic cell and the electrode. Since in printed circuit board electroplating, for reasons of the precision required (constancy of the layer thickness), it is necessary to use individually adjustable bath direct current sources for the front side of the printed board and the rear side of same, there is a doubling of the outlay which is necessary for realizing this method according to this form of embodiment, to four bath current sources altogether.
In addition to this high technical outlay, especially for the respective second bath current source per printed circuit board side, the electronic high current switches cause great energy losses. On each electronic switch, when it is switched on, a voltage drop occurs on the inner non-linear resistor when the current flows. This is true for all kinds of semi-conductor elements in the same way, however with varying sizes of voltage drop. With increasing current, this drop in voltage, also called saturation voltage or forward voltage U.sub.F, becomes greater. With the currents usually used in electroplating technology, e.g. at 1,000 amperes, the forward voltage U.sub.F on diodes and transistors amounts to approximately one volt and on thyristors approximately two volts. The power loss P.sub.V at each of these semi-conductor elements is calculated according to the for
REFERENCES:
patent: 3616434 (1968-04-01), Hausner
patent: 3959088 (1976-05-01), Sullivan
patent: 4208254 (1980-06-01), Mitsumoto et al.
patent: 4517059 (1985-05-01), Loch et al.
Atotech Deutschland GmbH
Gorgos Kathryn
Leaden Willaim T.
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