Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly single metal or alloy coating on...
Reexamination Certificate
1999-04-29
2001-01-30
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly single metal or alloy coating on...
C204S228100, C204S229500
Reexamination Certificate
active
06179984
ABSTRACT:
The invention relates to a circuit arrangement and a method for supplying pulse current to one or more electrolytic cells which are connected in parallel. The method is preferably used for the electroplating of printed circuit boards in vertical dipping systems and in vertical and horizontal feed-through systems.
Electroplating by means of pulse current makes it possible to influence certain properties of the metallic layers which are deposited. In this way, the physical properties of the electroplate layers can be altered within wide limits. In particular, throwing power is improved. In addition, the nature of the surface can be influenced. What is also particularly advantageous is that the resulting electroplating current density and the product quality can be considerably increased. But against the advantages in relation to the product to be electroplated is set the disadvantage of the outlay on equipment for the generation of the pulse current. The systems used for supplying the pulse current come quickly up against physical, technical and economic limits if the electroplating is to be done with current pulses which are brief. As “brief” are to be understood here pulse times in the region of 0.1·10
−3
to 10·10
−3
seconds. It is in this time range that electroplating by means of pulse current is particularly effective. Where bipolar pulses are used, the article to be treated is polarised alternately cathodically and anodically. If the article for treatment is to be electroplated, the cathodic current/time product (cathodic charge) must be greater than the anodic current/time product (anodic charge).
The advantages of pulsed electroplating can also be used in the electrolytic treatment of printed circuit boards. Vertical and horizontal electroplating systems are used in the manufacture of printed circuit boards. Systems of this kind are generally of very large spatial dimensions. Furthermore, the electroplating currents are large in this case. For this reason, galvanic rectifiers with correspondingly high capacity must be installed in the electroplating system.
Because there is practically always a lack of space, it is often not possible for the galvanic rectifiers with the pulse generators to be positioned very close to the electroplating tanks. This is the opposite of what is required with the application of pulsed technology: with known methods and systems, the distance between the galvanic rectifier and the bath, i.e. the electrolytic cell, must be very short in order to achieve the necessary edge steepness of the pulse currents. With the usual large pulse currents I
p
and in the case of low bath resistances R
bath
, the inductance L
L
of the conductors between the galvanic rectifier and the electrolytic cell must be kept small. In practice this can only be achieved with very short electric lines. Further measures for reducing the conductor inductance are known, such as for example the transposition of forward and return conductors. This current conductor layout is admittedly possible with cables which have a small conductor cross-section. On the usual conductor rails for high electroplating currents, on the other hand, transposition is impossible. The time constant Tau for the current rise in the bath resistance R
bath
is calculated according to the formula
Tau=L
L
/R
bath
.
If the distance between the galvanic rectifier with its pulse generator and the bath is only, for example, three metres, with a conductor inductance of, for example, 1·10
−6
Henry per metre this would be 6·10
−6
Henry for the forward and return conductor.
If a value of R
bath
=3·10
−3
Ohm is assumed for the bath resistance, this gives, with an admissible disregard of the ohmic conductor resistance, a time constant of
Tau=L
L
/R
bath
=(6·10
−6
Henry)/(3·10
−3
Ohm)=2·10
−3
seconds.
With an ideal voltage rise in the pulse generator the current in the bath resistance R
bath
thus rises within 2·10
−3
seconds to 63% of the maximum current. The speed of this pulse rise is insufficient for circuit board electroplating, for example. In that case the above-mentioned pulse lengths are worked with. The pulse rise time must be correspondingly shorter.
A horizontal electroplating system for printed circuit boards consists for example of twenty-five anodes connected in parallel on the underside of the circuit board and twenty-five anodes connected in parallel on the upper side of the circuit board. The pulse currents on each side are up to 15,000 amperes. The dimensions of an electroplating system of this kind amount, for example, to six metres in the direction of transport. The current conductors from the galvanic rectifier with its pulse generator to the anodes must be of a corresponding length.
A usual system is shown diagrammatically in FIG.
1
:
The circuit boards
1
to be treated are transported through the system in the direction of the arrow, between the upper anodes
2
and the lower anodes
3
of drive elements which are not shown. These anodes can be both soluble and insoluble anodes. In a feed-through system of this kind, each anode forms with its associated cathode (circuit board) and the electrolyte an electrolytic partial cell. By preference, all the upper anodes
2
form together with the upper side of the article to be treated and the electrolyte the upper electrolytic total cell which is supplied with bath current from the galvanic rectifier
5
. Correspondingly, the lower anodes
3
form together with the lower side of the article to be treated and the electrolyte the lower electrolytic total cell. Anodes
2
,
3
are each electrically connected with a common upper galvanic rectifier
5
and a common lower galvanic rectifier
6
via a switching contact
4
. Because of the large size of the rectifier, the distance between the galvanic rectifiers
5
,
6
and the electroplating system amounts in practice to at least a few meters. The current conductor
7
to the upper anodes and current conductor
8
to the lower anodes are of a corresponding length. The common current return conductor
9
closes the circuits of the galvanic rectifiers. The article to be treated is connected to electric line
9
by means of electrical contact elements, for example in the form of clamps
10
which are connected with a slip rail
11
so as to slide and be electrically conductive. The switching contacts
4
, as a rule electro-mechanical contactors, serve to switch the anodes individually when the first conductor boards are transported into the feed-through system and to switch off the anodes individually when the last circuit boards are brought out of the feed-through system or when gaps appear between the circuit boards. The function of switching contacts
4
is described in DE-A-39 39 681. Reference is made to this document.
In traditional technology, the necessary high current rise speeds, in conjunction with the high currents in the current conductors from the pulse generator to the bath, cause severe magnetic disturbance fields. The admissible field strengths for work in the surrounding field of such disturbance fields, are laid down in corresponding standards. Safety measures for protecting the personnel who operate the electroplating systems, as described for example in the German standard VDE 0848, also have to be observed. For this reason, screening measures are necessary which involve considerable technical outlay with correspondingly high costs. Where the electroplating currents are very high, even these measures are ineffective.
The described technical and economic problems with electroplating by means of quick pulses represent reasons why so far this pulse technology has not been used in commercial production.
In WO 89/07162 A1, an electrochemical process is described which uses a pulse method. With the reverse pulse method, at least one bath voltage source in the forward direction, i.e. electroplating, and at least one further bath voltage source in the reverse direction, i.e. etching or deplating, is
Atotech Deutschland GmbH
Gorgos Kathryn
Paul & Paul
Smith-Hicks Erica
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