Electrolytic Processing Apparatus and Method With Time Multiplexed Power Supply

The present invention relates generally to electrolytic processing of materials, for example, electroplating and, more particularly, to alloy plating.

In the field of electroplating an article intended to be plated is placed in an electrolytic bath. The plating material, such as gold, zinc, nickel, silver or other material or a combination of materials intended to be plated onto the article, is dissolved in or is otherwise conveyed into an electrolyte which forms the electroplating bath. Often the plating material is derived from one or more anodes positioned in the bath. The anode is coupled to a power supply and the article intended to be plated also is coupled to the power supply and serves as the cathode in the electrolytic plating system. By applying an electrical potential difference/voltage between the anode and cathode a current flows therebetween through the electrolyte and the plating material migrates anode to the article intended to be plated. The amount of plating material actually plated onto the cathode/article intended to be plated is a function of the applied voltage, current flow through the electrolyte and current density at the article intended to be plated. When the current density varies at different parts of a particular cathode/article, the degree of plating there also will tend to vary correspondingly.

Current typically is a function of applied voltage and the impedance, as is well known. The impedance typically is a function of the efficiency of the electrodes themselves, of the impedance characteristics of the plating bath, and of the spacing of the electrodes in the bath.

It would be desirable to maintain a controlled and uniform current density at the cathode/article intended to be plated in order to achieve a desired controlled, uniform plating thereof.

One type of alloy plating uses the anodes themselves to contribute to the composition of the bath. In such case plural plating materials are applied to the article intended to be plated as an alloy or mixture of such plating materials. The problems encountered with non-uniform current density are further complicated by the additional factor that the concentration of plating material ingredients may vary with time, voltage, plating that forms on a particular anode or cathode, efficiency of the anode and/or cathode, current density, etc. For example, in one alloy plating system with respect to which the invention will be described in detail below, there may be multiple anodes, each of which contributes a separate ingredient into the electrolytic plating bath to form the plating material alloy. The concentration of one plating material relative to the other or others must be maintained in the electrolytic plating bath to obtain the desired constituency or relative concentrations of plating materials on the article intended to be plated. For example, to obtain a plated coating of an alloy of zinc and nickel on a metal article, say of a constituency of 88% zinc 12% nickel, it is necessary that the relative amounts of such zinc and nickel ingredients in the electrolytic bath be at approximately a 62/38 ratio or some other known ratio that is altered according to some function, such as a function of the ionic nature of the respective ingredients that is representative of the tendency of such material to plate onto the cathode/article, the operative efficiency of the anode(s) and/or cathode, etc., as is known. For example, it is known that one material may plate more readily than another due to the fact that there are more free electrons available in one than in the other or there is some difference between the ions as they travel through the electrolytic plating bath from the anode to the cathode.

However, as the concentration of one ingredient relative to the other (others) in the electrolytic plating bath changes, the ratio of those ingredients in the finished plating coating will change, which may cause a variation from specifications for the finished plated article. This, of course, is undesirable.

To alter the concentration relationship of the electrolytic plating bath to bring it back to the desired specification, it is necessary to increase the amount of one ingredient relative to the other. Such increase sometimes is brought about by changing, say increasing, the current/potential difference between the anode that is supplying such ingredient and the cathode. Such a change in current, though may cause a change in current density at the cathode if a corresponding adjustment in the current/potential difference between the other anode and the cathode is not made, which changes the uniform plating thereat and also can cause a difference in the amount of one ingredient that is plated onto the cathode/article relative to the amount of the other ingredient that is plated onto the cathode/article. Such a change in current also causes a potential difference between the anode whose voltage has been changed by increasing current there and the other anode which is supplying the other ingredient to the plating bath, thus possibly causing undesirable plating of the first ingredient onto the other anode, which in turn can cause an undesirable shift in operation, anode efficiency, and/or concentration in the electrolytic plating bath.

Another type of electrolytic processing is known as electrolytic polishing or simply electropolishing. In electropolishing high points or roughness causing flaws in the surface of a material intended to be polished are removed to improve smoothness of the surface. In electropolishing the part intended to be polished is placed in an electrolytic bath and serves as one electrode of the electropolishing system. A second electrode also is placed in the bath. A potential difference of prescribed polarity is applied between such electrodes so that current flows from the part toward the other electrode. Due to the direction of current flow, the part is referred to as the anode, and the other electrode is referred to as the cathode. Since the high points on the part tend to concentrate current or at least tend to experience higher current density than the already smoother surface portions, material at the high points tends to be removed with the current flow, thus effecting polishing. The various problems encountered in electroplating also can detrimentally affect electropolishing.

The present invention helps solve the above problems and disadvantages encountered in prior electroprocessing systems and methods.

Another problem encountered in the past has been the tendency of some electroplating baths to grow or to increase in the amount of a particular ingredient therein. Sometimes additives have been added to the bath to control such growth, and sometimes inert electrodes (anodes) have been used to control such growth. The present invention described below may be used to control power to such control electrodes, i.e., the inert electrodes.

Further, in the past electrodes formed of an alloy material have been used to provide the desired concentration of alloy ingredients to an electroplating bath. One example is an alloy of tin and lead. Using the present invention, though, separate electrodes, one of tin and the other of lead, can be controlled to provide the desired concentration of ingredients to the plating bath.

Briefly, according to the present invention an electrolytic processing system and method provide and control power in a time multiplexed fashion.

Two examples of time multiplexing according to the invention include pulse width modulation of applied signals and frequency modulation (also known as pulse position modulation) of applied signals.

According to the invention the time multiplexing may be used to provide current control, voltage control, or both. A preferred embodiment described in detail below provides current control while maintaining substantially constant current density at the cathode.

According to another aspect of the invention the concentration of ingredients, such as those derived from electrodes, can be controlled using the time multiplexing control features.

Most desirably, the invention provides a variety of control functions and/or capabilities while maintaining constant or substantially constant current density at the cathode to help assure uniformity of plating.

Various other features of the invention, as are described herein, may be employed in electrolytic processing systems and processes, such as in electroplating, and especially in alloy plating, in electropolishing and in other systems.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent as the following description proceeds. It will be appreciated that while a preferred embodiment of the invention is described herein, the scope of the invention is to be determined by the claims and equivalents thereof.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described in the specification and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but several of the various ways in which the principles of the invention may be suitably employed.

In the annexed drawings,

FIG. 1 is a schematic illustration of an electroplating apparatus in accordance with the invention;

FIG. 2 is a schematic electric circuit block diagram of a power supply in accordance with the invention;

FIG. 3 is a schematic electric circuit block diagram of another power supply useful in accordance with the invention; and

FIGS. 4 and 5 are schematic electric circuit diagrams of the power supply of FIG. 3.

Referring, now, in detail to the drawings, wherein like reference numerals designate like parts in the several figures, and initially to FIG. 1, an electroplating apparatus in accordance with the present invention is generally designated 10. The apparatus 10 includes a tank 11 containing an electroplating bath 12 in which are located plural anodes 13a through 13f. More or fewer anodes may be used. For alloy plating at least two or more of the anodes may be of different materials so that such respective anodes deposit such respective materials into the electroplating bath 12. The quantity of material (usually metal) deposited by an electrode (typically the anode) in an electroplating bath is a function of the anode efficiency, the number of coulombs, the bath composition, and/or the electro-chemical nature of the material of which the electrode is constituted. The material of which such bath is formed may be a conventional electrolyte. The material of which such anodes are formed may be conventional materials used for electroplating; exemplary materials are zinc and nickel. Other ingredients, additives and the like also may be included in the bath as is conventional.

Placement of the anodes in the bath 12 may be according to usual convention as a function of deposition of a plated coating on a cathode 14. The cathode 14 in fact preferably is the article intended to be coated. The cathode 14 may be a single cathode or may represent several cathodes.

If the apparatus 10 were used for electropolishing, for example, then the part to be polished would be the anode and the other electrode(s) would be the cathode(s), as is well known.

In the preferred embodiment several of the anodes 13a-13c are supported from a common electrically conductive support bar 15; and several anodes 13d-13f are supported from a second common electrically conductive support bar 16. The support bars 15, 16 may be of copper or other conventional material. The anodes 13a-13c may be of nickel and the anodes 13d-13f may be of zinc. More or fewer anodes may be used and they may be of more than two materials when alloy plating, depending on the nature of the plating material. Also, if desired, the anodes may be of the same material when the plating is to be of only one material, i.e. an embodiment of the invention in which the plating may not necessarily be alloy plating.

The cathode 14 may be a single article or several articles intended to be plated. Such cathode 14 may be supported from a single electrically conductive support bar 17, for example of copper or other material, or from several such bars. As is well known, the anodes and cathode preferably are strategically located in the plating bath 12 relative to each other in order to obtain a particular current density at various portions of the cathode and, thus, particular plating characteristics at those portions. The support bars 15, 16, 17 may be suspended from rails 18 or by come other conventional mechanism used in an electroplating system to position the electrodes 13, 14 in the plating bath 12.

It will be appreciated that although the invention is described and illustrated in detail for electroplating an article 14 that remains placed in a single location in the bath 12, the features of the invention also may be employed for strip plating.

An electrical power supply 20 according to the invention is coupled to supply electrical power to the anodes and cathode. More specifically, such power supply is coupled by a conductor 21 to the cathode 14 and by respective conductors 22, 23 to the respective groups of anodes that are respectively supported from the electrically conductive support bars 15, 16, as is illustrated in FIG. 1. The power supply provides a potential difference or voltage between the cathode 14 and one group of anodes 13a-13c at one period of time and between the cathode 14 and the other group of anodes 13d-13f at a different period of time. Preferably the power supply 20 supplies current to flow between the cathode and the respective anodes to cause plating on the cathode. Since the power supply preferably is directly coupled to supply power to the electrically conductive support bars 15, 16, 17, such support bars also will be referred to below for convenience as the power outputs or power output terminals of the power supply.

According to the invention, the power supply 20 provides electrical power in a time multiplexing type of procedure. Such time multiplexing is carried out using pulse width modulation techniques according to one embodiment. Such time multiplexing can be carried out using frequency modulation techniques according to another embodiment. Other time multiplexing techniques that provide functions similar to those disclosed also may be used.

What preferably is meant by time multiplexing is that for a particular time period a voltage is applied between the conductors 21 and 22, for example, thereby providing voltage between the cathode 14 and the first group of anodes 13a-13c and causing current flow therebetween; and for a the same or a different length of time, but in any event at a different moment in time, a voltage is applied between the conductors 21 and 23 thereby providing a voltage between the cathode 14 and the second group of anodes 13d-13f and causing current flow therebetween. Also, according to the preferred embodiment of the invention, current control is provided so that the current flow preferably remains constant, regardless of whether it is flowing between the cathode and the first group of anodes or between the cathode and the second group of anodes. Therefore, the current density at the cathode remains constant.

Since the current density at the cathode remains substantially constant, uniform plating in a controlled fashion can be achieved. Indeed, depending on the set up, e.g., the positional arrangement, efficiency and size of the respective anode(s) relative to the cathode(s), different current densities and, thus, plating characteristics, can be obtained in substantially repeatable fashion.

The actual process or electrochemical mechanism of electrolytic plating at the cathode occurs as it does in conventional electroplating processes. Plating material in the bath plates on the cathode 14 when current flows between anode(s) and the cathode. The actual current and voltage, as well as current density, employed during electroplating using the invention may be selected according to conventional electroplating processes. For example, the actual current may be from less than one amp to thousands of amps or even in the million-amp range. The voltages would, of course, be a function of current and impedance between respective electrodes in the plating bath, as is well known.

Moreover, by changing the length of time that each anode or group of anodes is energized, i.e. has a voltage applied between such anode(s) and the cathode 14 and is the source of current in the bath 12, the concentration of the ingredient supplied to the plating bath 12 by such anode(s) relative to the ingredient supplied by the other anode(s) of a different material can be controlled. Especially, the ratio of such ingredients in the bath can be maintained substantially constant. Also, as the efficiency of one electrode changes, e.g., due to some plating material or the like depositing thereon, corrections can be made by the power supply 20 altering such respective lengths of time to continue maintaining uniformity of the plating bath composition. Therefore, the nature of the alloy plated coating on the cathode can be closely controlled.

As a corollary, if desired, over a period of time, the concentration of one ingredient in the bath 12 can be changed relative to the other or other ingredients, thereby to provide an alloy plated coating on the cathode that varies in a specified manner through the thickness of the coating, for example.

According to the preferred embodiment, the actual length of time that each anode or group of anodes is on, i.e., energized or receiving power, may be set by the user in order to obtain the desired plating operation and/or plating characteristics on the article being plated. However, the relative amount of time that each anode or group of anodes is on relative to the other(s) can be set or controlled manually or automatically to maintain a desired composition of the plating bath preferably while maintaining constant current density during plating, i.e., when power is on.

It will be appreciated that although the invention is described in detail with respect to alloy plating, features of the invention may be employed in other electroplating systems, electropolishing systems and other electrolytic processing systems.

Turning to FIG. 2, a block diagram of a power supply circuit 20 useful in the invention is illustrated. The circuit 20 includes an input rectifier 30 which receives input AC electrical power A. The various exemplary signals occurring in the circuit 20 are designated by alphabet letters. Such signals are illustrated as examples, but other shapes of signals and other signals also may be employed. Also, the signal wave shapes are provided only to facilitate the explanation of the circuit, and values are not indicated. The values would depend on the particular apparatus 10 and the power required to accomplish the desired plating function.

Preferably such rectifier 30 is a full wave rectifier which rectifies the input AC power A to provide full wave rectified power B. A filter 31 filters the full wave rectified power to provide a substantially continuous DC signal C having a particular amplitude. The rectifier 30 and filter 31 may be eliminated in the event that another source of power, such as a DC source, were provided to node 32 or even elsewhere in the power supply circuit 20. Also, although the invention is described in detail below relative to a circuit that operates on a DC power input supplied to node 32, the features of the invention may be employed in a circuit that operates based on AC power input supplied to node 32 by altering the below described various components and interrelationships thereof of the power supply switching circuits 33, 34, as will be evident to those having ordinary skill in the art.

Switching circuits 33, 34 preferably are identical or substantially identical. The purpose of the switching circuits 33, 34 is to provide voltage between the cathode 14 and the respective anodes 13. Accordingly, the switching circuit 33 provides voltage on conductor 22 to the power output terminal (support) 15 for energizing anodes 13a-13c; and the switching circuit 34 provides voltage on conductor 23 to the power output terminal (support) 16 for energizing anodes 13d-13f. Such voltage or energization is with respect to the cathode 14, which is coupled to ground or to some other source of reference voltage potential at power output terminal (support) 16 (not shown in FIG. 2).

The switching circuits 33, 34 are operated by a general control 35, which determines when and for how long each switching circuit will be enabled to energize the anodes connected thereto. Preferably the general control 35 assures that only one group of anodes will be energized at a particular time.

The general control 35 may be, for example, a conventional free running oscillator that is controllable to determine when and for how long each switching circuit is on. The general control 35 also may be some other device. The general control 35 may change the relative lengths of time that the switching circuits 33, 34 are enabled in order to maintain a desired bath composition, while the switching circuits are operative to maintain constant or otherwise controlled current density.

Each switching circuit 33, 34 preferably is the same. Therefore, only the circuit 33 will be described in detail. The circuit 33 includes a switching element 40 that is able to switch power on and off as it is received from the node 32. Such switching element 40 may be for example a conventional power field effect transistor, one type of which is sold under the trademark HEXFET. Whether the switching element 40 is passing electrical power to conductor 41 or not is determined by a conventional pulse width modulator control circuit (PWM) 42 when PWM 42 is selectively and sequentially enabled by the general control 35. The output signal produced by the general control 35 is delivered on lines 36, 37 to the PWM circuits 42 in the respective switching circuits 33, 34. Such output signal has frequency and duty cycle characteristics that can be adjusted or controlled automatically or manually, e.g., to determine and to control the composition of the bath 12. The signals on lines 36, 37 are preferably complementary so that when one is on to enable a respective PWM 42, the other is off, and vice versa.

The PWM 42 may be an integrated circuit part No. 3524. Such part includes an oscillator. Therefore, when the PWM 42 is enabled by the general control 35, such PWM produces an AC output signal on line 38 (line 39 in the switching circuit 34). An exemplary frequency for such AC output signal on lines 38 and 39 may be on the order of about 100 KHz. The duty cycle or pulse width of such AC signal can be controlled to control current or voltage at the anode(s) energized by the respective switching circuit 33, 34 in which the PWM 42 thereof is coupled as is described further below. When the PWM 42 in a particular switching circuit 33, 34 no longer is enabled by the general control 35, it stops providing the indicated AC signal; at that time preferably the other PWM 42 in the other switching circuit produces its AC signal. The respective PWM circuits 42 drive the respective switching elements 40 in the switching circuits 33, 34.

The output signal D from the switching element 40 is a square wave signal of relatively high frequency compared to the frequency of the input AC power A. The pulse width of such square wave signal is determined by the PWM 42; the frequency may be controlled by the general control 35. Such relatively high frequency square wave signal can be transformed by a power transformer 43 to a desired output voltage signal E of, for example, twenty-four volts peak to peak. As is well known a high frequency signal usually can be transformed more efficiently than can be a relatively lower frequency signal, and this preferably is taken into consideration in determining the frequency with which the switching element is switched.

An output rectifier 44 is coupled to receive the signal E from the transformer 43. The output rectifier 44 preferably is a full wave rectifier which provides a substantially constant magnitude full wave rectified output voltage F. The rectified voltage F is filtered by an output filter 45 to provide a substantially constant DC voltage on line 22 whenever the full wave rectified voltage F is received. Since the switching element 40, transformer 43, and output rectifier 44 are operating on AC generally square wave signals, the nature of the voltage at the output of the filter 45 supplied to conductor 22 will be a substantially constant, i.e. unvarying, DC voltage relative to the source of ground reference potential 46, for example taken back at the input filter 31. (The signal G' is shown at output 16 from switching circuit 34. Signals G and G' are out of phase so when one is on the other is off, as is seen by indicated times t.sub.o to t.sub.1 for signal G and times t.sub.1 to t.sub.2 for signal G'.)

The output filter 45 is coupled by a current sensor feedback line 50 to the PWM 42; and the line 22 is coupled by a voltage feedback line 51 to the PWM 42. Such feedback signals on lines 50, 51 provide control functions to the PWM 42 to help assure that the voltage at line 22 is maintained constant and that the current flow from the energized anodes remains uniform and constant as the respective switching circuits 33, 34 energize respective anodes coupled to respective power outputs 15, 16. Current and voltage adjustments 52, 53 may be set by the user and/or automatically to establish the desired current, current density and/or voltage that the PWM 42 attempts to maintain in the bath 12. For example, to maintain a particular current, the current sensor line 50 provides an input to the PWM 42 which in turn adjusts the duty cycle of the AC signal on line 38, i.e., the percentage or ratio of on time and off time in each on/off cycle of the switching element 40 as driven by the PWM 42. Current in the bath will be a function of such percentage or ratio.

The power supply 20 operates, as follows. Input power is supplied to the input rectifier 30. That power is full wave rectified by the filter 31 and is provided at node 32 to the switching circuits 33, 34. The general control 35 determines when and for how long each of the switching circuits 33, 34 will be operative to energize the respective anodes coupled to the outputs 15, 16 thereof. Ordinarily, when one switching circuit is energizing the anodes connected thereto, the other is off, and vice versa. The pulse width modulator control 42 in each switching circuit 33, 34 operates the switching element 40 thereof when energized to produce the signal D at a pulse width relation determined by the settings of the PWM and the current and/or voltage feedback on lines 50, 51 and over a period of time determined by the general control 35. That signal D is transformed by the high frequency power transformer 43 and is full wave rectified by the output rectifier 44. The full wave rectified signal is delivered via an output filter 45 in each switching circuit to the respective power output 15 (for switching circuit 33), 16 (for switching circuit 34).

In accordance with the invention the switching circuits 33, 34 are operative so that the current flow from the respective anodes coupled to the respective outputs 15, 16 will be constant and uniform so that the current density at the cathode remains constant regardless of the particular output 15 or 16 that is being energized at any given time. Further, by altering the amount of time that one anode or group of anodes is energized relative to the amount of time that another anode or group of anodes is energized, the concentration of ingredients supplied to the electroplating bath by the respective anodes can be controlled and/or altered.

The present invention is particularly suited to automated control, for example, of bath composition. Thus, a sampling tap 54 may obtain sample material in the bath 12 and deliver such sample continuously or periodically via a flow line or other means 55 to an analyzer 56. An exemplary analyzer may be a conventional X-ray fluorescence analyzer, which detects bath composition. Information concerning bath composition may be coupled electrically by electrical connection 57 to the power supply 20 as is seen in FIG. 1. Such electrical connection 57 may be provided as an input to the general control 35 to cause the general control to change or to maintain the relative amounts of time that the switching circuits 33, 34 are respectively enabled, thereby to control bath composition. If desired, conventional interface and/or control circuitry, possibly including a computer, may be coupled between the analyzer 56 and the power supply 20, as is represented at 58, to decode output information from the analyzer for control use at the power supply.

Although the circuit 20 uses a single input rectifier 30 and filter 31, it will be appreciated that each of the switching circuits 33, 34 may be supplied with electrical power from a separate input rectifier and associated filter or other circuitry from a common or separate supplies of power. Also, although separate power transformers 43 are used in each switching circuit 33, 34, the invention may use a single power transformer 43 shared by both circuits 33, 34, an example being in the power supply circuit 60 of FIG. 3. Therefore, the embodiment of FIG. 2 depicts switching of power before the power transformer and the embodiment of FIGS. 3 and 4 depict switching of power after the power transformer. The power supply 60 may be substituted for the power supply 20 in the apparatus 10 of FIG. 1. It will be appreciated that other types of circuitry may be employed in accordance with the present invention to accomplish the time multiplexed type of operation of the electrodes in an electrolytic materials processing system.

Turning to FIG. 3, another power supply 60, which may be used in accordance with the present invention, to supply power to plural groups of anodes coupled to power outputs 15, 16, respectively, such as anodes 13a-c connected to output 15 and anodes 13e-f coupled to output 16 in FIG. 1. The power supply 60 includes a DC power supply that has both current control and voltage control capability represented by potentiometers 62, 63, respectively. A current sensor 64 senses the current flow at the cathode 14, which is coupled to the conductor output 17, and provides an input on line 65 to the DC power supply 61.

The power supply 60 includes a dual output circuit control 70, which is described in greater detail below with respect to FIGS. 4 and 5. The dual output circuit control 70 is coupled to a pair of output channels 71, 72 and determines which is selected at any time to operate through respective driver output circuits 73, 74 to provide power via lines 75, 76 to the respective power outputs 15, 16. Details of the output channels 71, 72 and of the circuit 60 as a whole are described in detail with respect to FIGS. 4 and 5. Fundamentally, though, it will be appreciated that the DC power supply 61 provides a source of DC power to the dual output circuit control 70. Such circuit control 70 selects which of the output channels 71, 72 is to be energized and cooperates with the respective driver output circuits 73, 74 to supply power at a desired level to the respective power outputs 15, 16 and, accordingly, to the several anodes, respectively. The power supply 60 may include more than two output channels.

Referring to FIG. 4, a schematic electric circuit diagram of the power supply 60 is illustrated. The power supply 60 includes a DC power supply portion 61, the dual circuit control 70, driver outputs 73, 74, and a current sensor portion 64. Such components cooperate to provide desired power at power outputs 15, 16 coupled to the respective anodes relative to the power output 17 coupled to the cathode. In FIG. 5 is illustrated a portion of the driver output circuitry, as will be described further below.

The DC power supply 61 includes a high voltage power input 79, for example, of 150 volts or other voltage level as may be desired, such as a DC power input from a conventional source of DC voltage (not shown), and a chassis ground connection 80. The DC power supply 61 couples power via a transformer 81 to the output channels 71, 72. The current sensor circuitry 64 is coupled via a transformer 82 to the DC power supply 61 in order to control the same. The preferred form of control is in such a fashion that the current provided at the terminals 15, 16 relative to the terminal 17 will remain substantially constant and uniform regardless of which output channel 71, 72 is energized and, accordingly, regardless of whether current is flowing from the output terminal 15 relative to the cathode terminal 17 or from the output terminal 16 relative to the cathode terminal 17.

In the DC power supply 61 are included a pair of power field-effect transistors (FET), such as those sold under the trademark HEXFET, 83, 84 and a pair of diodes 85, 86, which are connected relative to each other and relative to the two secondaries in the transformer 82 and the primary in the transformer 81 in full wave bridge configuration. The FETs 83, 84 are operated such that both are on at the same time or both are off at the same time. As is evident from the illustration in FIG. 4, by periodically turning the FETs 83, 84 to conductive or on state, current flows through the primary of the transformer 81 in one direction, and when the FETS 83, 84 are turned off, diodes 85 and 86 demagnetize the transformer 81. Depending on the nature of the signal provided the primary of the transformer 82 from the current sensor circuitry 64, the magnitude of current in the secondary of the transformer 81 is controlled. Therefore, when more or less current is desired at one of the power outputs 15, 16, the DC power supply 61 increases or decreases the power coupled by the transformer 81 which in turn is provided at node 87 for coupling into the respective output channels 71, 72. The actual signal at node 87 is a pulsating voltage signal. The frequency of the voltage pulses at node 87 may be changed under control of the feedback current sensor 64 depending on whether more or less current or power is needed at the power output terminals 15, 16.

Zener diode circuits 88 may be coupled across the drain and source electrodes of the FETs 83, 84 for usual protective purposes, limiting the voltage drop therebetween. Also, resistors 89 may be coupled in circuit with the secondaries of the transformer 81 and the respective zener diode circuits for conventional protective purpose limiting current in the diodes.

To develop the current control signal provided by the current sensor circuitry 64 to the DC power supply 61, an input terminal 90 is coupled to a selected one of the power output terminals 15, 16. Either one may be used or both may be used. For illustrative purposes, a jumper 91 is shown connected to the power output terminal 15. An accurate current sensor resistor 92 is coupled between the input terminal 90 and the circuit ground 93, and a voltage then is developed on line 94 representative of the current flowing at the power output terminal 15. A conventional display 95 is coupled to the line 94 to display an indication of the actual current flow from the selected power output terminal 15 or 16. The display 95 preferably includes a calibration circuit to provide full scale calibration. A capacitor 96 conducts high frequency signals on line 94 to the circuit ground 93. Moreover, a coupling and isolation amplifier 97 amplifies the signal on line 94 and provides the same as a voltage on line 100 to a comparator 101, which is formed by an operational amplifier. The voltage on line 100 is coupled to the inverting input of the comparator 101. A reference voltage is supplied to the non-inverting input. The reference voltage is developed across a resistor 102 and a potentiometer 103, which are coupled between the circuit ground and a source of voltage, such as the V.sub.cc voltage of 12 volts provided form the conventional circuit power supply (not shown) coupled at terminal 104. A capacitor 105 stabilizes the input voltage supplied to the non-inverting input of the comparator 101. The potentiometer 103 is referred to as the current setting potentiometer, for by adjusting that potentiometer, the magnitude of current supplied at the output terminals 15, 16 can be adjusted (set), as will be evident from the following description.

The output voltage M on line 106 at the output from the comparator 101 ordinarily will be a logic 1 or a logic 0 level, i.e., on or off, depending on whether the current at the power output terminal 15, 16, which had been selected by the jumper 91, is greater or is less than the current setting adjustment of the potentiometer 103. As the current at the selected output terminal 15, for example, periodically exceeds or does not exceed that current established in the current setting potentiometer 103, the signal M will take a characteristic of sometimes being on and sometimes being off. The signal M is coupled via a resistor 107 and a resistor-capacitor circuit 108 to the input of a voltage controlled oscillator (VCO) 110. The resistor 107 and the RC circuit 108 cooperate to convert the signal M to a DC voltage level signal represented at N.

The voltage controlled oscillator 110 produces an AC output signal P on line 111. The frequency of the signal P is a function of the magnitude of the voltage of the signal N. Therefore, as the comparator 101 detects that the current at the selected power output 15, for example, is closer to or is further from the desired current as determined by the setting of the potentiometer 103, the voltage N at the input of the VCO will change and the frequency of the signal P on line 111 will vary correspondingly. Preferably, the voltage controlled oscillator 110 is a linear voltage controlled oscillator that always provides the signal P such that there is a 50% duty cycle of the output signal P even though the frequency of the signal P may vary. Therefore, in each full cycle of positive-going and negative-going portions of the signal P, the duration of each positive-going portion will be the same as the duration of each negative-going portion for the particular frequency of the signal P.

The signal P is coupled via a capacitor 112 to the input of a transistor circuit 113, which functions in a sense as a one-shot multivibrator. The purpose of circuit 113 is to prevent saturating the transformer 82. In particular, if the frequency of the signal P is large enough, it will operate via the transistor 114 and the pair of amplifying transistors 115 to be coupled through a capacitor 116 to line 65, thus providing an AC signal to the primary of transformer 82. The frequency of the signal on line 65 will be increased or decreased as a function of whether current at the power output 15, for example, requires increasing or decreasing.

The multivibrator function of the transistor circuit is achieved by a bias circuit 120 which includes a resistor 121 a capacitor 122, and potentiometer 123. The circuit 120 and the potentiometer 123 cooperate with the capacitor 112 to assure the length of any pulse delivered by the transistors 113, 115 to the transformer 82 will be short enough that the transformer 82 will not saturate.

When the output from the VCO is of high enough frequency, such signal passes through capacitor 112 and directly drives the transistor 114. However, when such frequency is relatively slow compared to the time constant of the capacitor 112 and the resistance setting of potentiometer 123, such capacitor and potentiometer cooperate to limit the pulse width of the signal delivered on line 65 to the transformer 82 to one that appears as having a frequency that is large enough to avoid saturation of the transformer. For example, transistor 114 is biased to conduction to hold transistors 115 off. When a negative going pulse begins on output 111 from VCO 110, transistor 114 turns off and transistors 115 turn on sending a signal to transformer 82. If the VCO output on line 111 does not return to a high or relatively positive level (relative to the mentioned negative going pulse), the capacitor 112 charges in time according to the RC time constant with resistance of potentiometer 123 to a level that again biases transistor 114 on and transistors 115 off. Such time constant is adequately fast to assure that transformer 82 will not saturate.

The pulsating voltage at node 87 is derived from the output of the secondary of transformer 81, RC filter 130 and diodes 131, 132. Such pulsations voltage is the source of current that will be provided drive outputs 15, 16. Voltage at the node 87 is provided via a conventional inductor 133 and capacitor 134 LC filter, which provides a DC voltage level at node 87. Due to the operation of the LC filter, the magnitude of the DC voltage at the node 87 will be a function of the frequency of the voltage pulses at node 87. The voltage at node 87 is supplied to the inputs of a pair of power FETs 135, 136. The control inputs of the respective FETs are coupled to control lines 137, 138. When a signal is on a respective line 137, 138, the respective FET provides to the respective power output terminal 15, 16, current flow to energize the respective anodes coupled to such power output terminals. The driver output circuits 73, 74 generate such control signals for delivery on lines 127, 138 to the FETs 135, 136, as now will be described.

Since the invention provides for operation of the circuit 60 in current control mode, whereby the current output at output terminal 15, for example, is maintained constant due to feedback through the current sensing resistor 92 and current sensor circuitry 64, the magnitude of the voltage at node 87 will be a function of the actual load, e.g., the load across the terminals 15 and 17 (and any additional load, such as that provided by the resistors 15a and 16a. The load across terminals 15, 17 is a function of the impedance of the electroplating bath 12 and the spacing of the anodes and cathode. Resistors 15a and 16a provide protection by supplying a finite load for the circuit in case the anodes and cathode are not in the bath when the power supply circuit 60 is on.

The voltage at node 87 is coupled via diodes 140, 141, resistors 142, 143, Zener diodes 144, 145, capacitors 146, 147 to a photosensitive module 150, 151 of a conventional opto-isolator and to a transistor output circuit 152, 153. Referring to the output channel 71, when a light input, which is represented by arrow 154, is supplied to the photosensor 150, the latter produces a signal on line 155 to provide a signal via the transistor circuit 152 and resistor 156 to the line 137 turning on the FET 135. The FET 135 then delivers current to the power output terminal 15. Similar operation occurs in the output channel 72 when light 164 is provided the photosensor 151, which then provides a signal on line 165 to cause the transistor circuit 153 to produce a signal via resistor 166 onto the control line 138. The FET 136 then turns on to provide current to the power output terminal 16.

The optical signals 154, 164 are developed in the dual output circuit control 70 portion illustrated in FIG. 5 and, more particularly, in fact are emitted by respective light emitting diodes 170, 171 of respective opto-isolators that include the respective photosensors 150, 151. The circuit 70 includes a pair of timing circuits 172, 173, each of which may include a conventional 555 integrated circuit timer 174, 175 connected generally in the manner illustrated. The timing circuit 172 determines the cycle frequency of the dual output circuit control 70. The timing circuit 173 determines the relative duty cycle of the two portions of the signal which makes up an entire cycle; specifically, the timing circuit 173 determines what proportion of each complete cycle the light emitting diode 170 emits light and what proportion of the complete cycle the light emitting diode 171 emits light.

To establish the cycle frequency, an input circuit 176 is coupled via line 177 to an input of the timing circuit 172. The input circuit 176 includes several resistors 180, 181 and a potentiometer 182, all of which are coupled in series between the V.sub.cc power supply 104 and the circuit ground 93. The wiper arm of the potentiometer 182 is connected to the base of a transistor 183, which in turn is connected in a charging circuit that includes a resistor 184 and a capacitor 185. Depending on the setting of the potentiometer 182 and the values of the resistor 184 and capacitor 185, such capacitor 185 will charge at a prescribed rate that will determine the frequency of the signal produced on the output 186 of the 555 timer 174. Current flow through the resistor 184 represents the frequency of such signal on line 186 and can be monitored and displayed by a conventional display, such as a liquid crystal display, represented at 187.

The input line 177 and the output line 186 for the 555 timer 174 are coupled to inputs of the 555 timer 175. Moreover, a control input circuit 190 is coupled to another input of the 555 timer 175 to determine the portion of each full cycle of output signal Q produced on output line 191 from the 555 timer 175, that is, at a logic 1 or "on" level and what portion is at a logic 0 or "off" level. The input control circuit includes a series connection of a pair of resistors 192, 193 and a potentiometer 194. By adjusting the wiper arm of the potentiometer 194, the percentage or proportion mentioned can be changed. Since the voltage drop between the potentiometer wiper arm and one other terminal of such potentiometer represents such percentage or proportion, that voltage can be monitored and displayed, for example, in a conventional liquid crystal display, which is represented schematically at 195.

Output line 191 is coupled to a complementary output circuit 200, which decodes the signal Q so as to turn on the light emitting diode 170 to emit light when the signal Q exhibits a logic 0 or off state and to turn on the light emitting diode 171 when the signal Q exhibits a logic 1 or on state. The complementary output circuit 200 includes a transistor 201, which is coupled via an RC circuit 202 to the line 191, a resistor 203, which is coupled to the V.sub.cc voltage source 104, and a resistor 204, which is coupled to the light emitting diode 170. When the signal Q on output line 191 from the 555 timer 175 is at logic 0 level, the transistor 201 is not conducting; therefore, current can flow from the V.sub.cc source 104 via resistors 203, 204 through light emitting diode 170 to cause the latter to emit light. Since the signal Q is at a logic 0 level, the light emitting diode 171 will not emit light. However, when the signal Q is at a logic 1 level, such signal will bias the transistor to an on or conducting condition so as to shunt current away from the light emitting diode 170, which then will not emit light. Additionally, when the signal Q is in a logic 1 condition, such signal will operate through resistor 205 to cause light emitting diode 171 to emit light.

In operation of the power supply 60, the potentiometer 182 (FIG. 5) can be adjusted to determine the frequency that signals are developed in the opto-isolators to drive the respective FETs 135, 136. Adjustment of the potentiometer 194 (FIG. 5) determines the proportion of time that FET 135 is on relative to the time that FET 136 is on and, therefore, the proportion that time current is supplied to respective power outputs 15, 16. Adjustment of the potentiometer 103 (FIG. 4) determines the magnitude of current actually delivered to the power output terminals 15, 16. The current sensor circuitry 64 and DC power supply 61 cooperate to maintain a substantially constant current level, as is set by the potentiometer 103, at the power output terminals during plating.

It will be appreciated from a review of the circuit portion 70 of FIG. 4 that the outputs to the opto-isolators are complementary. Therefore, when one light output is on, the other is off; and vice versa. An advantage to the complementary outputs is that the current density in the bath may be maintained substantially constant without regard to which anode is being energized at a particular time.

However, it will be appreciated that other means may be provided for developing the driving signals for the FETs 135, 136, either optical or otherwise. Moreover, the means for developing the driving signals for the FETs 135, 136 may either be coupled in the complementary manner illustrated or may be developed independently. An example of independent developing of such signals may be achieved by using separate monostable multivibrators independently controlled by the current control circuit 64 and/or by the circuit portion 70, as will be evident to those having ordinary skill in the art.

An advantage of the present invention is the ability to adjust the circuit 60 to compensate for a difference in the efficiency and electrochemical equivalent of the various electrodes. For example, relative to the cathode one anode may be relatively efficient, say 90 percent efficient, and a second anode may less (or more) efficient, say 80 percent efficient. Over time the ingredient added to the plating bath by the more efficient anode will increase in the bath relative to another ingredient supplied by the other less efficient anode.

The invention permits a change in the times that each of the anodes is energized thereby to maintain uniform composition of the bath.

As was mentioned above with respect to the power supply 20 in FIGS. 1 and 2, the invention lends itself to both automated and manual control. Manual control, e.g., by setting the current density at potentiometer 103, and relative amounts of energization times for the respective cathodes or groups of cathodes, e.g., by setting the potentiometer 194, has been described above for the power supply 60 of FIGS. 3 and 4. Automated control would include the use of a sampling tap 54 , analyzer 56 and possibly additional interface and/or control circuitry of conventional design 58, that would provide an input to the power supply circuit 60. An exemplary connection would be one shown at 57' in FIG. 5 to bias the 555 timer 175 causing it to change the relative amounts of time that the respective electrodes or groups of electrodes are energized by power supply 60. Another automated control connection may be to the VCO 110, as is described further below.

Another advantage of the present invention is that although the invention is particularly suited for use in alloy plating using electrodes of different materials, it is usefull, too, for plating using electrodes of the same material and even in circumstances in which the plating material is furnished the electroplating bath as an additive to the bath, i.e., not being directly supplied by a respective anode. Using the time multiplexing features of the invention, especially also with the constant current feature, one anode or group of anodes may be energized for one period of time, and subsequently a different anode or group of anodes may be energized. During the time that an anode or group of anodes is deenergized such anode or group may depolarize. The amount of time that the anodes are deenergized can be determined in the power supply circuit 20 by the general control 35; and the amount of time that the anodes are deenergized can be determined in the power supply circuit 60 by alteration of the frequency of the VCO 110. Further, if the power supply circuits of the invention were altered for use to energize respectively and sequentially three or more anodes or groups of anodes, then the amount of time that a particular anode or group is deenergized can be further increased to facilitate and/or to enhance depolarization. Moreover, since the invention can be use both for alloy plating and for single material plating, or for plating in which the relative amounts of time that each electrode or electrode group are energized is the same, a savings in equipment cost can be realized because only one power supply may be used to perform both types of plating functions.

Another advantage of the present invention is the ability to prevent so-called creep, growth or increase of a particular ingredient in an electroplating bath. This is a particular problem in alkaline zinc plating wherein the anode efficiency can be greater than the cathode efficiency. As is known, inert anodes sometimes are placed in an electroplating bath together with anodes that contribute material to the bath. When the concentration of an ingredient contributed by the latter anodes exceeds a prescribed level or is at least at a desired level, the inert electrodes are energized and the others are deenergized. In this way electroplating can continue while using up the mentioned ingredient from the bath after which the ingredient contributing anodes will be energized instead of the inert electrodes. The present invention is suited for use in such an environment because the respective power supplies may be switched in time multiplexed fashion to energize the ingredient contributing anodes and the inert anodes for respective amounts of time such that the particular ingredient that tends to grow in the bath will remain substantially at a stable concentration in the bath, while plating still is carried out at constant current. Moreover, the relative amounts of time that such electrodes may be energized can be changed, as was described above, further to change the concentration of the mentioned ingredient in the bath.

Various features of the invention shown and/or described with respect to a particular drawing figure or embodiment hereof may be used with the other drawing figures or embodiments hereof, as will be evident to those having ordinary skill in the art upon reading the instant disclosure.

From the foregoing it will be appreciated that the present invention may be used to plate objects in electroplating processes.