Apparatus With Power Source For Plating

Abstract

Apparatus and method for plating with a pulsating direct current variable in frequency so as to obtain desirable plating characteristics. The amplitude as well as the frequency of the plating current may be adjusted to obtain superior plating results.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates in general to plating, and in particular to a new and novel plating apparatus and method wherein an alternating current is superimposed on a direct current applied to a plating bath and the amplitude and frequency of superimposed alternating current produces improved plating characteristics.

2. Description of the Prior Art

My prior patent entitled "Electroplating," U.S. Pat. No. 2,824,830 discloses superimposing on a DC field in a plating bath at least two high-frequency fields of relatively high frequencies.

SUMMARY OF THE INVENTION

An improved apparatus and method of plating which uses semiconductor devices for producing a direct current plating signal upon which is superimposed alternating current pulses of varying amplitudes and which may be varied in frequency and in time on and time off to control the shape of the AC pulses and obtain much improved plating results over the prior art. An amplifier is provided which when connected in the circuit prevents feedback.

Other and further objects of this invention will be apparent to those skilled in this art from the following detailed description of the annexed sheets of drawings which, by way of a preferred embodiment of the invention, illustrate one example of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the plating apparatus of this invention.

FIG. 2 is a graph of the plating response in which the AC signal has a frequency of 450 hertz.

FIG. 3 is a graph of the plating response using an AC pulse of 550 hertz.

FIG. 4 illustrates a plating characteristic utilizing an AC pulse of 700 hertz;

FIG. 5 illustrates a plating characteristic utilizing a 600 hertz signal; and

FIG. 6 illustrates a plating response utilizing an AC signal of 1,800 hertz .

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a plating bath 10 which includes a cathode 11 and an anode 12 to which plating leads 13 and 14 are attached, respectively. A cathode-ray oscilloscope 16 connected across the leads 13 and 14 indicates to an operator the wave shape of the applied signal to the plating bath.

For example, plating may be carried out in a chrome plating aqueous electrolyte bath, having 250 grams per liter of CrO.sub.3 (as chromic acid), 1.8 percent sulfuric acid (H.sub.2 SO.sub.4), 3 percent boric acid (H.sub.3 BO.sub.4), and 1 percent oxalic acid. In this bath, the essential pH control is provided by the sulfuric acid, but superior results are obtained by the use also of substantially 0.5-5.0 percent boric acid and 0.1-3.0 percent oxalic acid. (Unless otherwise specified, the terms "parts" and "percentage" mean the same by weight.) The plating bath is maintained at 135.degree. F., while plating with a cathode current density of about 2-3 amperes per square inch. The anode may be lead or other conventional material. The specific cathode A.S.I. (DC) is measured when the pulsating current in operation is recited with other operating conditions in FIGS. 2 through 6, and under the conditions established in such figures.

A resistor R.sub.1 is also connected across leads 13 and 14. An ammeter 17 is connected in line 13 and is bridged by a suitable shunt R.sub.2. A diode D.sub.1 is connected to resistor R.sub.2 and a condenser C.sub.1 is connected from line 14 to diode D.sub.1. A voltmeter 18 is connected across condenser C.sub.1. A source of 3-phase 220 volts, for example, AC is connected to terminals 19, 20 and 21 and through a circuit breaker 22 to a Y primary 23 of a transformer. A light 24, for indicating that power is on, is connected between contacts 20 and 19 in series with a resistor R.sub.3. A delta-connected portion of the transformer is designated as 24 and is connected to adjustable wiper contacts 26, 27 and 28 which engage the Y-connected primary 23 to vary the voltage to the unit 24 of the transformer. A Y-connected output 29 is magnetically coupled to the portion 24 and is connected to a full-wave rectifier designated generally as 31, and which comprises the diodes D.sub.2 through D.sub.7. An inductor L.sub.1 is connected between the diodes D.sub.2, D.sub.3 and D.sub.4 and the diode D.sub.1. Thus the rectifier 31 supplies rectified direct current to the plating bath. The inductor L.sub.1 and capacitor C.sub.1 provide a smoothing filter. Lead 32 is connected to lead 14 and provides an input lead for superimposing an AC pulsing signal on the direct current supplied to the plating bath. A second lead 33 provides the second lead for providing the AC pulsating input to the plating bath. A pair of input leads 34 and 36 are connected to a suitable AC source as, for example, 120-volt single-phase alternating current power. The circuit breaker 37 is connected in circuit with input terminals 34 and 36. A resistor R.sub.4 and indicator light 38 are connected across the input terminals 34 and 36. Primary 39 of an auto transformer is connected to terminals 34 and 36, and has a secondary 41 which has a slide contact that may be controlled by the knob 42. A secondary 43 is coupled to the winding 41, and supplies an input to a rectifier designated generally as 44, comprising the diodes D.sub.8 through D.sub.11. Condenser C.sub.2 is connected across the output terminals of the rectifier 44. A variable potentiometer 46 is connected to point B and controls the time off of the alternating current output. The potentiometer 46 is connected to the base of a transistor T.sub.1 which has its collector connected to the base of a transistor T.sub.2. The collector of transistor T.sub.2 is connected to a tunnel diode D.sub.12. A time-on potentiometer 47 is connected to the emitter of transistor T.sub.1. A pair of diodes D.sub.30 and D.sub.14 are connected in series between the emitters of transistors T.sub.1 and T.sub.2. A resistor R.sub.5 is connected between the tunnel diode D.sub.12 and the base of a transistor T.sub.3. The emitter of the transistor T.sub.3 is connected to the base of transistor T.sub.4. The collector of transistor T.sub.4 is connected to the bases of transistors T.sub.5 through T.sub.9. A lead 48 is also connected to the potentiometer 46 and to resistors R.sub.6 through R.sub.11 which have their opposite sides connected to the emitters of the transistors T.sub.5 through T.sub.9. An input terminal for reverse bias 49 is connected to lead 48 and a terminal 51 is connected to the bases of the transistors T.sub.5 through T.sub.9 through the resistor R.sub.12. Lead 33 is connected to the collectors of the transistors T.sub.5 through T.sub.9 through a switch 52. The switch 52 is closed when the solenoid 53 is energized. A bias voltage is supplied by rectifiers 54 and 56 which respectively comprise diodes D.sub.13 through D.sub.16 and diodes D.sub.17 through D.sub.20. The rectifier 54 is connected across the secondary 57 of a transformer which has its primary 58 connected to the power leads 34 and 36. A primary 59 of a transformer is also connected to the input power leads 34 and 36 and has its secondary 61 connected to the rectifier 56. A resistor R.sub.13 is connected to one output of the rectifier 54 and condensers C.sub.5, C.sub.6 and C.sub.7 are connected in parallel between the other output lead of the rectifier 54 and resistor R.sub.13. A lead 62 is connected between the resistor R.sub.13 and point B of the rectifier 44. The time-on potentiometer is also connected to the condensers C.sub.5, C.sub.6 and C.sub.7. A lead 63 is connected to the output of the rectifiers 54 and 56 and a resistor R.sub.14 is connected between the lead 63 and the collector of the transistor T.sub.2. Lead 63 is also connected to the emitter of the transistor T.sub.4. A light 64 is connected across the output of the rectifier 56. A resistor R.sub.15 is connected in series with a diode D.sub.21 in one output line of the rectifier 56 and a pair of condensers C.sub.8 and C.sub.9 are connected in parallel between the leads 66 and 67. A resistor R.sub.16 and a variable resistor R.sub.17 and a resistor R.sub.18 and a capacitor C.sub.10 are connected between the collector of transistor T.sub.1 and lead 66. A resistor R.sub.19 is connected from resistor R.sub.16 to a unijunction transistor UT.sub.1 and particularly to the base 1 terminal. The emitter terminal of the unijunction UT.sub.1 is connected to the junction point between the resistor R.sub.18 and the capacitor C.sub.10. The base 2 electrode of the unijunction transistor UT.sub.1 is connected to resistors R.sub.20 and R.sub.21 which have their opposite sides connected to the lead 66. A diode D.sub.22 is connected from the base 2 of the unijunction transistor UT.sub.1 to a resistor R.sub.22 which has its opposite side connected to resistor R.sub.19 and a resistor R.sub.23 which has its opposite side connected to the lead 67. A capacitor C.sub.13 is connected between resistor R.sub.23 and a resistor R.sub.24 which has its opposite side connected to the lead 67. An SCR.sub.1 is connected between capacitor C.sub.13 and lead 66 and has its control electrode connected to the base 2 of the unijunction transistor UT.sub.1. A second SCR.sub.2 is connected between resistor R.sub.24 and lead 66 and has its control electrode connected to the base 2 electrode of a unijunction transistor UT.sub.2. A resistor R.sub.25 is connected between the control electrode of the SCR.sub.2 and lead 66. A resistor R.sub.26 is connected from resistor R.sub.24 to the first base electrode of the unijunction transistor UT.sub.2. Variable resistor R.sub.27 is connected in series with the resistor R.sub.28 between the resistor R.sub.26 and the emitter of the unijunction transistor UT.sub.2. The condenser C.sub.11 is connected between lead 66 and resistor R.sub.26. The solenoid 53 is connected across the output terminals of the rectifier 56.

In operation, power is applied to terminals 19, 20, 21, 34 and 36, and the potentiometers 46 and 47 are adjusted with knob 42 and contacts 26, 27 and 28 to obtain an input to the electrodes 11 and 12 of the plating bath to obtain a greatly improved plating characteristic. The AC pulses applied by leads 32 and 33 to the bath are superimposed upon the direct current from the rectifier 31 and the frequency of the pulses may be adjusted as well as the "on" and "off" duty cycle by controlling the potentiometers 46 and 47, for example. The transistors T.sub.1 through T.sub.9 operate as a pulse generator and a meter 70 is connected from lead 48 to lead 32 and indicates the amplitude of the pulses. The unijunction transistors UT.sub.1 and UT.sub.2 and SCRs, SCR.sub.1 and SCR.sub.2 prevent feedback and provide an improved output of the pulse generator.

FIGS. 2 through 6 illustrate the plating characteristics that may be obtained with different adjustments at the circuit of FIG. 1. FIG. 2, for example, illustrates the plating characteristic utilizing an output AC pulse of 450 hertz with an amplitude of 7 volts and with the direct current voltage read by meter 18 being 5 volts, and the meter 17 reading 3 amperes, and with the pulse current being equal to 11/2 amperes. It was found that the plating had a Rockwell hardness of C68, that the quality of the plating was poor, that the throwing power was poor and that the buildup was heavy, but that there were no bleeders. As shown in FIG. 2, the plating rate was relatively slow.

FIG. 3 illustrates the plating characteristic using a frequency output of the pulse generator of 550 hertz with meter 70 reading 7 volts, the meter 18 reading 5 volts, and the DC current read by meter 17 being 3 amperes, and the pulse amperage being 11/2 amperes. The Rockwell hardness was C72; quality of the plating was excellent; throwing power was excellent, and there were no bleeders or buildup. It is to be particularly noted that the plating rate was over twice as great as the plating which occurred under the conditions of FIG. 2. Specifically, in 4 hours of plating, the characteristics of FIG. 2 resulted in a thickness of 0.006 inch, whereas the characteristic of FIG. 3 resulted in a plating thickness of 0.014 inch.

FIG. 4 is a plating characteristic utilizing a pulse frequency of 700 hertz at an amplitude of 6 volts and with a DC voltage being equal to 5 volts and a DC amperage equal to 3 amperes, and the pulse amperage being 11/2. The Rockwell hardness for the plating was C70; the quality was good; the throwing power was good; there were no bleeders, and there was only slight buildup.

FIG. 5 illustrates a plating characteristic utilizing a pulse frequency of 600 hertz at an amplitude of 5 volts, with the DC voltage being 6, the DC amperage being 3 and the pulse amperage being 11/2. The Rockwell hardness was C72; the quality and throwing power were excellent, and there were no bleeders or buildup.

FIG. 6 illustrates a plating characteristic utilizing 1,800 hertz for the pulse frequency, with the amplitude equal to 5 volts. The DC voltage was 5; pulse voltage was 31/2; DC amperage was 2 amperes, and the pulse amperage was 11/2. The Rockwell hardness was C70; the quality and throwing power were fair; there were no bleeders, and there was only slight buildup.

It is seen that the characteristics illustrated in FIG. 3 give much improved results over the characteristics illustrated in the other figures -- 2, 4, 5 and 6. Thus, by controlling the amplitude of the pulses and the frequency of the pulses, much-improved plating results may be obtained. The pulse-on and pulse-off time controls the pulse current, and the conditions illustrated in FIG. 3 provide much-improved plating results.

It is interesting to note the plating speed variation as a function of frequency. Note that at 450 and 1,800 hertz, the plating thickness is 0.006 of an inch after 4 hours; whereas at 550 hertz the plating reaches 0.014 of an inch at 4 hours.

Although I have herein set forth my invention with respect to certain specific principles and details thereof, it will be understood that these may be varied without departing from the spirit and scope of the invention as set forth in the hereunto appended claims.

Claims

I claim as my invention:

1. In an electroplating apparatus having a plating bath and electrodes, an improved source of power comprising:

a source of direct current voltage attached to said electrodes,

an alternating current pulse generator producing a series of output pulses which are superimposed on the output of the source of direct current voltage, means for adjusting the length of each output pulse as a function of time forming a part of said pulse generator, means for adjusting the time off of each of said output pulses independent of the length of said pulses forming a part of said pulse generating, means for adjusting the amplitude of said output pulses forming a part of said pulse generator, and means for adjusting the amplitude of the output of said direct current source.

2. A source of power according to claim 1 wherein the ratio of the time on to time off of the pulse generator varies from on 10 percent of the time to on 80 percent of the time.

3. A source of power according to claim 1 wherein the frequency of said pulse generator is between 500 to 700 hertz.

4. A source of power according to claim 1 where the frequency of said pulse generator is in the vicinity of 550 hertz.

5. A source of power according to claim 1 including means for preventing feedback in the pulse generator.