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Pulse and pulse-reverse electroplating
PULSE AND PULSE-REVERSE ELECTROPLATING by N.V. Mandich HBM Engineering Co., Lansing, III. Electroplating with pulse current (PC) and pulse-reverse current (PRC) is a technique where standard, stationary DC current is replaced with its modulated (nonstationary) forms. Although it has been recognized for a long time electroplating with modulated currents was mostly dormant and used only occasionally for PRC plating of cyanide copper where better leveling was recognized early in the 1940s and 1950s. Lack of appropriate current sources and/or its prohibitive cost hampered the further interest for this otherwise promising technique for the next 4 decades. Although it was known that the morphology of gold and gold alloys would be influenced by periodically reversed or pulsed current, great interest was developed only when it was realized that pulse-plated gold deposits proved superior to DC-plated deposits for certain electronic applications. The age of electronics and materials shortages gave new importance to gold as a commodity, and a boom in gold prices led to a demand for ways of reducing gold consumption. Pulsing proved to be an answer and this justified the more expensive pulse rectifiers. After that theoretical contributions and industrial applications followed.
GENERAL CHARACTERISTICS OF UNIPOLAR AND BIPOLAR CURRENTS Before the effect of pulsed current on various aspects of metal deposition is elaborated it is necessary to examine the way in which pulsed current waveforms are characterized. The advent of modem electronics and microprocessor control has permitted great flexibility of the programming of the applied current waveform. Trains of pulses can be programmed to give very complex waveforms. Square waves are the easiest waveforms to produce because this requires only a switching arrangement rather than a specialized waveform generator. Current waveforms can be divided into two major groups: These are unipolar pulses, where all the pulses are in one direction, and bipolar pulses. where anodic and cathodic pulses are mixed. There are many variants on these, but as the complexity of the waveform increases so does the number of variables, which makes it more difficult to understand how a particular waveform affects the electrodeposition process. Consequently, the present discussion is restricted to the consideration of the simplest case of unipolar and bipolar pulses, i.e., pulse and pulse reverse. In order to characterize a direct current it is sufficient to know the current density. The characterization of a train of current pulses requires three parameters to be known. These are the cathodic peak pulse current density, i.. the cathodic pulse length, t.; and the interval between the pulses. tp • as shown in Fig. I. In practice average current densitY.jAv' is measured, and it is expressed in the case of PC as follows: (I)
A quantity frequently encountered is the duty cycle, T, representing the portion of time in each cycle when the current is on. It is defined by the following equation:
T = tj(te + lp)
(2)
The product of the duty cycle and the peak pulse current density gives the PC average current density. (3)
382
Unipolar Pulsing Pulse
Superimposed Pulse
Ie
Duplex Pulse
MfU. .JU1J1 Pulsed Pulse
Pulse-on Pulse
BipolarPulsing (PRC) Pulse Reverse
J-
la -;,-
Pulsed Pulse Reverse
Pulse Reverse (with offtime)
Pulse-On Pulse Reverse
o
Fig. I. Nonstationary current waveforms for unipolar and bipolar pulsing.
383
The characterization of a PRC waveform requires four parameters to be known and defined by the following equations:
JAV = Ue X i, + j.t.)/(te +
t.)
(4)
where t. = anodic pulse time. T PRe = Ue X te where j.
-
j.t.)\(je te )
(5)
=anodic current density.
THE EFFECT OF PULSED CURRENT ON DEPOSIT PROPERTIES Unipolar pulsed current justified its practical application mainly through its ability to affect the mechanisms of electrocrystallization, which in turn, controls the physical and mechanical properties of the electrodeposited metal. Because the nucleation rate of a growing electrodeposit is proportional to the applied current density, the use of high current density pulses can produce deposits with reduced porosity and, in most cases, a finer grain size. Whether a finer grained deposit is obtained in practice depends upon what happens during tp when the current is interrupted because this can allow the desorption of impurities and encourage renucleation with the formation of new, smaller crystal grains. The effects obtained in practice are also dependent on the specific electrochemical system to which a particular set of the current pulses is applied. A common example of different grain growth caused by the desorption of impurities is observed when current pulses are applied to an acid gold-cobalt alloy plating system. In this case deposits by pulse current have a very low carbon and nitrogen content and the grain size is either decreased or increased depending on duty cycle. There is also a significant increase in the ductility of these deposits and an increase in electrical conductivity and density, as well as a decrease in porosity. Decrease in porosity in practice means that a thinner gold deposit can be applied and still pass porosity tests, resulting in a significant cost saving. Another rather unexpected metal where pulse current plating has found its application is nickel, which was of practical interest due to the ability of pulsed current to control the levels of stress and ductility in nickel deposits. This has some obvious applications in the e/ectrofonning industry where stress control is of paramount importance.
LIMITATIONS OF PC IMPOSED BY DUTY CYCLE AND FREQUENCY The rate of metal deposition is dictated by the average current density and is equivalent to the applied current density in DC plating. Now, the first major limitation of pulse plating from a practical viewpoint becomes evident. In order to produce the same average deposition rate as for DC. as duty cycle is reduced, the peak pulse current density needs to be increased. For example, at a duty cycle of 10%, a peak pulsed current density of 40 Ndm 2 is required for an average current density of 4 Ndm 2 • In practical applications this would seldom be viable due to limitations of rectifier capacity. As duty cycle is increased it begins to approach DC; so a compromise situation needs to be achieved. If sufficient spare rectifier capacity is available a duty cycle of 33 to 50% is probably the minimum practical value. For applications using high current, such as chromium plating, a value of 50 to 75% would be the lowest usable duty cycle. After considering the practical limitations of duty cycle it is necessary to consider the effects of frequency, which is defined as the reciprocal of the cycle time, t:
f = I/(te + lp) = lit
(6)
At high f, the double layer does not have time to charge fully during the r, or fully discharge during the tp time. This has a smoothing effect on the applied waveform, which
384
begins to approach DC current, and this limits the maximum useful frequency to around 500 Hz for most applications; however, higher frequencies can be used where very high pulse current densities are employed because the double layer charge and discharge times become shorter as peak pulse current density is increased. Some manufacturers of pulse-plating equipment advocate the use of very high frequencies, but the practical maximum frequency that can be applied to a plating electrolyte is limited by the capacitance of the double layer at the interface between the plating electrolyte and the article being plated.
THE EFFECT OF PULSED CURRENT ON THE MAXIMUM RATE OF DEPOSITION Unsubstantiated claims have often been made that pulse plating can improve the maximum rate of deposition obtainable from plating baths by several orders of magnitude. The theory behind this reasoning is that as frequency is increased, the diffusion layer becomes thinner; however, the diffusion layer, which is encountered with pulse plating, also pulses at the high frequency applied to a plating system. The diffusion layer does not have time to disperse completely between pulses, and the total thickness of the diffusion layer approaches that obtained when plating with DC current. Consequently, the use of pulse current has very little effect on the limiting current density. Metal ions cannot be discharged faster than they are supplied to the cathode surface. The rate of supply of metal ions depends only on hydrodynamic and concentration factors and is little influenced by the applied waveform. The major factors governing the supply of metal ions to the cathode surface are hydrodynamic factors: rate of agitation, viscosity, diffusion coefficient, and the metal ion concentration. The rate of consumption of metal ions depends only on the average current density and is not influenced to any extent by the shape of the applied waveform. Although pulse plating cannot increase the theoretical limiting current density of a plating electrolyte it must be taken into account that the maximum practical current density at which a plating electrolyte is usually operated is only 20 to 30% of the theoretical limiting current density. This is because mass transport effects can produce burning at higher current densities areas. Obviously, there is room for improvement, and higher practical plating rates can sometimes be achieved with pulse current due to improved deposit properties. This effect is attributed to the influence of pulsed current on the electrocrystallization rather than on an enhancement of the rate of mass transport.
THE EFFECT OF PULSE CURRENT ON CURRENT DISTRIBUTION The cell geometry dictates primary current density distribution; therefore, it is not affected by electrochemical parameters and would be unaffected by the applied current waveform. The primary current distribution is modified in plating solutions by the secondary current distribution, which arises due to the effects of activation (chemical) overpotential. In general the greater the rate of change of potential with increasing current density the more the Overallcurrent density distribution tends toward a secondary distribution and, excluding mass transport effects, produces a more even metal distribution. As current density is increased the electrode resistance decreases and the rate of change of potential with current density also becomes less. Thus, as current density is increased the current distribution tends toward a primary distribution and the throwing power usually deteriorates. In electrolytes where metal ions are not strongly complexed and cathodic efficiency is high a low current density usually Produces a better throwing power than a high current density, With any form of modulated current, whatever the shape of the waveform and whether unipolar or bipolar pulses are used in order to maintain the same rate of deposition as for DC current, the cathodic current density dUring the cathodic pulse must be higher than the equivalent DC current density. This means
385
that the overall current density distribution tends toward a primary distribution when pulse or PRC plating are employed. Consequently. the deposit distribution in general would be expected to be less uniform using pulsed current than with direct current; however, in electrolytes where the cathodic efficiency is less than 100%. the use of pulsed current may change the relative efficiencies of metal deposition and hydrogen evolution and may under a particular set of conditions produce an increase in throwing power.
PRACTICAL APPLICATIONS OF PULSE PLATING The application of pulse current leads to the improvement in the quality of a number of industrial electrodeposits. By using pulse current in comparison with the deposits obtained by DC one can achieve smoother deposits of copper. nickel. and zinc from sulfate solution and gold and copper from cyanide solutions, with the average current densities and plating times being kept equal. Also, an equal or better quality of the deposits of copper, gold. and zinc is obtained at higher current densities under pulse current than under DC conditions. A decrease of grain size of the pulse-plated deposits is generally found to lead to an increased coverage of the substrate with the same quality of the deposited metal. resulting in decreased porosity and surface resistance. and increased density of the metals. It can be expected that this increase in compactness is associated with a decrease in internal stresses and increased ductility and change of hardness of metal deposits. In situations where hydrogen is codeposited (current efficiency less than 100%) the current distribution can be better in pulse current regimes as compared with DC current.
PULSE·REVERSE PLATING PRC plating is the bipolar electrodeposition process where DC current is continuously changing its direction (polarity). Quite popular in the 1950s. it then found its major use in improving leveling action in hot cyanide copper plating baths; however, the use of these baths was drastically reduced due to environmental problems and inherent build-up of carbonates. and PRC was relegated to secondary duties such as for very effective electrocleaning. as well as for derusting and desrnutting. Theoretical studies were. therefore. not pursued to any extent. In the I970s new theoretical contributions were introduced. and in the 1980s and 1990s it became quite obvious that PRC can give answers to many complicated electrodeposition problems. Theory became much more complex and intriguing. The essence of current reversal is that during metal electrodeposition the polarity automatically changes and the duration of the reverse (anodic) current is normally a small fraction, e.g.• 20% or less. of the duration of the current in the direct (cathodic) direction. The bottom part of Fig. I shows the different kinds of nonstationary PRe currents. Periodically reversed current is characterized by the following properties: a current in the forward direction (cathodic polarization current). ie; a time for the metal deposition (cathode period). te; anodic polarization (reverse) time period. ta ; and a total period of Ihe current reversal. I. + Ie' The average density is given by equation 4. In the electrodeposition of metals by a reversed current the electricity qe flowing through the electrodes during the cathode period, Ie' must be greater than the amount of electricity q. flowing during the time of the anode period I., i.e.• ie Ie > i. t.. Consequently, by this method of deposition, the whole quantity of electricity is not consumed in the formation of the deposit. A significantly smaller portion is expended for the anodic polarization (often called depleting, stripping, or reversing) of the plated products. During each total period of plating with current reversal a layer of metal of a determined thickness is built up on the surface of the products. and during anodic polarization (t.). part of the deposited layer is dissolved. In general. to increase the rate of metal deposition. conditions at the electrode surface must be created under which the limiting cathodic current density and passivating anodic
386
current are significantly increased. At the same time a high metal-to-current efficiency must be maintained and the protective properties of the electrodeposits must satisfy the requirements. As was noted earlier, until relatively recently, these conditions were met mainly by changing DC variables: the metal concentration, temperature and agitation of the bath, as well as by introducing various additives. Despite the fact that PRC deposition occurs with the incomplete use of the whole quantity of electricity applied on the electrodes, this method of metal deposition often permits acceleration of the electroplating rates. Faster deposition can be accomplished by using a higher working current densities than compared with a DC current. Initially, this is caused by the prevention of depletion of metal ions in the cathode double layer, to which the anodic dissolution of the deposit contributes. The upper limit of the working current densities during the metal deposition is a function of the parameters of the reversed current ta and ia and also of the length of the total period. The limiting cathodic current density usually increases with an increase of the ratio t.ftc and with a decrease in total time. The a higher working current density during the PRC deposition as compared with DC plating is a result of the periodic depolarization of the electrodes. Such an effect of the current reversal on the electrode processes, as well as the electropolishing action of the anodic current on the deposit, permits ensuing electrocrystallization on the cathode surface in the preferred direction (smaller crystal grains) under a higher working (practical) current density. During the cathodic period this leads to grain refinement usually associated with unipolar, pulse current pulse plating, which is now further supported by the disturbance of growth steps by the periodic inversion of the current. In a number of DC electroplating processes for example, during the electrodeposition of certain metals from solutions of their simple salts, the upper limit of the working current density can be, under certain conditions, held close in magnitude to the theoretical limiting current. Often, however, under real, actual plating conditions, the working current density of the cathode is limited to the admissible upper limit of the anodic current density. Exceeding this limit can cause anode passivation when the electroplating bath is operating in a DC mode (e.g., nickel, brass, and cyanide and acid copper baths). Now, current reversal can prevent passivation of the anodes during the period of high cathodic current and sometimes contributes to a significant increase of the upper limit of the anode working current density and, thus, in tum, increases the maximum working cathode current density.
PRACTICAL APPLICATIONS OF PULSE-REVERSE PLATING Electrolytes containing organic additives may undergo changes in the deposition mechanism when PRC current is used, which can produce changes in the uniformity of the deposition that have a beneficial affect on throwing power. When electrodepositing a copper from acid baths containing organic additives substantial improvements in hole throwing power can be achieved for deposition of copper on printed circuit boards. The use of a current of alternating polarity permits not only increasing the working current density, but as a result of the periodic action of the anodic current on the deposited layer of metal, the deposit acquires better protective properties as demonstrated in actual practice for acid copper, nickel, gold, chromium, and zinc. For a number of processes, current reversal is the means of achieving smoother and brighter deposits, better current distribution, and less porous deposits with lower internal stresses. In the case of chromium plating higher current efficiency is easily achieved, whereas for nickel plating, brightness and leveling can be obtained with much reduced concentration of organic additives with additional important benefits of increased ductility and reduced stress. It is obvious that PRC plating has more variables than DC plating. More variables mean more complex plating equipment and controls but also wider choice of deposit characteristics that can be tailored for a particular application, e.g.. mcreased corrosion protection, leveling, different hardness, ductility, stress values, and alloy composition and, in the case of chromium, results in higher cathode current efficiency.
387
GENERAL CHARACTERISTICS OF UNIPOLAR AND BIPOLAR CURRENTS Before the effect of pulsed current on various aspects of metal deposition is elaborated it is necessary to examine the way in which pulsed current waveforms are characterized. The advent of modem electronics and microprocessor control has permitted great flexibility of the programming of the applied current waveform. Trains of pulses can be programmed to give very complex waveforms. Square waves are the easiest waveforms to produce because this requires only a switching arrangement rather than a specialized waveform generator. Current waveforms can be divided into two major groups: These are unipolar pulses, where all the pulses are in one direction, and bipolar pulses. where anodic and cathodic pulses are mixed. There are many variants on these, but as the complexity of the waveform increases so does the number of variables, which makes it more difficult to understand how a particular waveform affects the electrodeposition process. Consequently, the present discussion is restricted to the consideration of the simplest case of unipolar and bipolar pulses, i.e., pulse and pulse reverse. In order to characterize a direct current it is sufficient to know the current density. The characterization of a train of current pulses requires three parameters to be known. These are the cathodic peak pulse current density, i.. the cathodic pulse length, t.; and the interval between the pulses. tp • as shown in Fig. I. In practice average current densitY.jAv' is measured, and it is expressed in the case of PC as follows: (I)
A quantity frequently encountered is the duty cycle, T, representing the portion of time in each cycle when the current is on. It is defined by the following equation:
T = tj(te + lp)
(2)
The product of the duty cycle and the peak pulse current density gives the PC average current density. (3)
382
Unipolar Pulsing Pulse
Superimposed Pulse
Ie
Duplex Pulse
MfU. .JU1J1 Pulsed Pulse
Pulse-on Pulse
BipolarPulsing (PRC) Pulse Reverse
J-
la -;,-
Pulsed Pulse Reverse
Pulse Reverse (with offtime)
Pulse-On Pulse Reverse
o
Fig. I. Nonstationary current waveforms for unipolar and bipolar pulsing.
383
The characterization of a PRC waveform requires four parameters to be known and defined by the following equations:
JAV = Ue X i, + j.t.)/(te +
t.)
(4)
where t. = anodic pulse time. T PRe = Ue X te where j.
-
j.t.)\(je te )
(5)
=anodic current density.
THE EFFECT OF PULSED CURRENT ON DEPOSIT PROPERTIES Unipolar pulsed current justified its practical application mainly through its ability to affect the mechanisms of electrocrystallization, which in turn, controls the physical and mechanical properties of the electrodeposited metal. Because the nucleation rate of a growing electrodeposit is proportional to the applied current density, the use of high current density pulses can produce deposits with reduced porosity and, in most cases, a finer grain size. Whether a finer grained deposit is obtained in practice depends upon what happens during tp when the current is interrupted because this can allow the desorption of impurities and encourage renucleation with the formation of new, smaller crystal grains. The effects obtained in practice are also dependent on the specific electrochemical system to which a particular set of the current pulses is applied. A common example of different grain growth caused by the desorption of impurities is observed when current pulses are applied to an acid gold-cobalt alloy plating system. In this case deposits by pulse current have a very low carbon and nitrogen content and the grain size is either decreased or increased depending on duty cycle. There is also a significant increase in the ductility of these deposits and an increase in electrical conductivity and density, as well as a decrease in porosity. Decrease in porosity in practice means that a thinner gold deposit can be applied and still pass porosity tests, resulting in a significant cost saving. Another rather unexpected metal where pulse current plating has found its application is nickel, which was of practical interest due to the ability of pulsed current to control the levels of stress and ductility in nickel deposits. This has some obvious applications in the e/ectrofonning industry where stress control is of paramount importance.
LIMITATIONS OF PC IMPOSED BY DUTY CYCLE AND FREQUENCY The rate of metal deposition is dictated by the average current density and is equivalent to the applied current density in DC plating. Now, the first major limitation of pulse plating from a practical viewpoint becomes evident. In order to produce the same average deposition rate as for DC. as duty cycle is reduced, the peak pulse current density needs to be increased. For example, at a duty cycle of 10%, a peak pulsed current density of 40 Ndm 2 is required for an average current density of 4 Ndm 2 • In practical applications this would seldom be viable due to limitations of rectifier capacity. As duty cycle is increased it begins to approach DC; so a compromise situation needs to be achieved. If sufficient spare rectifier capacity is available a duty cycle of 33 to 50% is probably the minimum practical value. For applications using high current, such as chromium plating, a value of 50 to 75% would be the lowest usable duty cycle. After considering the practical limitations of duty cycle it is necessary to consider the effects of frequency, which is defined as the reciprocal of the cycle time, t:
f = I/(te + lp) = lit
(6)
At high f, the double layer does not have time to charge fully during the r, or fully discharge during the tp time. This has a smoothing effect on the applied waveform, which
384
begins to approach DC current, and this limits the maximum useful frequency to around 500 Hz for most applications; however, higher frequencies can be used where very high pulse current densities are employed because the double layer charge and discharge times become shorter as peak pulse current density is increased. Some manufacturers of pulse-plating equipment advocate the use of very high frequencies, but the practical maximum frequency that can be applied to a plating electrolyte is limited by the capacitance of the double layer at the interface between the plating electrolyte and the article being plated.
THE EFFECT OF PULSED CURRENT ON THE MAXIMUM RATE OF DEPOSITION Unsubstantiated claims have often been made that pulse plating can improve the maximum rate of deposition obtainable from plating baths by several orders of magnitude. The theory behind this reasoning is that as frequency is increased, the diffusion layer becomes thinner; however, the diffusion layer, which is encountered with pulse plating, also pulses at the high frequency applied to a plating system. The diffusion layer does not have time to disperse completely between pulses, and the total thickness of the diffusion layer approaches that obtained when plating with DC current. Consequently, the use of pulse current has very little effect on the limiting current density. Metal ions cannot be discharged faster than they are supplied to the cathode surface. The rate of supply of metal ions depends only on hydrodynamic and concentration factors and is little influenced by the applied waveform. The major factors governing the supply of metal ions to the cathode surface are hydrodynamic factors: rate of agitation, viscosity, diffusion coefficient, and the metal ion concentration. The rate of consumption of metal ions depends only on the average current density and is not influenced to any extent by the shape of the applied waveform. Although pulse plating cannot increase the theoretical limiting current density of a plating electrolyte it must be taken into account that the maximum practical current density at which a plating electrolyte is usually operated is only 20 to 30% of the theoretical limiting current density. This is because mass transport effects can produce burning at higher current densities areas. Obviously, there is room for improvement, and higher practical plating rates can sometimes be achieved with pulse current due to improved deposit properties. This effect is attributed to the influence of pulsed current on the electrocrystallization rather than on an enhancement of the rate of mass transport.
THE EFFECT OF PULSE CURRENT ON CURRENT DISTRIBUTION The cell geometry dictates primary current density distribution; therefore, it is not affected by electrochemical parameters and would be unaffected by the applied current waveform. The primary current distribution is modified in plating solutions by the secondary current distribution, which arises due to the effects of activation (chemical) overpotential. In general the greater the rate of change of potential with increasing current density the more the Overallcurrent density distribution tends toward a secondary distribution and, excluding mass transport effects, produces a more even metal distribution. As current density is increased the electrode resistance decreases and the rate of change of potential with current density also becomes less. Thus, as current density is increased the current distribution tends toward a primary distribution and the throwing power usually deteriorates. In electrolytes where metal ions are not strongly complexed and cathodic efficiency is high a low current density usually Produces a better throwing power than a high current density, With any form of modulated current, whatever the shape of the waveform and whether unipolar or bipolar pulses are used in order to maintain the same rate of deposition as for DC current, the cathodic current density dUring the cathodic pulse must be higher than the equivalent DC current density. This means
385
that the overall current density distribution tends toward a primary distribution when pulse or PRC plating are employed. Consequently. the deposit distribution in general would be expected to be less uniform using pulsed current than with direct current; however, in electrolytes where the cathodic efficiency is less than 100%. the use of pulsed current may change the relative efficiencies of metal deposition and hydrogen evolution and may under a particular set of conditions produce an increase in throwing power.
PRACTICAL APPLICATIONS OF PULSE PLATING The application of pulse current leads to the improvement in the quality of a number of industrial electrodeposits. By using pulse current in comparison with the deposits obtained by DC one can achieve smoother deposits of copper. nickel. and zinc from sulfate solution and gold and copper from cyanide solutions, with the average current densities and plating times being kept equal. Also, an equal or better quality of the deposits of copper, gold. and zinc is obtained at higher current densities under pulse current than under DC conditions. A decrease of grain size of the pulse-plated deposits is generally found to lead to an increased coverage of the substrate with the same quality of the deposited metal. resulting in decreased porosity and surface resistance. and increased density of the metals. It can be expected that this increase in compactness is associated with a decrease in internal stresses and increased ductility and change of hardness of metal deposits. In situations where hydrogen is codeposited (current efficiency less than 100%) the current distribution can be better in pulse current regimes as compared with DC current.
PULSE·REVERSE PLATING PRC plating is the bipolar electrodeposition process where DC current is continuously changing its direction (polarity). Quite popular in the 1950s. it then found its major use in improving leveling action in hot cyanide copper plating baths; however, the use of these baths was drastically reduced due to environmental problems and inherent build-up of carbonates. and PRC was relegated to secondary duties such as for very effective electrocleaning. as well as for derusting and desrnutting. Theoretical studies were. therefore. not pursued to any extent. In the I970s new theoretical contributions were introduced. and in the 1980s and 1990s it became quite obvious that PRC can give answers to many complicated electrodeposition problems. Theory became much more complex and intriguing. The essence of current reversal is that during metal electrodeposition the polarity automatically changes and the duration of the reverse (anodic) current is normally a small fraction, e.g.• 20% or less. of the duration of the current in the direct (cathodic) direction. The bottom part of Fig. I shows the different kinds of nonstationary PRe currents. Periodically reversed current is characterized by the following properties: a current in the forward direction (cathodic polarization current). ie; a time for the metal deposition (cathode period). te; anodic polarization (reverse) time period. ta ; and a total period of Ihe current reversal. I. + Ie' The average density is given by equation 4. In the electrodeposition of metals by a reversed current the electricity qe flowing through the electrodes during the cathode period, Ie' must be greater than the amount of electricity q. flowing during the time of the anode period I., i.e.• ie Ie > i. t.. Consequently, by this method of deposition, the whole quantity of electricity is not consumed in the formation of the deposit. A significantly smaller portion is expended for the anodic polarization (often called depleting, stripping, or reversing) of the plated products. During each total period of plating with current reversal a layer of metal of a determined thickness is built up on the surface of the products. and during anodic polarization (t.). part of the deposited layer is dissolved. In general. to increase the rate of metal deposition. conditions at the electrode surface must be created under which the limiting cathodic current density and passivating anodic
386
current are significantly increased. At the same time a high metal-to-current efficiency must be maintained and the protective properties of the electrodeposits must satisfy the requirements. As was noted earlier, until relatively recently, these conditions were met mainly by changing DC variables: the metal concentration, temperature and agitation of the bath, as well as by introducing various additives. Despite the fact that PRC deposition occurs with the incomplete use of the whole quantity of electricity applied on the electrodes, this method of metal deposition often permits acceleration of the electroplating rates. Faster deposition can be accomplished by using a higher working current densities than compared with a DC current. Initially, this is caused by the prevention of depletion of metal ions in the cathode double layer, to which the anodic dissolution of the deposit contributes. The upper limit of the working current densities during the metal deposition is a function of the parameters of the reversed current ta and ia and also of the length of the total period. The limiting cathodic current density usually increases with an increase of the ratio t.ftc and with a decrease in total time. The a higher working current density during the PRC deposition as compared with DC plating is a result of the periodic depolarization of the electrodes. Such an effect of the current reversal on the electrode processes, as well as the electropolishing action of the anodic current on the deposit, permits ensuing electrocrystallization on the cathode surface in the preferred direction (smaller crystal grains) under a higher working (practical) current density. During the cathodic period this leads to grain refinement usually associated with unipolar, pulse current pulse plating, which is now further supported by the disturbance of growth steps by the periodic inversion of the current. In a number of DC electroplating processes for example, during the electrodeposition of certain metals from solutions of their simple salts, the upper limit of the working current density can be, under certain conditions, held close in magnitude to the theoretical limiting current. Often, however, under real, actual plating conditions, the working current density of the cathode is limited to the admissible upper limit of the anodic current density. Exceeding this limit can cause anode passivation when the electroplating bath is operating in a DC mode (e.g., nickel, brass, and cyanide and acid copper baths). Now, current reversal can prevent passivation of the anodes during the period of high cathodic current and sometimes contributes to a significant increase of the upper limit of the anode working current density and, thus, in tum, increases the maximum working cathode current density.
PRACTICAL APPLICATIONS OF PULSE-REVERSE PLATING Electrolytes containing organic additives may undergo changes in the deposition mechanism when PRC current is used, which can produce changes in the uniformity of the deposition that have a beneficial affect on throwing power. When electrodepositing a copper from acid baths containing organic additives substantial improvements in hole throwing power can be achieved for deposition of copper on printed circuit boards. The use of a current of alternating polarity permits not only increasing the working current density, but as a result of the periodic action of the anodic current on the deposited layer of metal, the deposit acquires better protective properties as demonstrated in actual practice for acid copper, nickel, gold, chromium, and zinc. For a number of processes, current reversal is the means of achieving smoother and brighter deposits, better current distribution, and less porous deposits with lower internal stresses. In the case of chromium plating higher current efficiency is easily achieved, whereas for nickel plating, brightness and leveling can be obtained with much reduced concentration of organic additives with additional important benefits of increased ductility and reduced stress. It is obvious that PRC plating has more variables than DC plating. More variables mean more complex plating equipment and controls but also wider choice of deposit characteristics that can be tailored for a particular application, e.g.. mcreased corrosion protection, leveling, different hardness, ductility, stress values, and alloy composition and, in the case of chromium, results in higher cathode current efficiency.
387