Plating bath control using ampere-time instruments

control, analysis, and testing PLATING BATH CONTROL USING AMPERE-TIME INSTRUMENTS DAVID A. STEWART S¥STEMATICS INC., BRISTOL, R.I.; www.systinc.com JOSEPH J. WERBICKI COPPERWELD METALLON DIV., PAWTUCKET, R.I. Plating takes place when an electrical force (voltage) causes the positive-charged metal ions in a plating bath to move towards the negative-charged cathode part of the plating circuit and combine with available electrons. At the cathode they are deposited as metal atoms. The movement of ions from one place to another (and electrons in the opposite direction) constitutes electrical current. The standard unit of measure for current is the Ampere. When we plate, we plate by amperage. An analogy can be drawn between amperage and voltage in a plating circuit. Voltage is like the force a bicycle rider exerts on the pedals. Amperage is like the speedometer reading. On a consistent road surface, the more force the rider exerts, the faster he goes; however, if he encounters a hill or a valley, he will have to adjust the force on the pedals to keep the same speed. Voltage is force. A change in voltage will change amperage. A change in the bath's resistance will also change the amperage. In this case the voltage will need to be adjusted to maintain the same amperage. Measuring ampere-hours is like measuring miles with an odometer. An ampere-time counter tells us the a m o u n t of current that has passed through a plating bath. The a m o u n t of metal deposited depends on three factors: amperage, plating time, and current efficiency of the bath. Amperage and plating time are integrated and tracked with an ampere-time counter. Current efficiency of a bath is determined by a simple laboratory-plating test. It is a fairly reproducible measure of the rate at which metal is deposited under a given set of operating conditions. These conditions include metal content, pH, temperature, agitation, and current density. Current density is the total n u m b e r o f a m p s passing through a plating bath, divided by the total area of plateable metal surface in the bath. Typically, current density is stated in amps per square foot or A/ft 2, though other units, such as amps per square Foot or amps per square decimeter, are often used. For a given set of operating conditions, a bath will have an o p t i m u m current density range within which acceptable deposits can be produced. This range is spelled out on technical data sheets for most proprietary plating systems. Current efficiency may be stated on a technical data sheet in a n u m b e r of ways. For precious metals you will often see it stated in milligrams per ampere-minute, or you may see it stated in terms of deposit thickness per unit of time. For example, at 40 A/ft 2, a Watts nickel bath will deposit 0.0001 in. in 3 minutes. These seemingly different ways of stating current efficiency are completely interchangeable, using b u t two basic formulas, either singly or in combination: 501

Ampere-time [times] Current Efficiency = Milligrams of metal deposited Milligrams of metal deposited " Factor Thickness (micro inches) --Plated area (in2) The factor to commit to memory is 3.162 for gold. For any other metal the gold factor is multiplied by 19.3 and divided by the specific gravity of the new metal. As an example, the factor for nickel is 3.162 [times] 19.3 divided by 8.9 or 6.86. The two formulas above can always be combined in order to calculate one of the variables that may not be known. For example: Thickness Area Ampere-time " Current Efficiency Factor Using the combined formula, you can determine ampere-time to plate a required thickness or thickness that will be deposited in a given amount of ampere-time. Area, current efficiency, and factor are constants. The area of work being plated and the plating range of the baths fix total amperage. To determine the required amperage multiply the recommended current density by the area to be plated. The ampere-time counter always shows the total accumulated ampere-time regardless of any changes in amperage that may have occurred during the time of accumulation. This is because the counter takes a signal from an electrical component called a shunt. The signal from the shunt is in milh'volts and varies from zero when there is no current flow, to a maximum signal (usually 50 millivolts) when the amperage is at maximum. This signal causes the counter to increment at a rate that is always equal to the amperage multiplied by the elapsed time. An added feature on some ampere-time counters is an interval preset. With this feature the operator can set the ampere-time that he wants to plate. When the instrument reaches the interval preset count, an alarm may sound, or the rectifier may be shut down. Equally as important as metal deposited is the amount of other constituents that is removed from a bath during plating. Chemical composition is one of the most difficult bath parameters to control. Analysis and replenishment is the only sure way; however, on-line chemical analysis is expensive, and periodic analysis may not be enough. The ampere-time instrument is the answer. SAtw'DA C O R P . i www.sanda.com

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If the current efficiency o f a bath is known metal deposited (or removed) from the bath is directly related to ampere-time by formula (1). For additives, such as brighteners, manufacturers' literature will give the consumption rate in terms of brightener volume per a certain n u m b e r o f amperehours. The consumption rate should be compensated for losses due to dragout. An improvement to the ampere-time counter is the automatic feeder. This instrument combines an ampere-time counter with an interval preset and a pump. The operator can set the device for an interval ampere-time count that will activate the feed p u m p for a preset time to feed the right a m o u n t o f replenishm e n t chemicals into the bath. Adding replenishments in smaller amounts and frequent intervals helps keep the bath composition consistent. Modern electronics has helped to make these instruments very reliable and easy to set up. Some are equipped with m u l t i p l e p u m p s for chemicals that can't be premixed. Others are configured to accommodate mulThree Must -Haves tiple rectifiers that operate in the For your Plating Line same bath. In summary we have seen that Amp Hour Meter an ampere-time counter and a few Essential for any simple formulas can give even the plating line. It smallest plating shops m o s t o f features both the information that is required a non-resettable to maintain their plating chemand a resettable istry. A few added features can conflict automate processes that are time consuming and prone to h u m a n New Amp Meter error. With Alarm Accurate control of plating thick~ ~ , ~,~................................~i!i~i!:¸:i! ness. Track chemical adds. i Veryoperator i i i i ¸ friendly. ~!!!1

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