Ozone Treatment: Plating Bath and Rinse Water Control

Ozone treatment is an oxidation step that uses dissolved ozone gas instead of a liquid oxidant, and in a plating shop it is used in two places. It breaks down organic material carried in rinse water, and it destroys the organics that accumulate in a bath from additives and from drag-in. Both uses rely on the same chemistry and the same weakness, which is that ozone must be dissolved before it can do anything. Neither duty works unless the gas is dissolved efficiently in the contactor.

The gas itself is generated on site from oxygen or from dried air, so there is no chemical to store and no oxidant to pump. That is the main attraction for a shop that wants to reduce the volume of liquid chemicals it keeps on the floor. The trade is that the equipment has to be run and maintained with more care than a drum of peroxide.

What Ozone Treatment Does in a Plating Shop

Ozone is a strong oxidising agent, and in water it reacts with organic molecules and with many inorganic species. In a plating context that means additive breakdown products, traces of brightener carried over, and organic contamination from oils and from resist residues. The reaction products are simpler molecules that the bath can tolerate or that the waste plant can handle. The same reaction also removes colour from the water, which is a visible sign of progress.

Its weakness is selectivity and speed. Some species react almost instantly while others need a long contact time, so a single dose rate cannot be expected to handle every load. The load also varies with production, which means the control has to follow the production rate rather than a fixed setting.

Oxidation of Organics in a Plating Bath

A plating bath accumulates organic material from the additives it is fed, and part of that material is spent. Left alone it builds to a point where the deposit becomes brittle, dark or poorly distributed. Carbon treatment removes much of it by adsorption, and oxidation removes a different fraction by breaking the molecules down.

The two methods are complementary rather than competing, and carbon treatment is described in the carbon treatment plating bath notes. Where oxidation is used on a working bath, the dose has to be small and controlled, because an excess will attack the good additives along with the spent ones. The dose should be set by analysis of the bath rather than by the calendar.

Rinse Water and Recycle Loops

Rinse water carries the chemistry that the work drags out of each tank, and the organic part of that load is what makes the water difficult to reuse. Oxidising it reduces the chemical oxygen demand and the colour, which makes the water a better candidate for recycling back to the rinse line. The recycled water still has to be clean enough for the process it returns to.

Treatment is normally placed on a holding tank rather than on the flowing rinse, so that the contact time can be controlled. Recycling of rinse water is described in the PCB rinsing water recycling notes, and the oxidation step sits alongside ion exchange and filtration in that scheme.

Contacting, Dosing and Mass Transfer

Ozone that stays in the gas bubble never touches the contaminant, so the contacting system decides how much of the gas is used. A fine bubble diffuser, a venturi injector or a packed contactor all work, and each has a different transfer efficiency. Undissolved gas leaves the tank as off-gas, which then has to be destroyed.

The dose should be expressed as a mass of ozone per unit volume of water or per unit of time, not as a percentage on a dial. A generator output that drifts with the temperature of the feed gas will deliver a different dose at the same setting. Feed gas dew point and flow should therefore be part of the record. The oxidation step also protects a downstream resin bed from organic fouling.

Monitoring and Control

There is no way to see ozone in water, so the process is controlled by instruments. Dissolved ozone is measured with a probe, and the residual in the treated water is the usual control point. The oxidation reduction potential of the tank is a cheaper proxy and is often used as a trend rather than as a setpoint.

The load also has to be estimated, and the production figures for the shift are the best available estimate. A treatment step sized for average load will under-treat on a heavy day unless the dose follows the load. Sampling downstream and checking the result is the only way to confirm that the pairing works. Recording the dose properly is what allows one month to be compared with the next.

Safety and Off-Gas

Ozone is toxic and it is a respiratory irritant, so the safety design is not optional. The generator area and the contactor headspace need to be ventilated, and an ozone monitor should alarm before the level becomes dangerous. Off-gas must be passed through a destruct unit before it leaves the plant.

Anyone working near the equipment should know the smell, which is sharp and noticeable at levels well below the exposure limit. That makes the nose a useful warning even though it is not a measurement. A leak on a generator or a contactor should be treated as an urgent repair rather than scheduled work.

Materials, Maintenance and Spares

Ozone attacks many common materials, including some gaskets, seals and flexible tubing, so wetted parts should be specified for the service. A component that is acceptable in a rinse line can fail in weeks on an ozone line. The materials question is one of the first things to settle when the equipment is ordered. Training should cover what to do when the alarm sounds as well as what the alarm means.

Maintenance follows the manufacturer’s schedule for the generator, the air dryer and the destruct unit. The dryer matters because humid feed gas reduces output and eventually damages the generator cell. Holding spares for the sensors protects the process, since a failed probe leaves the operator dosing blind.

Records and Verification

The record should carry the generator setting, the feed gas dew point, the residual and the treated volume. Where the water is returned to the line, the quality of that water belongs in the same record. Waste water quality is described in the PCB waste treatment and recycling notes.

Acceptance of the finished boards follows the published IPC documents, and a rinse water system that is in control is part of what makes that acceptance repeatable. An oxidation step that is run without records is a step that cannot be shown to work. A treatment step that is not measured is a treatment step that will drift.

FAQ

Is ozone treatment a replacement for carbon treatment? No. Oxidation breaks down organics while carbon removes them by adsorption, and most shops use the two for different duties.

Can ozone be dosed into a working plating bath? Only in small controlled amounts, because an excess will attack the useful additives as well as the spent ones.

What is the greatest risk with ozone equipment? Leakage of gas into the working area, which is why ventilation, monitoring and an off-gas destruct unit are required.