Pollution Prevention Techniques: Rinse Water Reduction

Estimated reading time: 3 minutes

The first step in any pollution prevention strategy is to minimize chemical wastes and their rinse waters. There are five general categories of common techniques for pollution prevention in a PCB fabrication facility: (1) new processes to replace sources of pollution; (2) extend the bath’s life; (3) rinse water reduction; (4) dragout reduction; and (5) ventilation reduction.

While this list is not all-inclusive, it provides an overview of the types of technologies used around the world that are important to consider. In this article, we will examine rinse water reduction.

Rinse Water Reduction

Most of the waste generated in the manufacturing of PCBs is from cleaning, plating, stripping, and etching. This section describes some of the techniques available for reducing the volume of rinse water used.

While we stress the need to reduce individual rinse water flow rates when a conventional wastewater precipitation system is anticipated, there may be at least one possible exception. If in excess of 95% of all chemicals are recovered, the chemicals do not enter the rinse water collection system, and a central membrane filtration or deionization system is used to produce a zero-effluent system; then, the need to reduce rinse water flow is reduced. However, one must also consider the capital and operating costs for the central system.

One author wrote the following concerning rinse flow rates in the case of a zero-effluent system [1].

“It should also be noted that in a well-engineered zero liquid discharge environment, water conservation is unnecessary. Instead, the focus is total dissolved solids (TDS) budgeting throughout the plant, along with an analysis of specific critical contaminants. Water supply is only limited by the size of the pumps. For instance, to maximize absorption in the fume scrubber, 10 gallons/minute of DI water can be fed continuously into and out of the scrubber system reservoir… With a closed-loop system, there is no sacrificing of rinse quality to save water. Lift stations integrated with conductivity sensors can automatically identify an out-of-control waste stream as it happens, allowing for quick corrections by maintenance. Also, fresh rinse-water conductivity is always DI quality.”

Particulate Filtration on Deburr and Panel-scrubbing Operations

Deburrers are used to remove stubs of copper formed after the drilling of holes in double-sided and multilayer panels before they enter the copper deposition process. Scrubbers are used to remove oxides from printed circuit laminates, clean the surface prior to a surface coating to provide better adhesion and remove residuals after etching or stripping. In deburring and board scrubbing, particulate materials are added to the water and are removed by various methods based on size and the weight of the copper particle such that the wash water becomes suitable for up to 100% recycling. The types of filtration available for this operation are cloth, sand, centrifugation, and gravity settling with filtration.

Etcher and Conveyorized Equipment Design Modifications

Etching machines can be the single largest source of copper waste in the discharge from a PCB facility. The amount of copper discharged, and the rinse flow rate from that machine, is a function of the machine design.

An older etching machine will contain a single-stage etchant replenishing module positioned between the etching chamber and its continuously flowing single or multiple stage water rinse chamber. Fresh etchant is fed to the replenishing module to wash the panels, and that etchant (now containing copper washed from the freshly etched panels) then flows (in a direction opposite to the direction of the panel movement) into the etching chamber. The continuous water rinse can contain from 100 to 500 milligrams per liter (mg/l or ppm) of copper, depending on the configuration of the rinse module. Companies that use this type of equipment, with a single-station rinse module, normally have floor space restrictions in their production area. The etcher design affects the rate of etchant solution dragout as does the volume of panels being processed and the quality of the etchant solution control.

By way of comparison, adding a second stage replenisher station will reduce the range of copper dragout in the following rinse from 50 mg/l to 300 mg/l. We have observed more than one etching machine–using a four-stage replenisher module and a combination of water recirculating with a single-stage rinse module–produce a rinse effluent containing from less than 1.0 mg/l up to 2.0 mg/l of copper. That is the option that we recommend to clients when etchant recycle, and copper recovery are not practical or economical.

To read the full article, which appeared in the August 2019 issue of PCB007 Magazine, click here.