DI Water Quality in PCB Wet Processes: Conductivity and Rinse
Deionised water is the rinse medium for almost every step in printed circuit fabrication, from the rinse after etching to the final rinse before the surface finish is applied. Its job is to carry away dissolved salts without leaving anything of its own behind, and that only works while the water is pure. DI water quality is therefore measured continuously, and the measurement is one of the few process variables that is monitored on every line at every hour.
What DI Water Is and Why It Matters
Deionised water has had its dissolved ions removed by ion exchange resin, so its electrical conductivity is very low. Tap water contains calcium, magnesium, chloride and sulphate at concentrations that would leave a visible residue when it dries on a board.
The residue is the problem. Salts left on the surface after a rinse are hygroscopic, they promote corrosion and electrochemical migration, and they interfere with the adhesion of the solder mask and of the final finish. Even a rinse that looks clean can leave enough ionic material to fail a cleanliness test.
Conductivity and Resistivity Measurement
Water purity is expressed either as conductivity in microsiemens per centimetre or as resistivity in megohm-centimetres, and the two are reciprocals. A common specification for a final rinse is above 2 megohm-centimetres at the point of use, with many processes asking for 10 or more, and the value quoted is meaningless unless the temperature is stated as well, since conductivity rises with temperature.
The measurement point matters as much as the value. Water that leaves the treatment plant at 18 megohm-centimetres can arrive at the rinse tank at a much lower figure if the pipework is long, the loop is not continuously circulated or the resin bed is exhausted. The instrument should be installed where the water is used rather than only at the plant.
Rinse Stages and Drag-Out
A rinse does not remove contamination in one step. Drag-out carries a film of process chemistry on the board into the rinse tank, and the first rinse becomes progressively more concentrated as the lot is processed. Most lines use two or three rinse stages in series, with the cleanest water introduced at the last stage and flowing back towards the first.
The counterflow arrangement is what makes a small volume of water do a large amount of work. Where the stages are fed independently, the rinse quality falls at the end of a shift as the tanks load up, and the board that happens to be last is the one that fails the cleanliness check. A drip board over the process tank, and enough time for the film to drain, reduce the load on the whole set.
Resin Bed Monitoring and Regeneration
Ion exchange resin is consumed by the ions it removes, and its capacity is finite. When it is exhausted the conductivity of the output rises, and if the bed is not regenerated or replaced the rinse water begins to carry the very ions it was installed to remove.
Beds are regenerated with acid and caustic, and the regeneration is incomplete if the chemicals are not in the right proportion or if the rinse after regeneration is too short. The conductivity of the output immediately after a regeneration is the check that the procedure worked.
Water Quality by Process Step
Not every step needs the same. A rinse after a mechanical operation tolerates lower quality than a rinse before a plating step, and the rinse before a final finish is the most critical of all, because the finish is deposited directly onto whatever the water left behind.
The specification should be written per step rather than as a single figure for the whole shop. Applying the highest specification everywhere wastes treatment capacity, and applying the lowest everywhere produces tarnish and staining on the finishes that are most sensitive to residual salts.
Ionic Contamination and Its Consequences
Ionic contamination on a finished assembly is measured by extracting the ions from the surface into a known volume of pure water and measuring the conductivity of the extract. The result is expressed in micrograms of sodium chloride equivalent per square centimetre, and the acceptance figure comes from the applicable standard rather than from habit.
Ionic contamination and electrochemical migration are directly linked. Residual salts dissolve in the moisture that condenses on a board, forming an electrolyte between adjacent conductors, and metal migrates across it under bias. The failure appears as a dendrite or as a leakage path that only shows up under humidity, and the cleanliness verification methods used at assembly are the same ones that apply to fabricated boards.
Distribution Loop, Biofilm and Pipework
Pure water is aggressive, and it will leach ions out of pipework, fittings and storage tanks. The materials used in the loop have to be compatible: PVC, polypropylene and stainless steel are common, and ordinary carbon steel is not. A new section of pipework is a common explanation for a sudden rise in conductivity at the point of use.
Stagnant water in a dead leg of the loop is a place where bacteria grow, and biofilm in a pure water system produces both contamination and a steadily rising conductivity. The loop should be designed without dead legs, and it should be flushed on a schedule even when the line is idle.
Verifying Rinse Effectiveness
The conductivity of the rinse water tells you what is in the tank, not what is on the board. Rinse effectiveness is verified on the product: a water break test shows whether the surface is free of hydrophobic residues, and an extraction test measures the ionic material actually left behind.
The two are complementary. A water break test is quick and can be done at the tank, while the extraction test is quantitative and slower. Using only the first risks accepting a board that wets but still carries salts, and using only the second means waiting for a result while the lot continues through the line.
Records and Alarms
The record should carry the conductivity or resistivity at each point of use, the resin bed status, the regeneration dates and the results of the periodic extraction tests. Alarms set at the point of use, rather than only at the plant, are what prevent a whole shift from being processed with water that is out of specification.
When a contamination failure appears at assembly, the DI water record is one of the first things to examine. The assembly cleanliness measurement gives the number; the water record explains where it came from, and the two together are needed before the rinse system can be corrected with any confidence.
FAQ
What resistivity should DI water have at the point of use? Above 2 megohm-centimetres is a common minimum for a final rinse, with many processes specifying 10 megohm-centimetres or more. Measuring at the point of use rather than at the plant is essential.
Why does the rinse water quality fall during a shift? Because drag-out from the process loads the first rinse tank and the stages are usually fed in series. Counterflow feeding keeps the last stage cleanest and spreads the load over the whole set.
Does pure water always rinse better? Up to a point, but very pure water is also aggressive and can leach ions from pipework and from some materials. Water quality, loop materials and flow all have to be considered together.
