Ionic Contamination Testing On Assembled Boards

Cleanliness is invisible, so it has to be measured rather than judged by appearance. A board with a heavy rosin residue may be perfectly clean in the electrical sense, while one that looks spotless may carry enough ionic material at a connector to promote migration. The measurement that resolves this is the ionic contamination test, and it is one of the few process checks whose result can be predicted from the physics rather than from experience.

This article explains what the test measures, how the methods differ, how the limits are set, and how a result that fails is traced to its source.

What The Test Measures

Water is a poor conductor when it is pure, and it conducts better when ions are dissolved in it. The test exploits that: the board is washed with a known volume of high purity water, the ions on the surface dissolve into it, and the change in conductivity of the water is measured. The result is expressed as an equivalent weight of sodium chloride per unit of board area.

The result is therefore a measure of the material that can be extracted from the surface under the conditions of the test, not of everything that is present. Residue that is insoluble in water, such as a cured polymer, contributes nothing to the reading even though it is present, and a residue that is ionic but chemically bonded to the surface may contribute less than expected. That is why the test is used together with the flux specification rather than instead of it.

<img src="https://www.gopcba.com/wp-content/uploads/2026/06/以太网交换机PCBA.jpg" alt="Board immersed in a resistivity of extract test cell” />

Resistivity Of Extract And The Older Method

The traditional method, still in use, immerses the board in a bath of a water and alcohol mixture, heats it and measures the change in resistivity of the bath. The result is a single figure for the whole board, and it is compared with a limit that depends on the product class. The method is simple and inexpensive, and its weaknesses are that it measures the total rather than the location and that the solvent mixture extracts a different set of materials from water alone.

Both the bath method and the spray method are subject to the same limitation: they give one number for the whole board, so a lot that is clean overall but has a local concentration under a large component will pass. That is why the test is often complemented by a visual inspection of the areas that are hard to clean and by a check of the rinse water from the cleaning machine, which is measured continuously and provides a faster indication of a drift.

Ion Chromatography

Ion chromatography separates the ions in the extract and measures each one, which turns a single figure into a fingerprint. Chloride, bromide, sulphate, weak organic acids and ammonium are reported separately, and the pattern identifies the source: a chloride signal points to an activator or to handling with bare hands, a bromide signal to a flame retardant or to a particular flux chemistry, and a weak organic acid signal to a flux that has not been fully consumed.

The method is slower and more expensive, and it is used for qualification, for a supplier dispute and for a failure investigation rather than for routine production. Its value is that it distinguishes a cleanliness problem from a chemistry problem, which the resistivity method cannot do. A board that fails the total figure with a high organic acid content is a board with a flux problem, while one that fails with a high chloride content is a board that has been contaminated after cleaning.

Sampling, Limits And Records

The test is destructive in the sense that the board has been washed, so it is performed on a sample rather than on every unit. The sample should include a board from the current lot and, where possible, a board that carries the most difficult area on the product. A limit of around 1.5 micrograms of sodium chloride equivalent per square centimetre is common for general purpose work, with values below 1 microgram for high reliability products.

The record matters as much as the figure. The extraction method, the volume of water, the temperature, the time and the instrument calibration all affect the result, and a figure without them cannot be compared with anything. The record also allows a trend to be seen, and a slow rise over several lots is the earliest warning that a cleaning bath or a rinse stage is losing its effectiveness.

Where Contamination Comes From

The flux is the largest single source, and within the flux the activators are the part that matters. A flux that is applied in excess leaves more residue, and a profile that does not consume the activators leaves them on the board in their active form. Solder paste contributes through its flux content, and a print that deposits more paste than the joint needs deposits more flux as well.

Other sources are less obvious. Handling without gloves transfers salts from the skin; a plating bath that is not fully rinsed leaves salts in the holes; a cleaning machine whose rinse water is recirculated concentrates what it removes; and packaging materials, paper and cardboard in particular, can transfer ions to a board in storage. Where the assembly process and the design interact, the rules are described under PCBA development process.

Troubleshooting A Failed Test

The first step is to divide the problem. A board that fails before cleaning has a flux or a plating problem; one that fails after cleaning has a cleaning or a rinse problem. Cleaning a board and testing it again is a quick way to make that distinction, and it costs one board.

The second step is to look at where the material is. A section or an extract taken from a specific area shows whether the contamination is under a component, in the holes or on the surface. Where the source is the rinse water, the conductivity of the water at the outlet is the measurement that identifies it, and the remedy is a change of water or of the flow rate rather than a change of chemistry. Related process decisions are described under conformal coating and board protection and under PCB design and fabrication.

Surface Insulation Resistance And The Electrical View

A cleanliness measurement answers a chemical question, and the electrical consequence is measured by a surface insulation resistance test. A test pattern with closely spaced conductors is coated with the flux under investigation, exposed to a controlled humidity and bias, and its resistance is monitored over days. A pattern whose resistance falls indicates that the residue is promoting migration under those conditions, which is the failure that cleanliness is meant to prevent.

The two tests answer different questions and are used together. Ionic contamination measures how much removable material is present, while surface insulation resistance measures whether what remains is harmful in the environment the product will see. A flux with a low measured contamination can still fail the insulation test if its residue is hygroscopic, and a flux with a higher figure can pass if its residue is inert.

FAQ

Does a clean looking board pass the test? It usually does, but appearance is not a measure. A board can look clean and carry a film of ionic material that is invisible, and a board with a heavy benign residue can pass comfortably.

How often should the test be run? On each lot for a product with a strict requirement, and at a defined frequency where the process is stable. The frequency is part of the control plan and should be justified by the history rather than chosen arbitrarily.

Can a coating hide a cleanliness problem? No, and it should not be used for that purpose. A coating applied over a contaminated surface traps the ions against the metal under the coating, which is a worse situation than leaving the surface bare and dry.