Ionic Contamination: Design Rules and Process Limits

Ionic contamination testing measures the material left on a board that can dissolve in moisture and carry current. The measurement is a conductivity, the result is expressed as an equivalent mass of salt per unit area, and the limit is written into the assembly specification. The test takes minutes, it is repeatable, and it turns a subjective judgement about cleanliness into a number that can be compared across batches.

What the Test Measures

The test measures the change in the resistivity of a solvent that has been in contact with the board. Ionic material dissolves into the solvent, the conductivity rises, and the change is converted into a mass of sodium chloride equivalent.

The formal name for the measurement is the resistivity of solvent extract, and it is often shortened to a contamination level. The equivalence to sodium chloride is a convention that lets very different ions be compared on one scale.

The result is a single number per unit area, which is convenient and also a simplification, because different ions have different conductivity and different harmfulness. A chloride and a weak organic acid are not the same hazard even though the test treats them the same.

The number is therefore an index rather than a complete description of the residue. It is still the most useful single measurement available, and it is the one that the standards use.

The measurement is also affected by the geometry of the board and by the extraction, so the method has to be repeated consistently for results to be comparable.

The Two Methods

The traditional method places the board in a bath of a solvent mixture, usually isopropyl alcohol and deionised water, and measures the conductivity of the bath as the ions dissolve. The board is immersed for a defined time and the result is calculated from the change.

The dynamic method, which is used in most modern equipment, circulates the solvent over the board and monitors the conductivity continuously until it stabilises. The curve shows how quickly the ionic material is released.

The static method is simpler and it is used for a small board or for a sample that can be cut. It has a larger uncertainty because the ions are not removed from the surface as quickly.

Both methods give a result in micrograms of sodium chloride equivalent per square centimetre, and both need a calibration with a known amount of salt before the bath is used.

Limits and Classification

The limits are quoted in the standard that the product is built to, and they are usually stated per class. A typical limit for a general product is around one and a half micrograms per square centimetre, and a high reliability product requires less.

The measurement is often paired with the solderability test on the same sample, since both are process checks that are cheap to run together. The coating stage that follows depends on both results.

The limit applies to the assembled board rather than to the bare board, although a bare board can be tested as an incoming check. A board that arrives contaminated cannot be cleaned by a process that is designed for the assembly residue alone.

The limit also depends on the environment. A board inside a sealed enclosure in an air conditioned room is at a lower risk than one in a humid outdoor housing, and the requirement should reflect that difference.

The measured value should be compared with the limit in the same units and with the same method, which is why the method belongs in the requirement.

Sources and Control

The flux is the main source, and the amount that remains depends on the flux type and on the profile. A water soluble flux leaves a large ionic load that must be removed.

The plating and the finish leave ionic material as well, and a board that is not rinsed properly at the fabricator carries it into the assembly.

Handling is the third source and the easiest to eliminate, since a glove that is clean and a board that is held by the edges cost nothing.

A bath that is not changed often enough becomes a source of contamination itself, since the ions that were removed from one board can deposit on the next. The rinse water has the same problem, and its quality should be monitored rather than assumed.

The cleaning process is the control, and its effectiveness depends on the temperature, the flow, the chemistry and the rinse. A rinse that is not monitored leaves as much residue as the wash removes.

Reading the Result

A single value above the limit is a problem, but a trend is more informative. A value that drifts upward over weeks points at a bath that is being used beyond its life or a rinse that is losing effectiveness.

A value that spikes and returns to normal points at an event, such as a batch of boards from a different supplier or a change of flux.

The curve from a dynamic test carries additional information, since a rapid release suggests surface residue while a slow release suggests material trapped under a component.

The result is usually reported with the quality record for the batch, and it is one of the checks that an inspection plan may reference even though the two methods look at different things.

The measurement should be recorded with the batch and with the method, and the record is what makes the result useful a year later.

Practical Use

The test is best used as a process control rather than as an inspection gate. It is run on a sample from each batch or each shift, and the value is plotted against time.

A board that fails should be cleaned again where that is possible, and the cause should be found before the next batch. Cleaning a sample until it passes does not fix the process.

The requirement should be agreed with the customer or with the reliability engineer, since a limit that is tighter than the process can hold becomes a source of scrap rather than of quality.

The method should also be checked against a surface insulation resistance test at the qualification stage, because the two measure different things and a board can pass one and fail the other.

Practical Rules

State the method, the solvent and the limit in the assembly document, and calibrate the bath before each session.

Test a sample from each batch and plot the result, since a trend is more useful than a single pass or fail.

Control the flux type, the profile and the rinse together, and treat the handling as part of the process.

Record the value with the batch and keep the record, and confirm the requirement with a surface insulation resistance test at qualification.

FAQ

What does the test measure? The ionic material that dissolves from the board into a controlled volume of solvent, expressed as an equivalent mass of sodium chloride per unit area.

How long does the test take? A few minutes on a dynamic instrument, plus the calibration of the bath. The static method takes longer and has a larger uncertainty.

Is a passing result a guarantee? No. The test measures ionic residue and does not detect a non ionic film, so it is combined with a surface insulation resistance test at qualification.