Ionic Contamination And Cleanliness Testing

A flux that is described as no clean leaves a residue on the board, and the description means that the residue is designed to be harmless rather than that no residue is present. Whether it is actually harmless depends on the chemistry, the process and the environment the product sees. Cleanliness testing exists to answer that question with a measurement instead of an assumption.

This article explains what ionic contamination is, how the standard test measures it, and how a requirement should be set for a product.

Why Residue Matters

Residue on a board can be harmless, cosmetic or damaging, and the category depends on what it contains rather than on how much of it there is. An organic residue that is electrically insulating and stable does not affect the circuit, while a residue that contains ionic species and can absorb moisture provides a conductive path between conductors.

The mechanism that follows is electrochemical migration, in which metal dissolves at one conductor and deposits at another under the influence of an electric field and moisture. It requires the ionic species to be mobile, which is why a residue that is dry and stable is not a problem and the same residue in a humid environment is.

What Ionic Contamination Is

Ionic contamination is the fraction of the residue that dissolves in water to produce ions. The common sources are the activators in the flux, the salts left by handling, and the residues from the plating and the fabrication of the bare board. The quantity is usually expressed as the mass of sodium chloride equivalent per unit area of the board.

The measurement is a proxy rather than a direct measurement of the harm. A given quantity of contamination may be harmless in one design and a problem in another, because the spacing between the conductors and the applied voltage determine whether the leakage matters. The number is therefore best used as a process control figure, with a limit established from a product that is known to work.

The Resistivity Of Solvent Extract Test

The test immerses the assembly, or a sample of it, in a measured volume of a solvent mixture of water and alcohol while the conductivity of the solution is monitored. The ionic species dissolve and the conductivity rises, and the result is converted to an equivalent mass of sodium chloride and divided by the surface area of the board.

The test measures only what dissolves, so it detects ionic contamination and not the neutral organic residue. A board with a heavy but electrically benign residue can pass, and one with a small amount of a highly mobile ionic species can fail. The test is also sensitive to the extraction conditions, including the temperature, the time and the agitation, which is why the result is comparable only between measurements made with the same procedure.

No Clean Flux And When It Is Not Enough

A no clean flux is formulated so that its residue is benign under the conditions for which it was designed, which normally means a defined reflow profile that fully activates and then immobilises the flux. If the profile is wrong, the flux is not fully reacted and the residue remains chemically active. If the board is then used in a humid environment, the residue can support leakage.

The cases where cleaning is still required include a high impedance circuit, a high voltage circuit, a product that will be used in a condensing environment, and an assembly that carries a conformal coating, because a coating over an active residue traps it against the surface. In each of those cases the cleaning step is part of the process rather than an option.

Cleanliness And Electrochemical Migration

The relationship between cleanliness and migration is not linear. A small amount of the wrong species is worse than a larger amount of a benign one, and the presence of moisture is what makes the difference. A board that passes a cleanliness test in a dry state can still fail if the design places fine pitch conductors in a location where condensation forms.

The design therefore contributes to the risk as much as the process does. Increasing the spacing between the conductors, avoiding the placement of fine pitch devices in the area where moisture collects, and providing a coating that keeps the moisture off the surface all reduce the sensitivity to the residue. The relationship between residue and surface leakage is the same one that governs the design rules described for protective coating and for the general quality characteristics of a product.

Setting A Cleanliness Requirement

The requirement should be written as a limit on the measured value and a stated test method, because a number without a method is not reproducible. It should also state the sampling frequency and whether the sample is the whole board or an extraction from a defined area.

Where the product is simple, the limit can be set generously and used as a process indicator rather than a pass or fail criterion. Where the product carries a high impedance or a high voltage, the limit should be tight and the process should include cleaning. The choice of flux and the decision to clean are linked to the alloy and the profile, which are covered in the comparison of leaded and lead free processes.

Process Control and Verification

On a design of this kind, cleanliness testing is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Process Control and Verification

On a design of this kind, cleanliness testing is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

FAQ

Does a no clean flux mean the board does not need cleaning? It means the residue is designed to be benign under the specified process conditions. If the profile is wrong or the environment is humid, the residue can still cause leakage.

What does the extract test actually measure? The ionic species that dissolve from the surface into a solvent. It does not measure the neutral organic residue, which is why a board can pass and still carry a heavy visible residue.

Is a coating a substitute for cleaning? No. A coating applied over an active residue traps it against the surface. Where a coating is used, the board should be clean first.