Flux Residue Qualification for No Clean Processes

A no clean process leaves flux residue on the board by design. The decision to leave it is a qualification decision, and it has to be supported by evidence that the residue is harmless for that product in that environment. The evidence comes from measurement rather than from the label on the flux container, because the residue depends on the paste or flux chemistry, the profile and the board. This article covers what has to be demonstrated.

What the Residue Contains

A flux contains activators, solvents, rheology modifiers and, in many cases, resins. During reflow the solvents evaporate and some of the activators are consumed, but a fraction of the material remains on the board. It can be a thin transparent film, a sticky layer, or a hard crust depending on the chemistry and the profile.

The residue is not neutral. Activators that remain are capable of promoting corrosion and electrochemical migration in the presence of moisture and a voltage, and the risk depends on how much remains and on how mobile it is. The qualification has to establish that the amount and the mobility are low enough for the product.

Ionic Contamination Measurement

The standard measurement extracts the ionic material from the board surface and measures the conductivity of the extract, which is reported as an equivalent quantity of a reference salt per unit area. It gives a single number that is easy to compare against a limit and easy to track over time.

The measurement has to be done on a representative assembly, not on a bare board, because the residue comes from the assembly process. The board size, the cleaning if any, and the test area all affect the result, so the method and the area should be recorded with the number. The technique is the same one used for the general assembly cleanliness measurement that applies to cleaned assemblies as well.

Surface Insulation Resistance Testing

Surface insulation resistance testing measures the resistance between two conductors on a coupon while a voltage is applied and the coupon is held at a controlled temperature and humidity. It is the test that most directly represents the risk, because it measures the actual leakage that a residue can cause.

The test conditions matter. A higher humidity and a higher voltage accelerate the effect, and the duration has to be long enough for the residue to hydrate. The result is normally reported as a resistance value that has to stay above a limit throughout the test, and a residue that passes for the first hours and then falls is a failure rather than a pass.

Electrochemical Migration and Dendrites

Where ionic material and moisture are present with a voltage between adjacent conductors, metal can dissolve at the anode and deposit at the cathode, forming a dendrite that grows across the gap and eventually shorts. The process is accelerated by a small spacing, a high humidity and a residue with a high ionic content.

Inspection after a humidity test can reveal the early stages of migration as a discolouration or a fine growth between conductors. The examination should be done under magnification on the narrowest spacing on the assembly, because that is where migration starts. The defect patterns that result are described alongside other assembly failures in solder defect assessment.

Process Variables That Change the Residue

The profile determines how much of the flux is consumed and how much remains. A profile with a longer time at temperature activates and evaporates more of the flux, while a short profile leaves more residue. A change of profile therefore changes the residue even if the chemistry is unchanged.

The paste volume and the stencil design also matter, because more paste means more flux. A deposit that is larger than necessary leaves more residue for no joint benefit. Where the process is qualified at one volume and later increased for a different reason, the residue qualification should be revisited.

Qualification Approach and Records

The qualification should include the ionic contamination measurement, a surface insulation resistance test at the worst case conditions the product will see, and an inspection for migration. Where the product will be coated, the coating adhesion should also be checked, because a residue that is acceptable on a bare assembly can prevent a coating from bonding.

The gopcb assembly group qualifies a no clean process for a product family rather than for a single board, and records the chemistry, the profile, the paste volume and the test results together. That record is what allows a change of paste or profile to be assessed against the original qualification rather than treated as a new process.

When Cleaning Is Still Required

Some products cannot be qualified as no clean regardless of the chemistry. A high impedance circuit, an assembly that operates in high humidity, a product with a fine conductor spacing and a voltage between the conductors, and any assembly that will be coated are all cases where the risk of leaving residue is high. In those cases the cleaning step is part of the process and has to be controlled rather than omitted.

The decision should be made with the measurement data rather than by preference, and it should be documented so that a later change in chemistry or profile does not silently change the answer.

Checks Before Release

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

Verification and Records

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

FAQ

Does a no clean flux really leave no residue? No. It leaves residue that has been qualified as acceptable under defined conditions. The name describes the process, not the board.

Why is surface insulation resistance testing needed if the ionic measurement passes? Because the ionic measurement does not capture the mobility of the residue or the effect of the applied voltage and humidity.

Can a coated assembly be left no clean? Only if the coating is qualified to adhere to the residue. In most cases the two decisions are made together.