Cleanliness Requirements: Design Rules and Process Limits

PCB cleanliness requirements define how much contamination may remain on a board after assembly. The contamination comes from the flux, the handling marks, the plating chemistry and the dust in the air, and it forms a conductive path when it dissolves in a film of moisture. The limit is a measured quantity, not a visual impression, and the measurement is what separates a clean board from a board that merely looks clean.

What Counts as Contamination

The first family is ionic contamination, which includes the salts from flux activators, the halides from plating and the chlorides from fingerprints. These dissolve in moisture and carry current, so they are the most damaging family and the one that is measured.

The second family is the non ionic residue, which includes the rosins, the waxes and the polymerised flux. It does not conduct on its own, but it absorbs moisture and it holds the ionic material against the surface where it can do harm.

The third is particulate contamination, which is dust, fibre and solder spheres. A particle that is conductive and that bridges two traces is a short waiting for a stimulus.

The fourth is the surface condition itself, because an oxide or a tarnish is not contamination in the usual sense and it still affects the insulation resistance.

Why the Residue Matters

The failure mechanism is electrochemical migration. A film of moisture forms on the surface, the ionic residue dissolves into it, and the voltage between two conductors drives a current that carries metal from one to the other.

The result is a dendrite, a thin metallic growth that bridges the gap. It grows slowly, it is invisible under a coating, and it eventually produces a short that is difficult to find because the board works again once it dries.

The rate depends on the voltage, the humidity, the temperature and the distance between the conductors. A fine pitch board with a high impedance node is the most sensitive case, and it is also the case where a small amount of residue has the largest effect.

The mechanism explains why cleaning matters more on a modern board than it did on an older one, since the spacing has fallen while the ionic load from the flux has stayed the same or risen.

Sources in the Process

The solder paste and the flux are the largest source, and the amount that remains depends on the type. A no clean flux leaves a defined residue, while a water soluble flux leaves an ionic load that must be removed.

Handling is the second source. A fingerprint contains salts and oils, and it is one of the most common causes of a cleanliness failure on a board that was otherwise processed correctly.

The third source is the fabrication itself, since the plating bath, the etching chemistry and the final finish all leave material that has to be rinsed away. A board that arrives contaminated cannot be made clean by the assembly process alone.

The fourth is the environment, because dust and airborne salts settle on an open board. The storage and the handling practice are therefore part of the cleanliness programme.

Cleaning Methods

Aqueous cleaning uses water with a saponifier or a detergent, at a temperature and a flow that remove the residue mechanically and chemically. It is effective and it needs a rinse and a dry stage that are as well controlled as the wash.

A semi aqueous process uses a solvent that is rinsed with water, which suits a flux that water alone cannot dissolve. It is more expensive and it brings its own disposal requirements.

A solvent process uses a non flammable solvent in a vapour or a spray machine, and it suits a product where water is not allowed, such as an assembly with a part that cannot get wet.

The no clean approach is not the absence of cleaning but the choice of a flux whose residue is safe to leave. It works only if the residue really is safe, which is a question for the flux specification and the measurement rather than for the process engineer.

Measuring the Result

The standard measurement is the resistivity of solvent extract, in which the board is washed with a controlled volume of a solvent mixture and the conductivity of the extract is measured.

The result is expressed as an equivalent mass of sodium chloride per unit area, and the limit depends on the product class. A typical limit for a high reliability product is below one and a half micrograms per square centimetre.

A surface insulation resistance test is the other method, in which a pattern of interleaved conductors is held at a bias in a humid chamber and the resistance is monitored. It measures the consequence rather than the contamination.

[[insulation resistance|Surface insulation resistance]] testing takes days and is used for qualification, while the extract method takes minutes and is used for process control.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/tupian5.png" alt="Close view of flux residue around solder joints” />

Setting a Requirement

The cleanliness requirements for a product come from its class and from its environment. A board in a sealed dry enclosure has a different requirement from one in a humid outdoor housing.

The requirement should state the method as well as the limit, because a number without a method cannot be verified. The solvent, the volume and the extraction time all affect the result.

The requirement should also state where it applies, since a board with a connector or a sensitive part may not be washable in that area and a local cleaning method is needed.

The conformal coating applied afterwards is a barrier and not a substitute for cleanliness, since a coating applied over a contaminated surface traps the residue against the conductors.

Practical Rules

Decide whether the flux residue can stay before choosing the flux, and confirm the decision with the measurement rather than with a visual check.

Control the handling as carefully as the wash, because a fingerprint defeats a good cleaning process.

State the method and the limit in the assembly documentation, and check the first article with the extract method.

Record the result with the batch, and treat a drift as a process signal rather than as an isolated board.

Process Control and Verification

On a design of this kind, cleaning is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. 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.

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

FAQ

What is ionic contamination? Residue that dissolves in moisture and carries current, such as flux activators, halides and salts from fingerprints. It is the family that is measured and limited.

Is a no clean flux really clean? The residue is designed to be safe to leave rather than absent. The claim has to be supported by a measurement for the product class and the environment.

How is cleanliness measured? Usually by the resistivity of solvent extract, reported as an equivalent mass of sodium chloride per unit area. Surface insulation resistance is used for qualification.