Contamination and Cleanliness Measurement on Assemblies

Contamination on an assembly is not only a cosmetic problem. It changes the surface insulation resistance, it provides the ions for electrochemical migration, and it can block a connector or a coating. The question is how much is acceptable and how it is measured.

What Counts as Contamination

The residue from flux is the largest contributor by volume, but it is not the most damaging. Salts from plating and etching, particles from machining and the oils from handling all matter by their ionic content rather than their mass.

A residue that is chemically benign and a residue that is active can look identical. That is why the acceptance is expressed as a measured quantity rather than as an appearance. Our migration notes describe the failure the active residue causes.

Ionic Contamination Measurement

The standard method extracts the residue from the assembly into a solution and measures its conductivity, which is converted into an equivalent mass of sodium chloride per unit area.

The result is a single number that describes the whole board, so it does not tell you where the residue is. Where a local problem is suspected, a local extraction or a surface insulation resistance test is used instead. Our test coupon notes describe the structures used for the resistance measurement.

Surface Insulation Resistance

The resistance test applies a voltage between two conductors on a test pattern and measures the leakage, usually at an elevated temperature and humidity.

The test is closer to the application than the extraction method, because it measures the effect rather than the quantity. It is also slower and it requires a dedicated pattern. Our fabrication notes notes list the attributes that should be stated.

Cleaning Processes

An aqueous process removes ionic residue well and needs a rinse and a dry that leave no water behind. A solvent process suits the fluxes that water does not dissolve and needs its own recovery and disposal.

A no-clean process is not a cleaning process. It relies on the flux residue being benign and on the amount being small, which is only true for a defined flux and a defined profile. Our paste notes describe how the flux is controlled.

What the Process Cannot Reach

Cleaning works where the fluid can flow. Under a low clearance package, inside a connector, or in a blind area, the fluid may not reach the residue and the residue may not be rinsed out.

Where the geometry prevents cleaning, the design should prevent the contamination instead, by using a flux that can be left in place or by sealing the area before the process. Our coating notes describe how the surface is prepared before it is sealed.

Verification

The verification is the measurement described above, performed on a sample of the production output rather than on a hand cleaned sample.

Where the measurement is close to the limit, the process is not controlled; the limit should be met with margin so that routine variation does not take the board out of specification. Our quality notes describe how the result is recorded.

Specifying the Requirement

The drawing should state the cleanliness requirement, the method of measurement and the limit. A requirement stated only as a process name leaves the result undefined.

Where the product is coated afterwards, the cleanliness becomes a prerequisite for the coating. Coating over a contaminated surface seals the contamination in and defeats the purpose of the coating.

Process Control and Verification

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.

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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Verification and Records

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

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

FAQ

Is a no-clean flux the same as a clean board? No. It leaves less active residue by design, and it does not leave nothing. Where the application is sensitive, the surface should be measured.

Can a board be cleaned twice? It can, and each cycle is another chance for the fluid to be trapped. The number of cycles should be defined and the result measured.

What does gopcb provide for contamination control? We provide a stated cleanliness requirement with its measurement method and limit, aqueous or solvent cleaning processes with rinse and dry control, design rules for areas the fluid cannot reach, sampling of production output rather than hand cleaned samples, and verification before any coating is applied.