What the C3 Corrosivity Index™ Means For Contamination Testing

By Terry Munson

The C3 Corrosivity Index™ is an indicator of the cleanliness of a test site, and is calculated by dividing the maximum current seen during the test by the elapsed time of the test. It provides a single test result value that can be plotted over time to track trends in product cleanliness—the lower the C.I. the less likely a tested product site will suffer performance problems due to the presence of a detrimental ionic residue.

 

C3 Corrosivity Index™ vs. ROSE Testing

At first glance, the C3 Corrosivity Index™ may appear very similar to the single value returned by bulk resistivity testing, but there are distinct advantages inherent to the C3/C.I. system that help the user detect localized areas of contamination and anticipate field reliability.

 

The actual electrical measurement method is basically the same between the C3/C.I. system and a typical ROSE tester; both are applying a voltage across a known spacing and measuring the resultant current. This is a standard resistivity measurement. The primary differences lie in both the data graph and the electrode itself.

 

The Electrode

The single-use electrode used in a C.I. test is an integral part of the electrical evaluation. It’s FR-4 with exposed copper traces, 50 mil spacing and a ten volt bias. This allows a location for corrosion or electrochemical migration to occur during the electrical test and can have a substantial impact on the test results if corrosive or conductive residues are present in sufficient quantities. In cases of heavy contamination, signs of corrosion and electrochemical migration (dendrites) can be observed on the electrode upon completion of the electrical test. C3 localized extraction allows for direct analysis of individual component areas to determine the level of corrosivity. Depending on how fast a dendrite or leakage event occurs, we can predict if residues present in humid conditions will cause performance problems.

 

The Data Graph

The very small volume of test solution coupled with rapid, high-accuracy electrical measurements results in a data graph that is more helpful to manufacturers than bulk resistivity testing. Corrosion and electrochemical migration of the electrode during the test cause detectable variations in current measurement and the resultant graph.

 

The C3 Corrosivity Index™ is a single cleanliness number that facilitates statistical process control and makes it easier to track cleanliness trends. This helps manufacturers more accurately predict reliability with resistivity and corrosivity evaluation.

 

Detecting Localized Contamination

ROSE, or any other cleanliness evaluation method that relies on submerging the entire sample, is incapable of detecting localized pockets of contamination in critical areas that can lead to field failures. The small volume and sample area of the C3 allow for evaluation of specific, critical areas or architectures of an electronic assembly. The heated, ultra-pure DI water extracts easily ionizable residues that would react under humid conditions, but not residues that are chemically bound and benign. Process monitoring, with careful selection of critical sample locations, provides user-friendly results for SPC and evaluation of real-world product performance.

 

ROSE Testing

C3 Corrosivity Index™ Testing

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Large volume of extraction solution dilutes test results, particularly when analyzing small component areas on a PCBA.
Specific, small component area analyzed using the C3