Choosing Between Ionic Cleanliness and SIR Evidence

A board can look clean while retaining residues that matter to its application. Conversely, a visible residue is not enough to quantify electrical reliability. Choose the test according to the question: process contamination, the identity of extractable species, or electrical behavior under defined environmental conditions.

Separate the measurements

IPC-hosted research compares ROSE testing, ion chromatography, and surface-insulation-resistance evaluation and reports that their results do not always correlate. Kester’s work on residues beneath low-standoff components further illustrates why local assembly geometry and processing history matter. These studies support combining evidence when the risk warrants it, rather than treating one number as a complete reliability verdict. IPC technical paper: cleanliness comparison; Kester: residues under bottom terminations.

ROSE evaluates an extract’s conductivity using a defined procedure. Ion chromatography can identify and quantify particular extracted ionic species. SIR evaluates insulation resistance on a specified test structure under specified exposure and electrical conditions. Each result belongs to its method, specimen, and test setup.

Write the question before requesting a test

• For production monitoring, define the process change or contamination trend that must be detected.
• For a suspected local residue, identify the location and whether whole-board extraction could obscure it.
• For electrical reliability, define the representative materials, geometry, bias, and environment.
• For a customer requirement, use the agreed method and acceptance criteria rather than substituting a convenient test.

For example, a cleaning change might lower a whole-board extraction result while leaving residue trapped under one package family. That result is useful but does not alone show that the local risk was removed. A targeted investigation should retain the relationship between residue location, process history, and electrical behavior.

Make test samples representative

Record flux and paste products, board finish, component standoff, reflow conditions, cleaning parameters, drying, and handling. Include appropriate controls and avoid contaminating samples during preparation. If a test vehicle is used, explain which features of the real assembly it represents and which it does not.

Use the result within its limits

Do not invent a universal acceptance threshold from an unrelated published study. The product’s requirements, test method, and qualified process determine the applicable criteria. Preserve raw data and the laboratory procedure so comparisons across lots remain meaningful. A useful conclusion states what the evidence supports, what uncertainty remains, and which process controls maintain the accepted condition. That is more actionable than labeling an assembly simply “clean” or “dirty.”

Keep extraction volume, sample area, and any normalization method visible when comparing reports from different laboratories or product sizes.