Localized Extraction and Cleanliness Testing System
This test system utilizes a steam head (Figure 2) to extract a sample of deionized water and flux residue effluent from a localized region of approximately 0.1 in2 (6.45 mm2). The test system is designed to extract the flux residue effluent by heating deionized water and delivering it to the test site (red arrow in Figure 2), followed by a vacuum aspiration of the solution into the collection reservoir (blue arrow in Figure 2). This cycle is performed nine times to produce a volume of effluent sufficient to fill the collection reservoir to a point where the exposed traces on the test coupon are covered. The test system applies a known bias (10 VDC) across the test coupon electrodes and measures the resulting current every 0.25 seconds.
The test system provides for two different acceptance criteria schemes. The more stringent criteria are recommended for use on IPC Class 2 and Class 3 assemblies and the less stringent criteria are recommended for use on IPC Class 1 assemblies. It should be noted that no IPC specification refers to this test, and the selection of IPC classes as representative of the two test limits is arbitrary.
The Class 2/3 acceptability limit is a measured current below 250 μA for a minimum of 120 seconds. The Class 1 acceptability limit is a measured current below 500 μA for at least 60 seconds. For both conditions, test cycles that meet the acceptance requirements
are reported as CLEAN and test cycles that do not meet the acceptance requirements are reported as DIRTY.
In addition, the localized cleanliness test system reports a measure that is called the Corrosivity Index (CI). The index is calculated by dividing the maximum current seen during the test by the elapsed time of the test, or by dividing the maximum test current limit by the time required to reach the limit.
The calculation for determining the CI for any test indicates that a lower CI is preferable. For reference, the Class 2/3 limit of 250 μA at 120 seconds can be converted to a CI of 2.08; this implies that a CI higher than 2.08 indicates a DIRTY test result for the Class 2/3 limit. The CI at the Class 1 limit of 500 μA at 60 seconds is 8.33.