26 May 2011
Contaminometers, omegameter, and ionographs are all well-known commercial instruments used to measure ionic residues on the surface of electronic assemblies.
They operate by immersing the sample bare board or assembly into a pre-determined volume of water and alcohol with a known resistivity to determine the relative cleanliness. The test method is now often referred to as Resistivity of Solvent Extract (ROSE) testing and quoted in many international standards.
Depending on the specification, this test may be conducted in a test solution of 75% or 50% IPA with the remainder quantity of demineralised water. These instruments can be used to assist in the development of a cleaning process or monitor any changes in the systems’ performance.
The equipment records and measures the drop of resistivity due to ionic residue in a measured amount of time. The results are reported in µ gram of NaC1 per unit area as per MIL STD 28809, IPC 600, IPC TM 650 or MOD Standards. These measurements do not differentiate between ionic species or confirm non-ionic contamination. They simply measure conductivity or resistivity, which can then be related to amounts of ionic materials present in solution. There is no identification of the contribution to the total conductivity readings of any individual ionic species that may be present in the solution.
Some suppliers in the past have used the amount of contamination and the speed of solubility via software to try to determine the possible reasons for the contamination and where it may have come from.
This test method has been used successfully in the industry for many years for monitoring the level of flux residues on the surface of the board after cleaning. Each machine can have a slightly different way of working, and depending on the solution, temperature, and fluid agitation, have a different capability for small stand-off heights.
Ionic contamination testing is often used in the electronics industry in conjunction with other forms of contamination assessment. Ion Chromatography and Surface Insulation Resistance SIR are two other test methods. SIR can simulate the accelerated environment impact on reliability or in the case of Ion Chromatography measure and define the types of contamination.
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