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30 SMT Magazine • August 2015 pads. They are constructed out of ionic com- pounds that were permitted to remain on a PCB and, under voltage bias, create a fully continu- ous chain of electrically conductive molecules between terminals. The result is current leakage and irregular performance in the final product. Hypothesis 1: Some ionic constituents within solder flux residues are more detrimental than others. Species Identification The most commonly present ionic constitu- ents within solder flux residues have been nar- rowed down to a handful of ionic compounds. Some compounds are more detrimental than others; however, the impact of each can be quantified. Below is a list of compounds, bro- ken into groups of ionics (anion and cation) and weak organic acids. Hydration/Carrier System Regardless of the compound, a fluid carrier system must be present in order to allow the ionic compound(s) to mobilize. The most viable carrier system is water. Water, without contami- nants and in a normal air environment, tends toward a resistance on the order of 1–5 MΩ. The presence of sufficient moisture allows for the ionic species to mobilize in unique ways that provide repeatable resistance values that are often greater than 10 MΩ. Overhydrated com- pounds tend toward a resistance equivalent to water, masking the effects of the contaminant. A consistent hydration method is necessary to provide repeatable hydration levels to the ion- ics present on the PCB. Hypothesis 2: Reduced spacing between test locations correlates to lower SIR values. Conductor Spacing It is theorized that increasing the spacing between test locations will demonstrate a corre- sponding increase in SIR resistance values. The coupon used to conduct this testing incorpo- rates parallel, exposed sensor traces. These trac- es are .005" (.127 mm) wide and are separated by .005". Therefore, testing the adjacent sensor traces (7 to 8, 8 to 9, etc.) provides insight into the SIR over a .005" (.381 mm) gap. Skipping a FeAture HOW CLEAN IS CLEAN ENOuGH TO ACHIEvE RELIABLE ELECTRONIC HARDWARE? continues Table1: ionic constituents of concern on pcbs. Figure 5: Sir Sensors under BTCs.