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August 2015 • SMT Magazine 33 a second time yielding a total of four resistance measurements per sensor trace pair. The resis- tances for each sensor distance were average for each board. The data findings are presented in the figures below. Zero on the chart represents boards that tested the resistance of the solder paste residue before cleaning. Species Results For the species testing, one drop of the ionic species at the desired concentration was placed on the sensor traces. The board was then placed in a 110°C oven for 30 minutes and then al- lowed to cool in a desiccator for one hour. The results of the species testing are presented in Figures 10 and 11. HOW CLEAN IS CLEAN ENOuGH TO ACHIEvE RELIABLE ELECTRONIC HARDWARE? continues FeAture figure 7: low-activity boards cleaned and measured. figure 9: High-activity boards cleaned and measured. figure 8: Medium-activity boards cleaned and measured. Figure 10: naCl species data findings at PPM levels. Figure 11: Benzoic acid species data findings at ppM levels.