David,
You just provided enough material to start an endless debate in an unexplored field (acceptable impurities, levels and effect on reliability in high tin content (lead free alloys). High tin content alloys have a high affinity to metals like Cu and Ag (even stainless steel is not spared).
Until now no standards have been set due to the lack of experience and the amount of different alloys used in our industry. Your concerns in the young and unexplored lead free field are extremely valid. But looking at the picture of the low cost impurity tester I don't think this tester will be able to quantify impurities accurately but just signal the presence of them. Accurate equipment to measure and quantify impurities is atomic absorption and spectrum analysis in order of accuracy.
Solder manufacturers can justify the high cost of this equipment not the user of bar solder. With 10 waves and the cost of lead-free solder your supplier will be able to provide frequent analyses at no charge to you as a service, if not find one who will.
Nice to know someone trying to locate the trees in the forest, or shall we say lead free jungle.
Good Luck Patrick
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