This is more an academic/interest question than a particular question on any meter.It arose from a discussion on the volt-nuts mailing list.
I'm just interested if anyone from Keysight would tell us the sort of solder used in instruments like the 3458A where you want to reduce the effect of all the little thermocouples you create. There's a reference to cadmium/tin (70%/30%) allow in a paper about a 1.018 V secondary reference, where a Josephson Junction array is used as the primary standard.
Someone on the volt-nuts list has been trying various solders, with some better than others. I note the 3458A page says the 3458A is not RoHS compliant. Maybe avoiding materials like lead and cadmium would present serious redesign issues in a multimeter of the quality of a 3458A.