About one year we have that problem with some of pcbs. Our customers told us about that. Because they can't mounting components on those pcb reason of flux residue. As you know flux not conductive chemical. We didn't change anything before having that. Now we think that we must change operating system of rinsing process. Sorry for late reply.
(http://www.smtnet.com/Forums/Index.cfm?CFApp=1&Message_ID=46412)