Excess flux residue on Beaglebone Black PCBs

We have received a large batch of BBB and they all have excessive left over flux residue on the bottom side of the PCB! In order to determine if we should return the BBB, I would like to know what type of flux was used and if it corrosive. We don’t want issues to creep up over time due to corrosion or pin-to-pin conduction.