It sounds like an oxy-moron but it is a common misconception that no-clean flux never needs to be cleaned. Many in the industry have experienced the conductive residue that can be left behind by no-clean flux. This residue can be extremely hard to remove, which becomes a big problem if you are working with sensitive boards. Because the no-clean fluxes are designed to not need to be cleaned they do not come off easily, which can be problematic.
The amount of residue that is left on the board is dependent on several factors including the amount of solids-resins, gelling agents and activators contained in the flux. There is less flux residue left on the boards when the solids content is lower. Most of today’s no-clean flux contain between 50 and 70 percent solids.
Despite all of this, no-clean remains the most widely used type of flux. Manufacturers are in the habit of running all of their boards with no-clean, and simply cleaning only those boards that require it.
It may be tempting to not clean your dirty boards, simply because you are using a “no-clean” flux but this can have significant adverse effects down the road. In the past, assemblers saw problems arise with in-circuit testing on these un-cleaned boards. The residues from these no-clean fluxes also lead to malfunctions in circuits with clock speeds.
While in most cases not every board being soldered with no-clean flux will need this extra step, you do need to be on the lookout for the tell-tale residue left behind and clean these boards to prevent problems down the road. One great product for doing this is ACL Staticide 8623-12 No Clean Precision Flux Remover. This product can be purchased online at KIMCO Distribution https://gokimco.com/search?form_key=OhbYqHgT3MbeEp53&q=ACL+Staticide+8623-12+No+Clean+Precision+Flux+Remover.