We commonly think of flux itself as the reason it should only be used for 8 hours at a time. As the workable life of new fluxes increase, we should also consider any effects the equipment might have. In recent work with extended working life ball attach fluxes, I have found a disturbing issue with certain pin transfer machines, a problem that is not noticeable under typical conditions.
The picture above shows some ball attach flux removed from a pin transfer flux reservoir after 72 hours of continual usage. The dark area is ball attach flux with imbedded aluminum particles from the doctor blade assembly. The friction during usage must not have been accounted for in this machine’s design. Under normal use (changing the flux after only 8 hours) this issue has not been seen, it becomes pronounced sometime during the second shift.
This additional strain on the equipment should be realized before the making the decision to extend the time between cleanings. I wouldn’t want BGA’s assembled with aluminum particles suspended in flux and I’m sure you feel the same way. The fix for this issue may be simply adjusting the squeegee pressure, or as involved as having a new reservoir / blade assembly machined out of different wear-resistant materials. That really depends on the level of adjustability your equipment has.