The Flux Capacitor

[Ken Forgettable]

Can anyone point me to some evidence that excessive flux can cause a sound PCB to malfunction please

Around these areas please:
+ analog/digital ground plane splitting.
+ fundamental clock harmonics
+ floating I/O lines
+ or even ground bounce?

[rockdude]

Interesting question.
Not the areas you're asking about but it's a start
http://www.freepatentsonline.com/6524398.html

Many of the existing low-solids, no-clean soldering fluxes cause an excessive number of solder balls to be left on the PCB surface. These fluxes also cause solder bridging because their surface tensions are too high. In addition, due to their weak activity levels, the fluxes are unable to thoroughly remove the tarnish and oxides from the surfaces to be joined. Solder balls are unwanted balls of solder occurring randomly or non-randomly on the solder mask and/or between the leads of the components on the board; the solder balls can bridge a gap between two conductors resulting in an electrical short. Solder bridges are connections of unwanted solder that can form a short circuit between two traces or leads that were not designed to be connected. Solder bridges and/or solder balls may cause electrical failure of the board. Excessive solder balls and bridging also require costly solder touch-up operations for their removal. Even tiny solder balls, often referred to as micro-solder balls, because they are only visible with magnification (e.g., 10 times magnification), can result in electrical shorting of very-closely-spaced board lines and pads as well as component leads and terminations. Another problem with existing fluxes is that they can leave visible residues on the surfaces of the PCB, which in addition to being unsightly, can cause false rejects with in-circuit pin testing.