This depends on the type of flux that is being used. If you are using a no-clean rosin-based flux, these types of residues are often softened during a reheating process that elevates the board temperature to 125 C. The softening of the residue does mean that it will become more mobile and could easily travel down a connector pin. However, just softening the residue does not necessarily mean that something will become reactivated.
If the residues are not rosin based, they don't soften and become mobile in the same way. Non-rosin no-clean residues from liquid fluxes are primarily dicarboxylic acids that stay in solid form beyond 125 C. These types of residues would not be reactivated unless they go into some sort of liquid solution, which can happen if water condensation forms on the board.
IPA is usually not the best cleaning method for most no-clean fluxes. This is highly dependent on the exact flux formula and should be verified with the manufacturer of the flux.
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The main effect is that the flux residues will soften and may flow under gravity. This depends a little on the exact formulation but some fluidity is to be expected. There will be some residual activator in the residues and the resin which makes up the bulk of the residue also has some fluxing action. The metal surfaces under the residues will therefore see some cleaning action to reduce any oxide that has formed but this will be minimal and of no practical consequence. Once the residues cool down, they will behave just like the residue from the original soldering process except that they may be darker in colour and have a slightly higher softening temperature (i e harder and more brittle).
Cleaning residues is not normally thought to be necessary unless the physical flow would cause a problem (e g spread onto contact surfaces which seems to be the problem here). IPA is fairly effective but it does depend on the exact chemistry of the residues and the process used for cleaning. Poor cleaning can actually make the system less reliable by removing the resins, which provide an inert conformal coating, while leaving behind insoluble activators. Specific recommendations from the flux supplier should be followed.
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