No-clean ball attach and package-on-package fluxes are usually designed to reach a certain temperature before the post-reflow flux residue acts as an encasement for the unused activators. Below that temperature, most fluxes fail SIR testing due to electromigration. A unique flux (PoP Flux 30b) passes SIR in both the normal reflowed, and less common un-reflowed state. This is important to note, since the flux can be used in low temp applications with peak temperatures of only 100 degrees C. Most fluxes start working around 150 degrees C and need a peak temperature around 200 degrees C to pass IPC SIR testing.
I think I’ve found my new favorite low temperature flux.