I am prototyping a board that uses one of two modules (that is another PCB with components already on it). One module - all pads on that module are underneath (not exposed and not castellated), rather small and tightly packed:
Another module is less problematic as most pads are castellated holes and only a few pads are underneath (bigger and more dispersed compared to first module):
While 1st module is more complex, it is most preferable to be used as it is more reliable one as far as I know. And even with 2nd one I still would need access to those hidden pads. I only have 1 of each module for prototype, so I would highly prefer not to screw this up.
Tools I have access to:
- Simple soldering iron with a few different tips
- Quite good hot air station
- Solder wire with rosin core
- Syringe of solder paste (low temp ~130 C) and syringe of flux
I am thinking I should be able to solder it with the hot air using low temperature solder paste. However I will have no way to verify that solder has melted and created reliable joints.
Another concern is that during the pre-heating flux in the paste drops in viscosity and spreads solder balls all around the area where it has been applied. On open joints the surface tension usually brings melted balls together into a nice and clean joint, but with this being sandwiched between two surfaces I am very concerned about making bridges.
I guess my question is what techniques I could use for increased chances of a good quality soldering of those hidden pads?
PS: as a workaround for bridges I was thinking to prime the pads with paste and melt it using hot air, then apply extra flux from syringe, put module atop and reflow this using hot air. Would that work, is there any issues with this approach?