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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:

module 1

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):

enter image description here

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?

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  • \$\begingroup\$ @SpehroPefhany thanks for correction, you are right. non-native speaker here :) \$\endgroup\$ Apr 4 '20 at 13:19
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Personally I would have a framed stainless-steel stencil made, apply low temperature Sn 42 Bi 58 fine 138°C solder paste (if necessary repeat to get it perfect under the microscope) and then bake the board in an inexpensive benchtop T962 SMT oven. That's not much equipment but it does cost more than zero.

Those pads are pretty tiny, you might also want to consider having someone do it for you.

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  • \$\begingroup\$ Thanks for the input, I can have a stencil made with the PCB house (where I did order before). I am in the process of designing prototype boards so can add that when ordering them. It is an extra US$10 usually, but could be useful. I can have a reflow oven based on electric baking oven as well (was thinking on making one based on plenty of arduino based examples with temperature sensors that I have already). But what do you think of the last bit about priming pads on a board first with low temp solder paste and then reflowing module on top of that (same as BGA reflow I guess)? \$\endgroup\$ Apr 4 '20 at 13:39
  • \$\begingroup\$ I would just reflow. Clean the bottom and maybe put some flux on the modules, but the flux probably can't be properly cleaned out afterward so keep that in mind in choosing the flux. With low temperature solder the reflow should be a piece of cake (even if you got it so hot the modules reflowed a bit as long as you don't jar them they would survive- sometimes I run boards through a second time to get the other side). \$\endgroup\$ Apr 4 '20 at 13:42
  • \$\begingroup\$ Then what do you think about using hot air station instead of oven? I have done a fair amount of reflow on open joints with it, and usually verify that solder has melted by making components "dance" with the air. Not sure how easy would it be to make a module "dance" like that with the air. It shouldn't reflow modules themselves though considering low temp paste. \$\endgroup\$ Apr 4 '20 at 13:46
  • \$\begingroup\$ I have had so-so results with hot air for prototype and rework purposes (seems very dependent on PCB design such as presence of ground planes) for handheld. Better to solicit others' experience on that. I also don't have one of those fancy hot air rework stations for BGA work like the Hakko FR-811. \$\endgroup\$ Apr 4 '20 at 13:51
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    \$\begingroup\$ I've got Quick 861DW and that works really well for me for different types of work (reflow and desolder mostly). Though I never needed to reflow BGA chips in general, I've seen people use this one a lot for that purpose. \$\endgroup\$ Apr 4 '20 at 14:14

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