DIY paste mask for SMD PCB fab

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Try this

I use solder paste, very fine, that is put onto the SMD board with a syringe. The 5cc syringe is filled with 500-625 mesh eutectic solder and RMA flux. I use a 27 GA hypo. needle. Now, that setup costs over $200. With it and a Met-Cal iron with a .016 inch tip I build SMD pcbs with components as small as 0201. Met-Cal are outstanding soldering irons, not cheap and available from Digikey and others. Of course I have a stereo microscope. Get some solder paste with flux from Digikey. Put it on the PCB with a fine needle. 0603 is as small is I can build without a microscope. Best tweezers I've found is a Wiha AAA IND ESD from Digikey. You will have to hold the component down to solder them.
Begin by putting solder in the PCB on one of the SMD pads. Melt it to the PCB to cover the SMD pad. Apply a liquid flux to the solder bump on the PCB. Pick the SMD part up with the tweezers and place it over the pads. It will stick up on the end where the SMD pad has the solder. Heat up the solder on the pad and the component will be attached to the one pad by the solder. It should be flat on the PCB since the end of the component melted into the solder on the board. You can let go of the component. Apply some solder paste the unsoldered end of the component with a fine needle. Heat up the solder paste and the component soldering is done except for cleaning. Use Isoprop. alchohol or tolulene and a q-tip and small stiff brush, if necessary.
 
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