I understand that solder paste layers define the stencil openings used during PCB assembly. For conventional through-hole components, are paste apertures required on either the top or bottom layer, or are they only needed for SMD components?
Does this change when using pin-in-paste (intrusive reflow) assembly, where solder paste is deposited into the through-holes?
For leaded components using conventional wave soldering or selective wave font soldering - no paste apetures are needed or desired.
For leaded components which are pin-in-paste reflow capable, then paste apertures are generally desired.
One additional case is a mixed-technology board with SMD components on the bottom side. These components are often held in place using adhesive dots before wave soldering. The adhesive keeps them attached to the PCB as it passes over the solder wave.
In this assembly process, the bottom-side SMDs don’t require solder paste apertures. However, any SMDs assembled through reflow on the opposite side will still need a normal solder paste stencil. The assembly sequence determines which side needs paste apertures.