Is it practical to solder an SMD component directly over through-holes if the pad pitch matches? What are the usual methods for preventing solder from wicking into the holes and ensuring a reliable solder joint?
For a one-off DIY project, I wouldn’t be too concerned if the pad pitch matches and you can get a good solder joint. The situation changes for production, where solder wicking and assembly defects become much more important to control.
I’d also consider what the SMD component is doing. If it’s a power device that needs an exposed pad or nearby copper for thermal management, the through-holes underneath become a much bigger concern, as solder will wick away and ruin the thermal/mechanical connection. As for preventing wicking in general, the usual methods are ensuring the holes are tented with solder mask or filled with epoxy.
Another thing I’d check is the annular ring and solder-mask clearance around the through-holes. If the SMD pad overlaps the hole too closely, controlling solder volume becomes difficult even with tenting. For production, I’d have the PCB fabricator confirm the pad/hole geometry and solder-mask capability before committing to the layout.
The assembly process also matters here. With reflow, the stencil aperture and paste volume can have a big effect on how much solder gets pulled into the holes. For a production design, I’d have the assembler review the stencil and pad design rather than relying on the PCB geometry alone.
One thing I’d clarify first is whether the through-holes need to remain open for their electrical or mechanical function. If they’re only there as unused holes, plugging or filling them gives you more control over the solder joint. If they’re functional plated through-holes, you’ll need to preserve that connection and work around the solder-wicking issue (e.g., via tenting) rather than simply filling them and risking the loss of their intended function.
Trapped moisture in the via barrel can expand under heat and rupture the solder joint. Baking boards beforehand prevents this. Specify IPC-4761 Type VII; here, the hole is filled, planarized, and copper-capped to preserve connectivity.