Paste-in-Hole (PIH) printing, a.k.a. through-hole printing, pin-in-paste, intrusive reflow, etc., has always been a way to accommodate traditional through-hole components on a mixed-technology SMT assembly using reflow soldering rather than wave to make the through-hole connections. Paste is printed in such a manner to as to fill the through-holes, the through-hole components are inserted, and the assembly is reflowed. Assuming that the TH components can take the heat, successful PIH boils down to three basic issues: 1.) determining the amount of metal needed in the hole to create a good solder joint; 2.) Getting the metal in there; and 3.) Keeping it in there without loss when the through-hole component’s pins are inserted. Over time, changes in technology have made PIH even trickier. For example, early-on, an important issue was the heat tolerance of the through-hole components subjected to reflow temperatures in an oven. Nowadays the shrinking real estate of the PCB with fine-pitch SMT pads closely adjacent to paste-hungry through-holes has raised deposition issues, thus affecting design considerations, and the list goes on from there.
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