PASTE-IN-HOLE (PIH) PRINTING (aka through-hole printing, pin-in-paste, intrusive reflow) is 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 as to fill the through-holes; the components are inserted, and the assembly is reflowed. Assuming that the PTH 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.
There are as many well-established guidelines for calculating how much actual metal, and thus actual solder paste, must be deposited or printed to properly fill a through-hole. Since paste is generally about 50 to 55% metal by volume, overprinting the pad area to ultimately deliver the calculated volume of solder to the connection may be required; this overprinting is typically 10% on the annular pad.
One other approach suggests the use of preforms to add pure metal volume to the printed paste, where enough paste might not be able to be applied due to PCB design or considerations. This has become somewhat popular in some quarters, with the preforms available in tape-and-reel packaging, just as chip capacitors and resistors. However, preforms are expensive and add an extra (placement) step to the process.
Once the paste is printed, keeping the ideal volume in the hole without loss during pin insertion is not necessarily easy. Although there is no requirement for special paste types, paste formulations do vary, including such attributes as tackiness and viscosity, and this can
affect the propensity for paste to be dragged out of the hole by the pin, dripping, or other means. Pin shape may also be a factor.
The biggest issue with PIH is, of course, getting enough metal into the hole. Metal gets into the hole two ways: first, by being printed effectively, and second, by coalescing at the surface in overprinted areas and subsequently wicking down into the hole. Volumes have been written about different approaches to PIH over the past decade, and there are many different formulas and approaches to the implementation of the process. But PIH has become tougher with shrinking board topographies because, often, high paste volume PIH holes are too close to low-volume...