Pin-in-paste is the technique of intrusive soldering through-hole components to a circuit board using reflow soldering. Typically, solder paste can be printed and this will provide enough solder for complete barrel fill. Occasionally, more solder is needed than can be printed, and solder preforms can be used.
Phil Zarrow and Jim Hall of ITM Consulting have a recorded discussion forum called "Board Talk" in which they discuss common board assembly issues and solutions. One of the topics they have discussed is called, "Through Hole Reflow - Pin in Paste." Here, they discuss the typical techniques used for soldering through-hole components to a circuit board. These include printing solder paste around the through-hole in the board, inserting the through-hole connector, and reflowing the assembly in a convection oven.
They mention that one way to apply more solder is to overprint the solder paste. For many applications, this is an excellent technique. The overprinted solder paste will wick into the barrel during the reflow stage. This is a natural process because the over-printed area is typically a non-solderable solder mask surface outside the plated through-hole annular ring. The solder de-wets from the solder mask and surface tension pulls it into the pool of solder, onto the annular ring, and down the plated barrel of the board.
Occasionally, overprinting solder paste still does not provide enough solder to fill the through-hole. On these occasions, a good alternative is solder preforms.
There are a couple types of solder preforms to consider. The first is a standard 0402 or 0603 size preform supplied in carrier tape. These preforms are 100% metal compared with solder paste, which is only approximately 50% metal by volume. To incorporate preforms, print solder past around the through-hole. Place a preform just outside the annular ring, but touching the solder paste. Just like overprinting, the solder preform supplies extra metal, but this time, the metal from the solder preform will wick down the through-hole barrel and provide much more solder than the overprint was able to. The flux in the solder paste will be enough to remove oxides from the preform, so no additional flux is needed.
A second type of preform to consider for the most difficult-to-solder connectors are integrated preforms. These preforms are custom-designed for each connector type and include an array of washers. One washer will fit over every pin. Flux is applied and the connector is inserted. The assembly is reflowed in a convection oven, and more solder is available to fill the barrel than could be supplied using solder paste at all. This is the most manual technique, however, may be the only option to achieve a complete barrel fill without re-designing the board.
