As early as the 1990s, people were predicting the end of through-hole components, but they are alive and well with the numbers of dual in-line packages (DIPs) and connectors still measured in the tens of billions per year. Many of these components are assembled by wave soldering; however, in mixed technology (SMT and through-hole on the same board) where the through-hole count is low, it is often advantageous to consider selective soldering or the pin-in-paste process (PIP). PIP is a process in which solder paste is printed over or near the PWB through-holes. The through-hole components are then placed and the solder joint is formed during the reflow process. PIP has the advantage of eliminating the wave soldering process step. In many cases it is difficult to print enough solder paste to make an acceptable through-hole solder joint. Solder preforms were developed to meet this need.
These solder preforms are typically shaped in the form of 0402, 0603, or 0805 passive components. The preforms are placed on the appropriate printed solder paste deposit by a component placement machine. Preforms come in tape & reel packaging.
Today solder preforms are also used in other “solder starved” applications such as radio frequency (RF) shields, connectors, and under QFN thermal pads. In all cases, the extra solder delivered by the preform is vital to the reliability of the assembled product.
In this paper, process, design, and assembly methods for solder fortification using preforms will be discussed. Four successful solder fortification examples will be presented along with the associated defect reductions.
solder preforms, pin-in-paste, solder fortification, solder starvation, mobile phone shields, QFN packages, flux
