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Choosing the right solder paste is a vital step in PCB assembly. The paste is made up of minute tin and copper spheres held within a specialized form of solder flux. The spheres are deposited onto the circuit board surface during assembly, where they melt and join together to create electrical contact and mechanical strength. Several factors must be considered in selecting the correct paste for the job. These factors include:
solder paste particle size is a critical factor in the quality of printed circuit boards (PCBs). The spherical shape of the metal particles reduces surface oxidation, improves print performance and helps to ensure impeccable joint formation with adjacent spheres during reflow. Additionally, the spherical geometry of the particles assists in reducing stencil aperture clogging. Solder pastes are typically classified by size using IPC standards such as J-STD-005 Requirements for Soldering Pastes. Table 3-2 details the main particle size ranges for each type.
As stencil aperture / pad dimension decreases, the number of average spheres in the paste needs to increase. The normal rule of thumb is that the average sphere in the paste should be at least five times smaller than the smallest stencil aperture dimension.
This translates to a typical Type 3 paste needing an aperture at least 20 microns (9 mils) in diameter to function properly. This requirement makes it important to use a good stencil with exceptional release characteristics and a reflow profile that will allow the paste to be deposited without excessive spreading. In general, the reflow temperature profile should be gentle enough to prevent explosive expansion and excessive reflow coalescence, but fast enough to enable quick initiation of flux activity.