Material Grade
A specialized category of solder paste uses tiny, highly consistent metal powder particles to enable the printing of very closely spaced component pads. Specifying an ultra fine pitch solder paste is necessary when printing for micro-BGA or 01005 metric passive components.
Particle Distribution
Solder powder is classified into types by size, with smaller types used to prevent the clogging of small stencil apertures. In ultra fine pitch solder paste, Type 5 or Type 6 powder is used because the smaller spheres can pack together tightly within narrow stencil openings. If the powder size is too large, the paste cannot flow through the micro-apertures, leading to insufficient solder volume and open circuits.
These smaller particles however have a larger surface area, which increases the susceptibility of the paste to oxidation during reflow.
Flux Formulation
Advanced flux systems are designed to protect the tiny metal particles from oxidizing at high temperatures while maintaining excellent rheological performance. For ultra fine pitch solder paste, the flux chemistry must provide strong wetting action without causing excessive slump or voiding. If the flux is too aggressive, it can leave behind corrosive residues that harm the reliability of the board.
Finding the right balance between powder size and flux chemistry ensures high yield on complex assemblies.
