Particle Distribution
Screen printing of solder paste requires the metal powder particles to be small enough to flow easily through the stencil apertures without clogging. Using type 4 solder paste refers to selecting a solder formulation with a metal particle size distribution between twenty and thirty-eight microns. This classification is standard for many general-purpose surface mount applications, where component pitches are down to 0.5 millimeters.
It offers a good balance between printability, print definition, and resistance to oxidation. According to industry standards, at least eighty percent of the powder particles by weight must fall within the specified size limits. Using particles that are too large leads to stencil clogging, while using particles that are too small increases the risk of solder balling during reflow.
Aperture Fill
The five-ball rule dictates that the minimum aperture width of the stencil must be at least five times the diameter of the largest solder powder particle. For this grade of paste, the minimum recommended aperture opening is approximately one hundred and ninety microns. This dimension permits reliable printing on standard fine-pitch components without risking paste clogging.
Rheological Advantages
The lower surface area of these larger powder particles reduces the risk of paste oxidation during the reflow process. This resistance allows for more stable storage and longer stencil life on the production floor. This stability makes it the preferred choice for high-volume, cost-sensitive assemblies.
