Powder Particle
A grade of metal powder is classified by the size range of its spherical solder particles according to international electronics standards. The type 7 powder specification requires particle sizes to range between two and eleven micrometers. Using such fine spheres allows for the printing of extremely small apertures on high-density circuit boards.
The increased surface area of these fine particles demands a specialized flux formulation to prevent oxidation. Particle size distribution must be strictly controlled to ensure that at least eighty percent of the powder falls within the specified range. Excessive amounts of sub-micron particles can lead to premature flux exhaustion during the reflow process.
Assembly Application
Fine-pitch and ultra-fine-pitch component assembly relies on the printability of miniature solder paste deposits. Solder pastes containing type 7 powder are used for advanced packaging applications such as system-in-package and flip-chip attach. These assemblies utilize stencil apertures that are too small for coarser powders to pass through without clogging.
The smaller particles ensure that the paste flow remains consistent during printing.
Quality Challenge
Visual and x-ray inspection systems verify the quality of reflowed joints on ultra-fine-pitch assemblies. Solder paste formulated with type 7 powder is more prone to solder balling and voiding due to the high surface-to-volume ratio of the powder. This issue is managed by utilizing nitrogen reflow atmospheres and optimized thermal profiles.
Storing and handling the paste under strict temperature controls minimizes oxidation.
