Solder Materials Science Gets Small as Miniaturization Challenges Old Rules
by Neil Poole, Ph.D. and Brian Toleno, Ph.D., Henkel
Abstract: As use of ultra fine-pitch devices grows and the industry moves from 0201s to 01005s and from 0.4mm CSPs to 0.3mm CSPs, prevailing Type 3 solder pastes will no longer be sufficient to address smaller deposit volume requirements. Simply moving from Type 3 to Type 4, however, will not necessarily deliver the desired result either. It is critical that the Type 4 materials are optimized for today’s miniaturization demands.
In this instance, optimizing means tightly controlling not only the particle size but the distribution of those particles within the material as well. While current industry standards tend to be a bit unclear as to allowable particle size in the upper end of the range, the published IPC J-STD-006A is fairly liberal with the distribution range of particle sizes. But, recent testing has suggested that a tighter distribution range and a smaller upper limit particle size may prevent some problems down the line.
Current work has focused on not only condensing the distribution and size range of the Type 4 particles, but also on producing the powder in such a way that the integrity of the surface finish is maintained, as this is also essential to lowering oxidation risk. The smaller particles of Type 4 materials make for a higher surface are to volume ratio which, in turn, introduces more opportunity for oxidation. Left uncontrolled, the oxidation can lead to a variety of performance issues including non-coalescence, poor wetting, and/or graping (more on that later), just to name a few. New powder production technology, however, has delivered consistent, smooth surfaces even on powder spheres less than 35 microns in diameter.