Present and future solder technologies: how new powders, activator chemistries and epoxy fluxes are evolving for production use.

Author:Poole, Neil
Position:SOLDER MATERIALS
 
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History shows that the electronics assembly industry is always up for a good challenge. This was proved with the successful move from through-hole to SMT assembly, the elimination of CFCs from the cleaning process and implementation of Pb-free, to name just a few. Now, the industry is arguably at one of its biggest--er, smallest--challenges to date: extreme miniaturization. Although device footprint reduction has been an ongoing process over the past two decades, recent developments are some of the most exigent to date. Although designing much smaller packages presents its own unique set of hurdles (a topic for another article), the ability to incorporate these microscopic components into a high-volume, high-reliability production environment is at issue for assembly specialists.

Placing 0201s and 0.4 mm CSPs in a lab environment is one thing; achieving this feat reliably in high-volume manufacturing is quite another. A plethora of process variables are impacted by this reality, none likely as complex as the soldering process. Not only must solder materials accommodate much tighter pitches and smaller geometries, they also must maintain all the previously established requirements for modern manufacturing, including Pb-free capability, compatibility with higher reflow temperatures, humidity resistance, wide process windows and much more.

These new conditions pressure tried-and-true rules for solder materials such as stencil aspect ratios and surface-area-to-volume requirements. Here, we describe several developments on the solder materials front--from new powders to activator chemistries to epoxy flux technologies--to meet miniaturization trends.

As use of ultra fine-pitch devices grows and industry moves from 0201s to 01005s and from 0.4 mm CSPs to 0.3 mm 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. Type 4 materials must be optimized for miniaturization demands.

In this instance, optimizing means tightly controlling not only the particle size, but the distribution of those particles within the material. While current industry standards tend to be a bit unclear as to allowable particle size in the upper end of the range, J-STD-006A (TABLE 1) is fairly liberal with the distribution range of particle sizes. But, recent testing has suggested a tighter distribution range and a...

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