A low-melting and magnetically-responsive alloy could be the key to soldering the components of three-dimensional microelectronics according to researchers in the USA [Ramirez et al., Proc Natl Acad Sci (2010), doi: 10.1073/pnas.1001410107].

Modern microelectronics are essentially two-dimensional devices created using photolithographic processes. Adding a third dimension involves stacking together two or more such layers. However, the potential for truly 3D microelectronics lies in being able to construct vertical architectures that exploit the possibility of logical shortcuts between micro-components.
Unfortunately, while the industry advances rapidly every 18 months following the infamous Moore's Law, it is generally not geared up for a paradigm shift in manufacturing. As such, Ramirez and colleagues have focused on developing a technology that could exploit current manufacturing equipment with the kind of minimal disruption plants commonly see.
The team has developed metal alloys, based on tin and silver, that comprise a magnetic iron particles dispersed in a metal matrix. These can be manipulated remotely and with precision using a magnetic field and be placed at strategic connection points as a device is constructed vertically from the base substrate. Such manipulation will allow the materials to be channeled into hard-to-reach locations via channels in the electronic assemblies. The materials have proven to have enhanced mechanical strength and so would make strong interconnects for a 3D device.
Additionally, they can be melted readily by an alternating magnetic field through a localized heating effect that “solders” together the components. The low melting point of the materials is critical to their utility as they melt at temperatures well below the temperatures at which the sensitive components found in optoelectronics would be damaged.
The new technology uses tin-silver alloys that are already increasingly familiar to the industry, and avoids toxic and environmentally harmful lead, which is used in conventional soldering materials.
In their proof-of-principle studies, the team used a strong magnetic field, 3000 Gauss, to draw the material through nanoscopic channels with great control. The magnetic field could produce the stereotypical spiky field patterns seen with ferrofluids, but with the advantage that solidification occurs at room temperature and so sharp tips can be sustained. The iron-dispersion does not affect the melting point of the tin-silver alloy so that it can be manipulated just as readily as the non-iron containing solder.
“These lead-free alloys are useful both for current challenges in microelectronics and have novel utilities for additional applications,” the team concludes.