The creation of new, 21st Century manufactured products gives us exciting possibilities. However, the number of complex devices and components that consist of one piece of a single material is negligible; almost all manufacturing involves the joining of materials.
Joining technology is extensive, but is still challenged by novel designs and new advanced materials. Frequently, these needs could be met by soldering, where a low melting point alloy is introduced in liquid form into the joint, where it solidifies, making a bond. Many people will associate soldering with the electronics industry, where it is widely used, reliably, effectively and at low cost.
Yet current soldering is not good at forming bonds with many materials, (for example metals with tenacious oxides and ceramics) and it does not form strong joints which can resist exposure to elevated temperatures where applications demand it. To do this may need an approach used for brazing (very much like soldering, but at higher temperature) of adding an element to the alloy, whose role is to chemically interact with surfaces and improve wetting when liquid and bonding once solidified. Adapting the terminology from brazing, this would be "active soldering".
Such a process is not simple however. First we must identify the correct active elements, which may not be the ones used in brazing. These must produce sufficient reaction at low temperatures and be adapted to the materials being bonded. Secondly, a way to introduce a large enough amount of these elements into the solder is required. Solders are based on tin, which may react with the active elements itself if too large quantities are present. Finally, such joints that have been attempted have very poor mechanical properties, and these must be improved.
To resolve these challenges, we will deposit the active elements (selected with the aid of thermodynamic modelling) onto a metallic carrier, a Ni or Cu sponge or foam, with fine (~0.5mm) pores, and infiltrate the Sn into this, creating a composite solder. This will keep the active elements and the Sn separate until soldering, when the Sn will begin to dissolve the foam and progressively release the active material to aid in bonding. The residual network of the foam structure across the joint seam will also be effective in increasing the joint strength. We will make and test these composite solders and the joints, and we will also probe the reactions occurring in great detail, to ensure we understand the key step of this new technology.
Of immediate use, this approach will improve the strength of bonds achieved in current applications (such as in antennae, heat exchangers and semiconductor devices), give them higher temperature resistance in service and reduce the environmental impact of the process, by removing the need for polluting chemical fluxes or electroplating to prepare the joint and aid bonding. The benefits certainly do not stop there, as the technology would also allow new applications. For example, metals like stainless steel are brazed in vacuum at high temperature; achieving the same goal at lower temperatures and in air would be a much less expensive process. Low process temperatures save energy and cost; for example, some electroceramics (important for, e.g. capacitors) can be processed by cold sintering at temperatures as low as 200degC, but the advantages would be lost without low temperature means to join them in electronic devices. Advanced materials such as graphene also hold much promise in areas like touchscreens and circuitry, and a technique like that developed here would be an essential part of making this a reality.
The simple, mass manufacturing nature of solder means that, with our research partners including end users and processors of solder materials, the scalability of the new method created, and the chances of realising these benefits, will be very high.
