To put it simply: joining means to connect things in a permanent way, e.g. different materials or components which then form a new product. A well-known joining process is soldering, which is usually performed in a furnace. There, the components are heated together with the solder alloy, the solder melts, and creates a permanent bond between the components. But heating is not viable for all areas of application. For example, sensitive sensors can be damaged by the temperatures required for soldering. Until now, the preferred alternative for temperature-sensitive materials or components was gluing, which in terms of quality and durability is far from being able to compete with classical soldering.

Joining with reactive nano-multilayers (RNMS) appears to be a promising alternative, producing significantly better joints than adhesives, and this without need of a furnace. For this joining process, an RNMS foil, a so-called reactive foil, is used. The nano-scaled metallic layers of the foil are able to react with each other under strong heat development. For joining, the reactive foil is placed between the components together with the solder. The reactive foil is ignited by means of an electric spark or a laser. Heat of reaction is generated, the solder melts and the components are joined within a fraction of seconds (see video). The heat is generated within a very short time and exclusively in the area of the reactive foil. Thus it cannot damage or destroy the components. Joining with RNMS is therefore considered an excellent way to join heat-sensitive materials. The actual difficulty lies in the exact dosage of the heat produced by the reactive foil: On the one hand, it must be sufficient to melt the solder, but on the other hand, it must not be too high to avoid damaging of the materials or components.

This heat development is difficult to control and is one reason why reactive joining has no widespread use yet. The more different the properties of the materials to be joined, the more inconsistent the result: Copper, for example, is an excellent heat conductor and allows large parts of the heat generated by the foil to drain away rapidly. This heat-loss can hinder the solder from complete melting. Glass, on the other hand, is a poor heat conductor and behaves in the opposite way: it can literally provoke a pile-up of the reaction heat, which can lead to cracking of the glass due to local overheating.

Hence, there is no universal approach when joining with RNMS. Researchers from Empa's "Joining Technology and Corrosion" department around Bastian Rheingans and Jolanta Janczak-Rusch have now succeeded in joining materials with vastly different properties with this technology. For this purpose, the process is specifically adapted to the properties of the respective materials, for example by carefully choosing the type and thickness of the reactive foil, or the amount of solder. "We can even join copper with glass without damage from heat or thermal stress," explains Rheingans. The Empa researchers now want to make this know-how available to industry and give support in the development of complex processes.

In the recent months, Empa researchers have worked together with the Hahn Schickard Society for Applied Research in Villingen-Schwenningen in an "interreg" project - the EU regional programme for the promotion of cross-border cooperation - to increase their combined knowledge for joining sensitive components with the RNMS method. “We now understand how a certain material behaves during reactive joining, and can thus adapt the reactive joining process accordingly," says Rheingans. Reactive joining allows the assembly of complex structures, also in multiple joining steps, without major technical effort or investments. The Empa researchers therefore anticipate great application potentials especially for sensor technologies, e.g. for the assembly of advanced sensor systems, the on-site installation of sensors, or the production of small series with high value. However, due to its simple and flexible approach reactive joining also offers many opportunities for other fields of application.