RMS Technology

With so-called reactive multilayer systems (RMS), Fraunhofer IWS has developed heat sources whose composition can be tailored to their application purpose. RMS consist of at least two materials, which are stacked in several hundred individual layers and react in a self-propagating exothermic manner after exposure to an activation energy, for example a short-term temperature increase. The heat of reaction can be used to melt the base material or a solder to produce a joint. RMS can be deposited directly on components or produced as a standardized, free-standing film. They achieve overall thicknesses ranging from a few micrometers to over 100 μm.

The deposition of the individual layers for the production of RMS is carried out by magnetron sputter deposition (MSD). Via a rotation of the substrate carrier in front of the coating sources, the individual layers can be deposited in the range of a few nanometers and over a hundred nanometers, thus building up the RMS as a whole.

RMS offer the potential for joining to precisely adjust the amount of heat required. Thus, low, medium and high energy RMS material combinations have been developed at Fraunhofer IWS, which allow to melt plastics up to brazing alloys with melting temperatures of more than 700 °C.

Joining of metals and ceramics

The application of reactive film for joining has been demonstrated so far for soft soldering for materials of the same and different types, such as metals, ceramics, semi-metals and hard metals. The necessary solders with melting temperatures in the range of 200 - 300 °C were applied either to the RMS or to the components.

A significant advancement in reactive joining technology was achieved by doubling the amount of energy provided by the RMS. As a result, solders with melting temperatures just above 700 °C (e.g. Incusil ABA (Wesgo), AlSi10) can now also be used. This permits the joining of components that are subject to high temperature stresses in service. In addition, such solders can also be used to improve the strength of the joints.

Joining polymers

Within a very short time, remarkable results have also been achieved in joining polymers (plastics) with reactive films. The energy provided by the films is used to melt the surfaces of the polymers directly, so that the polymer components are then welded together. The fact that the amount of heat supplied by the films can be precisely controlled by their nanolayer structure is particularly advantageous in this application. Thus, on the one hand, burning of the polymers can be avoided and, on the other hand, a defined liquid phase can be generated.

Applications of RMS are diverse in areas such as: 

Microsystem technology and electronics

• Hermetic sealing of cavities
• Electrical and thermal contacting of sensors
• Bonding of Si wafers to metals and ceramics
• Electrical contacting and bonding of diamond
• Electrical and thermal contacting of power electronics

Polymer technology and Lightweight construction

• Gentle reactive joining of fiber-reinforced and unreinforced plastics in fractions of a second
• Metal-plastic hybrids
• Body construction, housing construction
• Welding of difficult to access joining zones (pneumatics, housings)

Mechanical and systems engineering

• Metal-ceramic joints
• Medical technology
• Joining of temperature-sensitive structures and components
• Low-stress joining of a wide variety of materials

RMS technology offers enormous advantages, especially for the gentle joining of components. In particular, the RMS properties enthalpy and reaction front velocity can be precisely adapted to the application via the RMS structure (layer thicknesses, stoichiometric ratio). The resulting internal joining zone and tailored heat source enables not only the joining of standard components but also the joining of the smallest filigree and, above all, sensitive components in extremely short joining times (< 1s) with outstanding strength.

The technology makes it possible:

• Perform joints of widely differing and similar materials,
• To create hybrid joints and
• Join thermoplastics and thermoplastic composites (CFRP, GFRP) without pretreatment or activation of the surfaces.

If two components are to be joined in a way that is gentle on the material, thermosensitive, low-stress and also fast, the choice of the right joining technology is crucial. The use of RMS offers the solution. Metals, ceramics, semiconductors, diamond and polymers can be joined effectively with this method.

In the method developed at Fraunhofer IWS Dresden, a reactive multilayer system (RMS) is inserted between the two components to be joined. Pressure is then applied to the complete joining assembly and an electrical pulse or a laser pulse initiates the RMS reaction. Activation of the RMS initiates a chemical reaction that briefly and locally releases energy in the form of heat, resulting in a strong bond between the components if the RMS type is selected appropriately. For thermoplastics, direct melting of the joining partners takes place, whereby bonding of the materials used can be achieved relatively easily without having to carry out pretreatments or introduce wetting layers. For higher-melting materials, solder layers and possibly also dampening layers are necessary, since direct melting of the base material is not possible. In this case, the energy released during the reaction is used to melt the solder and thus initiate the brazing process.

Soldering or welding processes used for joining generally require heating of larger areas of the components to be joined that are adjacent to the joining zone. In order to avoid changes in the material properties and the occurrence of stresses in the joining zone due to the thermal load, as well as to reach joining points that are difficult to access, a heat source is required that provides heat with pinpoint accuracy only directly in the joining zone. The solution is provided by the RMS.

Using the simulation software COMSOL Multiphysics, an RMS joining process can be simulated prior to practical implementation in order to obtain specific information on the structure of the RMS and the components. For this purpose, the experimental setup is reproduced in the software and an RMS reaction is modeled. The RMS properties enthalpy and reaction velocity required for this are identified as input variables and incorporated into the model. Finally, statements on the temperature distribution in the joining zone and in the component, the heat influenced zone, and the RMS design can be made in advance without having to carry out an elaborate trial and error principle. This saves time and expenses.