## 1 Introduction  
Thermal mechanical stresses are one of the most crucial issues in joining processes in microelectronics. These stresses are usually caused by two factors. Factor one is the differences in thermal expansion of the materials to be joined. Factor two is the high process temperatures that are needed for most joining processes. Eliminating one of the two factors helps to significantly reduce the thermal-mechanical stress after bonding. One idea to achieve this goal is the usage of localised heat in the joining zone without a global heating of the entire assembly. There are different approaches for low-temperature localised bonding techniques. One such method of realising this is to use nanoscale reactive multilayer technologies to deploy a heat source on a targeted area. Established reactive multilayer systems consist of an alternating stack of layers of different metals, such as Fe and Ni, which can create a self-propagating exothermal reaction. One targeted area for the application of these reactive multilayers in microelectronics are additive bonding processes in LTCC technology, in order to prevent critical thermal-mechanical stresses in sensitive structures within the LTCC multilayer. LTCC provides special 3D-structuring abilities, which enables liquid cooling system integration [1], special biomedical sensors [2; 3; 4] and fluidic structures [2; 5]. Such LTCCarchitectures are sensitive to thermo-mechanical stresses. Another strength is its specific dielectric properties, that make LTCC a very beneficial substrate for specific areas, such as high frequency or millimetre wave circuits [6; 7; 8] or RF-antennas (radio-frequency antennas) and radars [7; 9]. Other applications of LTCC substrates are power LED (light-emitting diode) packaging [10] and fibre optical applications [11]. In addition to this multifaceted functionality, LTCC packages are capable of offering a high reliability [8; 12], with respect to superior thermal mechanical integrity, a high level of hermiticity [8; 13] and excellent chemical resistance [13; 14; 15]. The requirements for such characteristics make the LTCC technology an excellent candidate for automotive applications [9; 12]. However, due to high costs, LTCC applications are presently best suited to specific high-performance applications, such as military, space, biomedical and millimetre wave communication [12]. While the deposition of reactive multilayers on silicon substrates is well-established [16], it is somewhat more challenging to achieve deposition on LTCC substrates. The main hindrance is the intrinsic roughness of the LTCC substrate, the roughness of which ranges from 0.4 to 1 \(\upmu\)m [17; 18; 19]. The reactive multilayer thin films are constructed from well-defined energetic materials that are heterogenous in their structure and contain stored chemical energy [20]. These multilayers have thin layers of reactants which alternate between layers of different metastable solids, each of which have a thickness in the range of 10-300 nm [21]. These alternating layers are typically combined up to a total thicknesses in the range of \(-\)0.1-300 \(\upmu\)m [20]. The layers are chosen based on their ability to make a reaction and to release heat, and the most straight-forward method for obtaining such a multilayer system is by using layer-by-layer magnetron-assisted deposition [22], however alternative approaches such as vacuum deposition and more cost-effective mechanical methods can also be used [23]. Should the reactants by sufficiently perturbed, e.g., through heat, then they begin to spontaneously intermix on the atomic level, as per Figure 1, where the reaction propagates from left to right, releasing heat in the process [24], and once the reaction is initiated, the subsequent heat release is sufficient such that the reaction continues to self-propagate [20]. Alternate methods for initiating the reaction include the means of using an electric spark or laser pulse [22], whereafter the reaction can also continue to self-propagate.