Investigation of the Solder Thickness Effect on the Propagation Properties of Reactive Multilayer Systems

This study examines the influence of solder thickness on the propagation characteristics of reactive multilayer systems (RMS). RMS serve as a fast and uniform bonding technique for the integration of macro and microsystems. RMS consist of a multilayer material stack formed of nanoscale bilayer thin-film materials. By applying external local energy to the RMS, such as mechanical, thermal, or electrical pulse, an exothermic reaction is initiated creating atomic diffusion at elevated temperatures of the materials. The heat released is then sustains and self-propagates the reaction front to the rest of the unreacted parts. RMS are used in many applications varied from controlled micro-ignitors in pyrotechnic devices or aerospace and defense systems, to selfassembly and joining of micro and macro-scale applications such as electronic packaging, MEMS/NEMS devices integration, microfluidic devices sealing, joining of dissimilar materials, and of thermally sensitive components, where extended heating would cause damage. Numerous parameters significantly influence the features of self-sustained reaction propagation, including the RMS material compositions and their geometrical characteristics, such as bilayer thickness, roughness, and atomic ratio. Nevertheless, the impact of the solder layer on propagation characteristics for joining applications has yet to be studied. This work examines the influence of solder layer thickness on propagation characteristics, including propagation velocity and peak temperature of the RMS deposited using PVD process on LTCC substrates. The study demonstrates the link between the solder layer thickness, its roughness and the propagation characteristics of the RMS. Increasing the solder layer to $\mathbf{1 6} \boldsymbol{\mu} \mathbf{m}$ leads to $\mathbf{8 0 \%}$ reduction in propagation velocity (decreases from $3.65 \mathrm{~m} / \mathrm{s}$ to $0.7 \mathrm{~m} / \mathrm{s}$ for samples without and with a $16 \mu \mathrm{~m} \mathrm{Sn}$ layer thickness, respectively). The reaction temperature exhibited also a decline with the increase in solder layer thickness, decreasing from $854^{\circ} \mathrm{C}$ to $753^{\circ} \mathrm{C}$ for samples without and with a $16 \mu \mathrm{~m} \mathrm{Sn}$ layer thickness.