Reactive Bonding of LTCC Substrates Using Different Types of Nanolayer Stacks

Conventional soldering processes in microsystem packaging often induce critical thermo-mechanical stresses, caused by mismatches in the coefficients of thermal expansion (CTEs) of materials such as silicon dies, molding compounds, and substrates. Reactive multilayer systems (RMS) offer a localized heat source that can significantly reduce such stresses, thereby enabling the bonding of heat-sensitive components. To expand the technological possibilities in fabricating heterogeneous microsystems, this study investigates RMS-based bonding of low-temperature co-fired ceramics (LTCCs) substrates. Two experimental series were conducted. In the first, both reactive foils and deposited RMS structures were used to bond relatively large chips measuring <inline-formula> <tex-math notation="LaTeX">$10\times 10$ </tex-math></inline-formula> mm under a bonding pressure of 1.5 MPa, with ignition triggered either thermally (up to <inline-formula> <tex-math notation="LaTeX">$350~^{\circ }$ </tex-math></inline-formula>C) or resistively, supported by a temperature of <inline-formula> <tex-math notation="LaTeX">$200~^{\circ }$ </tex-math></inline-formula>C. In the second series, smaller chips measuring <inline-formula> <tex-math notation="LaTeX">$3\times 3$ </tex-math></inline-formula> mm were bonded using a reactive foil to achieve a bonding pressure of 100 MPa. Here, ignition was triggered by an electrical spark, supported by a temperature of <inline-formula> <tex-math notation="LaTeX">$160~^{\circ }$ </tex-math></inline-formula>C. The bonding interfaces and metallurgical reactions were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The results reveal significant differences between bonding LTCC substrates using prefabricated reactive foils and deposited RMS structures: prefabricated foils generate higher peak temperatures, often exceeding the thermal limits of the metallization system, whereas deposited RMS structures produce lower peak temperatures and offer better compatibility due to the ability to tailor specific parameters.