Abstract
Reactive nano-multilayers (RNMLs), which are able to undergo a self-heating exothermal reaction, can, e.g., be utilised as a local heat source for soldering or brazing. Upon joining with RNMLs, the heat produced by the exothermal reaction must be carefully adjusted to the joining system in order to provide sufficient heat for bond formation while avoiding damaging of the joining components by excessive heat. This heat balance strongly depends on the thermal properties of the joining components: a low thermal conductivity leads to heat concentration within the joining zone adjacent to the RNML, while a high thermal conductivity leads to fast heat dissipation into the components. The quality of the joint is thus co-determined by the thermal properties of the joining components. This work provides a systematic study on the influence of the thermal properties upon reactive joining for a set of substrate materials with thermal conductivities ranging from very low to very high. In particular, the evolution of the microstructure within the joining zone as a function of the specific time-temperature-profile for the given component material is investigated, focusing on the interaction between solder, RNML foil and surface metallisations, and the associated formation of intermetallic phases. Finally, the specific microstructure of the joints is related to their mechanical performance upon shear testing, and suggestions for optimum joint design are provided.
Highlights
Reactive joining utilises the heat produced by reactive nano-multilayers for soldering or brazing.The reactive nano-multilayers can be employed either as free-standing reactive foils (RFs) [1,2] or as nano-multilayer coatings which are directly deposited onto the joining components [3]
The time-temperature profile evolving in the joining zone depends on the properties of the RF, i.e., the total amount of heat and the rate of heat production, and on the thermal properties of the components to be joined, i.e., on their thermal conductivity and heat capacity [11]: For a given amount of heat produced by the reactive foil, a high thermal conductivity of the components can lead to rapid heat dissipation through the joining components, while a low thermal conductivity of the joining parts can lead to a pronounced heat pile-up within the joining zone [10]
Reactive joining was first performed for all four substrates using the standard joining configuration with a reactive foil of 60 μm thickness and Sn solder foils of 10 μm thickness
Summary
Reactive joining utilises the heat produced by reactive nano-multilayers for soldering or brazing. The time-temperature profile evolving in the joining zone depends on the properties of the RF, i.e., the total amount of heat and the rate of heat production, and on the thermal properties of the components to be joined, i.e., on their thermal conductivity and heat capacity [11]: For a given amount of heat produced by the reactive foil, a high thermal conductivity of the components can lead to rapid heat dissipation through the joining components (in extreme cases, even the self-heating reaction within the reactive nano-multilayer can be quenched), while a low thermal conductivity of the joining parts can lead to a pronounced heat pile-up within the joining zone [10] This problem is similar to other joining processes which utilise a localised, dynamic heat source, e.g., electron-beam welding or laser welding [12,13]. The microstructural features observed for the different component materials as a function of their specific thermal properties, time-temperature-profiles, are related to the mechanical performance of the reactive joints upon shear testing
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