Influence of Substrate Material Properties and Process Control in Reactive Joining

Current research in joining technology focuses on effective soldering of temperature sensitive materials. In this regard nanotechnology shows promising possibilities as scale effects and their resulting thermophysical effects such as high diffusion rates can be used for providing a Self-Propagating High-Temperature Synthesis (SHS) at room temperature. By using Al/Ni systems with alternately arranged nanoscaled layers, the resulting multilayer foil can be used as a local heat source for melting solder material within the joining zone without any external preheating. After ignition the reaction provides high process velocities up to 30 m/s and temperatures up to 2000 °C. Significant thermal influences on the joined parts are not detectable.

The influence of substrate properties as well as substrate-depending process parameters in reactive joining is still not well understood. It is estimated that a long duration of melting is beneficial for high strength bonding due to enlarged diffusion processes. Also high applied process pressure is shown to enhance the flow of molten solder and thus improve wetting and bonding.

Within these investigations, substrate-depending process parameters e.g. applied pressure and surface treatment are analysed on joining copper, aluminium, ceramics and their dissimilar joints with Tin-coated reactive multilayer foils. In this context a joining mechanism was constructed making it possible to apply pressure in different modes, (1) continuously by using a spring or (2) by using a stationary plunger. Pressure data are recorded using a load cell and the influence of the mode and amount of pressure on the joint is studied. The properties of the resulting joints were characterised by shear tests and nanoindentation.

This research also aims at analysing solder wetting as a function of duration of melting which depend on substrate properties such like thermal conductivity and capacity. The wetting behaviour of Tin-solder was studied in static-like as well as in fast-tracking experiments and compared with cross-sections of the reactive joints.

It is shown that an enhanced duration of melting can affect de-wetting and pore formation. Contrary short duration of melting leads to poor bonding strength. Mechanical properties and the quality of the joint zone strongly depend on optimized duration of melting and aligned applied pressure for each substrate material. Unwanted influences of substrate materials can be minimised by process (pressure) control and surface treatment.