In-situ microscale cold welding using a focused ion beam-scanning electron microscope

Microscale cold welding is an efficient method for achieving high-quality joints, particularly in electronic components, as its low-temperature process preserves the mechanical and electrical properties of the base metals. However, the limited fundamental understanding of successful bonding mechanisms for dissimilar metal joining has significantly restricted its industrial adoption. This is especially true at the microscale, where heat-assisted methods are still generally preferred. Therefore, this work presents a test bed procedure for cold welding of dissimilar metals at the microscale. A Focused Ion Beam (FIB) -scanning electron microscope was employed to design, monitor and characterise the technique, allowing complete control over the welding parameters, such as speed, geometry, and superficial oxides. Successful bonding was achieved without preliminary surface preparation by pushing a tapered copper wire into a pre-made hole in a soft aluminium alloy. The copper wire diameter was larger than that of the hole, promoting shear stresses and plastic deformation. Cross-sectional analysis of joints revealed severe grain refinement near the bonded interface. Elemental mapping highlighted that shear forces removed most contaminants mechanically as soon as contact began. Bonding defects originated from microscopic residuals of oxides or contaminants from the FIB, while a uniform interface was observed in their absence. A four-probe setup for in-situ electrical resistance measurement across the Al-Cu interface was tested, and it was indirectly used to testify the bond quality. Transmission electron microscopy investigations revealed that interdiffusion occurred across the joint interface, forming a thin intermetallic Al-Cu layer, even in the presence of a nanoscopic fragmented oxygen layer.