Selected properties of aluminum ultrasonic wire bonded joints with nickel-plated steel substrate for 18650 cylindrical cells

Abstract

Ultrasonic wire bonding is a process of connecting two or more surfaces using a metal wire. In battery manufacturing heavy wire bonding is used to connect battery cells with each other or more commonly with printed circuit board or metal (aluminum, plated or bare copper) busbar by means of metal (aluminum, copper) wire with diameters ranging 100–500 µm. Single sided battery cell bonding means that connections are made from one side on both positive (cap) and negative (crimp) battery terminals. The main advantages of this method are easy process automation, low production time, easier recyclability, self-fusing properties of the wire and high process flexibility allowing for more space efficient battery holder design and different material thermal expansions (in comparison with direct busbar to cell joining using resistance or laser welding). This study is focused on analysis of selected properties of 400 µm aluminum wire ultrasonic wedge heavy wire bonding on 18,650 Lithium-Ion cylindrical battery cells. Bonding process was conducted using a RBK03 bond head equipped on a Hesse Bondjet BJ985 CNC wire bonder. As part of the work, the transversal and the longitudinal cross-section profile of the weld, the microstructure of the weld and the heat-affected zones of the dissimilar joint were described using electron and optical microscopy, the microhardness distribution in the joint was characterized and the joint sheer tests were performed according to DVS-2811. Materials of the aluminum bonding wire and the battery cell cylinder used for joining were also characterized. Bonding process on the machine used in this study is controlled by its duration, wire deformation or both of these parameters and consist of wire touch down and several (usually 2 or 3) bonding phases. Parameters of each bonding phase consist of bonding force, bonding force ramp, ultrasonic generator current/voltage, ultrasonic generator current/voltage ramp and bonding time. Each wire has at least 2 bonds connected by wire loop and is ended with wire pre-cut and tear off.