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Automotive suppliers of electronic components are jumping on the bandwagon of multiple-wire termination. Ultrasonic welding gives a higher-quality weld and conserves cost.
By Tony DiFinizio
As more and more electronic features appear in vehicles, automotive companies strive to reduce the cost and complexity of the wire harness. One of the focal points of these efforts has been multiple-wire termination. Multiple-wire termination brings numerous wires, traditionally ground leads, to a single point where they can be attached to a terminal (see Fig. 1). This reduces the complexity of the harness by eliminating single terminations in favor of fewer grouped locations. The resulting simplification of the harness leads to cost reductions, performance gains, and improved quality.
 FIGURE 1. Multiple ground leads are ultrasonically welded to a single point and attached to a single terminal instead of multiple terminals. |
Ultrasonic welding is the key to making this process so successful. With traditional crimping or soldering, it is difficult or impossible to make good-quality bonds with more than one wire. Crimping multiple wires will often lead to inconsistent terminations in which some wires are held properly while others may come loose and give intermittent connections. Increasing the crimp pressure leads to cut strands and improper extrusion of the wires. Soldering requires a skilled operator, and quality is difficult to monitor.
Ultrasonic welding is ideally suited for this application. For more than 20 years, ultrasonic welding has been used in the wire harness industry to weld wires to wires or terminals. Most of the knowledge and experience gained over this period is directly transferred to multiple-wire termination. In fact, ultrasonic welding makes it possible to weld over 20 wires to a single terminal.
Ultrasonic metal welding is a low-power type of friction welding that generates little heat and can be controlled to assure process quality. The principle of ultrasonic metal welding involves scrubbing two materials across each other while clamping them under pressure to create a metallurgical bond (see Fig. 2).
 FIGURE 2. Ultrasonic metal welding works using friction rather than heat to create a metallurgical bond between materials. |
The first material to be welded (called the substrate) is set upon a stationary tool called an anvil. A blacksmith uses an anvil as a solid backing for the pounding of metal into various shapes. Similarly, an ultrasonic anvil forms a solid foundation for creating a weld. The ultrasonic anvil is typically knurled with fine ridges that bite into the material holding it in place during the weld process. The second material to be welded (called the superstrate) is placed on top of the substrate, overlapping where a lap joint is to be made. The ultrasonic vibrating tool, called the sonotrode, or horn, is then lowered onto the layers, clamping them under pressure. Fine knurls on the horn surface grip the superstrate and move it in time with the vibrating tool. That creates a scrubbing action between the two layers of material.
This scrubbing motion disperses the layer of oxides and impurities at the boundary until the base metals are in direct contact. The excited atoms from each material then diffuse into each other creating a pure metallurgical bond. The horn vibrates at a frequency of 20 kHz for most applications and 35 or 40 kHz for some low-power applications. The amplitude or stroke of vibrations typically varies from about 6 to 36 µm (half wavelength).
Note that heat is not a factor with ultrasonic welding. The two materials described above are not melted and in fact never reach more than 15% of the melting temperature of the parent materials. The metallurgical bond is a true migration of materials.
Ultrasonic advantages
In the past, numerous techniques have been used to terminate the multiple ground leads of a wire harness (see Fig. 3). One technique was to use a special terminal housing with many connection points such as crimped-spade terminals on one end and a single bolt hole on the other end. With this technique, each individual wire would be terminated with a female terminal and manually plugged into the housing during vehicle assembly. This technique required many extra steps, including fabrication of the special terminal (which is normally in a molded housing), termination of all the individual crimps, and manually plugging in all the wires.
 FIGURE 3. Different termination techniques include crimping single wires to a complex terminal and splicing wires together to ground one wire instead of many. |
Another method was to make a splice where many wires were welded to a single large wire. This large wire was then crimped to a large ground terminal. By using ultrasonics and multiple-wire termination, these steps can be eliminated, saving money and simplifying the harness assembly.
One of the primary advantages of ultrasonic welding is the ability to weld dissimilar materials. Materials such as copper, aluminum, and noble metals can be welded in combinations of foil, mesh, and strands of wire. Many layers of foil or mesh can be welded to a tab in one operation, thus eliminating extra steps and cost.
Since the oxides have been dispersed and the base metals have been joined together, the resulting junction has a lower resistance than other bonding techniques. The contact area can be customized to provide the ideal current path for different applications.
This unique joining process uses no consumable materials, thereby cutting costs by eliminating process steps and the headaches associated with materials (such as lead-based solder) that are hazardous to the environment. The ultrasonic vibrations dissipate surface impurities, and in many cases the chemical cleaners required for many other processes are unnecessary. Combined with the low power consumption of the controller, ultrasonic welding is clearly the most environmentally friendly welding process.
Finally, one of the more critical benefits of the ultrasonic weld is that it is a true metallurgical bond. Typical crimps are easily corroded over time by moisture that wicks into the joint. The ultrasonic weld is impervious to corrosion and offers superior conductivity and longevity, crucial in demanding under-hood applications.
Machine capabilities
Recently developed ultrasonic welding machines have expanded the capability of multiple-wire termination to make this technology a reality. Machines are capable of welding over 20 wires to a single terminal. In fact, over 20 mm² of wire of various gauges can be welded at once. In some applications even larger cross sections are common. This includes combinations of 22-gauge wire up to and including large battery cables. Combinations that include many small wires and several larger-sized wires can be welded with this technique.
Ultrasonic welding has also made terminal inventory reduction possible. Welds as small as 4 mm² up to 20 mm² can be made on the same exact terminal with no tooling changes. After making sample welds, an operator simply stores all the weld parameters and quality-control settings in the ultrasonic controller as a part number. Later, these part numbers can be recalled with the press of a button or the swipe of a bar-code scanner. Up to 1,000 part numbers can be stored.
The weld machine has a built-in crimp feature that welds and performs an insulation crimp at the same time. This quickly creates a superior electrical bond while forming a strain relief for added security. These welds are impervious to liquid because a pure metallurgical bond exists. Ultrasonic welds are therefore ideally suited for use in vehicles in high-stress under-hood locations.
 FIGURE 4. Ultrasonic welding machines enable welds of many different sizes without a tooling change. |
The machine is easy to operate and provides a deep pocket in which to place the wires (see Fig. 4). Every effort has been made to make the awkward loading of so many wires easy on the operator. The wires are positively located and held in place during the weld, preventing loose strands and misplaced wires.
Multiple-wire termination is just one of the solutions made possible by ultrasonic metal welding. Working closely with manufacturers, we are able to solve difficult problems that enable future technology advances.
TONY DIFINIZIO is engineering manager at STAPLA Ultrasonics, 375 Ballardvale St., Wilmington, MA 01887. Tel: (978) 658-9400; email: tdifinizio@staplausa.com.
