Discussion on Ultrasonic Welding and U-shaped Terminal Crimping

Wire harness bundle is the product reflection of the electrical schematic diagram, and it is the carrier for transmitting or exchanging the power signal or data signal of the electrical system, realizing the functions of electrical components and meeting the real-time control of vehicle driving safety, reliability and entertainment. The conductor occupies an extremely important position in automotive Wiring Harness and the electrical system of the whole vehicle. Ideally, the connection between wires, terminals and consumers should be a carrier with zero potential and zero impedance. Due to the physical characteristics, environmental characteristics and material characteristics of the connection terminal, there is always a certain voltage drop in the automotive electrical system. Generally speaking, the voltage drop of the line should not exceed 3% of the circuit voltage. For example, in a 12 V circuit, the maximum voltage drop should not exceed 0.36 V, and the remaining 97% voltage (11.64 V) should be effectively used for electrical loads. As the wires used in automotive cable harness are directly connected by multiple wires, it is necessary to reduce the voltage drop caused by wire factors to the greatest extent by means of different tooling equipment and process methods in our factory, Kable-X. Usually, voltage drop less than 3mV is qualified.

In the manufacturing process of automotive wiring harness, for the wires that need to be connected together, the U-shaped terminal punch connection, ultrasonic welding machine for welding the connection points, and terminal double-wire and multi-wire co-voltage connection are generally adopted. In this paper, the contact voltage drop between punch-in connection of U-shaped terminal and ultrasonic welding is analyzed and compared.

1. Two Ways Are Usually Adopted When Wires Are Connected in A Centralized Way.

At present, there are usually two ways for centralized connection of wires, one is cold stamping of U-shaped connecting terminals, and the other is ultrasonic welding. The brief introduction of these two methods and the comparison of voltage drop are briefly described in this paper. My humble opinion is to discuss with you.

1.1 U-shaped Connection Terminal Cold Stamping Connection

Cold stamping connection of U-shaped connecting terminals is abbreviated as U-shaped crimping, which is a traditional way of wire connection crimping. U-shaped crimping is the process of selecting the U-shaped terminal and crimping machine according to the sum of contact wire diameters, making a special crimping die and jaw for each type of U-shaped terminal, and then cold stamping two or more wires together with the aid of the U-shaped terminal crimping equipment.

The welding machine is equipped with a pressure monitoring device. When there is an abnormal situation (shallow drilling, deep drilling, wire leakage, copper wire interruption, core wire sticking out, etc.), it will automatically alarm and lock the crimping equipment, which must be unlocked manually. Crimping parameters and testing basis shall meet the requirements of QC/T 29106. The closed height can be continuously adjusted, the accuracy can reach 0.03 mm, the transmission structure is simple, the operation is stable and reliable, and work can only be done at the moment of crimping in the working state. Energy saving, unique overload protection function. Nominal pressure 80 kN, adjustment 2 mm(0.079 in), stroke 40 mm(1.575 in). The contact resistance is large, and if the mechanical strength can't be guaranteed, soldering operation is needed; Low production efficiency, and soldering flux and solder are needed for electric welding; The welding process is polluted, and the resistance of the welding point is large, so the phenomenon of virtual welding is easy to occur.

1.2 Ultrasonic Welding

1.2.1 Working Principle of Ultrasonic Welding

People's hearing can only hear the sound with vibration frequency of 20 Hz~16 kHz, and the vibration beyond this range is called ultrasonic. The ultrasonic welding machine and welding area convert the current electric frequency of 50/60 Hz into high-frequency electric energy of 20 kHz or 40 kHz by a transistor function device, and supply it to the converter.

The converter converts electrical energy into mechanical vibration energy for ultrasonic waves, and the pressure regulating device is responsible for transmitting the converted mechanical energy to the welding head of the ultrasonic welding machine. Welding head is an acoustic device that directly transmits mechanical vibration energy to the products to be pressed. The vibration is transmitted to the bonding surface by welding the workpiece, and the thermal energy generated by vibration friction melts the plastic. The vibration will stop when the molten material reaches its interface. Holding the pressure for a short time can make the melt produce strong molecular bonds when the bonding surface is solidified, and the whole cycle is usually completed in less than one second.

The tool is mainly composed of welding head, anvil connecting block, anvil top block and polymerization module. During welding, the wires are vertically arranged tightly and attached to the anvil connecting block. After the foot switch is turned on, the polymerization module moves toward the anvil top block, and the anvil connecting block moves downward together with the anvil top block, pressing the wires tightly in the welding area. The welding head vibrates and transmits energy to the copper wire, thus welding the wire bundles together. During welding, except the vibration of the welding head, other tool heads are stationary. After welding, the polymerization module is retracted, the anvil top block is retracted (XL machine), and the anvil connecting block is raised, so that the Wire Harness can be taken out. As the welding head vibrates, while other tool heads are fixed, in order to prevent the welding between each tool head and the welding head from damaging the welding machine, a gap of 0.025 mm is left between the upper surface of the welding head and the bottom surface of the polymerization module, the side of the anvil top block and the side of the anvil connecting block, so that the welding head cannot contact with other tool heads. Broken copper and other sundries can't be left between these gaps, otherwise the working surface of the tool head will be eroded during welding, and the circuit board will be damaged in severe cases. As the ultrasonic vibration is generated by the welding head, its energy is transmitted from the welding head to the anvil top block, so the closer it is to the welding head, the greater the energy will be, and the energy will be transmitted from top to bottom. Therefore, when placing the wire, the thick wire should be placed at the lowest end, close to the welding head, and the thinner wires should be vertically arranged upward in turn, so that the thick wire can obtain large energy, thus preventing over-welding or under-welding. The vertical arrangement can prevent side welding, thus ensuring the welding quality.

1.2.2 Ultrasonic Welding Requirements for Wire Placement

During ultrasonic welding, it is necessary to set relevant parameters, such as wire cross-sectional area, wire alignment, pressure, welding distance, amplitude, width, pressure, energy, etc. During the welding process, the wires should be vertically overlapped, and the large cross-section line should be close to the welding tool head below, so as to make the welding sufficient; The conductors should be placed close to the anvil surface and close to each other, so as to have sufficient firmness after welding; Generally, the length of conductor overlap is set to 13~15mm. If the overlap length is too short, the welding strength is not easy to guarantee. If the overlap length is too long, the welding end is prone to become warped, which is inconvenient for the next working procedure. Oxidation, broken wires, defects and insulation layer melting are not allowed on the surface of the welding place.

1.2.3 Four Important Parameters and Advantages of Ultrasonic Welding

1) Amplitude, in the direction of vibration, is the maximum distance from the starting point of vibration, in microns. When welding, they interact with each other, which directly affects the welding quality of wires. Different wires have different settings.

2) Width: The distance between the surface of the polymerization module and the opposite surface of the anvil connecting block, in millimeters, determines the welding width.

3) Pressure: The pressure exerted on the copper wire in the welding area by the anvil top block. Its magnitude is related to the air pressure, and the acting direction is perpendicular to the vibration direction. The unit is pounds per square inch.

4) Energy During welding, the total energy released by the welding machine, in joules. That is, when the energy released during welding reaches the set value, the welding is completed.

Ultrasonic welding has its unique advantages:

(1) High fusion strength, excellent electric conductivity after welding, and extremely low or nearly zero resistance coefficient;

(2) The welding material has the characteristics of non-melting, non-fragile conductor;

③ The welding time is short, the efficiency is greatly improved, and it is fast and energy-saving;

④ Stable welding process, online detection and control;

⑤ No gas, solder or flux is needed;

⑥ Welding is free of sparks and smoke, which is both environmentally friendly and safe;

⑦ Improve the welding quality and ensure the reliability of product conductor performance.

2 Wire Voltage Drop

The so-called voltage drop refers to the potential difference formed at both ends of the resistor when the current flows. According to Ohm's Law U=R×I, when the current of the circuit is constant, the voltage is directly proportional to the resistance, that is, the voltage drop formed on the resistance with large resistance is large, and the voltage drop formed on the resistance with small resistance is small.

The voltage drop u of the insulated wire is calculated by the following formula

U=IρL/A

Where: U-voltage drop, V; ρ-resistivity, ω mm2/m; L-- conductor length, m; A-- cross-sectional area of conductor, mm2.

The author quoted the voltage drop at the crimp between the terminal and the wire in QC/T 29106-2004 standard.

3 Voltage Drop at Wire Connection Point

In many previous data, there has always been empirical estimation for the calculation of voltage drop. Through many years' working experience, the author adopts the correct measurement method to provide powerful data for readers. Readers can reasonably adopt the wire connection method according to the different design requirements of wire harness products, the capital investment of the company's operating equipment and other factors.

3.1 Test Parameters of Voltage Drop at Wire Connection Point

Test samples: 20 pieces of ultrasonic welding and 20 pieces of U-shaped crimping respectively, with the left wire 0.75 mm², the right wire 0.5 mm² and the other wire 1.0mm², all of which are 200mm long.

Testing equipment: 50 A DC regulated power supply and multifunctional tester.

Standard QC/T 29106-2004, ambient temperature 22.6℃, ambient humidity 70%, atmospheric pressure 100 kPa, visual acuity above 1.0, illumination 200~300 lx, visual distance 0.3 ~ 0.5m.

Measure the voltage drop parameters of the above two wire connection modes, and the measurement method shall meet the requirements of QC/T 29106-2004.

Test results show that the voltage drop of ultrasonic welding is 0.116~0.348mV lower than that of U-shaped crimping, and that of high-frequency welding is smaller than that of punch-in connection of U-shaped terminals. The electrical conductivity and signal transmission performance of the wiring harness with high-frequency welding contacts are better.

3.2 Service Life of Ultrasonic Welding

The service life of ultrasonic welding depends on the following points.

1) Ultrasonic welding of welding head materials requires that metal materials have good flexibility (small mechanical loss in the process of sound wave transmission), so the most commonly used materials are aluminum alloy and titanium alloy. However, ultrasonic metal welding requires the welding head to be wear-resistant (requiring higher hardness), which makes the selection of materials more difficult, because hardness and toughness are inherently opposite, which requires the selection of high-quality steel materials with very high requirements, so as to maximize the effective life of the welding head. Therefore, the cost is very high.

2) Welding head processing technology includes processing technology and subsequent processing technology, heat treatment and parameter trimming. After each welding head is manufactured, parameters should be measured and adjusted separately to ensure welding quality.

3) Unreasonable welding parameters and wrong operation methods will also greatly reduce the service life of ultrasonic welding head.

The cost of high-frequency welding is slightly higher, but in order to occupy the high-end market and expand the market share, the application of ultrasonic welding in wire harness has become the most economical and reliable design feature.

With the continuous improvement of people's requirements for the safety and comfort of automobiles, the requirements of OEMs for automobile parts are becoming higher and higher. Compared with the cold stamping of U-shaped connecting terminals, ultrasonic welding, as a new advanced welding technology, has obvious advantages such as superior conductivity, rapidness, energy saving and environmental protection, and is the development direction of high-end equipment in automotive wiring harness manufacturing industry.