Ultrasonic Welding: Basics You Should Know

Ultrasonic Welding Basics You Should Know

Case Study

In the manufacturing process of solar silicon photovoltaic cells, ultrasonic welding has replaced precision resistance welding, achieving the connection between a 0.15mm thick silicon wafer with a coating and a 0.2mm thick aluminum wire. Using ultrasonic welding, hundreds of nodes of 25~50μm diameter aluminum or copper wires can be interconnected on a 1mm2 silicon chip.

The ultrasonic welding machine used on the assembly line has a power of 0.02~2W, a frequency of 60~80kHz, a pressure of 0.2~2N, and a welding time of only 10~100ms.

Principles and Characteristics of Ultrasonic Welding

Ultrasonic Welding (USW) is a solid-state welding method that uses high-frequency oscillations of ultrasonic waves to generate intense friction on the contact surface of the workpieces under pressure, removing surface oxides and heating the workpieces to achieve welding.

This method does not require an external heat source, the workpieces do not melt, and there is no gas or liquid phase pollution. Therefore, it can weld the same or different metals, as well as semiconductors, plastics, metals, and ceramics.

Welding Principle

Figure 6-1 shows a schematic diagram of the principle of ultrasonic welding. During welding, the workpiece 6 is clamped between the upper sound pole 5 and the lower sound pole 7. The upper sound pole inputs elastic vibration energy of ultrasonic frequency to the workpiece and applies pressure; the lower sound pole is fixed, used to support the workpiece and bear the applied pressure.

Figure 6-1 Schematic Diagram of Ultrasonic Welding Principle
  • 1 – Ultrasonic Generator
  • 2 – Transducer
  • 3 – Booster
  • 4 – Coupling Rod
  • 5 – Upper Sound Pole
  • 6 – Workpiece
  • 7 – Lower Sound Pole
  • A – Amplitude Distribution
  • I – Ultrasonic Oscillating Current
  • F – Static Pressure
  • v1 – Longitudinal Vibration Direction
  • v2 – Bending Vibration Direction

The heat energy required for ultrasonic welding is obtained through a series of energy conversion and transmission links. The ultrasonic generator 1 is a frequency conversion device, which converts the power frequency current into an oscillating current of ultrasonic frequency.

The transducer 2 uses the reverse piezoelectric effect to convert electromagnetic energy into elastic mechanical vibration energy. The booster 3 is used to amplify the amplitude and couple the load. The transducer, vibrating rod, booster, coupling rod, and upper sound pole form a whole, called the acoustic system.

The natural frequencies of each component in this system will be designed at the same frequency. When the oscillation current frequency of the generator is consistent with the natural frequency of the acoustic system, the system produces resonance and outputs elastic vibration energy to the workpiece. Figure 6-2 shows a schematic diagram of the energy conversion and transmission process during ultrasonic welding.

During ultrasonic welding, the ultrasonic generator 1 produces high-frequency vibrations tens of thousands of times per second, and transfers elastic vibration energy of ultrasonic frequency to the workpiece through the transducer 2, booster 3, and coupling rod 4.

Under the joint action of static pressure and elastic vibration energy, the elastic mechanical vibration energy is converted into friction work, deformation energy, and the resulting temperature rise between the workpieces. The oxide film or other surface attachments are destroyed, and the metal atoms between the pure interfaces are infinitely close, achieving a reliable connection.

When welding metal materials, the physical metallurgical process at the interface accompanies the bonding and diffusion between atoms. The entire welding process does not have current flowing through the workpiece, nor the action of heat sources such as flames or arcs. The welded material does not melt and does not need to be filled with metal and protection.

It is a special type of solid-state pressure welding method. When welding plastics, the acoustic resistance at the interface of the workpiece is large, which generates local high temperatures, causing the contact surface to quickly reach a molten state. Under the action of pressure, they merge into one.

After the ultrasonic action stops, the pressure continues for a few seconds to make it solidify and form a strong welding joint. The joint strength is similar to that of the parent material.

Figure 6-2 Schematic Diagram of Energy Conversion and Transmission Process in Ultrasonic Welding

Joint Formation Process

The ultrasonic welding process is similar to resistance welding, consisting of three steps: “pre-pressure”, “welding”, and “holding” to form a welding cycle. Analyzing from the microscopic mechanism of joint formation, ultrasonic welding goes through the following three stages:

(1) Vibration Friction Stage

At the beginning of ultrasonic welding, friction is created between the upper sound pole and the upper workpiece due to the ultrasonic vibration of the upper sound pole, resulting in a temporary connection. The ultrasonic vibration energy is then directly transferred to the contact surface between the workpieces, generating intense relative friction.

This friction gradually expands from the initial individual protrusions to surface friction, simultaneously destroying, extruding, and dispersing the surface oxide film and other attachments.

(2) Temperature Rise Stage

In the subsequent reciprocating ultrasonic friction process, the contact surface temperature increases. When the temperature of the weld zone is about 35% to 50% of the metal melting point, the deformation resistance decreases.

Under the joint action of static pressure and alternating shear stress caused by elastic mechanical vibration, the plastic flow of the contact surface between the workpieces continues, further dispersing the shattered oxide film, even deep into the welded material, allowing pure metal surface atoms to approach infinitely close to the range where atomic attraction occurs.

This results in atomic diffusion and mutual bonding, forming common grains or recrystallization.

(3) Solid Phase Bonding Stage

As the friction process continues, the microscopic contact area increases, and the plastic deformation of the contact part also increases continuously. A vortex-like plastic flow layer even forms in the welding area, as shown in Figure 6-3, leading to mechanical interlocking between the surfaces of the workpieces, causing physical metallurgical reactions.

Co-crystalline grains are produced on the bonding surface, resulting in recrystallization, diffusion, phase changes, and bonding between metals, forming a solid joint.During ultrasonic welding, the formation of the joint depends on the vibration shear force, static pressure, and temperature rise in the welding area.

These are related to the thickness of the workpiece, surface condition, and room temperature performance.

Figure 6-3 Vortex-like Plastic Flow Layer in the Ultrasonic Weld Spot Area

Features of Ultrasonic Welding

1) Ultrasonic welding can be used for a wide range of materials, including the same metals, metals with significant differences in physical properties such as thermal conductivity, hardness, melting point, thickness, metals and non-metals, and different materials like plastics.

2) It is especially suitable for welding metal foils, thin wires, and micro-devices. It can weld foils and aluminum foils with a thickness of only 0.002mm, and can also weld multilayer laminated aluminum foil and silver foil.

As it is solid-state welding, there will be no high-temperature oxidation, pollution, or damage to microelectronic devices, making it ideal for precise welding of semiconductor silicon wafers and metal wires (Au, Ag, Al, Pt, Ta, etc.).

3) The workpieces do not melt, the welding temperature is relatively low, the deformation of the workpiece is small, and the physical and mechanical properties of the weld metal do not undergo macroscopic changes. The static load strength and fatigue strength of the weld joint are higher than those of the resistance weld joint, and the stability is good.

4) The effect of the oxide film or coating on the surface of the welded metal on the welding quality is small, so the requirements for cleanliness of the workpiece surface in ultrasonic welding are not high. It can even weld metals coated with paint or plastic film.

5) Compared with resistance spot welding, the power consumption is small. For example, when welding an aluminum plate with a thickness of 1.0~1.5mm, the electric power of ultrasonic spot welding is 1.5~4kVA, while resistance spot welding requires at least 75kVA.

6) Easy operation, fast welding speed, and high production rate.

The main disadvantages of ultrasonic welding are:

  • The power required for welding increases exponentially with the increase in workpiece thickness and hardness, currently limited to welding thin parts such as wires, foils, and plates, and only in the form of lap joints;
  • The surface of the weld spot is prone to fatigue damage at the edges due to high-frequency mechanical vibration, which is not conducive to welding hard and brittle materials;
  • Due to the lack of precise non-destructive testing methods and equipment, it is difficult to accurately test the quality of ultrasonic welding online, making it difficult to achieve mass mechanized production in actual production.

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