The thermosonic bonding technology used in multifunctional ball bonding machines achieves highly reliable connections between metal leads and pads by precisely controlling the synergistic effects of heat, force, and ultrasound. The key to controlling energy input lies in dynamically balancing various parameters to ensure atomic-level diffusion at the bonding interface while avoiding material damage. This process involves multi-dimensional technologies, including energy form coupling, timing control, feedback regulation, and equipment hardware support.
The energy input for thermosonic bonding is primarily ultrasonic vibration. A transducer converts high-frequency electrical signals into mechanical vibrations, which are amplified by a horn and transmitted to a wedge. Under vertical pressure, the wedge drives the metal lead against the pad surface in high-frequency friction. The generated frictional heat, combined with the applied preheating, raises the contact surface temperature above the metal's recrystallization temperature. At this point, the metal oxide layer is destroyed, exposing the pure metal matrix, and a metallurgical bond is formed through atomic diffusion. Controlling ultrasonic energy requires a balanced balance of frequency, amplitude, and duration. Excessive frequency can lead to lead breakage, while excessive amplitude can damage the pad, and insufficient duration prevents the formation of a stable bond. Multifunctional ball bonding machines typically utilize a tunable frequency transducer, supporting frequency optimization ranging from tens of kilohertz to hundreds of kilohertz to accommodate various wire diameters and material properties.
Heat input is dually controlled via a heating stage and a built-in resistance wire in the wedge. The heating stage provides a base ambient temperature to soften the metal on the pad surface, while the wedge's localized heating precisely controls the temperature at the bond point to prevent expansion of the heat-affected zone. Temperature control must match the material's thermal expansion coefficient. For example, when bonding gold wires, the pad temperature is typically controlled between 150°C and 250°C to activate metal atoms while preventing wire oxidation or pad deformation. Some high-end models utilize an infrared thermometer to monitor the bond point temperature in real time. A PID algorithm dynamically adjusts the heating power to ensure a temperature fluctuation within ±5°C.
Pressure input is controlled via a force sensor and closed-loop control system, achieving millinewton-level precision. Bonding pressure is adjusted based on wire diameter, pad hardness, and bond type (ball bond/wedge bond). For example, when bonding 25μm gold wire balls, the initial pressure is typically set at 30gf-50gf to avoid crushing the metal ball. However, due to the smaller contact area during wedge bonding, the pressure needs to be increased to 80gf-120gf. The multifunctional ball bonding machine combines an electrically driven locking mechanism with a pneumatic pressure source to achieve rapid pressure response and stable maintenance. It also features a force compensation algorithm that automatically corrects for pressure deviations caused by mechanical vibration or material deformation.
The timing of energy input is critical to bond quality. The typical process consists of four stages: ball burn-in, first bond formation, wire arc pull-in, and second bond formation. During the ball burn-in stage, a high-voltage electric spark is used to melt the wire end into a ball. The energy generated is determined by the capacitor discharge parameters, and the current and time must be controlled to avoid excessively large or small ball diameters. During the first bond formation, ultrasonic energy and pressure are activated simultaneously, typically for 10ms-30ms to ensure sufficient metal diffusion. During the wire arc pull-in stage, ultrasonic input is paused, and only pressure is maintained to prevent wire breakage. When the second bond is formed, ultrasonic energy and pressure work together again to complete the wedge bond and break the tail wire. The start and stop of energy input at each stage is precisely scheduled by a PLC or motion controller, with timing errors controlled to the millisecond level.
A feedback control system adaptively optimizes energy input by monitoring multiple parameters. The vision system captures bond topography in real time, such as ball diameter, necking ratio, and pad deformation. When anomalies are detected, ultrasonic power or pressure is automatically adjusted. An acoustic emission sensor captures acoustic signals during the bonding process to assess interface quality. If the signal strength falls below a threshold, energy compensation is triggered. Some models also incorporate a tensile testing module for online spot checks of bonds. If the tensile force deviates from the standard range, the system recalibrates the energy parameters. This closed-loop control mechanism enables the multifunctional ball bonding machine to adapt to variations caused by different material batches, ambient temperature fluctuations, and equipment aging.
The equipment hardware design provides fundamental support for energy input control. The transducer is made of piezoelectric ceramic, whose resonant frequency matches the wedge mass, ensuring efficient ultrasonic energy transmission. The horn's tapered structure amplifies the amplitude while isolating mechanical vibrations from interfering with other components. The heating platform is made of ceramic, offering both thermal conductivity and insulation, eliminating the risk of short circuits. The pressure actuator utilizes a high-precision servo motor coupled with a ball screw drive to achieve linear pressure output. The coordinated operation of these hardware components provides the physical foundation for precise control of energy input.
