Thermosonic Flip Chip Bonding Research Articles

Robustness Improvement of Thermosonic Flip Chip Bonding to Device Chip Tilt

In this paper, we describe the robustness of thermosonic flip chip bonding when the bottom chip is tilted in the chip-on-chip assembly. Experiments were carried out using a bottom chip with 12 Au ball bumps and a top chip with Al pads. A bonding condition that causes gross sliding at the bonding interface was chosen to improve the robustness to chip tilt. The bottom chip was tilted around the axis perpendicular to the direction of the ultrasonic vibration. We found that die-shear force and the sum of the contact area between bumps and the top chip electrodes were constant in spite of tilting of the bottom chip. Although the contact area between each bump and top chip electrode depended on different bonding forces generated by bottom chip tilting, the fractional area of Au-Al alloy of each bonding area became nearly equal. These were caused by the deformation stress of the gold ball bump under the application ofultrasonic vibration being constant in the deformation range in this study.

HTS and PCT Reliability of Chips and Flex Substrates Assembled Using a Thermosonic Flip-Chip Bonding Process

This study assesses the high-temperature storage (HTS) test and the pressure-cooker test (PCT) reliability of an assembly of chips and flexible substrates. After the chips were bonded onto the flexible substrates, specimens were utilized to assess the HTS test and PCT reliability. After the PCT and HTS tests, the die-shear test was applied to examine changes in die-shear forces. The microstructure of the test specimens was analyzed to evaluate reliability and to identify possible failure mechanisms. When the duration of the HTS test was increased, the percentage of gold bumps that peeled off from the surface of the copper pads on the chip side increased, and a crack was present at the bonding interface between the gold bumps and chip bond pads. This crack was due to thermal stress generated during the HTS test, and degraded the die-shear force of the assembly of chips and flexible substrates. After the PCT, the crack was present at the interface between deposited layers of copper electrodes after the specimens were subjected to the PCT for various durations. Moisture penetrated into the deposited layers of the copper electrodes, deposited layers lost their adhesion, and the crack progressed from the corner into the central bond area as the test duration increased. To improve the PCT reliability of assemblies of chips and flexible substrates using the thermosonic flip-chip bonding process, one must prevent moisture from penetrating into deposited layers of copper electrodes and prevent crack formation at the interface between nickel and copper layers. Underfill would be an effective approach to prevent moisture from penetrating into deposited layers during the PCT, thereby improving the reliability of the samples during the PCT.

Effects of Ultrasonic Power and Time on Bonding Strength and Interfacial Atomic Diffusion During Thermosonic Flip–Chip Bonding

In this paper, bonding strength of thermosonic flip-chip (FC) bonding was tested by a Dage 4000 bonding tester, atomic diffusion of bonding interfaces was detected by a scanning transmission electron microscope, and deformation of the bumps was observed by X-ray inspection system. Experimental results show that moderate ultrasonic power (about 4 W) results in higher shear strength of FC interfaces, while excessively high ultrasonic power induces larger displacement of vibration to damage bonding performance. Appropriate ultrasonic bonding time (about 100 ms) leads to the optimal thickness of atomic diffusion at the interfaces (about 200 nm) and better bonding bump shear strength with an average of 75 g. Overlong ultrasonic bonding time obviously aggravates the deformation of the bumps, which reduces bonding precision, thickens the thickness of atomic diffusion at the interfaces and damages the bonding strength. So, optimized parameters of ultrasonic power of 4 W and ultrasonic bonding time of 100 ms are proposed to improve the quality and performance of FC bonding.

Dynamics of Ultrasonic Transducer System for Thermosonic Flip Chip Bonding

Thermosonic flip chip bonding is used in certain fine pitch IC packaging for its unique features. By using this bonding process in this paper, 1 mm times 1 mm chip with 8 gold bumps has been bonded onto a silver-coated substrate. Dynamical properties of transducer system, which is the key component for providing the ultrasonic energy, have been investigated using finite element model (FEM) and measurement using impedance analyzer and laser Doppler vibrometer (LDV). The simulation results show that the ultrasonic transducer vibrates by coupling with all excited modes, therefore resulting in complicated motions during bonding. The third axial mode, which includes 1.5 wavelengths and 3 nodes, is the dominant working vibration. However, this axial mode is severely disturbed by undesirable non-axial modes such as flexural modes. There are some other unwanted parasitic modes close to dominant mode. Measured velocities of the transducer horn show that the system vibrates under several modes simultaneously. The impedance measurements reveal additional frequencies overlapping the working frequency. All non-axial modes of the ultrasonic transducer disturb the bonding process and degrade the bonding quality. A subtle control is needed to obtain unique axial mode and stable vibration for high bonding quality.

Viration transfer process at thermosonic flip chip bond interface

A laser Doppler vibration (LDV) measurement system was setup to monitor the real time vibration of bonding tool and flip chip in thermosonic flip chip bonding. By analyzing the vibration of chip and comparing with the vibration of tool tip, the "stall" phenomena of chip is elucidate, which is significant critical phenomena indicating bonding states changing during bonding process. Stall means that the initial bonding strength is formed at the bonding interface. The chip was vibrated with tool tip before "stall" . After "stall" , relative movement produced between them, a part of tool energy is propagated to the bonding interface via chip and formed bonding strength, and a part of energy is consumed at the tool/chip interface friction, which caused damage to bonding strength and bonding interface structure, caused damage on the surface of chip and tool. Finally, a novel bonding parameter apply method is developed to reduce the friction and to enhance the bonding strength.