| Summary: | 碩士 === 國立成功大學 === 工程科學系 === 103 === In recent years, the Cu wire has gradually replaced the Au wire and becomes the popular material for the wire bonding. Thus the process parameters of wire bonding should be affected by the material properties of the Cu wire, i.e., the hardness of the Cu wire is significantly higher than that of the Au wire. In order to achieve the bonding quality, the adjustment and setting of the process parameters in the bonding process are required.
First of all, the Finite Element Analysis software ANSYS12.0/LS-DYNA is adopted for 3D transient nonlinear dynamic simulation. The dynamic and mechanical behavior is analyzed and the reliability of the model is verified. Next, the effects of process parameters including bonding force, ultrasonic amplitude, frequency, and contact velocity on the formation of defects are investigated. For instance, the structural inefficiency of the bonding pad resulting from neck reduction of Cu ball, residual of Al pad and Al push-out, and the stress on the low-K layer are investigated. It is found that the stress on the low-K layer at the bonding impact moment can be effectively reduced with the contact velocity of 27.5mm/s. Besides, the ultrasonic amplitude and the bonding force are recognized as significant parameters.
Secondly, effects and advantages of the double load technology are applied and analyzed. The results show that the application of the double load technology can effectively reduce the stress on the peripheral low-K area and the neck reduction of Cu ball. In particular, at the large ultrasonic amplitude, the stress on the peripheral low-K area as well as the neck reduction of Cu ball are found to reduce from 202.24MPa to 160.97MPa and 1.8997μm to 1.1749μm, respectively. Furthermore, an optimal second stage bonding force of 31.5gf is found so that the width of Al push-out can be reduced.
Finally, a set of guidelines for tuning the process parameters to achieve the reliability of the Cu wire bonding process is proposed. The results show that lower ultrasonic amplitude and impact force, and second stage bonding force can improve the behavior of bonding break due to the neck reduction of Cu ball. In other words, with the reduces of the ultrasonic amplitude from 2μm to 0.25μm, the impact force from 45.5gf to 24.5gf and the second stage bonding force from 35gf to 21gf, the necking reduces from 1.8997μm to 0.0282μm in the ultrasonic stage, from 3.6299μm to 2.1754μm at bonding impact moment, and from 0.835μm to 0.7482μm in the ultrasonic stage, respectively. Furthermore, reduce of ultrasonic amplitude and impact force can improve the problem of the weaker reliability of electricity caused by over thinness of Al pads. In addition, by reducing the ultrasonic amplitude from 2μm to 0.25μm and the impact force from 45.5gf to 24.5gf, the depth at the lowest point of Al pads can be decreased from 0.8495μm to 0.6553μm and from 0.8598μm to 0.5023μm, respectively. Also, the damage of SiN PSV wall caused by aluminum push-out can be improved by reducing the ultrasonic amplitude and the impact force, then looking for an optimal second stage bonding force. Therefore, the ultrasonic amplitude reduces from 2μm to 0.25μm, the width of Al push-out reduces from 56.689μm to 51.804μm. Also, an optimal value 53.389μm is found with the second stage bonding force of 28gf. In addition, reducing the impact force and looking for an optimal contact velocity can improve the damage on the central low-K area caused by larger impact force. For example, reducing the impact force from 45.5gf to 24.5gf can decrease the stress from 233.822MPa to 140.201MPa. Meanwhile, an optimal value 176.47MPa is obtained at the contact velocity of 22.5mm/s. However, the damage on the peripheral low-K area usually occurring at the ultrasonic stage can be improved by reducing the ultrasonic amplitude and second stage bonding force. Hence, by reducing the ultrasonic amplitude from 2μm to 0.25μm and the second stage bonding force from 35gf to 21gf, the stress can be decreased from 202.241MPa to 126.354MPa and from 159.774MPa to 144.243MPa, respectively.
Keywords: Copper wire bonding, 3D transient nonlinear dynamic simulation, Necking, Aluminum push-out, Stress Analysis
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