Author: Lam, Kui-kam
Title: Supervisory system for epoxy dispensing process monitoring in die bonding
Degree: M.Sc.
Year: 1998
Subject: Electronic packaging
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Department of Manufacturing Engineering
Pages: xi, 95, xiv leaves : ill. ; 30 cm
Language: English
Abstract: Dispensing of adhesives is the most widely used technology in the high-speed, automated die-bonding process, and it is expected to grow and expand even faster as chip size grow and quality expectations increase. The application of adhesive to substrates is one small but critical step in the manufacturing of a packaged IC. The process directly affects the overall quality of the finished product. The distinctive advantages of dispensing over other application methods include its fast operating cycle, no degradation of adhesive materials, minimal wastage, as well as a relatively friendly process. On the other hand, an adhesive that has been optimized for maximum workability often results in the deterioration of the significant properties such as insufficient dispensing volume, non-optimal distribution, no epoxy/too much epoxy when starting up, and etc. The difficulties of dispensing conductive silver adhesive start with to have a stable and reliable control system. At the present technology, a pneumatic control system is most often employed for epoxy dispensing because of its simplicity in control and low cost. Its basic operation is achieved by activating a solenoid and apply a regulated pneumatic high pressure into a syringe tube in a specified time period to squeeze out the epoxy. Due to the compressibility property of air, the system exhibits nonlinear characteristics. The amount of epoxy to be squeezed out is controlled by the parameters of the squeeze time, high pressure and the vacuum pressure applied. Another critical parameter affect the dispensing performance is the dispensing level above the lead frame bond pad surface where the process is performed. The higher and higher requirement of the applications demand for a comprehensive investigation of the epoxy dispensing process in order to optimize and improve the process. In addition, the development of an on-line dispensing process monitoring system is also essential for early detection of any faults in the process so as to avoid the production of a large volume of defective parts unnoticed. In the dissertation, the dispensing process was experimentally investigated and an on-line monitoring system was also built for on-line detection of the failure dispensing process. In the project, a three-axes testing jig was built for dispensing process study. The testing jig equipped with step motors with encoder feedback for X, Y, and Z movements. Process investigations included the effect of the control parameters of squeeze time, squeeze pressure, as well as the nozzle size. A study of the dispensing level with respect to the epoxy size and volume was conducted in a range of dispensing level, while the quality of the epoxy was also taken in to the analysis. Based on the experimental result. It was found that the optimum range of the squeeze time, squeeze pressure and dispensing level are from 60 to 80msec, 0.14 to 0.18 bar, as well as 40 to 100um respectively. Processing parameters within these range provides the most consistent epoxy dot size. Whereas the processing parameters out of this range will cause unstable and fluctuation results, It was also noted that dispensing level does not significantly affect the epoxy volume. Other experiments were also carried out to study the effects of the epoxy amount inside the syringe and the dispensing pressure built-up characteristics with respect to the dispensing process. In the experiment, epoxy size and volume were taken for the measurement and total area under the pressure built-up curve was taken for analysis. Investigation found that the whole dispensing process from a fully filled epoxy to 20% of epoxy left behaved non-linear characteristic at both ends, and a relatively linear characteristic at the middle. This characteristic was referred to as run-in, steady, and empty processing states. Same characteristic was also found in the pressure area build up plotting. These process behavior were due to the gradually decreasing amount of the epoxy inside the syringe during dispensing. And the result agreed with the nature of the dispensing process. The results of the investigation concluded that precision control of dispensing level is critical. More advanced motion control system have to be employed for the application to achieve the accuracy in the dispensing level. Secondly, changing in epoxy level inside the syringe during dispensing process brings the system into different process states with different process characteristics in both epoxy volume and pressure response. On-line monitoring on both epoxy volume and pressure area build have to be built into the system to monitor the process at any point of time. To achieve this, a on-line dispensing process monitoring system based on the adaptive time-series modeling method was developed. In the system, an autoregressive (AR) time-series model is used to predict the pressure area built up every time a new data is sampled. A Modified Least Mean Square adaptive Algorithm (MLMS) was adopted to modify the coefficients of the AR model to adapt to the real-time dispensing process. The MLMS algorithm make use to the initial values of AR initial coefficient vector, initial value of the adaptive value, as well as the adaptive gain so as to minimize the initial prediction error. Pattern recognition technique of Least Mean-Square-Error classifier was also used for classifying the normal and failure dispensing processes. Experimental results also confirmed that the AR model coefficients show a reasonably strong correlation to the epoxy dispensing process and the system error estimation of the classifier shows the successful classification rate The system has been implemented on the die attach machine for on-line dispensing process monitoring. The performance of the system under production environment was found to be satisfactory.
Rights: All rights reserved
Access: restricted access

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