Published in Proceedings of the 41st International Symposium on Microelectronics: Providence, RI, November 2, 2008. Copyright 2008 International Microelectronics And Packaging Society.
This paper presents a systematic study on the effect of 120KHz ultrasonic frequency on the bondability and reliability of fine pitch gold wire bonding. The study was carried out on a thermosonic ball bonder that is allowed to easily switch between ultrasonic frequency of 60KHz and 120 KHz by changing the ultrasonic transducer and the ultrasonic generator. Bonding parameters were optimized through design of experiment methodology for four different cases: 25.4 μm wire at 60 kHz, 25.4 μm wire at 120 kHz, 17.8 μm wire at 60 kHz, and 17.8 μm wire at 120 kHz. The integrity of wire bonds was evaluated by six response variables. The optimized bonding process was selected according to the multi-attribute utility theory. With the optimized bonding parameters developed on one metallization for each of the four cases, 8100 bonds were made on five different metallizations. The samples were then divided into three groups. The first group was subject to humidity at 85°C/85% RH for up to 1000 hours. The second group was subject to thermal aging at 125°C for up to 1000 hours. The third group was subjected to temperature cycling from -55°C to +125°C with one hour per cycle for up to 1000 cycles. The bond integrity was evaluated through the wire pull and the ball shear tests immediately after bonding, and after each 150, 300, 500, and 1000 hours time interval in the reliability tests. Results show that 120 kHz frequency requires less ultrasonic power than 60 kHz when all other parameters are equal. Results also indicate that bonding at 120 kHz frequency is less sensitive to different metallizations than bonding at 60 kHz. All three reliability tests do not negatively affect the bond integrity of Au wire bonds on a variety of Au metallizations for both frequencies. Furthermore, as the reliability test time increases, both pull and shear strengths of Au wire bonds on Au pads increase.
Industrial Engineering | Manufacturing