The Effect of Ultrasonic Frequency on Gold Wire Bondability and Reliability

�This paper presents a systematic study on the effect of 120 KHz ultrasonic frequency on the bondability and reliabili­ ty of fine pitch gold wire bonding to pads over an organic sub­ strate with gold metallizations. The study was carried out on a thermosonic ball bonder that is allowed to easily switch between ultrasonic frequencies of 60 KHz and 120 KHz by changing the ultrasonic transducer and the ultrasonic generator. Bonding pa­ rameters were optimized through the design of experimental methodology for four different cases: 25.4 mm wire at 60 kHz, 25.4 mm wire at 120 kHz, 17.8 mm wire at 60 kHz, and 17.8 mm wire at 120 kHz. The integrity of wire bonds was evaluated by six response variables. The optimized bonding process was selected according to the multiattribute utility theory. With the optimized bonding parameters developed on one metallization for each of the four cases, 8,100 bonds were made on five different metalliza­ tions. The samples were then divided into three groups. The first group was subjected to humidity at 85�C/85% RH for up to 1,000 h. The second group was subjected to thermal aging at 125�C for up to 1,000 h. The third group was subjected to tem­ perature cycling from �55�C to +125�C with 1 h per cycle for up to 1,000 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 1,000 h time interval in the reliabili­ ty tests. Results show that 120 kHz frequency requires less ultra­ sonic power than 60 kHz when all other parameters are equal. The 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 frequen­ cies. Furthermore, as the reliability test time increases, both pull and shear strengths of Au wire bonds on Au pads increase. Keywords�Bondability, organic substrate, reliability, ultrasonic frequency, wire bonding