Reduction of underpad stress in thermosonic copper ball bonding

Abstract

Ball bonding processes on test chips with Al metallized bonding pads are optimized with one Au and two Cu wire types, all 25 µm diameter, obtaining average shear strengths of more than 120 MPa. The process temperature is about 110 °C. The results demonstrate that ball bonds made with Cu wire show at least 15% higher shear strength than those made with Au wire. The estimated maximum shear strength c pk value determined for Cu ball bonding (c pk = 3.7 ± 1.2) is almost 1.5 times as large than that of the Au ball bonding process (c pk = 2.3 ± 0.9), where LSL is 65.2 MPa. However, the ultrasound level required for Cu is approximately 1.3 times that required for Au. Consequently, about 30% higher ultrasonic forces induced to the bonding pad are measured using integrated real-time microsensors. The higher stresses increases the risk of underpad damage.One way to reduce ultrasonic bonding stresses is by choosing the softer of the two Cu wire types, resulting in a measured ultrasonic force reduction of about 5%. A second way is to reduce the ultrasound level. While this causes the average shear strength to fall by 15%, the ultrasonic force falls by 9%. The c pk value does not change significantly, suggesting that a successful Cu ball bonding operation can be run with about 0.9 times the conventionally optimized ultrasound level. The process adjusted in this way reduces the extra stress observed with Cu wire compared to that observed with Au wire by 39%. Hence, significantly lower than optimized ultrasound levels can be used in a Cu wire bonding process to obtain c pk values higher than that of a comparable Au wire bonding process while reducing the risk for underpad damage.

Fine and ultra‐fine pitch wire bonding: challenges and solutions

“…Electronic devices with high‐circuitry integration of IC chips require high I/O numbers for power, thermal and signal management, and thus the BPPs must be reduced (Goh and Zhong, 2007b). The demand for reducing manufacturing costs and improving performance is also driving low‐cost packaging for fine pitch and high I/O devices (Shah et al , 2008).…”

“…Wire bonding using copper wire is a solution to wire sweep because of its high stiffness (Sivakumar et al , 2002), besides other solutions such as using insulated wire (Lyn et al , 2006; Lee, J., et al , 2007b; Lyn and Crockett, 2007; Zhong, 2008b) and having varying loop heights (Zhong et al , 2007a). Compared to gold wire, thinner and longer copper wires can be used for finer pitch packages because of its higher mechanical strength (Shah et al , 2008).…”

Copper Wire Bonding: A Review

“…Having a large process window is crucial for stable Cu processing, as it ensures optimal quality production, process portability, and stability [ 10 ]. Studies have shown that the hardness of Cu wire decreases with increasing purity levels [ 11 ]. Hence, using high-purity Cu wire in the bonding process can alleviate damage to the bonding pad caused by Cu wire.…”

Effect of EFO parameters and superimposed ultrasound on work hardening behavior of palladium coated copper wire in thermosonic ball bonding

“…Several different approaches have been explored to reduce underpad stress: Softer wire, producing softer FABs, optimizing the bonding parameters, and modifying the bond pad design are used to limit underpad damage [1,8,9]. Cu bonded ball [9] and Cu FABs [10] with different hardness can be obtained by changing EFO parameters and temperature of the shielding gas.…”