Microelectronic wire bonding is a manufacturing process used to electrically connect integrated circuits with circuit boards or other substrates. Conventionally, balls are molten at the end of a Au bonding wire and subsequently bonded on Al metallization of a integrated circuit. However, Pd/Ni metallization has recently been used for its improved mechanical properties. The bondability, bonding mechanism, and reliability of Au ball bonds on Pd are studied in this thesis. The substrates were produced in this project using three different materials. The base material is polished Cu in the shape of a coupon (1.0 cm × 1.0 cm × 0.5 mm). Cu coupons are plated with Ni (1.0 μm) using an electroless process, followed by electrolytic plating of a layer of Pd (0.3 μm), resulting in an arithmetic mean roughness of the surface of 0.08 μm (baseline sample, sample 0). Higher roughness values of 0.2, 0.4, and 0.5 μm are artificially produced by rolling (sample 1), sanding (sample 2), and sandblasting (sample 3), respectively, on the Cu surface before plating Ni and Pd. A 25 μm diameter Au wire is used for bonding on the polished and roughened substrates with a process temperature of T = 220 °C, and it was found that ≈ 4 % to ≈ 18 % less ultrasonic amplitude was required for successful bonding on the roughened substrates compared to the polished substrate. Bondability is measured by shear testing the ball bonds. An average ball bond strength achieved on the polished substrate is 130 MPa. This value is lower on the roughened substrate with the exception of the sandblasted substrate. Long-term thermal aging at 250 °C was performed with ball bonds on samples 0-3 for durations of ≈ 300 h. The reliability of the bonds is characterized by non-destructive contact resistance analysis during aging and destructive cross section analysis after aging. Contact resistance values for the ball bonds range from 1.6 to 3.5 mΩ at 20 °C before aging, and does not correlate with roughness. For the baseline sample, contact resistance of the ball bonds decreases during aging by -6 % (median value), which indicates electrical integrity of the interconnections at high temperature. This decrease possibly is due to interfacial gap filling by Au or Pd diffusion. In contrast, the contact resistance increases for the roughened samples 1-3 and changes are 0.4, 5, and 14 %, respectively (median values). A conclusive explanation for this increase has not yet been found. After 250 h of aging, a TEM analysis showed Au to Pd diffusion in the baseline sample with a diffusion depth of ≈ 0.1 μm Au. No intermetallics, voids, or contamination is found on the interfaces after aging according to nanohardness, SEM, and TEM analyses. No bond lift-offs or electrical opens were found for the aging temperature and durations chosen. No conclusive evidence for the presence of Au-Pd intermetallics or voids is found.

Microelectronic wire bonding is a manufacturing process used to electrically connect integrated circuits with circuit boards or other substrates. Conventionally, balls are molten at the end of a Au bonding wire and subsequently bonded on Al metallization of a integrated circuit. However, Pd/Ni metallization has recently been used for its improved mechanical properties. The bondability, bonding mechanism, and reliability of Au ball bonds on Pd are studied in this thesis. The substrates were produced in this project using three different materials. The base material is polished Cu in the shape of a coupon (1.0 cm × 1.0 cm × 0.5 mm). Cu coupons are plated with Ni (1.0 μm) using an electroless process, followed by electrolytic plating of a layer of Pd (0.3 μm), resulting in an arithmetic mean roughness of the surface of 0.08 μm (baseline sample, sample 0). Higher roughness values of 0.2, 0.4, and 0.5 μm are artificially produced by rolling (sample 1), sanding (sample 2), and sandblasting (sample 3), respectively, on the Cu surface before plating Ni and Pd. A 25 μm diameter Au wire is used for bonding on the polished and roughened substrates with a process temperature of T = 220 °C, and it was found that ≈ 4 % to ≈ 18 % less ultrasonic amplitude was required for successful bonding on the roughened substrates compared to the polished substrate. Bondability is measured by shear testing the ball bonds. An average ball bond strength achieved on the polished substrate is 130 MPa. This value is lower on the roughened substrate with the exception of the sandblasted substrate. Long-term thermal aging at 250 °C was performed with ball bonds on samples 0-3 for durations of ≈ 300 h. The reliability of the bonds is characterized by non-destructive contact resistance analysis during aging and destructive cross section analysis after aging. Contact resistance values for the ball bonds range from 1.6 to 3.5 mΩ at 20 °C before aging, and does not correlate with roughness. For the baseline sample, contact resistance of the ball bonds decreases during aging by -6 % (median value), which indicates electrical integrity of the interconnections at high temperature. This decrease possibly is due to interfacial gap filling by Au or Pd diffusion. In contrast, the contact resistance increases for the roughened samples 1-3 and changes are 0.4, 5, and 14 %, respectively (median values). A conclusive explanation for this increase has not yet been found. After 250 h of aging, a TEM analysis showed Au to Pd diffusion in the baseline sample with a diffusion depth of ≈ 0.1 μm Au. No intermetallics, voids, or contamination is found on the interfaces after aging according to nanohardness, SEM, and TEM analyses. No bond lift-offs or electrical opens were found for the aging temperature and durations chosen. No conclusive evidence for the presence of Au-Pd intermetallics or voids is found.

Thermosonic ball bonding is a key technology in electrical interconnections between an integrated circuit and an external circuitry in microelectronics. Although this bonding process has been extensively utilised in electronics packaging industry, certain fundamental aspects behind all the practice are still not fully understood. This thesis is intended to address the existing knowledge gap in terms of bonding mechanisms and interfacial characteristics that are involved in thermosonic gold and copper ball bonding on aluminium pads. The research specifically targets the fine pitch interconnect applications where a thin metal wire of approximately 20 µm in diameter is commonly used. In thermosonic ball bonding process, a thin gold or copper ball formed at the end of a wire is attached to an aluminum pad through a combination of ultrasonic energy, pressure and heat, in order to initiate a complex solid-state reaction. In this research, the mechanisms of thermosonic ball bonding were elaborated by carefully examining interfacial characteristics as the results of the bonding process by utilising dual-beam focused ion beam and high resolution transmission electron microscopy, including the breakdown of the native alumina layer on Al pads, and formation of initial intermetallic compounds (IMCs). The effect of bonding parameters on these interfacial behaviours and bonding strength is also investigated in order to establish an inter-relationship between them. Interfacial evolution in both Au-Al and Cu-Al bonds during isothermal annealing in the temperature rage from 175ºC to 250ºC was investigated and compared. The results obtained demonstrated that the remnant alumina remains inside IMCs and moves towards the ball during annealing. The IMCs are formed preferentially in the peripheral and the central areas of the bonds during bonding and, moreover, they grow from the initially formed IMC particles. Growth kinetics of Cu-Al IMCs obey a parabolic growth law before the Al pad is completely consumed. The activation energies calculated for the growth of CuAl2, Cu9Al4 and the combination (CuAl2 + Cu9Al4) are 60.66 kJ/mol, 75.61 kJ/mol, and 65.83 kJ/mol, respectively. In Au-Al bonds, Au-Al IMC growth is controlled by diffusion only at the start of the annealing process. A t^0.2-0.3 growth law can be applied to the Au-Al IMC growth after the Al pad is depleted. The sequence of IMC phase transformation in both Au-Al and Cu-Al bonds were investigated. Voids in Au-Al bonds grow dramatically during annealing, however, only a few voids nucleate and grow very slowly in Cu-Al bonds. The mechanisms of void formation, including volumetric shrinkage, oxidation and metal diffusion were proposed and discussed.

Thermosonic ball bonding is the most popular method used to create electrical interconnects between integrated circuits (ICs) and substrates in the microelectronics industry. Traditionally gold (Au) wire is used, however with industry demands for lower costs and higher performance, novel wire materials are being considered. Some of these wire materials include Cu, insulated, and coated wires. The most promising of which being Cu wire. Some of the main issues with these wire materials is their performance in the electrical flame off (EFO) step of the wire bonding process. In the EFO step a ball called the free air ball (FAB) is formed on the end of the wire. The quality of the FAB is essential for reliable and strong ball bonds. In Cu wire bonding the hardness of the FAB and oxidation are the main issues. A hard FAB requires larger bonding forces and US levels to make the bond which increases the likelihood of damage to the IC. Excessive oxidation acts as a contaminant at the bond interface and can also influence the shape of the FAB. Shielding gases are required to reduce oxidation and improve FAB quality. This thesis focuses on the EFO process and the influence of EFO parameters and shielding gases on Au and Cu FABs. The primary focus of this thesis is to provide a better understanding of the EFO process in order to expedite the introduction of novel wire materials into industry. Several different experiments are performed on an automated thermosonic wire bonder with 25 µm Au and Cu wires to investigate the EFO process during ball bonding. The effects of EFO parameters on the hardness and work hardening of FABs and the effects of shielding gas type and flow rates on the quality of the FABs are determined. The EFO discharge characteristics in different shielding gases is also studied to better understand how the composition of the atmosphere the FAB is formed in influences the energy input via the EFO electrical discharge. Using the online deformability method and Vickers microhardness testing it is found that the EFO current (IEFO) and EFO time (tEFO) have a large influence on the hardness and work hardening of Au and Cu FABs. A harder FAB produced with a large IEFO and low tEFO will work harden less during deformation. The bonded ball will be softer than that of a FAB produced with a lower IEFO and higher tEFO. The online deformability method is found to be twice as precise as the Vickers microhardness test. An online method for characterizing the quality of FABs is developed and used to identify shielding gas flow rates that produce defective FABs. The EFO process for an Au wire and two Cu wire materials is investigated in flow rates of 0.2-1.0 l/min of forming gas (5 % H2 + 95 % N2) and N2 gas. All three of the most common FAB defects are identified with this online method. It is found that good quality FABs cannot be produced above flow rates of 0.7 l/min and that H2 in the shielding gas adds a thermal component to the EFO process. It is recommended that the gas flow rate be optimized for each new wire type used. The EFO discharge power is measured to be 12 % higher in a N2 gas atmosphere than in a forming gas atmosphere. The lower ionization potential of the forming gas leads to a higher degree of ionization and therefore lower resistance across the discharge gap. It was found that the discharge power does not determine the energy transferred to the wire anode because the Au FAB produced in forming gas has a 6 % larger diameter than that of the FABs produced in N2 gas. Other factors that effect the voltage of the EFO discharge include the controlled EFO current, the discharge gap, and the wire anode material.

Thermosonic ball bonding is the most popular method used to create electrical interconnects between integrated circuits (ICs) and substrates in the microelectronics industry. Traditionally gold (Au) wire is used, however with industry demands for lower costs and higher performance, novel wire materials are being considered. Some of these wire materials include Cu, insulated, and coated wires. The most promising of which being Cu wire. Some of the main issues with these wire materials is their performance in the electrical flame off (EFO) step of the wire bonding process. In the EFO step a ball called the free air ball (FAB) is formed on the end of the wire. The quality of the FAB is essential for reliable and strong ball bonds. In Cu wire bonding the hardness of the FAB and oxidation are the main issues. A hard FAB requires larger bonding forces and US levels to make the bond which increases the likelihood of damage to the IC. Excessive oxidation acts as a contaminant at the bond interface and can also influence the shape of the FAB. Shielding gases are required to reduce oxidation and improve FAB quality. This thesis focuses on the EFO process and the influence of EFO parameters and shielding gases on Au and Cu FABs. The primary focus of this thesis is to provide a better understanding of the EFO process in order to expedite the introduction of novel wire materials into industry. Several different experiments are performed on an automated thermosonic wire bonder with 25 µm Au and Cu wires to investigate the EFO process during ball bonding. The effects of EFO parameters on the hardness and work hardening of FABs and the effects of shielding gas type and flow rates on the quality of the FABs are determined. The EFO discharge characteristics in different shielding gases is also studied to better understand how the composition of the atmosphere the FAB is formed in influences the energy input via the EFO electrical discharge. Using the online deformability method and Vickers microhardness testing it is found that the EFO current (IEFO) and EFO time (tEFO) have a large influence on the hardness and work hardening of Au and Cu FABs. A harder FAB produced with a large IEFO and low tEFO will work harden less during deformation. The bonded ball will be softer than that of a FAB produced with a lower IEFO and higher tEFO. The online deformability method is found to be twice as precise as the Vickers microhardness test. An online method for characterizing the quality of FABs is developed and used to identify shielding gas flow rates that produce defective FABs. The EFO process for an Au wire and two Cu wire materials is investigated in flow rates of 0.2-1.0 l/min of forming gas (5 % H2 + 95 % N2) and N2 gas. All three of the most common FAB defects are identified with this online method. It is found that good quality FABs cannot be produced above flow rates of 0.7 l/min and that H2 in the shielding gas adds a thermal component to the EFO process. It is recommended that the gas flow rate be optimized for each new wire type used. The EFO discharge power is measured to be 12 % higher in a N2 gas atmosphere than in a forming gas atmosphere. The lower ionization potential of the forming gas leads to a higher degree of ionization and therefore lower resistance across the discharge gap. It was found that the discharge power does not determine the energy transferred to the wire anode because the Au FAB produced in forming gas has a 6 % larger diameter than that of the FABs produced in N2 gas. Other factors that effect the voltage of the EFO discharge include the controlled EFO current, the discharge gap, and the wire anode material.