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Process Development of Al-alloy Wire Bonding for High-Temperature Power Electronics
Keywords: wire bonding, Al alloy wire, high-temperature power electronics
As a cost-effective, flexible, and reliable interconnect method, Al wire bonding remains the mainstream interconnect technology in power electronics. Unfortunately, the conventional Al wires are not suitable for high-temperature applications since pure Al wires become soft when operation temperature is higher than 150 C. With innovation of new devices made of silicon carbide (SiC) or gallium nitride (GaN) and Ag sintering die attach process, power electronics devices can be operated over 400 C. New topside interconnects have to be developed to accommodate high- temperature applications. Cu wire has high mechanical strength and higher electrical and thermal conductivities and is a promising candidate for high- power, high-temperature power electronics. Several companies have been using Cu wire in their high-end products. However, due to its high hardness, Cu wire requires special die-top metallization and has a few challenges in bonding process. Those challenges have to be overcome before Cu wire can be widely adopted in power electronics industry. Al-alloy wires such as AlX wire from Heraeus [1] and TALF wire from Tanaka [2], with modified hardness and microstructure, have improved performance and can be operated up to 300 C. Those wires generally have 99% Al purity and <1% alloying particles. With finer grain size and alloying particles acting as obstacles for crack propagation, those wires have higher heat resistance, stronger tensile strength, and higher thermal fatigue resistance than conventional Al wire, thus are suitable for high- temperature power devices. Unlike Cu wires, Al-alloy wires do not require special die-top metallization and are easier to implement in current mass production. Due to their higher strength and hardness, however, those wires present higher risk of die damage than conventional Al wires during wire bonding process. Bonding process development and optimization is critical to minimize die damage and maximize device reliability. In this paper, the bonding process of 300 m diameter TALF wire bonded on Semikron IGBT devices was studied and optimized. The process optimization was conducted in 3 steps by using Design of Experiment (DOE). A regular bonding process has 8 bond parameters Touch Force, Start Force, Bond Force, Start Power, Bond Power, Start Ramp Time, Bond Ramp Time, and Bond Hold Time. The process optimization was to define an optimal value for each bond parameters so that an optimal bond quality would be achieved. The criteria to evaluate bond quality were pull strength, pull failure mode, shear strength, shear nugget coverage, die damage rate, and bond reliability. Since bond reliability was tested by power cycling and took several weeks, we only used it in the end, after all DOE runs, to confirm the result. Die damage was initially tested by measuring the electrical resistance between the emitter, collector and gate every time after an IGBT module was bonded. In the confirmation step, more accurate measurement of die damage was conducted with a Gate Tester, which detects gate leakage current. In the first step, factor screening, 4 parameters were identified as the most significant factors out of the 8 bonding parameters. This step started with defining lower and upper limit of each of the 8 bonding parameters. 12 runs of Planckett-Burman screening design were created when the range of each bonding parameter was defined. The aforementioned mentioned criteria except bond reliability were used to make a Pareto Chart of Effects. Combining the 5 Pareto Charts, a ranking list of effect was generated. The four most significant factors were: Bond Force, Bond Power, Start Power, and Bond Hold Time. They were optimized in the next step. In the second step, optimization, an optimal value was identified for each of the important bonding parameters. A central composite response surface design of the 4 parameters was created, which contained 31 runs of tests. Pull and shear strengths were used as the two responses for optimization, and two fitting models were created. Analysis of Variance was used to make sure the fitting models were good. From each of the fitting models, an optimized set of parameters considering pull or shear strength was obtained. By combining the two models and maximizing both pull and shear strength simultaneously, a third set of optimized bonding parameters was obtained. In the third step, confirmation, 3 runs of tests were conducted to confirm that the optimized parameters produce optimal bond quality. Both pull and shear test results showed that with the optimized parameters, the bond quality was improved and >1,100 g pull force and >2,200 g shear force were achieved. The tested strengths were very close to or higher than the optimal strengths predicted from the response surface results. 250 bonds were made in each run to have enough sample size to evaluate die damage rate. No die damage was detected with both measurement methods. 2 optimized bonding parameters were selected to make samples for power cycling test. Each power cycling will run several weeks. Initial results showed the TALF wire has longer lifetime than conventional Al wires. The complete power cycling data will be presented in the full paper.
Tao Xu,
Kulicke and Soffa
Santa Ana, California

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