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Analysis of Ultrasonic Welding of Mechanically Coupled Small Area Contacts for a Silicon Carbide Power Module
Keywords: Ultrasonic welding, Power modules, Mechanically coupled bonds
Ultrasonic welding has been widely used in industry to weld similar or dissimilar materials using ultrasonic energy, pressure, and time. Its counterpart, ultrasonic wire bonding, continues to be the premier interconnect technology in microelectronics and power electronics to conduct currents ranging from a few milliamps to tens of amps. However, the need for reliable and efficient interconnects from a power substrate to power contacts has been in increasing demand as the power density continues to increase. The current carrying capabilities of materials that are needed to create the joint are strained as more current flows through the interconnect materials. Ultrasonic welding is already utilized in industry for both power and signal contacts in place of solder [1]. One difficulty in achieving high quality ultrasonically welded small area contacts is driven by the high sensitivity of the process parameters. ‘Small’ is here defined as a bond area at or below the bottom of the range of the welding equipment specification, typically in the range of 1-10 mm2. Additional difficulty is encountered when bonding parts which require multiple mechanically coupled welded contacts. In the case of the coupled contacts, there could be one or several components that are already mechanically coupled and then are welded at multiple separate points into a module. The issue arising with coupled contacts has been a varying mechanical strength of individual bonds. The goal is to optimize a set of parameters that will consistently create strong bonds. In this paper, the shear strength of small area welded contacts was optimized. Using design of experiments optimization techniques copper power contacts with 2 mm by 3 mm feet were welded to both copper and nickel/gold plated copper substrates where pressure, amplitude and deformation were the control factors. For the design of experiment, the combinations of settings were chosen based on the concept of classical Screening Design. This design was best suited because it would allow for an elementary exploration of interaction relationships of a wide range of factors. The pressure was varied from 1.4 to 1.8 bar, the amplitude from 90 to 100%, and the deformation from 0.06 to 0.1 mm. These limits were chosen based on prior welding experience with the equipment and similar parts, which had previously not fully explored the interactions of these parameters. In addition, the shear strength pre- and post-thermal cycling was performed to verify optimal settings. The quality of each bond was optically inspected to confirm high quality bonds. Using common shear techniques, over 200 bonds were sheared, and strengths recorded as the dependent variable in the design of experiment. The initial step found that pressure was the most significant factor. Pressure and deformation were also proven to affect strength inversely to each other. Interestingly, the planarity of the contacts had no measurable effect on the welding strength. Consistent bond strengths were achieved regardless of the bonds’ place within the established bond order. For both copper-to-copper and copper-to-nickel/gold plated bonds the optimal settings were: pressure at 1.8 bar, Amplitude at 90%, and deformation of 0.1 mm, while achieving shear strengths of 48.5 kgf and 42.2 kgf respectively.
Kuldeep Saxena, Process Development Engineer IV
Fayetteville, AR

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