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Au Planar Bumps for Flip Chip Bonding
Keywords: Ball Bonding, Ball Bump, Flip Chip
The semiconductor packaging industry continues a relentless pursuit to make smaller packages with a higher throughput. The industry demand for smaller components requires manufacturers to adapt by developing new processes. This presentation will explore the shrinking specifications of wire bonding requirements in regards to ball bump mashed ball diameter and top heights, and how to achieve them. The demand was to bond 60 micron diameter, 17 micron tall ball bumps on a wafer to be used for flip chip applications. Ball bumps are made by bonding the ball then shearing the top of the bump along an axis with the bonding tool. The shear motion is responsible for the top height and the top levelness. Typical ball bumps are sheared at a height where the bump is near the wire diameter transition height. Producing wide and flat ball bumps with heights less than 2/3 the diameter require shearing at thicker parts of the bump. Ball bumps with a height to diameter ratio of only 1/3 are typically very difficult to create; bonding tools clog, they are out of specification, or there are frequent user assists. When the bonding tool clogs, the user is required to interact with the machine to fix the wire, which can take minutes. These issues were mitigated through proper capillary and wire selection, utilizing a multi-step shear motion, and process parameters. Another method to avoid these issues is to perform a second pass on the wafer with a coining tool. This process takes ball bumps that may not be acceptable and bonds them again to flatten them so they reach a certain top height (In some cases all of the bumps on the die can be coined together to produce bumps that are coplanar to each other. For small die sizes they can be accomplished directly on the ball bump bonder). With the coining process, the volume deposited on the wafer by the ball bump pass is important so they can be coined to the correct shape. While both these methods are slower in their operations, they provide a better overall throughput due to the reduced user interaction caused by clogged bonding tools.
Trent Nash, Applications Engineer
Palomar Technologies
Carlsbad, CA

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