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Extraneous “Microball” Formations Encountered in Leading Edge, High-Pb Flip Chip on Organic Packaging Applications
Keywords: flip chip, organic, microball
While the movement to Pb-free microelectronic packaging continues at a steady pace, this change has not been an overnight flip of the switch, especially at the chip to substrate interconnect level. In addition to the usual challenges of developing a fundamental technology, other packaging system drivers, such as current density, interconnect pitch and device dielectric material, present themselves as moving targets to the Pb-free transition. As such, many leading edge applications continue to design and specify, in line with current RoHS compliance standards, a “high Pb” solder alloy for the flip chip interconnection, even for organic packages. In the latter case, the more temperature sensitive substrate accommodates the high Pb solder flip chip bump by having eutectic Pb-Sn solder deposits on the substrate flip chip receiving pads. That said, it can not be assumed that high Pb flip chip on organic will a “drop-in” solution for all such applications, most notably from a volume manufacturing perspective. One particular issue, and the subject of this paper, is known as “microballing”, where a portion of the eutectic solder on the substrate receiving pad breaks off to form an extraneous solder ball rather than residing within the flip chip solder joint. This paper not only explains the microballing phenomenon and its potential impacts, but also strives to emphasize the complexity of such an issue and the rigors required to fully address it. Manufacturing and experimental data are presented to demonstrate the interactions between a number of component attributes and processing variables. A mechanism for the formation of these microballs is proposed and actions taken to mitigate its occurrence are detailed. Finally, the relationship between the microballing phenomenon and continuing technology trends, including Pb-free, are discussed.
David Danovitch, Senior Engineer

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