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Cu Core Column Application for Fine Pitch 3D Mounting
Keywords: Cu cored solder joint, Cu core column, Package-on-package
 Along with the high density mounting of semiconductors used in portable devices such as smartphones and wearable devices, the improvements in many characteristics such as heat dissipation, electric conductivity, and dimensional accuracy are required in the industry. Furthermore, in 3D package mounting structures such as Package-on-Package (PoP) and embedded package, solder bumps may collapse due to the weight of top package. In order to improve these problems, Cu core solder balls, which consist of spherical copper as core material in the center with Ni and solder plating, have been developed and introduced to the market. [1-2] To prevent the diffusion of Cu atoms into the solder, Ni plating was embedded between the layer of Cu and solder. Cu core solder balls have excellent conductivity and have ability to control and maintain a consistent stand-off height to prevent bump collapsing because of these three physical properties of Cu: high electrical conductivity, high thermal conductivity, and high melting temperature.  In recent years, Cu core columns, a cylindrical-shaped copper core material with Ni and solder plating, are also attracting attentions as next generation joint material because it is highly suitable for narrow pitch designs while utilizing the desirable properties of Cu core solder balls. Mounting by conventional solder ball mounter is also being considered, not only as a replacement of conventional solder balls or Cu core solder balls, but also for Cu plating pillars and even TSV (Through Silicon Via). A significant reduction in manufacturing time and cost can be possible with this new advanced material.  However, it is unclear whether it can be mounted stably like normal solder balls because of its cylindrical shape. In the case of Cu core solder ball, previous research [3] have demonstrated that heating rate in the melting solder temperature area; 483-501(K) of reflow profile impacts the position of Cu balls in Cu core solder bumps. Slower melting of solder affects the Cu balls to maintain their center position. In the case of Cu core columns, we observed the tilt of Cu core columns after the reflow process due to alignment force caused by the surface tension of molten solder. This may potentially cause non-wet issue at the package assembly process or the degradation of package-level reliability.  Therefore, in this paper, we set our goal to establish Cu Core Column application for next generation bumping material. We examined the melting behavior of Cu core columns against the heating rate. It turned out that lower heating rate of 1 deg. C/sec is effective for suppressing tilts and voids of Cu core columns. Then, we performed package-level reliability testing: drop test and thermal cycle test. The results exhibited that the shape of Cu core column bumps is beneficial in preventing the stress concentration of drop impact. In terms of thermal stress, higher stand-off height, which is the key feature of this material technology, achieves outstanding results. In conclusion, Cu core columns can demonstrate adequate performance as compared to the conventional solder balls and Cu core solder balls, and be utilized for next generation bumping technology.
Hiroki Sudo,
Senju Metal Industry Co., Ltd.
Adachi-ku, Tokyo

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