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Flip Chip Joining with Quaternary Low Melting Temperature Solder Bump Fabricated with Injection Molded Solder (IMS)
Keywords: Injection molded solder (IMS), wafer bumping, low temperature melting solder
Fine pitch interconnect with micro solder bump is a key technology to achieve high density chip joining for 2.5D/3D package as well as conventional flip-chip plastic-ball-grid-array (FC-PBGA)/flip-chip chip-scale-package (FC-CSP). As the solder bump size is getting smaller for fine pitch applications, the control of interconnect material’s properties is becoming ever more important for chip-package-interaction (CPI) and electro-migration (EM) performance. One of the approaches for obtaining desired mechanical properties and EM resistance at the same time is the adoption of solders with some minor alloy additives. Electro-plating is widely used to fabricate micro solder bumps. However, their solder compositions are limited to pure Sn or some binary solders such as Sn-Ag, Sn-Cu, etc. Hence, a bumping technology with fine pitch capability and flexible solder alloy composition has been demanded. IBM has developed and has been enhancing the injection molded solder (IMS) technology [1-6] as an advanced solder bumping technology with flexible solder alloy composition even at fine pitch and small diameter. IMS is a simple bumping technology that can form solder bumps on various substrates (wafer, interposer or organic laminate) by injection of molten solder into via holes patterned in a photoresist layer. IMS is applicable to formation of solder caps for Cu pillar bumping which is a technology widely used for fine pitch applications with various solder alloy compositions. One of the advantages of IMS is the capability of using ternary, quaternary, or more compositions solder alloys for bumping, which is not achievable by current plating technology. In this study, we evaluated the feasibility of SnBi based quaternary alloy solder bumping using IMS and the flip chip joining using low melting temperature (135°C) solder materials. Reliability of assembled packages in a 2.5D package form factor as compared to the packages using standard lead free solder alloy (Sn-3.0Ag-0.5Cu: SAC305) is also investigated. The experimental results presented in this paper are the mechanical properties of evaluated solder alloys, the evolution of microstructures of solder alloys through post bumping, post flip chip joining and post reliability stress tests. Also, the results of 1000 cycles of thermal cycling test, which passed the criteria, are shown. In addition, the thermo-mechanical stress induced after chip joining for packages using quaternary low temperature melting solder relative to that for packages using SAC305 is discussed.
Takashi Hisada,
IBM Japan, Ltd.
Kawasaki, Kanagawa

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