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Low cost UBM for lead free solder bumping with C4NP

: Ruhmer, K.; Laine, E.; Hauck, K.; Manessis, D.; Ostmann, A.; Toepper, M.


International Symposium on High Density Packaging and Microsystem Integration, HDP '07 : 26 - 28 June 2007, Shanghai
Piscataway, NJ: IEEE Service Center, 2007
ISBN: 1-424-41253-6
ISBN: 1-424-41252-8 (print)
Art. 4283564
International Symposium on High Density Packaging and Microsystem Integration <2007, Shanghai>
Fraunhofer IZM ()

The under bump metallurgy (UBM) structure is a critical component of any solder interconnect system. The UBM typically provides three functions: adhesion to underlying dielectric and metal, barrier to protect the silicon circuitry, and a solder wettable surface. For lead-free solder bumps, the barrier layer is key to reliability due to their higher Sn content. A common barrier layer used in the industry is electroplated nickel. This layer provides good protection from degradation of the silicon metallurgy by tin rich lead free solders. Controlled Collapse Chip Connection - New Process (C4NP) provides an opportunity to eliminate electroplating, and its associated costs for plating chemistry, analysis, supply and waste treatment. This paper analyzes electroless Ni/immersion Au (ENIG), with and without Pd, as an alternative UBM structure. Wafers were fabricated with these UBM structures, solder applied with C4NP, and chip level stressing performed to determine the robustne ss of these alternative stack-ups. Analysis of these structures following multiple reflows and thermal cycling is presented. In addition, the paper also reviews production cost analysis for various UBM stackups and solder bump processes, based on a specifically developed cost model. The ENIG UBM structures in combination with C4NP solder bumping provide a significant cost reduction over alternative structures. C4NP is a unique solder bumping technology developed by IBM which addresses the limitations of existing bumping technologies by enabling low-cost, fine pitch bumping using a variety of lead-free solder alloys. It is a solder transfer technology where molten solder is injected into pre-fabricated and reusable glass molds. The glass mold contains etched cavities which mirror the bump pattern on the wafer. The filled mold is inspected prior to solder transfer to the wafer to ensure high final yields. Filled mold and wafer are brought into close proximity/soft contact at reflow temperature and sol