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The Advances of the Electro-less Ni/Pd/Au Metal Stack as UBM for the Flip Chip Technology
Keywords: Flip Chip , Electro-less , Wafer Level Packaging
As the wafer level packaging sector of the semiconductor industry is continuously investigating new and cost efficient technologies for the miniaturization process of the chip package, it becomes more and more important to integrate the wet chemical metal deposition into consideration. Especially the increased demands on the component density and the reduced space requirements, driving the industry towards the two alternative bumping technologies : electroplated and printed solder joints. Several aspects and customer individual considerations leading towards the process of choice. This presentation is focusing on the advances of an electro-less Nickel, Palladium and Gold metal stack, used as “under bump metallization” process for the subsequent printing process. By it’s chemical nature, the electro-less metal deposition is driven by electrochemical metal potential differences. Thus, the metal deposition is only occurring on the pad substrate. Therefore any cost intensive photolithographic process sequence can be avoided. The deposition technology is qualified as single- wafer and batch- wafer processes. The pros and cons are discussed. Additional information will be shown how dimensions contribute to the stability of the formed inter-metallic phases (IMP). The UBM process sequence for Al- alloys and Cu substrates will be compared as well. Today the majority of UBM in flip chip technology uses Ni/Au while new requirements ask for less corrosion at smallest pad geometries and solder volumes. The process of Ni/Pd/Au will be compared against the standard by data from in-line testing to reliability performance. The Palladium step is improving the inter-metallic layer formation and thus is resulting in improved life time and temperature cycling behavior. Investigations will be shown and discussed on the different aspects of inter-metallic corrosion (e.g. nickel layer) incl. the impact on the reliability performance of several systems (e.g. HTS tests according automotive or MIL standards).
Mr. Mike Boyle , Technical Manager
Atotech USA
Rock Hill, SC


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