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Continuous Au-Sn-Ni-Cu-Pb (ASN-CP) Layer Formation on Double Tin and Wicked Thick-Au/Ni Plated Printed Circuit Boards
Keywords: Au-Sn intermetallic, double tin and wick, high reliability packaging
Utilizing mixed interconnect methods such as Au wirebonding and Sn-Pb eutectic soldering on Au/Ni plated Printed Circuit Card Assemblies provides a robust design solution for High Reliability Packaging applications. Because the surface finish requirements for Au wire bonding and Sn-Pb eutectic soldering are incompatible, combining these interconnect methods onto a single printed circuit requires Au removal from solder interconnect locations. The standard process for localized Au removal, referred to as “Double Tin and Wicking”, involves several iterations of molten solder application and withdrawal at the pad location. This application process acts to dissolve some amount of the Au plating into the solder away from the surface and is followed by a wicking process to remove the Au contaminated solder, leaving a tinned surface for making the final solder interconnects. Double Tin and Wicking is an industry standard process which has been implemented for decades as a preventative measure to inhibit the formation of detrimental Au-Sn intermetallic phases identified as a potential cause for premature failure of solder joints. This paper describes the discovery of a thin (less than 1 um), continuous intermetallic layer at the Ni-solder interface containing Au, Sn, and Cu identified after completion of the Double Tin and Wicking process on pads. The Au/Sn/Cu layer was characterized by SEM/EDS analysis of soldered pad cross sections. A detailed evaluation of the thermal conditions encountered during the tinning portion of the Double Tin and Wick process was conducted to assess the impact of these thermal gradients on the Au/Sn/Cu layer formation and growth. Finally, a thermal cycle life test was performed to assess any potential reliability risk associated with the presence of the Au/Sn/Cu layer.
Jeffrey Jennings,
Harris Corporation
Palm Bay, FL

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