The suitability of various metallic printed wiring board surface finishes was assessed for new technology applications that incorporate assembly with Lead-free solders. The manufacture of a lead-free product necessitates elimination of lead (Pb) from the solder, the circuit board as well as the component lead termination. It is critical however for the selected interconnect Pb-free solder and the corresponding printed wiring board (PWB) and component lead finishes to be mutually compatible. Baseline compatibility of 'select' Pb-free solders with Pb containing PWB surface finish and components was assessed. This was followed by examining the compatibility of the commercially available CASTIN? (SnAgCuSb) Pb-free solder with a series of PWB metallic finishes: Ni/Au, Ni/Pd, and Pd/Cu. The compatibility was assessed with respect to assembly performance, solder joint integrity and long term attachment reliability. Solder joint integrity and mechanical behavior of representative 50 mil pitch 20I/O SOICs was determined before and after thermal stress. Mechanical pull test studies demonstrated that the strength of SnAgCuSb solder interconnections is notably greater than that of SnPb interconnections. With the exception of electroless Pd surface finishes, Pb-free solder interconnections to peripheral leaded packages survived 5000 temperature cycles of 0°C to 100°C. Metallurgical cross-sections were used to examine the structure of the joints made to Ni/Au, Ni/Pd, and Pd surface finishes. For the Ni/Pd finishes, intermetallic compounds were found floating neat the interface between these Pd-Ni compounds and the interface, correlating to lower pull strengths.
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