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Microstructure and chemistry of Pb-Sn solder/ENIG interconnections

  The recent directive of EU concerning the restriction of the use of certain hazardous substances, like lead, in electrical and electronic devices, does not apply to such an equipment as missiles, battlefield computers, satellites space probes, computers installed in aircraft, production and processing lines cranes, lifts, conveyor transport, cars, commercial vehicles, aircraft, trains, boat systems, hydraulic excavators, fork-lift road maintenance equipment, harvester, pacemakers, solar arrays and watt balances [1]. One of the very important issue is the plating system of electroless nickel with immersion gold (ENIG) which has been widely used to finish solder pads of printed circuit boards (PCBs), as well as ball-grid array (BGA) and flip chip substrates in many mentioned above devices [2÷4]. The goal of the present study was to provide more details about the microstructure and chemistry of the solder joints on ENIG finish obtained with widely used Pb-Sn alloy. EXPERIMENTAL STUDY Copper pad (35 m thick) with 4÷6 m of deposited Ni-P layer and 0.075 to 0.125 m thick plating of immersion gold was covered with Pb-Sn solder paste (Alpha Metals, 62Sn36Pb2Ag, wt %). The Pb-Sn/ENIG samples produced in such a way were subjected to the sessile drop method by contact heating procedure described in [6] at 503 K for 5 minutes. Then, samples were crosssectioned and examined using the FEI E-SEM XL30 Scanning Electron Microscope (SEM) equipped with the EDAX energy X-ray dispersive spectrometer (EDS). The thin foils for the transmission electron microscopy (TEM) observations were all cut using the Quanta 3D Focused Ions Beam (FIB). The TEM studies were performed on the TECNAI G2 FEG super TWIN (200 kV) microscope equipped with High Angle Annular Dark Field (HAADF) detector integrated with the EDS manufactured by EDAX. RESULTS AND DISCUSSION The SEM image of the cross-sectioned plating after interaction with Pb-Sn so[...]

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