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Microstructure and mechanical properties of the Cu/SnAuCu/Cu joints

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The European Union regulations forced the manufactures to limit the use of certain hazardous substances such as cadmium and lead in the electrical and electronic equipment and also make them responsible for its storage and recycling [1, 2]. In a consequence the lead-free interconnection technology is one of the most active branches of the nowadays materials science. PbSn replecements are mainly the tin alloys, among which a special class are those consisted of tin and noble metals such as copper, silver and gold. They are possesing two characteristic features - the high melting point and high concenteration of Sn compared to that of eutectic PbSn. Moreover, the composition of the lead-free solders can be described as eutectic or near-eutectic, which ensures the best manufacturability. The microstructure of these solders is a mixture of tin and the intermetallic phase (IP). Since both mechanical and electrical properties of the tin are anisothropic they are also anisothropic in the case of eutectic solders. Therefore, the intermetallics may take a form of the inhomogenous structures (for example Ag3Sn in the shape large plate-like crystals). To avoid of such IP creation the concentartion of silver in SnAg solder should be less than 3% wt. and copper should not go over than 0.7% wt. in SnCu solders. In the case of SnAu solders gold must not exceed the value of 5% wt. otherwise it leads to the formation of AuSn4 phase which causes britlle “cold" joint. Ternary solders are mainly SnAgCu solders (SAC) where the amounts of silver and copper (corresponding to the eutectic composition) are 3.5±0.3 and 0.9±0.2 (wt. %), respectively [3÷5]. Other ternary solder is SnAuCu alloy which may be used as a joining material for the joints in so-called noble electronics. They are applied in devices with the highest degree of reliability, such as biomedical devices. It should be noted that such products after their exploitation und[...]

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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