Wettability and interfacial morphology of Sn-3.0Ag-0.5Cu solder on electroless nickel plated ZnS transparent ceramic

ZnS has been widely used as infrared windows in defense applications and the ZnS glass with high purity has shown high transmittance in mid-infrared and near-infrared lights. The fabrication temperature of the ZnS ceramic was suggested to be reduced to avoid phase transformation of ZnS and sacrifice its good transparency. Similarly, the joining process of ZnS should be at a lower temperature. Traditionally, mechanical assembly and spicing techniques were used to assembly ZnS transparent windows with metal frames at the aircraft. Nevertheless, the assembly strength was still too low and might increase the risk for the safety of aircraft. Recently, low temperature bonding or soldering process has gradually become an opinion to reduce the safety risk. In this study, we utilized a pre-metallization method coating Ni-P on ZnS transparent ceramics and soldered with Sn-3Ag-0.5Cu at a low temperature. In this paper, we evaluated the solder wettability, bonding temperature, holding time and coating thickness of Ni-P layer in terms of the interfacial morphology and joining mechanism. Finally, the most failures of mechanical tests happened at Ni-P/ZnS interface due to long time reaction and weak interface coating forces between Ni-P/ZnS.