Research On Soldering Process And Mechanism Of Zinc Sulfide By Low-melting Solder

ZnS is the most widely used full-wave band infrared window material,because it has the excellent characteristics of high infrared transmittance,wide transmission range and so on.The conventional infrared window manufacturing process is to connect the ZnS to the metal frame in the form of a mechanical connection or bonding.Due to the metal frame barrier,the performance of the whole system is reduced.In order to solve the above problems,this paper proposes indirect and direct brazing to connect ZnS.Indirect brazing is the surface pre-metallization of ZnS with electroless nickel plating and ion sputtering gold,followed by SnAgCu soldering.Direct brazing is brazing ZnS directly with low temperature composite glass solder.With both two processes,reliable joints are obtained.The influence of process parameters on the microstructure of the joint was explored,and the mechanism of the interface bonding and the evolution of the interface reaction layer were analyzed.Through the study of electroless nickel plating process and mechan ism,Ni-P layer with appropriate phosphorus percentage composition and controllable thickness,smooth surface is obtained,and it combines well with ZnS.The bonding force o f ZnS to Ni-P layer can be improved by ZnS roughening treatment.It is found that a Ni-P layer having a mass fraction of phosphorus of about 8.5% which is suitable for brazing was obtained at 90°C.At the same temperature,the thickness of the Ni-P layer and the plating time are a certain degree of positive linear correlation.Plating rate is about 20?m per hour at 90°C.As the temperature increases,the plating rate increases while the phosphorus decreases.Based on the analysis of interface microstructure and element diffusion,it is considered that the Ni-P layer and ZnS are mainly mechanically bonded and also have a certain intermolecular interaction mechanism.The process and mechanism of SnAgCu soldering pre-metallization of ZnS were studied.SnAgCu can well wet the ZnS coated with Au/Ni-P layer and its wetting angle is 15 degrees at 250°C.The microstructure of the joint are analyzed.The interface reaction layer is(Ni,Cu)6Sn5 or(Ni,Cu)3Sn4,and Ni-P undergoes a crystalline transition to produce Ni3 P layer and Ag3 Sn particles precipitates in the SnAgCu solder.As the soldering temperature increases or the holding time is prolonged,the Ni-P layer is continuously consumed,the thickness of Ni3 P increases and the reaction layer grows gradually.The Kirkendall voids are formed due to the difference in the diffusion rates of Ni and Sn.The fracture occurred in the ZnS in the mechanical performance test of joint.The process and mechanism of soldering ZnS by PbO-B2O3-Zn O enhanced with Pb Ti O3 particles composite glass were studied.By testing the characteristic temperature of glass solder,the soldering temperature was determined to be 400-450°C.By testing the thermal expansion coefficient of glass solder,it was found that it matched the thermal expansion coefficient of ZnS very well.The interface reaction layer is Pb S,and the Pb Ti O3 particles are dispersed in the glass solder.The spontaneous nature of the interfacial reaction was verified by thermodynamic calculations.Sulfur diffused from ZnS and Pb2+ in the melting glass react to produce Pb S.As the temperature increases and the holding time is prolonged,the thickness of the reaction layer increases from nothing.The use of sintering remelting can reduce the defects in the joints.Due to the second-phase diffusion strengthening of Pb Ti O3 particles in joint and the interfacial chemical bonding of interfacial reactions,the joint strength test fracture occurred on the ZnS.The mechanical properties of the joint can meet the requirements.