Research And Development On Low-Melting Point Sn-Pb Based Solder Applied In Photovoltaic Ribbon

With the rapid development of the world economy,the traditional energy demand continues to grow.The traditional energy reserves have dropped dramatically.which leads to countries arc facing energy crises.Solar energy with rich reserves and low development costs has become the best alternative energy for traditional energy.The cells can convert solar energy into direct current electricity via the photovoltaic effect.Thus,governments all over the world regard the development of solar energy as an important energy development strategy.Photovoltaic ribbon(PVR)plays a critical role in solar power modules,which is used to transport the electrical energy produced from silicon solar cells.It is well known that the temperature resistance of silicon is 620 K beyond which photovoltaic effect will not work.The melting point of the solder cannot be too high because of the negative correlation between the efficiency of solar cells and soldering temperature.Furthermorce,the occurrence of residual stresses between the silicon and PVR is the major challenge that easily leads to breakag,e of the cells,It is mainly due to the great differences in coefficient of thermal expansion between these two materials.It is clear that reducing soldering temperature is a very efficient method in order to reduce the debris rate of solar cells,which will indireectly increase the efficiency of solar cells.Therefore,it is urgent to develop new low-melting poinl solders applied in PVR.According to the principle of alloy composition design and solidification theory.the Sn-Pb-In,Sn-Pb-Bi solder alloys applied in PVR are respectively prepared by adding In,Bi element to the Sn60Pb40 binary eutectic alloy in this work.The effect laws of In,Bi element on the comprehensive properties of the Sn60Pb40 alloy arc revealed by the testing of the microstructures,electrical conductivity,melting characteristics.spreading behaviorm and mechanical properties of solder joints.Here the Sn60Pb40 alloy has been studied as a reference to comlpare with the Sn-Pb-In,Sn-Pb-Bi solder alloys The microstructures and properties of the Sn-Pb-In,Sn-Pb-Bi alloys are sensitive to the composition.The melting points of the Sn-Pb-In,Sn-Pb-Bi alloys gradually fall with the content of the In and Bi increasing.The intcrmctallic compound(IMC)InSn4 and Bi5Pb3 with inherent brittle nature are found in the microstructures of the Sn-Pb-In,Sn-Pb-Bi alloys,which are bad for the properties of the alloys,So the content of the In,Bi element in the alloys should be controlled strictly.To sum up,the comprehensive properties of the Sn60Pb37In3 and Sn55.5Pb39.5Bi5 alloys arc relatively well among all solder alloys.In order to optimize the preparation and soldering process of PVR,the Sn60Pb40/Cu,Sn60Pb37In3/Cu,and Sn55.5Pb39.5Bi5/Cu soldering joints are designed and studied respectively at aging different temperatures and times.The types and growth characterizations of IMCs arc analyzed so as to illuminate the effect mechanism of the In and Bi elements on the growth rate of IMCs at the interface of different soldering joints.The results show that the In element can promote the growth rate of IMCs at the interface of different soldering joints while Bi element can effectively inhibit their growth.Finally,according to the aging test results,the growth schematic of IMC at the interface of different Cu/Solder soldering joints is established by means of thermal and kinetic models:Cu/?-Cu3Sn/?-Cu6Sn5/Solder,which can provide an effective theoretical guide for the preparation and soldering process of PVR.On this basis,the Sn60Pb40 and Sn55.5Pb39.5Bi5 systems PVR are prepared by hot-dip costing method,the comprehensive properties of which are characterized by the microstructures,resistivity.peeling strength,mechanical properties and aging resistance tests.The results show that the Sn55.5Pb39.5Bi5 system PVR with low manufacturing cost possesses excellent comprehensive performance compared with the Sn60Pb40 system PVR.Therefore,the Sn55.5Pb39.5Bi5 system PVR becomes a new generation of low-melting point PVR applied in the field of solar cell module packaging,which can completely replace the traditional the Sn60Pb40 system PVR.