Study Of The Microalloying Effect On The Microstructure And Properties Of Low Temperature Sn–Bi Solder Alloys And Their BGA Structure Joints

With the tendency of continuous miniaturization,portability and multi-functionality of electronic products,the packaging density increases gradually and the process becomes more complicated.The high soldering process temperature in electronic packaging due to use of high melting point solders is more likely to incur soldering process problems such as warpage.Notably,low temperature Sn–Bi solders have recently drawn a lot of attention in the field of electronic packaging mainly due to advantages of low melting temperature,high strength,as well as relatively low cost.However,the brittleness and poor ductility caused mainly by segregation of the Bi-rich phase,and the performance deterioration of solder joint in service?such as thermal cycling or aging?,are the major problems impeding the widespread application of Sn–Bi based solders in electronic packaging industry,in particular for the application which requires high reliability of solder joints.Therefore,it is highly significant and indeed valuable to develop low-temperature Sn–Bi based solders with stable performance and higher service reliability.In the present thesis study,firstly,the effects of adding trace amounts of alloying elements?e.g.,Ag,In and Sb?in Sn58Bi based solder on the microstructure,phase composition,melting characteristics,wetting properties and mechanical properties of the solder are investigated systematically,and the alloying mechanisms of three elements in the Sn–Bi solder are studied deeply.Then,the effects of element Sb on the microstructure and shear performance of BGA structure Sn58Bi–xSb/Cu joints before and after aging are studied in detail.The metallurgical reaction at the interface between the solder and Cu substrate,growth kinetics of interfacial IMC and fracture modes of solder joints are characterized.Finally,the combined effects of Ag,In and Sb on the microstructure and shear properties of Sn–Bi–X/Cu solder joints are investigated.The formation of IMC at the interface between the solder and Cu substrate,and the fracture mode of the solder joints are analyzed.The results show that adding trace amounts of Ag,In and Sb elements in Sn58Bi based solder has different degrees of refinement effect on the microstructure of the solder,and the elongation of Sn58Bi–M solders is obviously improved.The elongation of Sn58Bi–0.5Ag,Sn58Bi–0.5In and Sn58Bi–1.5Sb alloys is increased by 44.6%,54.0%and 37.2%,respectively,compared with that of the eutectic Sn58Bi alloy.Addition of Ag has little influence on melting temperature and the melting range of the Sn58Bi alloy,and addition of Sb can increase melting temperature of the alloy;in contrast,addition of In seems to have the opposite effect.The shear strength of Sn58Bi–xSb/Cu?d=600?m?solder joints increases largely with the increase of Sb content.For Sn58Bi–xSb joints with Sb content less than1.0%,the fracture occurs dominantly in the bulk solder matrix of Sn58Bi–xSb/Cu joints,while for Sn58Bi–xSb joints with Sb content over 1.0%,the fracture mode of solder joints changes from ductile fracture in the solder matrix to partially ductile and brittle fracture either in the solder matrix or in the IMC layer.With prolonging isothermal aging time,the microstructure of Sn58Bi–xSb/Cu?d=250?m?solder joints gradually becomes coarsened,and the shear strength of the joints gradually decreases,but the decrease rate of shear strength of Sn58Bi–xSb/Cu solder joints is lower than that of Sn58Bi/Cu solder joints.Adding trace amounts of Sb into the Sn58Bi has no obvious effect on growth of the interfacial IMC of reflowed Sn58Bi–xSb/Cu joints,but it can slow the growth of the interfacial IMC during aging,and meanwhile increase nucleation rate of Cu₆Sn₅ and reduce both the rate-constant D and the driving force of Cu-Sn IMC formation.The shear strength of Sn–Bi–X/Cu solder joints is slightly higher than that of Sn58Bi/Cu solder joints.