Study On Boron Containing Sn-Ag-Cu Low-silver Lead-free Solders For Electronic Packaging

With the vigorous development of the electronic industry,there is a high demand for electronic packaging micro-interconnection solders.Up to now,the Sn-Ag-Cu(SAC)lead-free solder with good comprehensive properties has been widely applied in electronic industry.Therefore,the SAC solder has become a competent substitute for traditional,environmentally unfriendly Sn-Pb solders.With its near eutectic Sn-3.0Ag-0.5Cu(SAC305)lead-free solder has the advantages of low melting point and good wettability.However,the higher silver content(3.0wt%)of SAC305 solders brings out high cost and makes the increasing resource burden of precious silver metal.Furthermore,coarse intermetallic compounds(IMC)in SAC305 have also become potential failure causes in practical applications.Thus,the current researches focus on low silver solder alloys.However,low silver lead-free solders have several problems,such as high melting point temperature,poor wettability,and poor mechanical properties.During the application process,the excessive growth rate of brittle IMC layer at the interface will lead to poor soldering performance and interfacial crack initiation,causing the decline of reliability.In view of the facts mentioned above,the element B(boron)was used to prepare trace B-containing Sn-1.0Ag-0.5Cu(SAC 105)low-silver lead-free solders,and thus improve the performance of SAC 105 solders.Since B is an immiscible element with a high melting point,SAC 105 high B content(10wt.%)master alloys were prepared by mechanically alloying initially,and its alloying progress was analyzed.Subsequently,the SAC 105+10B master alloy after 72 hours ball-milling,which is considered as the high alloying degree,was applied as a raw material to prepare the novel solder alloys:SAC 105-0.01B and SAC 105-0.02B.The microstructure of the solder alloy matrix and SAC105/Cu solder joint interface was comprehensively observed and analyzed,and the influence of B addition was determined concurrently.Also,the various properties of solders were systematically investigated and tested.Most notably,the mechanism of B was emphatically expounded through thermodynamics analyses and first-principles calculations.The main results of this work are as follows:The addition of B can refine the microstructure of solders and disperse in the solder matrix,improving the tensile properties and the elongation of solder alloys.The higher strength,stiffness,and plasticity have been obtained by introducing B into solders.At the same time,it can also improve the mechanical properties of solder joints significantly.Moreover,with B addition,the solidus and liquidus lines of solders have been decreased,but its melting range has been increased.The addition of trace B has little effect on the wettability of the solder alloy,but it can improve the oxidation resistance of solder alloys,which is mainly because of B segregating on the Sn/O surface(tin-based solder surface facing with oxygen).As a result,a thin and dense oxide layer can be formed by the preferential oxidation of B,slowing down the further oxidation of solders.First-principles analysis also demonstrates that the energy of B at Sn/O surface is lower than that of B in bulk,which provides a basis for its surface enrichment.By analyzing the distribution status of B,an enrichment phenomenon can be found at IMC/(Cu substrate)interface and inside interfacial IMCs.As a small atomic radius element,B is easier to segregate at the solder joint interfaces through the intergranular channel between IMC grains.The size of B-rich phase particles is close to spherical,reaching nanometer level(about 10 nm).For the segregation at IMC/(Cu substrate),B can inhibit the diffusion between C.u and Sn and modify the morphology of solder joints.With the prolongation of aging time,the growth of brittle interfacial IMC can be slowed down obviously,and the interfacial morphology tends to be flat,which ensures the reliability of solder joints.For the segregation inside IMC,B can be considered as an effective pinning center and make solder joint interfaces firmer.Furthermore,the B-containing solder alloys can maintain a good grain refinement effect even after aging,which is conducive to sustain the stability of solder joints in the application.Thermodynamics revealed the mechanism of interface segregation of B at SAC105/Cu solder joints.For SAC105-0.01/0.02B solder joints,by calculating Gibbs free energy of B and Cu systems,it is found that at reflow temperature(260℃),aging temperature(150℃)and room temperature(25 ℃),the energy value is negative(AG<0).By calculating the thermodynamic model of Sn-Cu-B ternary system,B meets AH<0 in Cu and Sn system and the critical content of B in the formation of the ternary alloy was determined to be 0.107 mol.It provides a theoretical basis for interface segregation and B enrichment in solder joints.To summarize,the SAC 105-0.02B solder alloy has smaller grain size,uniform microstructure,excellent performance,and high reliability compared with SAC 105 and SAC 105-0.01B solder alloys,which is the preferred scheme.In conclusion,the addition of trace B in SAC 105 solders can not only improve the performance of the solders but also suppress the rapid growth of IMC layer at the interface,further improving the reliability of solders.The B-containing Sn-Ag-Cu solders have good engineering application prospects.