Microstructure And Properties Of Sn-9Zn-xNd Lead-free Solder

In the modern manufacturing industry, solders used in electronic packaging are graduallytransforming from the traditional Sn-Pb alloys to lead-free solders. Due to the advantages of lowmelting point close to that of Sn-37Pb, low cost and excellent mechanical properties, Sn-Zn alloywas considered as one of the most potential lead-free solder. However, the existence of element Zncaused poor performances in wettability and oxidation resistance, thus, its application was hindered.In order to solve these problems, one way is to develop a kind of flux which can greatly help toimprove the wettability of Sn-Zn, the other way is to improve the properties of solder by alloying.In this thesis, the flux for Sn-9Zn lead-free solder was developed and found that the flux ishelpful for the wettability. Based on the designed orthogonal experiments, the optimum contents ofepoxy resin, glutaric acid, stannous methane sulfonate, triethanolamine and anhydrous alcohol inthe flux is10wt.%,5wt.%,20wt.%,10wt.%and55wt.%, respectively. Then, the effects of rareearth element Nd addition on the wettability, oxidation resistance, mechanical properties andreliability of the soldered joints were studied and the mechanism of action of Nd in the solder wasalso analyzed.The experimental results indicated that at the temperature of235℃, matching the threedifferent fluxes (the flux mentioned above, NH4Cl-ZnCl2water solution and non clean flux), thechange rule curve of the wettability of Sn-9Zn was similar by the addition of Nd. The wettabilitywas improved significantly when the content of Nd was0.06wt.%and the flux developed byourselves displayed better performance than the rest two. The addition of Nd could improve theoxidation resistance and the wettability of solder on the Cu substrate, however, excessive Nd insolders will be harmful for the oxidation resistance and wettability because of the enriched rareearth oxides on the surface of molten solder. The addition of rare earth Nd could refine the grainsof microstructure and suppress the increase of intermetallic layer thickness, so as to improvemechanical properties of soldered joints. When the content of Nd exceeded0.1wt.%, Nd-Sn phasewas prone to be formed near the interface, the fracture was gradually shifting to the interfacebetween solder and the intermetallic compounds and the mechanical properties of the solderedjoints decreased.During the process of aging treatment, the thickness of intermetallic layer formed at theinterface of Sn-9Zn/Cu and Sn-9Zn-0.06Nd/Cu increased due to the mutual diffusion of Zn and Cu, the mechanical properties of soldered joints decreased with the time of aging treatment extended,but the addition of Nd reduced the activity of Zn and slowed down the diffusion of Zn inSn-9Zn-0.06Nd. Compared with Sn-9Zn/Cu joints, Sn-9Zn-0.06Nd/Cu soldered joints formed athinner intermetallic layer and exhibited better mechanical properties. After being aged for360hand put in the room temperature for20h, many apparent white stick-like tin whiskers were found inthe microstructure of Sn-9Zn-0.5Nd and the interface of Sn-9Zn-0.5Nd/Cu. Comprehensivelyconsidering the properties of Sn-9Zn-xNd, the optimum content of Nd in Sn-9Zn solder is0.06wt.%.