Inhibition Of Ga And Nd On The Sn Whisker Growth Of Sn–Zn Solder

The Sn–Zn eutectic lead-free solder has advantages of relatively low melting temperature, higher mechanical properties and relatively low cost. However, the poor wettability limites its applications in practice use. It is reported that small amount of additions like Ga、Ce、Pr could improve the wettability of Sn–Zn lead-free, but the excceding amout of RE element would induce the growth of Sn whisker. However, the addition of Nd represents special performance. In this paper, the evoluation regulation of Sn–Zn lead-free solder bearing Nd and Ga were systemically investigated.The wetting balance method was used to evaluate the wettability of Sn–9Zn–0.5Ga–xNd solders.The results indicated that trace amount of Nd addition could obviously improve the wettability of the solder, the optimal wettability was achieved when the Nd addition was 0.08 wt.%. When measured at 240℃, the wetting time of the solder was descreased by 21% and the wetting force was increased by 43%. The wettability of solder soldering with Sn–Zn private flux performed better than water-soluble flux.The wettability of Sn–9Zn–0.5Ga–0.08 Nd solder can be improved with the increasing of reflow temperatures, the wetting time reduced and wetting force increased obviously. The wetting time of solder was lower than 1s under three different coatings at 240℃, which archived the industrial standards in automatic soldering technology. The wettability of solder under Sn and Sn–Bi coatings performed better than Au/Ni coatings. The wettability of solder under Sn coatings performed better with Sn–Zn private flux, while the wettability of solder under Sn–Bi coatings performed better with water-soluble flux.The addition of rare earth Nd could refine the microstructure of the solder due to the adsorption effect of Nd. The microstructure of Sn–9Zn–0.5Ga–0.08 Nd solder was finer and more uniform compared with that of other solder alloys. The mechanical properties of Sn–9Zn–0.5Ga solder were evidently improved, and when the content of Nd is at 0.08 wt.%, the comprehensive properties of solder joint were obtained. The shear force of Sn–9Zn–0.5Ga–0.08 Nd solder joint after aging treatment for 1200 h was twice the amount of Sn–9Zn joint and approached the origin as-soldered joint of Sn–9Zn.The addition of rare earth Nd can decrease the thickness of the IMC layer of Sn–Zn/Cu solder joints. Large amounts of dimples can be observed in solder joints, which indicates that the fracture types of solder joint represent toughness fracture. The fracture type of solder joint still performd ductile since multiple dimples observed even after aging treatment for 1200 h.It was found that spontaneous growth of Sn whiskers occurred on the surface of solder with excess Nd addition, and the longest Sn whisker was outside the range safety after aging treatment. No Sn whisker was observed in solder with optimal Nd addition. Sn whiskers in solders under room temprature results in morphologies including needle-like and fillforrm-like while the Sn whiskers under high temprature results in morphologies including plate-like and hillock-like and other complex forms.The mechanism of whisker growth in NdSn3 phase was discussed. With the oxidation of NdSn3 phase, fresh tin atoms were reacted and compressive stress was accmulated. The compressive stress continuously driven the fresh tin atoms out from the fracture in NdSn3 phase during the oxidation process until the NdSn3 phase was totally oxidized.The Sn whisker growth from IMC in solder joints was due to the diffusion-induced stress in the IMC layer between the Cu pad and the solder. With the release of stress, voids and cracks formed in IMC layer and finally the Sn whiskers grew from these fractures after aging treatment, which further decreased the mechnical property of solder joints.It was found that the formation of GaNd and GaNd3 compounds replacing SnNd compound is the key factor. Because the Gibbs formation free energies of GaNd and GaNd3 are more negative than that of NdSn3, the GaNd and GaNd3 compounds are relatively more stable, no free Sn atoms released during exposure and oxidizing reaction to feed the Sn whisker growth.The combined action of Nd and Ga can further decrease the growth of the IMC layer during aging process. The IMC layer still perfomed continuous and regular shape after aging treatment. The voids and cracks in IMC layer were decreased which inhibited the growth of Sn whikser.After aging treatment at 150℃, it was found in Sn–9Zn–0.5Ga–2Nd and Sn–9Zn–1Ga–2Nd solder joints, when the atom ratio of Nd and Ga addition was approaching 1:2, almost all the additon of Nd reacted with Ga, otherwise NdSn3 phase was formed in the solder joint. The result is available for guiding the study of Sn whisker growth inhibition.