| As fast development of electronic packaging technology and the inevitable trend of lead-freesoldering, the research focusing on lead-free solder alloys becomes more and more important in theelectronic industry. SnAgCu solder is considered as the best one to substitute for Pb-containingsolders because of their acceptable comprehensive property. However, they still have some problemsfor widely industrial application compare to conventional SnPb alloy, such as microstructural stability,insufficient wettability and large solidification undercooling. Moreover, the growth of intermetalliccompounds in the SnAgCu solder joints is more complicated than that of SnPb due to the higher Sncontent and soldering temperature of SnAgCu alloys. Microalloying is an effective way to improvethe properties of the solder alloys. To improve these drawbacks, rare-earth elements RE (RE=Pr, Nd)were selected to improve the properties and microstructures of Sn3.8Ag0.7Cu solder alloy. Thisdissertation systematically studied the effects of different amounts (0~0.5wt.%) of Pr and Ndadditions on the microstructures and properties of Sn3.8Ag0.7Cu alloy. Moreover, the mechanism ofthe effects of Pr and Nd on the microstructures and properties of SnAgCu solder was also discussed.The wetting balance method was used to evaluate the wettability of Sn3.8Ag0.7Cu-xRE solders,and the results indicated that trace amount of Pr and Nd addition could obviously improve thewettability of the SnAgCu solder, which the optimal wettability was achieved as the RE content isabout0.05wt.%due to the lower surface tension caused by RE elements. When measured at250℃,the wetting force of of SnAgCu solder was increased by6.7%and5.5%with0.05%Pr or0.05%Ndand the wetting time of SnAgCu solder was descreased by14.5%and10.0%, respectively under thesame condition. However, an excessive amount of RE addition into the solder would deteriorate thewettability due to the oxidization of the RE elements.The effects of RE additions on the melting and solidification behavior of SnAgCu solder wereinvestigated and the melting point of SnAgCu alloy changed little with the incorporation of REelements. However, the RE additions reduced the amount of solidification undercooling of SnAgCusolder thus reduced the probability of primary Ag3Sn phase formation. The undercooling of SnAgCu,SnAgCu-0.05Pr and SnAgCu-0.5Pr solder was20.6,5.0and5.5℃, respectively. During solidification,Pr and Nd atoms preferentially reacted with Sn atoms to form RESn3compound, the primary RESn3compounds played a role as the nucleation sites thus promoted the solidification of the solder alloy.The microstructure of SnAgCu-0.05RE alloy was finer and more uniform compared with that of other solder alloys. However, RESn3compounds were found in the solder as RE was added up to0.1wt.%.Nanoindentation was adapted to characterize the creep stress exponent n of SnAgCu-xRE solderalloys. Both RE bearing solders exhibited larger n than that of SnAgCu solder, with a creep stressexponent8.79for SnAgCu,10.19for SnAgCu-0.05Pr,9.26for SnAgCu-0.5Pr,10.97forSnAgCu-0.05Nd and9.84for SnAgCu-0.5Nd. It can be concluded that RE-bearing solder alloysperform better creep resistance than SnAgCu solder, which can be atttributed to the strengthen effectof Ag3Sn particles.The results showed that the mechanical property of SnAgCu solder joint were improved with Prand Nd addition, the maxium shear force was obtained with about0.05%Pr or Nd addition andexhibited a19.4%and23.6%increase compare with that of SnAgCu solder joint, respectively. Duringthermal aging process, the shear strength of all solder joints decreased with the increase of aging time,the SnAgCu-0.05RE solder joint exhibited the highest shear force all through the aging process. Afteraging for1440h, the shear force of SnAgCu-0.05Pr and SnAgCu-0.05Nd were37.3%and46.0%higher than that of SnAgCu solder joint. Trace amounts of Pr and Nd additions suppressed the growthof Cu6Sn5layer, but had little influence on Cu3Sn layer. Part of the Cu6Sn5/solder interface wasreplaced by Cu6Sn5/RESn3interface, the existence of RESn3compounds at the interface would inhibitthe reaction of6Cu+5Snâ†'Cu6Sn5that take place at Cu6Sn5/solder interface thus restrain the growthof Cu6Sn5layer. With an excessive amount of RE addition, the RESn3phase exposed in room ambienttransformed into brittle oxide. Consequently, the beneficial effect due to the suppression of interfaciallayer compounds was weakened.Three-dimensional morphology of the intermetallic compounds (IMCs) in the solder joints wasstudied by deep etching method. The results showed that the Pr and Nd addition could reduce theprobability of primary Ag3Sn phase formation in the SnAgCu solder joint, the existence of primaryAg3Sn plates with large size had an adverse influence on the mechanical property of the joint. Duringaging process, the Cu6Sn5grains at the interface showed both horizontal coarsening and longitudinalgrowth. After aging, the feature distinctions of primary Ag3Sn compound were inclined to be vague,Ag3Sn nano-particles adsorbed on the primary Ag3Sn compound would retard the morphologyevalution of primary Ag3Sn compound. It could be considered as that the presence of Ag3Sn particlesat the surface of Cu6Sn5layer could retard the migration of Cu6Sn5layer. The addition of Pr and Ndwould increase the volume fraction of Ag3Sn particles in the solder joint thus inhibit the growth of theinterfacial layer.When an excessive amount of RE was added in SnAgCu solder alloy, the risk of forming Sn whisker was greatly increased due to the induced compressive stress and released Sn atoms caused byoxidation of RESn3compound. It was found that there was no Sn whisker with visible size growthoccurred on the surface of SnAgCu-0.1RE samples after room temperature storage for six months.However, SnAgCu-0.5RE and Sn-RE alloys showed obvious Sn whisker growth after roomtemperature storage for24h. It can be concluded that the growth of Sn whisker was mainly resulted inthe RE amount, the micro-cracks shape and the local stress filed in Sn whisker root area. |
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