Investigation Of Solder Modification Of Rapidly Solidified And Microalloyed Sn-Zn System Alloys

The development of lead-free solders has been an essential and urgent task in theelectronics packaging industry because of the restriction of lead use by legislations.Sn-Zn alloys, which have some advantages, such as relatively low melting point,cost-saving and superior mechanical property, is considered as one of candidates thatcould replace Pb-containing solders in microelectronic packaging and interconnects.However, because Zn element is quite active, Sn-Zn system solders have worse wettingand corrosion behavior. The high temperature oxidation resistance and embrittlementbehavior of the Sn-Zn alloys is also of low quality.The subject aims at the questions existing in Sn-Zn alloys and studies the effect ofrapid solidification process and/or micro-alloying on microstructure and characteristicsof solder alloys. The interface behavior of solder/Cu and the mechanical properties ofsoldering joints are also investigated. The as-solidified eutectic Sn-Zn solder isemployed as the reference system.The studies showed that a CuZn intermetallic compound （IMC） layer was formedpreferentially within the Cu substrate at the Sn-Zn/Cu interface and the Cu₅Zn₈IMCformed by the reaction of CuZn with Zn atoms diffused from the solder. The diffusionand aggregation of the Cu atoms which crossed the IMC layer into the solder presented“pulsation" in the form, and the granular Cu₅Zn₈IMCs were formed in the solder. TheIMC at the interface was present an appearance of three types: coarse IMC layer at theinterface, compact cellular Cu₅Zn₈layer, and tabular CuZn IMC layer.Microstructure analysis of solder alloys showed that a netlike dendrite structure wasformed due to the rapid branch of β-Sn, the growth of Zn phases was suppressed anddistributed in β-Sn matrix in granular form with a size of0.5²μm. By contrast to theas-solidified Sn-9Zn alloy, the addition of0.1wt.%Cr in Sn-9Zn alloy promoted thedissolution of Zn in Sn during the rapid solidification process. The microstructure of thesoldering seam was notably fined and the Zn phases were not distinctly greater than1μm. The Zn phases in rapidly solidified Sn-8Zn-3Bi presented granular or strip andblock pattern, with a size of2⁴μm. Bi was completely dissolved in Sn and formedsupersaturated solid solution.The analysis results of melting characteristic of various solder alloys indicated thatthe melting temperatures of Sn-9Zn and Sn-6.5Zn were almost not changed after rapid solidification. By contrast to the as-solidified alloys, after rapid solidification, themelting point of the Sn-9Zn-0.1Cr alloy was lower by about9°C and the Sn-8Zn-3Bisolder was rise by about7°C. The pasty range of rapidly solidified alloys obviouslydecreased. The metastable phase in the rapidly solidified alloys could release the crystallatent heat during the soldering process and then promote the fusion of the solder.The analysis of IMC at the interface of Sn-Zn/Cu showed that the fine Zn phasesand precipitates in rapidly solidified solder promoted the homogeneity of the interfacialreaction. The Cu₅Zn₈intermetallics at the interface by using the rapidly solidifiedSn-9Zn alloy were finer and more uniform than that of the as-solidified Sn-9Zn alloy,and the tensile-shear strength of the joint was obviously improved.As aging time prolonged at150°C, the thickness of the IMC layer changed from anincrease to a reduction and the continuity and compactability of this layer was destroyeddue to the decomposition of the Cu-Zn IMC layer. A discontinuous layer of Cu6Sn5IMC was present within the Cu substrate near the decomposed region. By comparisonwith as-solidified solders, the compact and uniform IMC layer at the interface by use ofthe rapidly solidified solder was more stable during aging at150°C.The growth kinetics analysis of the interfacial IMC showed that the growth of IMCat the interface of solder/Cu during the soldering process could be suppressed due to asuitable addition of the elements Bi and Cr. In the tests, the growth rate constant k ofthe IMC at the interface of solder/Cu during soldering at240°C by using the Sn-9Znalloy and the Sn-9Zn-0.1Cr or Sn-8Zn-3Bi alloys was3.5,2.8³.1, respectively. Theeffects of various initial process conditions on the growth rate were puny. The effects ofsolder state on the growth kinetics of the interfacial IMC during solid aging wereslightly.The addition of0.1wt.%Cr in Sn-9Zn, the wettability and the bonding strength ofsolder/Cu were improved obviously. The IMC layer became relatively uniform and thetensile-shear strengths of the joints using the rapidly solidified solder were markedlyhigher than those of the joints using the as-solidified solders. The former exhibited goodsolderability and excellent joint strength. Due to the addition of0.1wt.%Cr in Sn-9Zn,the growth rate of the IMC was decreased during the solid aging process and theexcessive growth of the IMC layer at the interface can be suppressed.The addition of minor rare earth （RE） elements in Sn-9Zn alloy has distinctmetamorphism, but the RE compounds could be formed due to the overmuch addition.The improved properties of the joints of the Cu/Sn-9Zn-0.1Nd/Cu might result from the formation of the uniform, fine-grained microstructure at the interface.Studies in this project show that a suitable application of the rapid solidificationtechnology and alloying method independently or simultaneously can significantlyimprove the combination property of the Sn-Zn system lead-free solders. Then, thesoldering process can complete in a shorter period of time or/and a lower temperaturecompared with the as-solidified solders and the joints are of high quality. The adverseeffect of the IMC at the interface on the microstructure and mechanical properties issuppressed. The research extends the fields of the study and application of Sn-Znlead-free solder alloys with high-performance.