Massive Spalling of Intermetallic Compound in Lead-Free Solder Joints
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0968-V03-01
Massive Spalling of Intermetallic Compound in Lead-Free Solder Joints Su-Chun Yang1, Cheng-En Ho1, Chien-Wei Chang1, and C Robert Kao2 1
Department of Chemical and Materials Engineering, National Central University, No.300, Jhongda Rd., Jhongli City, 32001, Taiwan 2
Department of Materials Science and Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei, 10617, Taiwan ABSTRACT Spalling of intermetallic compounds in a massive scale has been reported in the literature for several solder/substrate systems, including SnAgCu soldered on Ni substrate, SnZn on Cu, high-Pb PbSn on Cu, and high-Pb PbSn on Ni. In this work, a common mechanism based on thermodynamic arguments is proposed to explain this rather peculiar phenomenon that occurs across several systems. According to this mechanism, two necessary conditions must be met. This first is that the most reactive element must be present in a limited amount, and the second is that the soldering reaction must be very sensitive to the concentration of this element. With the growth of intermetallic, more and more of the most reactive elements are extracted out of the solder and incorporated into intermetallic. As the concentration of this element decreases, the local equilibrium phase at the interface changes. This changing of the equilibrium phases caused the non-equilibrium phase to spall. INTRODUCTION In electronic packages, solder joints play an important role serving both as electrical interconnections between components and as mechanical support for components. Unfortunately solder joints have been recognized as the weak links and the reliability of each individual joint can control the overall lifespan of an electronic product. In order to ensure the quality of electronic product, in the first place we should make sure the good properties of solder joints. To meet the trend toward miniaturization in consumer electronics, the size of electronic packaging and thus the solder joints are being continuously scaled down. More integrated, fine-pitch solder joints are required and therefore the reliability of solder joints becomes more critical. One important factor influencing the solder joint reliability is the interfacial reaction between the solder and the substrate during soldering and the subsequent aging. Thus, knowledge of the solder/conductor metal interactions and phase evolution in the solder interconnections is necessary for the
understanding of the reliability of the solder interconnections. One reliability issue is the spalling behavior of intermtallic compound at the interface. A classical example is the spalling of Cu6Sn5 during the reaction of PbSn solder with Au/Cu/Cu-Cr thin film [1,2]. Spalling here was due to the exhaustion of the Cu film, and the poor wetting between Cu6Sn5 and the remaining Cr caused the compound to detach itself from the interface. However, a new spalling mechanism was reported which is not caused by the exhaustion of a reaction layer but the consumption of one element inside the reacting phase
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