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Please use this identifier to cite or link to this item: http://hdl.handle.net/2031/4447

Title: Study of interfacial reactions in ball grid array (BGA) solder joints for advanced integrated circuit (IC) packaging
Other Titles: Xian jin jing pian (IC) feng zhuang qiu shan zhen lie (BGA) han dian zhong de jie mian fan ying yan jiu
先進晶片 (IC) 封裝球栅陣列 (BGA) 焊點中的介面反應研究
Authors: Alam, Mohammad Ohidul
Department: Dept. of Electronic Engineering
Degree: Doctor of Philosophy
Issue Date: 2004
Publisher: City University of Hong Kong
Subjects: Ball grid array technology
Microelectronic packaging
Solder and soldering
Notes: CityU Call Number: TK7870.16.A43 2004
Includes bibliographical references (leaves 188-205)
Thesis (Ph.D.)--City University of Hong Kong, 2004
xxi, 209 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: One of the important directions of scientific research in the last decade has been device miniaturization. Perhaps the most important driving force for miniaturization has been the demands from the electronics market. Integrated Circuit (IC) packaging has been recognized as an important “enabler” for the revolution in the electronics industries. Among the most advanced IC packages, ball-grid-array (BGA) technology is expected to have increasing application because of its higher input–output connection density achieved through area-array solder joints. However, if any one of these solder joints fails, the whole package malfunctions. Thus, it is crucial to understand the reliability of a BGA solder joint, which is in turn related to interfacial reaction phenomena during soldering as well as during service life. This dissertation focuses on an understanding of interfacial reaction phenomena of BGA solder joints which are very critical from an industrial point-of-view. BGA solder balls of Sn-37wt%Pb, Sn-3.5wt%Ag and Sn-3.5wt%Ag-0.5wt%Cu have been selected for reflow soldering and solid state aging with typical BGA solder bond pads (also termed ‘ball pad’) of Cu, Au/electroless Ni/Cu, and Au/electrolytic Ni/Cu. After having been reflowed at 240℃ for 0.5 minutes, prolonged isothermal reflow up to 180 minutes and solid state aging up to 1000 h have been carried out to produce the ultimate interfacial reactions. Cross-sectional studies of interfaces have been conducted by scanning electron microscopy (SEM) equipped with an energy dispersive X-ray (EDX) analysis to investigate the interfacial reaction phenomena. X-ray Diffraction (XRD) analysis has been carried out to obtain the crystal structures of the newly-formed crystalline phases at the Au/electroless NP/ Cu interface. Ball shear tests have been carried out to obtain the interfacial strength and to correlate with the interfacial reaction products. After the shear tests, fracture surfaces have also been investigated to understand the fracture modes. Because of the finite/limited solder volume of a BGA solder ball and the finite/limited thickness of the Cu, Ni layer and Au layer on the BGA bond pad used in this study, complex reactions at the solder interface have been noticed. Depending upon the combination of the solder alloy and the metallization of the solder ball pad, these may lead to a better solder joint or an unreliable solder joint. It has been found that Auembrittlement in the BGA Sn-Pb solder joints on Au/Ni/Cu BGA bond pads can be reduced by using a thin Ni layer. During aging at 150℃, the thin Ni layer facilitates Cu diffusion from the bond pad and the Cu changes the brittle continuous layer of (Au,Ni)Sn4 to a nodular-shaped AuSn4. New interfacial reaction products with the modified morphology improve the interfacial joint strength significantly. The interfacial reaction rate between Sn-Ag and Au/electroless Ni-P/Cu has been found to be higher than that for the Sn-Pb solder. A P-rich Ni-layer is formed on the electroless Ni-P deposit due to the solder reaction. For short reflow times, this P-rich Ni layer consists only of Ni3P compound, however during prolonged reflow, because of the cumulative P accumulation, new crystals of Ni2P, Ni5P4 and NiP2 are formed from the amorphous electroless Ni-P layer. It has been found that the formation of the crystalline P-rich Ni layer at the solder interface of Au/electroless Ni-P/Cu bond pads deteriorates the mechanical strength of the joints drastically. It has also been noticed that this weak P-rich Ni layer appears quickly at the interface of high-P content electroless Ni-P deposits as well as at the Sn-Ag solder interface. However, this P-rich Ni layer disappears due to prolonged reflow reaction, the shear strength again increases. 0.5 wt% Cu addition in the Sn-Ag solder has been found to retard the growth of this brittle P-rich Ni layer. Sn-3.5Ag-0.5Cu solder is likely to be the best choice for the Au/electroless Ni-P/Cu bond pad. 0.5 wt% Cu addition in the Sn-Ag solder has also been found to be beneficial for reducing Cu dissolution from the Cu bond pad. Therefore, Sn-3.5Ag-0.5Cu solder is again a better choice for Pb-free soldering on BGA solder ball pads of bare Cu. On the other hand, 0.5 wt% Cu addition enhances the reaction rate between the Pb-free solder and the Ni metallization both in the liquid and the solid state. While Ni3Sn4 is formed at the Sn-3.5Agsolder interface, (Cu,Ni)6Sn5 has been detected at the Sn-3.5Ag-0.5Cu solder interface. Because of the typical needle-shaped morphology, (Cu,Ni)6Sn5 does not grow by a diffusion controlled process. A compact (Cu,Ni)6Sn5 has also been detected at aging temperatures below 175℃, which grows parabolically with time, however, it is not yet clear what the mechanistic differences are between these two morphologies. The effect of a high electric current density on the interfacial reactions of μBGA solder joints has been studied at room temperature and at 150℃. Four types of phenomena have been reported. Along with electromigration-induced interfacial intermetallic compound formation, dissolution at the Cu bond pad has also been noticed. With a detailed investigation, it has been found that the narrow and thin metallization at the component side produces “Joule heating” due to its higher resistance, which in turn is responsible for the rapid dissolution of the Cu bond pad near to the Cu trace. During an “electromigration test” of a solder joint, the heat generation due to Joule heating and the heat dissipation from the package should be considered carefully. When the heat dissipation fails to compete with the Joule heating, the solder joint melts and molten solder accelerates the interfacial reactions in the solder joint. The presence of a liquid phase has been demonstrated from microstructural evidence of solder joints after different current stressing (ranging from 0.3A to 2A) as well as an in-situ observation. Electromigrationinduced liquid state diffusion of Cu has been found to be responsible for the higher growth rate of the intermetallic compound on the anode side.
Online Catalog Link: http://lib.cityu.edu.hk/record=b1928982
Appears in Collections:EE - Doctor of Philosophy

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