This paper highlights the interfacial structure of tin-silver (Sn-3·5Ag) solder on nickel-coated copper pads during aging performance studies at a temperature of 150°C for up to 96 h. Experimental results revealed the as-solidified solder bump made from using the lead-free solder (Sn-3·5Ag) exhibited or showed a thin layer of the tin-nickel-copper intermetallic compound (IMC) at the solder/substrate interface. This includes a sub-layer having a planar structure immediately adjacent to the Ni-coating and a blocky structure on the inside of the solder. Aging performance studies revealed the thickness of both the IMC layer and the sub-layer, having a planar structure, to increase with an increase in aging time. The observed increase was essentially non-linear. Fine microscopic cracks were observed to occur at the interfaces of the planar sub-layer and the block sub-layer

Kovacevic, R.

Lin, D. C.

Srivatsan, T. S.

Wang, Guo-Xiang

English

The University of Akron

The University of AkronIdeaExchange@UAkronMechanical Engineering Faculty Research Mechanical Engineering DepartmentSummer 6-30-2008A Study Aimed at Characterizing the InterfacialStructure in a Tin-Silver Solder on Nickel-CoatedCopper Plate during AgingD. C. LinR. KovacevicT. S. SrivatsanUniversity of Akron, Main CampusGuo-Xiang WangUniversity of Akron Main CampusPlease take a moment to share how this work helps you through this survey. Your feedback will beimportant as we plan further development of our repository.Follow this and additional works at: http://ideaexchange.uakron.edu/mechanical_ideasPart of the Mechanical Engineering CommonsThis Article is brought to you for free and open access by Mechanical Engineering Department atIdeaExchange@UAkron, the institutional repository of The University of Akron in Akron, Ohio, USA. It has beenaccepted for inclusion in Mechanical Engineering Faculty Research by an authorized administrator ofIdeaExchange@UAkron. For more information, please contact mjon@uakron.edu, uapress@uakron.edu.Recommended CitationLin, D. C.; Kovacevic, R.; Srivatsan, T. S.; and Wang, Guo-Xiang, "A Study Aimed at Characterizing the InterfacialStructure in a Tin-Silver Solder on Nickel-Coated Copper Plate during Aging" (2008). Mechanical EngineeringFaculty Research. 341.http://ideaexchange.uakron.edu/mechanical_ideas/341Sa¯dhana¯ Vol. 33, Part 3, June 2008, pp. 251–259. © Printed in IndiaA study aimed at characterizing the interfacial structure in atin–silver solder on nickel-coated copper plate during agingD C LIN1, R KOVACEVIC1, T S SRIVATSAN2∗ and G X WANG21Research Center for Advanced Manufacturing, Department of MechanicalEngineering, Southern Methodist University, 1500, International Parkway,Suite No. 100, Richardson, Texas 750812Department of Mechanical Engineering, The University of Akron, 302 E,Buchtel Avenue, Akron, Ohio 44325e-mail: tsrivatsan@uakron.eduAbstract. This paper highlights the interfacial structure of tin-silver (Sn-3·5Ag)solder on nickel-coated copper pads during aging performance studies at a tem-perature of 150◦C for up to 96 h. Experimental results revealed the as-solidifiedsolder bump made from using the lead-free solder (Sn-3·5Ag) exhibited or showeda thin layer of the tin–nickel–copper intermetallic compound (IMC) at the sol-der/substrate interface. This includes a sub-layer having a planar structure imme-diately adjacent to the Ni-coating and a blocky structure on the inside of the solder.Aging performance studies revealed the thickness of both the IMC layer and thesub-layer, having a planar structure, to increase with an increase in aging time.The observed increase was essentially non-linear. Fine microscopic cracks wereobserved to occur at the interfaces of the planar sub-layer and the block sub-layer.Keywords. Interconnections; printed circuit board; electronic packaging;solder bumps; thermal aging performance.1. IntroductionThe electronic industry is consistently seeking for advanced, genuinely innovative, intercon-nection technologies for components and fabrication of printed circuit (PC) boards for large-scale manufacturing. The solder bump flip-chip technology (Andy 2005) has been widely usedfor components to interconnect, both electrically and mechanically, to the substrate throughthe solder bumps (Lau 1995; Lau & Erasmus 1993). These interconnections are easily formedin an area on the surface of the printed circuit (PC) board. In this area, or region, an underbump metallurgy, referred to henceforth as UBM, facilitates in the formation of a bond thatis receptive to soldering. The nickel coating in the under bump region (UBM) acts as a bar-rier for the occurrence of diffusion between copper and the solder. In essence, (a) the overallquality of the solder bump, (b) the bond of the solder, and (c) the intrinsic properties of the∗For correspondence251252 D C Lin et alsolder/substrate interfaces, are both important and essential for long-term product reliabil-ity analysis. In the fabrication of electronic packaging, the manufacturing of printed circuitboards (PCBs) with connection bond pads is critical from the standpoint of reliability. A typ-ical under bump region (UBM) essentially consists of three layers: (i) a copper base layer,(ii) a nickel layer, and (iii) a thin layer of gold (Andy 2005). The thin layer of gold on top ofthe pad helps in facilitating protection against both corrosion and oxidation of the connectionpads. A thin layer of nickel on the copper base helps in slowing down the diffusion of copperinto the solder with the purpose of limiting the copper-rich, tin-copper, and the scallop-likeintermetallic compounds at the interface of the solder with the substrate.Gold is well known to have a high dissolution in molten tin-dominated solder. For example,approximately 18 wt% of gold at the soldering temperature (225–250◦C) can be dissolvedinto tin-dominated solder (Ochoa et al 2003). This is one of the key advantages in using goldduring a reflow process. The nickel layer tends to interact directly with the molten solder oncethe gold has dissolved. This results in the formation of a sound metallurgical bond betweenthe solder and the pad. Formation and presence of the intermetallic compounds at the interfaceof the solder and the substrate is considered necessary for a sound solder joint (Tu et al 2003).The types of intermetallic compounds formed and their resultant growth are critical to theoverall reliability of electronic products.This paper attempts to focus on the interfacial structure of a Sn3·5Ag solder on nickel-coated copper pads, which find extensive use in packaging assembly. A homogeneous thermalaging process was performed for studying and rationalizing the growth behaviour of the Sn–Cu–Ni intermetallic compound at and near the interface.2. Experimental procedures2.1 Fabrication of solder bumpsThe materials used in this study include: (i) A commercial no-clean Sn3·5Ag solder paste.(ii) A PC board having copper bond pads covered with an electroplated Ni/Au coating.A thermocouple was positioned in the solder paste so that any temperature variations withrespect to time, during the reflow process, can be detected and monitored. The board hasaround forty numbers of the Cu/Ni/Au pads arranged in four rows, with each row having atleast ten pads. The procedure used to produce the solder bumps for further analysis is:(a) Using acetone solution the connection pads and the stencil were thoroughly cleaned.(b) Print solder paste on the connection pads using a stencil. The depositions of a near uniformsolder volume, without appreciable misalignment, bridging, or missing bumps, is thedesired objective. The overall uniformity of the solder deposit is critical to the overallquality of the assembly. This uniformity is related to both the type of solder paste used andthe print process parameters. The volume of solder deposit depends upon the following:(i) size of the stencil, (ii) thickness of the stencil, and (iii) size of the connection pads.(c) The stencil is removed with care.(d) Reflow of the solder paste was initiated on a temperature-controlled hot plate in a con-trolled environment with the peak soldering temperature reaching as high as 250◦C.(e) When the peak temperature was reached, the coupon was immediately removed fromthe hot plate and placed in ambient air (298 K) with the purpose of facilitating rapidsolidification of the molten solder. The cooling rate was estimated to be about 15◦C/sec.The solidified coupon was subsequently taken to a clean facility to enable in the removalof the flux and residue from the solder bumps.Interfacial structure in a tin–silver solder 253Figure 1. An overhead view of the solder bumps on a printed-circuit (PC) board.(f) The as-solidified solder bump is shown in figure 1.2.2 Homogeneous thermal aging performanceThe as-solidified solder bumps were then placed in a furnace whose temperature was main-tained at 150◦C. The samples were left in the furnace for four different time intervals: 24,48, 72 and 96 hours. During the aging process, the solder bumps were subjected to thefollowing:(i) A heating process (from room temperature (25◦C) to a homogeneous 150◦C).(ii) An aging process (at 150◦C at the four different time intervals).(iii) A cooling process (from 150◦C to room temperature).2.3 Sample preparation for microstructure analysisCross sections of the as-solidified and aged solder bumps were prepared for micrographicexamination using the standard metallographic technique. The ground and polished sampleswere subsequently etched in a solution mixture of 25% nitric acid, 2% hydrochloric acidand 93% methanol. The resultant microstructure of the interfacial regions was examined ina JEOL scanning electron microscope (SEM) in the backscattered mode. The distribution ofkey elements at the interfacial region was analysed with the aid of energy-dispersive X-rayspectroscope (EDS).To prepare the coupons for observation in the scanning electron microscopes, the followingprocedure was used to protect the solder bumps from detachment:(a) Graphite was initially applied as a dry film spray lubricant over the solder bumps.(b) The entire solder bump was wrapped in an aluminum foil with the purpose of securingeach of the solder bumps connected to the neighbour bump.254 D C Lin et al(c) The solder bumps were cold mounted using a mixture of resin and hardener at the center ofthe solder bump. This was followed by standard metallurgical preparation of the samplesfor observation in the scanning electron microscope.3. Results and discussion3.1 Microstructure of as-solidified Sn3·5Ag solder bumpA typical cross section of an as-solidified solder bump is shown in figure 2. The magnifieddetails at the interface between the solder and substrate of the nickel-coated copper padsare shown in figure 3. In figure 3a backscattered electron micrograph is shown, taken at thesolder/substrate interface. Figure 3b reveals the distribution of key elements in the nickel-layerthrough EDS analysis using the point mode technique. Figure 3c reveals the key elements inthe copper-layer while figure 3d is an EDS of the Sn–Cu–Ni compound. The distribution of keyelements was obtained using the attached X-ray detector with analysis made possible usingthe EDAX software. The elements present in the various layers are summarised in table 1.From both figure 3 and table 1, it is observed that the composition of the Ni-layer shows adistribution of 91·08% Ni, 4·17% Sn and 4·75% Cu. Further, in this layer, both tin and copperare believed to come from the neighbouring solder and the copper-layer. In the copper-layer,the EDS analysis shown in figure 3c reveals a distribution of 96·07% Cu, 1·24% Ni, and2·69% Sn. Further, in this layer the nickel and tin are believed to come from the neighbouringnickel layer and the solder by way of diffusion. On the solder side, i.e. near the solder/substrateinterface, EDS analysis revealed (figure 3d) a distribution of 60·86% Sn, 33·72% Ni, and5·42% Cu. This indicates the presence of the Sn–Ni–Cu ternary phase at the interface. Thiswas identified to be the (NixCuy)3Sn4 in the published literature (Kang et al 2002).From the data summarised in table 1, it is observed that the amount of copper in thenickel-layer (4·75%) is far more than the amount of nickel in the copper layer (1·24%).Figure 2. Cross-section view of a single solder bump.Interfacial structure in a tin–silver solder 255Figure 3. Backscattered SEM image and EDS analysis for the Sn3·5Ag solder bump. (a) Interfacialstructure, (b) EDS map of the nickel layer, (c) EDS map of the copper layer, (d) EDS map of theSn–Ni–Cu compounds.This suggests that the copper readily diffuses more atoms into the nickel layer than nickel iscapable of diffusing into the copper layer. The nickel layer in the overall structure physicallyseparates the copper and the solder. However, it cannot completely block or prevent thediffusion of copper into the solder. Once the solder reaches the molten state, the copper inthe nickel layer reacts with the tin-dominated solder and forms the Sn–Ni–Cu intermetalliccompound.The morphology at the interface shown in figure 3a is not characterized by the presence ofscallop-like Sn–Cu intermetallic compounds (Chuang et al 2004). It includes two sub-layers:(i) A planar looking Sn–Ni–Cu sub-layer, which is adjacent to the nickel layer. (ii) A blockylooking Sn–Ni–Cu sub-layer, which is immediately adjacent to the solder.Table 1. The distribution of key elements in the different layers.Sn (%) Ni (%) Cu (%)Ni-layer 4·17 91·08 4·75Cu-layer 2·69 1·24 96·07Sn–Ni–Cu compounds 60·86 33·72 5·42256 D C Lin et alFigure 4. Backscattered scanning electron micrographs showing the interfacial structure when sub-jected to various aging times: (a) 24 h, (b) 48 h, (c) 72 h and (d) 96 h.Figure 5. The variation of thickness of the IMC layer as a function of aging time.Interfacial structure in a tin–silver solder 257Table 2. The thickness of the IMC as a function of aging time for the Sn3·5Agsolder.Thickness (micron meter (µm))Solder 0 h 24 h 48 h 72 h 96 hSn3·5Ag solder 2·8 4 6 6·5 cracks3.2 Microstructure of the aged Sn3·5Ag solder bumpThe microstructure of the Sn3·5Ag solder bumps on subjecting to a thermal aging treatmentat 150◦C is shown in figure 4. The interfacial structure following 24 h treatment is as shownin figure 4a. To quantify the influence of aging time on microstructure, the thickness of theIMC is measured using the definition shown in figure 3a. The thickness of the IMC was4 microns (µm) following exposure to the temperature for 24 h and is shown in figure 4a.The thickness of the IMC was 2·8 microns (µm) for the as-solidified case and is shown infigure 3a. Results using the same measurement technique for the other coupons that were agedfor 48 h (figure 4b); for 72 h (figure 4c), and for 96 h (figure 4d) are summarised in table 2.The variation of thickness of the IMC as a function of aging time is shown in figure 5. It isobserved that the thickness of the IMC increases with an increase in aging time at temperature.Further, it was observed that the highest increase in the thickness of the IMC occurred duringthe first day or 24 h of aging at the chosen temperature. Following the initial increase thereoccurs a reduced rate of increase in the thickness of the IMC. After a certain time interval ofexposure, the thickness of the IMC cannot increase any further since both microscopic cracks(figure 6a) and macroscopic cracks (figure 6b) appear at the solder/substrate interface.The presence of fine microscopic cracks along the grain boundaries is caused by a gradualaccumulation of stress and resultant strain during aging at the higher temperature. Further, anoticeable difference in the thermal expansion coefficient of the various layers, i.e. copper,nickel and solder causes the microstructure of the Sn–Ag solder that was subjected to agingFigure 6. Scanning electron micrographs identifying the presence and location of the: (a) microscopiccracks, and (b) macroscopic crack.258 D C Lin et alTable 3. Thickness of planar Sn–Ni–Cu sub-layer in the IMC versus agingtime.Thickness (micron meter (µm))Solder 0 h 24 h 48 h 72 h 96 hSn3·5Ag solder 0·5 1 1·5 1·6 1·65at 150◦C for 96 h reveals a transformation of the microscopic cracks to macroscopic cracksto occur along the boundary line of the blocky-like compounds and the planar-like lookingcompounds.Further, it is seen from figure 4 that the microscopic cracks in the samples that occurred ina region between the planar Sn–Ni–Cu sub-layer and blocky-like non-planar sub-layer of Sn–Ni–Cu, in particular at the root of the blocky sub-layer (see figure 6). The overall thickness ofthe planar Sn–Ni–Cu sub-layer, measured from figure 3a, figure 4a, figure 4b, figure 4c, andfigure 4d, increases with an increase in aging time as summarised in table 3 and shown in fig-ure 7. It was found that the thickness of the planar sub-layer and the whole or entire IMC layer,as a function of aging time at temperature of 150◦C, increases with time of exposure to tem-perature. The macroscopic cracks occurred upon exposing the solder bumps to the high tem-perature environment 150◦C for 96 h aging and are conveniently located along the boundaryline, which separates the planar-like and the blocky-like Sn–Ni–Cu intermetallic compound.4. ConclusionsIn this paper, the results of a recent study on understanding the influence of thermal aging,i.e. time of exposure to temperature, on microstructural development of the solder/substrateinterface are presented. Based on the exhaustive microscopy examination and the resultantfindings the following are the key highlights:Figure 7. Variation of the thickness of the planar Sn–Ni–Cu layer with aging time (hours).Interfacial structure in a tin–silver solder 259(i) The interfacial structure between the solder Sn3·5Ag and nickel-coated copper padsincludes a layer of the Sn–Cu–Ni intermetallic compound having a planar morphologyfacing the nickel layer and a blocky structure facing the solder.(ii) The thickness of the layer of intermetallic compound for the Sn3·5Ag solder over theAu/Ni/Cu pads increased with an increase in aging time, i.e. time of exposure to temper-ature. The highest rate of increase was observed to occur during the first 24 h of aging.(iii) The macroscopic cracks were found to be present along the boundary of the planarstructure and the blocky structure for the Sn3·5Ag solder for the sample that was agedto 96 h at a temperature of 150◦C.The authors thank to Mr. Terrance Collier (President of CV Inc) for his continuous guidanceand timely assistance during experimental design and subsequent testing. Thanks and appre-ciation to Mr. Mithun Kuruvilla (Graduate Student; Department of Mechanical Engineering,The University of Akron) for his understanding, timely attention and assistance with format-ting the figures.ReferencesAndy L 2005 Chip packaging challenges, a roadmap based overview. Microelectron. Int. 22(2): 17–20Chuang T H, Wu H M, Cheng M D, Chang S Y, Yen S F 2004 Mechanisms for interfacial reactionsbetween liquid Sn3·5Sg solders and Cu substrates. J. Electronic Mater. 33(1)Kang S K, Yim M J, Advocate G G Jr., Henderson D W, Gosselin T A, King D E, Konrad J L, Sarkhel A,Puttlitz K J, Shih D Y, Fogel K, Lauro P, Griffin M, Goldsmith C 2002 Interfacial reaction studieson lead (Pb)-free solder alloys. IEEE Trans. on Electronics Packaging Manuf. 25(3): 155–161Lau J H 1995 Flip Chip Technologies. McGraw-Hill companies Inc.Lau J H, Erasmus S 1993 Review of packaging methods to complement IC performance. ElectronicPackaging and Production pp 51–56Ochoa F, Williams J J, Chawla N 2003 The effects of cooling rate on microstructure and mechanicalbehaviour of Sn-3·5%Ag solder. J. Metals pp 56–60Su P, Korhonen T, Korhonen M, Li C Y 2000 Intermetallic phase formation and growth kinetics atthe interface between molten solder and Ni-containing under bump metallization. Proc. ElectronicComponents and Technol. Conf. pp 1712–1718Tu K N, Gusak A M, Li M 2003 Physics and materials challenges for lead-free solders. J. Appl. Phys.93(3): 1335–1353

A Study Aimed at Characterizing the Interfacial Structure in a Tin-Silver Solder on Nickel-Coated Copper Plate during Aging

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A Study Aimed at Characterizing the Interfacial Structure in a Tin-Silver Solder on Nickel-Coated Copper Plate during Aging

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