Fabrication and Characteristics of SnAgCu Alloy Nanowires for ... - MDPI

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Fabrication and Characteristics of SnAgCu Alloy Nanowires for Electrical Connection Application Jung-Hsuan Chen 1, *, Shen-Chuan Lo 2 , Shu-Chi Hsu 2 and Chun-Yao Hsu 3, * 1 2 3

*

Department of Industrial Education, National Taiwan Normal University, 106 Taipei, Taiwan Department of Electron Microscopy Development Application, Industrial Technology Research Institute, 310 Hsinchu, Taiwan; [email protected] (S.-C.L.); [email protected] (S.-C.H.) Department of Mechanical Engineering, Lunghwa University of Science and Technology, 333 Taoyuan, Taiwan Correspondence: [email protected] (J.-H.C.); [email protected] (C.-Y.H.)

Received: 15 November 2018; Accepted: 3 December 2018; Published: 5 December 2018

Abstract: As electronic products become more functional, the devices are required to provide better performances and meet ever smaller form factor requirements. To achieve a higher I/O density within the smallest form factor package, applying nanotechniques to electronic packaging can be regarded as a possible approach in microelectronic technology. Sn-3.0 wt% Ag-0.5 wt% Cu (SAC305) is a common solder material of electrical connections in microelectronic devices. In this study, SAC305 alloy nanowire was fabricated in a porous alumina membrane with a pore diameter of 50 nm by the pressure casting method. The crystal structure and composition analyses of SAC305 nanowires show that the main structure of the nanowire is β-Sn, and the intermetallic compound, Ag3 Sn, locates randomly but always appears on the top of the nanowire. Furthermore, differential scanning calorimetry (DSC) results indicate the melting point of SAC305 alloy nanowire is around 227.7 ◦ C. The melting point of SAC305 alloy nanowire is significantly higher than that of SAC305 bulk alloy (219.4 ◦ C). It is supposed that the non-uniform phase distribution and composite difference between the nanowires causes the change of melting temperature. Keywords: SAC305; solder; nanowires; intermetallic compound; electrical connection

1. Introduction Electronic packaging is the final manufacturing process to transform semiconductor devices into functional products and advanced packaging has a direct impact on product performance and success rates [1,2]. As electronic and mobile devices continue to develop, the devices are required to meet the demand of ultra-thin, ultra-light, high performance, and low power consumption. Therefore, advanced 3D microelectronic packaging technology is becoming the industry trend [3–6]. More and more Input/Output (I/O) pins are needed in electronic packaging of integrated circuits while pitch is getting smaller and smaller. In order to achieve fine pitch requirement, applying nanotechniques to electronic packaging can be regarded as a possible approach in microelectronic technology. In recent years, nano-sized interconnections for advanced packaging have drawn much attention from researchers and scientists. Nanosolder is necessary to performed the soldering or bonding at the nanoscale. Among the numerous solder materials, tin-silver-copper (SAC) based alloys are widely used to replace the conventional tin-lead solders and have advantages such as low melting temperature, superior mechanical properties, and good solderability [7,8]. Nanoscale SAC solder alloys [9–16] such as nanoparticles and nanowires had been prepared in these years. SAC alloy nanoparticles could be produced by chemical methods and physical methods, such as electrodeposition, chemical reduction technique, consumable-electrode direct current arc technique, etc. These nanoparticles were easy

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to technique, aggregate consumable-electrode yet difficult to handle and current perform SAC alloy nanowires such direct arcsoldering technique,directly. etc. These nanoparticles were easy to as aggregate handle and perform soldering directly. SAC alloy nanowires such as Sn88 Ag5 Cu7 ,yet Sn93difficult Ag4 Cu3to , Sn Ag Cu , Sn Ag Cu , Sn Ag Cu , Sn Ag Cu alloy nanowires 58 18 24 78 16 6 90 4 6 87 4 9 Sn88fabricated Ag5Cu7, Sn 93Ag 4Cu3, Sn58Ag18Cuusing 24, Sn78 Ag16Cu6values , Sn90Agof4Cu 6, deposition Sn87Ag4Cu9 potential alloy nanowires were were by electrodeposition various the [16]. However, fabricated usinglead-free various solder values alloy, of thehas deposition [16]. SAC305 alloy,bya electrodeposition common commercial not beenpotential produced in However, the form of SAC305until alloy,now. a common commercial lead-free solder alloy, has not been produced in the form of nanowire nanowire until In this study,now. we fabricated SAC305 alloy nanowires inside the porous alumina membrane by the Incasting this study, we fabricated SAC305 alloy nanowires inside theofporous alumina membrane were by pressure method. The structural and thermal characteristics SAC305 alloy nanowires the pressure casting method. The structural and thermal SAC305 alloythe nanowires investigated and the differences from SAC305 bulk alloy characteristics were studied.ofFurthermore, nanowire were investigated and the differences from SAC305 bulk alloy were studied. Furthermore, the array in the soldering application was also discussed in this work. nanowire array in the soldering application was also discussed in this work.

2. Materials and Methods

2. Materials and Methods

Solder alloy nanowire array was fabricated in a porous alumina membrane with a pore diameter Solder alloy nanowire array was fabricated in a porous alumina membrane with a pore diameter around 50 nm by the pressure casting method. The processes were divided into three parts: production around 50 nm by the pressure casting method. The processes were divided into three parts: of the porous alumina template, smelting of the solder alloy, and fabrication of the alloy nanowires. production of the porous alumina template, smelting of the solder alloy, and fabrication of the alloy The detail process flow is shown in Figure 1. Firstly, the porous alumina template was fabricated nanowires. The detail process flow is shown in Figure 1. Firstly, the porous alumina template was using a two-step anodization. An Al sheet (purity 99.7%) was anodized in 0.3 M oxalic acid solution fabricated using a two-step anodization. An Al sheet (purity 99.7%) was anodized in 0.3 M oxalic acid (H2solution C2 O4 ) at(H 202C◦2C h. °C Then, film formedfilm by the first anodization was removed in a O4for ) at120 for the 1 h.alumina Then, the alumina formed by the first anodization was o solution, which was composed of the phosphoric acid (H PO ) and the chromic acid (H CrO ) at 60 3 4 2 4 removed in a solution, which was composed of the phosphoric acid (H3PO4) and the chromic acid C. The(Hsecond anodization was performed using the condition of the first anodization. After removing 2CrO4) at 60 oC. The second anodization was performed using the condition of the first anodization. theAfter Al substrate and layer bythe 0.1barrier M sodium solutionhydroxide (NaOH), solution the porous alumina removing thethe Albarrier substrate and layerhydroxide by 0.1 M sodium (NaOH), membrane was produced. the porous alumina membrane was produced.

Figure 1.1.Schematic alloy nanowires nanowiresfabrication fabricationininthis thisstudy. study. Figure Schematicprocess processflow flow of of SAC305 SAC305 alloy

Secondly, pure tintin (Sn) silver (Ag) (Ag)particles particles(purity (purity99.99%), 99.99%), and Secondly, pure (Sn)particles particles(purity (purity99.99%), 99.99%), pure pure silver and pure cupper (Cu) particles (purity 99.99%) were well mixed in a quartz crucible. The weights of pure cupper (Cu) particles (purity 99.99%) were well mixed in a quartz crucible. The weights of thethe three pure metals Cu)were wereininthe the ratio 96.5:3.0:0.5. the quartz crucible three pure metals(Sn, (Sn,Ag, Ag, and Cu) ratio of of 96.5:3.0:0.5. AfterAfter that, that, the quartz crucible was was heated 1100 vacuum. The alloy, Sn-3.0 wt% Ag-0.5 wt% (SAC305), was formed after heated toto 1100 °C◦ C ininvacuum. The alloy, Sn-3.0 wt% Ag-0.5 wt% CuCu (SAC305), was formed after cooling room temperature. cooling to to room temperature. Finally, SAC305alloy alloynanowire nanowirearray array was was produced produced using casting Finally, SAC305 using the thehigh highvacuum vacuumpressure pressure casting technique.The Theexperimental experimental equipment equipment and technique. and principle principle were weredescribed describedinindetail detailininour ourprevious previous studies [17,18].The The porous porous alumina produced in this work had had a thickness of 10 μm and studies [17,18]. aluminamembrane membrane produced in this work a thickness of 10 µm 2 and the area of the membrane was 1cm . A disc shaped piece of SAC305 alloy with dimensions

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of 1 × 0.1 cm was placed on the top of the porous alumina membrane inside the chamber. the area of the membrane was 1cm2. A disc shaped piece of alloy with dimensions of 1 cm2 × −6SAC305 The vacuum pressure of the chamber was maintained at 10 Torr to prevent the metal oxidation. After 0.1 cm was placed on the top of the porous alumina membrane inside the chamber. The vacuum ◦ the chamber heating to 300 C for 20 min, a hydraulic force was to SAC305 alloy −6 Torr to prevent pressure was of the chamber was maintained at 10 the applied metal oxidation. After the melt. During casting process, the molten alloy was injected into the nanochannel of the porous alumina chamber was heating to 300 °C for 20 min, a hydraulic force was applied to SAC305 alloy melt. During membrane. Solidification wasalloy performed using water quenching of at the theporous bottom of the membrane. chamber. After casting process, the molten was injected into the nanochannel alumina Solidification was performed using water the bottom of the chamber. After cooling to cooling to room temperature, SAC305 alloy quenching nanowire at array was produced. roomcrystal temperature, SAC305 alloydistribution, nanowire array wascomposition produced. The structure, phase and of SAC305 alloy nanowires were crystaldiffraction structure, phase and composition SAC305electron alloy nanowires were(SEM, examinedThe by X-ray (XRD,distribution, Bruker, Billerica, MA, USA) of scanning microscope examined by X-ray diffraction (XRD, Bruker, Billerica, MA, USA) scanning electron microscope (SEM, JEOL, Tokyo, Japan), energy dispersive spectroscopy (EDS, Oxford, Abingdon, UK), and transmission JEOL, Tokyo, Japan), energy dispersive spectroscopy (EDS, Oxford, Abingdon, UK), and electron microscope (TEM, JEOL, Tokyo, Japan). Moreover, the melting point and thermal properties transmission electron microscope (TEM, JEOL, Tokyo, Japan). Moreover, the melting point and of SAC305 alloy nanowires were measured by differential scanning calorimetry (DSC, TA, New Castle, thermal properties of SAC305 alloy nanowires were measured by differential scanning calorimetry DE, USA) analysis. (DSC, TA, New Castle, DE, USA) analysis. 3. Results and and Discussion 3. Results Discussion 3.1. Fabrication of SAC305 Alloy Nanowires 3.1. Fabrication of SAC305 Alloy Nanowires In this SAC305 nanowires werewere fabricated in the alumina membrane by the In study, this study, SAC305 nanowires fabricated inporous the porous alumina membrane by pressure the pressure Figure 2a surface shows that the surface morphology of the membrane porous alumina casting process.casting Figureprocess. 2a shows that the morphology of the porous alumina produced in the oxalic acid solution reveals thatand theuniform nanopores are order and uniform in themembrane oxalic acidproduced solution reveals that the nanopores are order arrays. The SEM image of the arrays. The SEM image of the porous alumina membrane is further analyzed with the image pro plus pore porous alumina membrane is further analyzed with the image pro plus software. The corresponding software. The corresponding pore size distribution was obtained by the histogram analysis. As size distribution was obtained by the histogram analysis. As presented in Figure 2b, the histogram was presented in Figure 2b, the histogram was plotted by measuring at least 400 pores in Figure 2a and plotted by measuring at least 400 pores in Figure 2a and the average pore diameter is about 50.19 nm. the average pore diameter is about 50.19 nm. Figure 2c shows the plane view of SAC305 alloy Figure 2c shows the plane view of SAC305 alloy nanowires in the porous alumina membrane. A high nanowires in the porous alumina membrane. A high filling ratio of nanowire could be achieved by fillingthe ratio of nanowire could be achieved by the pressure casting process. pressure casting process. 180

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Figure 2. (a) SEM image of the porous alumina membrane; (b) The pore size distribution which was calculated from image (a) and the average pore diameter denoted in the figure; (c) SEM image of SAC305 alloy nanowires in the porous alumina membrane.

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Figure 2. (a) SEM image of the porous alumina membrane; (b) The pore size distribution which was calculated from image (a) and the average pore diameter denoted in the figure; (c) SEM image of SAC305 alloy nanowires in the porous alumina membrane.


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3.2. Characteristics of SAC305 Alloy Nanowires

3.2. Characteristics of SAC305 Alloy X-ray diffraction analyses of Nanowires SAC305 bulk alloy and nanowires are shown in Figure 3. The signals of Sn in SAC305 bulk alloy and nanowires are identified The phase X-ray diffraction analyses of SAC305 bulk alloy and nanowiresas arebeta-Sn shown (β-Sn). in Figure 3. β-Sn The signals crystallizes in a tetragonal crystal structure (I4 1/and, space group = 141, JCPDS No. 04-0673) with the of Sn in SAC305 bulk alloy and nanowires are identified as beta-Sn (β-Sn). The β-Sn phase crystallizes lattice constants,crystal a = 0.5831 nm and = 0.3182 nm. The major of β-Sn in SAC305 bulk in a tetragonal structure (I41 c/and, space group = 141, crystal JCPDSplane No. 04-0673) with the lattice alloy and nanowires is the (200) lattice plane locating at the angle of 30.6°. Some slight signals form constants, a = 0.5831 nm and c = 0.3182 nm. The major crystal plane of β-Sn in SAC305 bulk alloy ◦ . Some slight Ag3Sn are alsoisobserved. The results indicate at that crystal structure of SAC305 is and nanowires the (200) lattice plane locating the the angle of 30.6 signalsbulk formalloy Ag3 Sn preserved in the nanowires after heating processes. Additionally, the diffraction are also observed. The results indicate thatand thehigh-pressure crystal structure of SAC305 bulk alloy is preserved in peak of Al in SAC305 alloy nanowires is coming from the residual Al substrate of the porous the nanowires after heating and high-pressure processes. Additionally, the diffraction peakalumina of Al in membrane. SAC305 alloy nanowires is coming from the residual Al substrate of the porous alumina membrane.

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2 Figure 3. XRD spectra of SAC305 alloy and SAC305 alloy nanowires. Figure 3. XRD spectra of SAC305 alloy and SAC305 alloy nanowires.

The low-magnification cross-sectional structure of SAC305 alloy nanowires is presented in the The low-magnification cross-sectional structure of SAC305 alloy nanowires is presented in the bright field TEM image as shown in Figure 4a. Figure 4b, which is the enlarged image of the red box in bright field TEM image as shown in Figure 4a. Figure 4b, which is the enlarged image of the red box Figure 4a, reveals a significant contrast difference and interface in the upper region of the nanowire. in Figure 4a, reveals a significant contrast difference and interface in the upper region of the EDS analysis was performed with field emission TEM to determine the composition of nanowire nanowire. EDS analysis was performed with field emission TEM to determine the composition of marked in Figure 4b. From the EDS spectrum, the crystal on the top of the nanowire (area a) is judged nanowire marked in Figure 4b. From the EDS spectrum, the crystal on the top of the nanowire (area to consist of Ag and Sn in the ratio of 3:1 with a statistical error of 5%. The gold signal is coming a) is judged to consist of Ag and Sn in the ratio of 3:1 with a statistical error of 5%. The gold signal is from the TEM gold grid. Area b is mainly composed of Sn, but a low Cu content around 0.5 wt% coming from the TEM gold grid. Area b is mainly composed of Sn, but a low Cu content around 0.5 is also observed. High-resolution TEM analyses were performed to determine the crystal structure wt% is also observed. High-resolution TEM analyses were performed to determine the crystal of the nanowire. Figure 5b is taken from the top of the nanowire. It indicates that the lattice fringes structure of the nanowire. Figure 5b is taken from the top of the nanowire. It indicates that the lattice (d = 2.59 Å) observed in the high-resolution TEM image is identical with the distance between the (201) fringes (d = 2.59 Å ) observed in the high-resolution TEM image is identical with the distance between lattice plane and confirms that the nanocrystal is Ag3 Sn. The high-resolution TEM (Figure 5c) of the the (201) lattice plane and confirms that the nanocrystal is Ag 3Sn. The high-resolution TEM (Figure region under the Ag3 Sn crystal in the nanowire shows that the lattice fringes, d = 2.92 Å and d = 2.79 Å, 5c) of the region under the Ag3Sn crystal in the nanowire shows that the lattice fringes, d = 2.92 Å and are identical to the distance between the (200) lattice plane and (101) lattice plane of β-Sn, respectively. d = 2.79 Å , are identical to the distance between the (200) lattice plane and (101) lattice plane of β-Sn, Fast Fourier-Transformation (FFT) analysis was performed on the lattice fringes of Figure 5c and the respectively. Fast Fourier-Transformation (FFT) analysis was performed on the lattice fringes of spots with a zone axis [101] match well with the tetragonal structure of β-Sn as shown in Figure 5d. Figure 5c and the spots with a zone axis [101̅] match well with the tetragonal structure of β-Sn as According to the TEM and EDS results, the nanowire is mainly composed of β-Sn. Furthermore, shown in Figure 5d. According to the TEM and EDS results, the nanowire is mainly composed of βa Ag3 Sn crystal always appears on the top of the nanowire and has a non-uniform distribution in Sn. Furthermore, a Ag3Sn crystal always appears on the top of the nanowire and has a non-uniform the nanowire arrays. However, the intermetallic compounds composed of Sn and Cu such as Cu6 Sn5 distribution in the nanowire arrays. However, the intermetallic compounds composed of Sn and Cu and Cu3 Sn were not found in our experiment. It is supposed that the reason for this is that Cu6 Sn5 or Cu3 Sn has a smaller crystal size and much lower content than Ag3 Sn, causing difficulty in observation.

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such as Cu6Sn5 and Cu3Sn were not found in our experiment. It is supposed that the reason for this is such as Cu6Sn5 and Cu3Sn were not found in our experiment. It is supposed that the reason for this is Micromachines 2018, 644 has a smaller crystal size and much lower content than Ag3Sn, causing difficulty 5 of 8 that Cu6Sn5 or Cu9,3Sn that Cu6Sn5 or Cu3Sn has a smaller crystal size and much lower content than Ag3Sn, causing difficulty in observation. in observation.

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Figure 4. (a) Low magnification TEM image of SAC305 alloy nanowires; (b) The enlarged image of Figure 4. Lowinmagnification magnification TEM imagedispersive SAC305 alloy nanowires; The enlarged image of the red square the image (a);TEM (c) Energy X-ray spectra recorded from the circular areas Figure 4. (a) (a) Low image ofofSAC305 alloy nanowires; (b)(b) The enlarged image of the in (b). the square in image the image (a);Energy (c) Energy dispersive spectra recorded from the circular areas red red square in the (a); (c) dispersive X-rayX-ray spectra recorded from the circular areas in (b).

in (b).

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β-Sn β-Sn ~2.79 Å (101) ~2.79 Å (101)

𝟏𝟐𝟏 𝟏𝟐𝟏 𝟏𝟎𝟏 𝟎𝟐𝟎 𝟏𝟎𝟏 𝟎𝟐𝟎

𝐙𝐨𝐧𝐞 𝐀𝐱𝐢𝐬 𝟏𝟎𝟏 𝐙𝐨𝐧𝐞 𝐀𝐱𝐢𝐬 𝟏𝟎𝟏

(c) (c)

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Figure TEM imageofofthe theinterface interfacebetween between Ag Ag33Sn and TEM image of of Figure 5. 5. (a)(a) TEM image and β-Sn. β-Sn.(b) (b)High-resolution High-resolution TEM image Figure 5. (a) TEM image of the interface between Ag 3Sn and β-Sn. (b) High-resolution TEM image of Ag 3 Sn crystal. (c) High-resolution TEM image of β-Sn crystal. (d) FFT analysis of the lattice fringes in Ag3 Sn crystal. (c) High-resolution TEM image of β-Sn crystal. (d) FFT analysis of the lattice fringes (c).3Sn crystal. (c) High-resolution TEM image of β-Sn crystal. (d) FFT analysis of the lattice fringes in in Ag (c). (c).

The DSC curvesininFigure Figure66show showthat that there there is a variation SAC305 The DSC curves variation in inthe themelting meltingpoint pointbetween between SAC305 The DSC curves in Figure The 6The show that there is a variation in thefrom melting point between SAC305 ◦C bulk alloy and thenanowires. nanowires. melting temperature increases 219.4 °C◦ Cto °C as as thethe bulk alloy and the melting temperature increases from 219.4 to227.7 227.7 bulk alloy and the nanowires. The melting temperature increases from 219.4 °C to 227.7 °C as the alloy’s crystal size decreases. For nanocrystals embedded in a matrix, they can melt below the melting alloy’s crystal size decreases. For nanocrystals embedded in a matrix, they can melt below the melting alloy’s crystalbulk sizematerials decreases.when For nanocrystals embedded a matrix, they can meltand below the melting point theinterfaces interfaces between in embedded the matrix areare point of of thethe bulk materials when the between embeddednanocrystals nanocrystals and the matrix point of the[19]. bulkMoreover, materials the when the interfaces between embedded nanocrystals and the ratio matrix are incoherent melting point depends strongly on the surface to volume in thethe incoherent [19]. Moreover, the melting point depends strongly on the surface to volume ratio in incoherent [19]. Moreover, the melting point depends strongly on the surface to volume ratio in the nanoscale range [20]. Our experimental result does not agree with the theoretical prediction but is nanoscale range [20]. Our experimental result does not agree with the theoretical prediction but is nanoscale range This [20]. could Our experimental withthe thecrystal theoretical prediction but is just the reverse. be explainedresult by thedoes fact not thatagree not only size decreases but also just the reverse. This could be explained by the fact that not only the crystal size decreases but also just the reverse. This could distribution be explainedchange by theduring fact that only theof crystal size decreases but EDS also the composition and phase thenot formation the nanowire. TEM and the composition and phase distribution change during the formation of the nanowire. TEM and EDS the composition andthe phase distribution change during the formation of the and EDS results indicate that intermetallic compound, Ag3Sn, always appears onnanowire. the top of TEM the nanowires results indicate that intermetallic compound, Ag33Sn, Sn,always alwaysappears appearsononthe the top of the nanowires results thatthe the top nanowires and theindicate composition of intermetallic the nanowirecompound, except Ag3Ag Sn crystal is mainly composed ofof Snthe and Cu (~0.5 and the composition of the nanowire except Ag Sn crystal is mainly composed of Sn and Cu (~0.5 wt%). 3 3Sn and theAccording composition of Sn–Ag–Cu the nanowire except Ag crystal[21], is mainly composed and (~0.5 wt%). to the ternary phase diagram the melting point of of Sn SAC 305Cu alloy is According to the ternary ternary phase diagram [21], the melting of SAC is around wt%). to thethe Sn–Ag–Cu phase [21], the melting point of305 SAC 305content. alloy is aroundAccording 220 °CSn–Ag–Cu and melting point of SACdiagram alloy increases aspoint the decrease of alloy Ag ◦ C and the melting point of SAC alloy increases as the decrease of Ag content. Furthermore, 220around 220 °CAgand the melting SAC process alloy increases as the decrease Ag content. Furthermore, diffusion duringpoint the of heating might become difficult of because of the AgFurthermore, diffusion during the heating process might become difficult because of the during theporous heating process might become difficult becausebehavior of of thethe confinement ofAg thediffusion nanochannel in the alumina membrane. Therefore, the confinement melting nanochannel inofthe porous alumina membrane. Therefore, therather melting behavior of SAC305 alloy confinement the nanochannel in the porous alumina membrane. Therefore, theSn–Ag–Cu melting behavior of SAC305 alloy nanowire is more similar to the Sn–Cu system than the system, nanowire is more similar to the Sn–Cu system rather than the Sn–Ag–Cu system, leading to the higher of SAC305 alloy nanowire is more similar to the Sn–Cu system rather than the Sn–Ag–Cu system, leading to the higher melting point than SAC305 bulk alloy. leading to the higher melting point than SAC305 bulk alloy. melting point than SAC305 bulk alloy. HeatHeat flowflow

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Figure 6. Differential scanning calorimetry (DSC) thermographs of SAC305 alloy and SAC305 alloy Figure 6.6. Differential scanningcalorimetry calorimetry (DSC) thermographs of SAC305 alloy and SAC305 Figure Differential scanning (DSC) thermographs of SAC305 alloy and SAC305 alloy nanowires. alloy nanowires. nanowires.

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Fine pitch and submicron bump bump technology technology is aa possible possible way way to to achieve achieve aa higher higher I/O I/O density within the smallest form factor package. In this study, we propose a new joint structure with solder within the smallest form factor package. In this study, we propose a new joint structurethe with the alloy array as shown in Figure 7 for the submicron electronic packaging. The solder alloy soldernanowire alloy nanowire array as shown in Figure 7 for the submicron electronic packaging. The solder nanowires are theare bridges between the solder on the and the pre-solder of the substrate alloy nanowires the bridges between the cap solder capCu onpillar the Cu pillar and the pre-solder of the or another chip. Some advantages such as offering more overlay budget and reducing the common substrate or another chip. Some advantages such as offering more overlay budget and reducing the soldering problems problems like cold joints andjoints solderand bridges be obtained the using new joint structure. common soldering like cold soldercould bridges could be using obtained the new joint Although there are still concerns about reliability and mechanical stress issues, this technique could be structure. Although there are still concerns about reliability and mechanical stress issues, this helpful to develop interconnection for the advanced packaging where high-density technique could bethe helpful to develop technology the interconnection technology for the advanced packaging joint interconnection required. Additionally, more stable and more flexible templates where high-density is joint interconnection is required. Additionally, more nanostructured stable and more flexible with the suitable coefficient of thermal expansion (CTE) would be a good candidate reducing the joint nanostructured templates with the suitable coefficient of thermal expansion (CTE) would be a good stress for the practical candidate reducing theapplication joint stress[22,23]. for the practical application [22,23].

Figure 7. 7. Schematic Schematic illustration illustration of of an an electrical electrical connection connection structure structure with Figure with the the solder solder alloy alloy nanowires. nanowires.

4. Conclusions 4. Conclusions SAC305 alloy nanowires have been successfully fabricated inside the porous alumina membrane SAC305 alloy nanowires have been successfully fabricated inside the porous alumina membrane by the pressure casting method. The nanowires with high aspect ratio are arranged uniformly. by the pressure casting method. The nanowires with high aspect ratio are arranged uniformly. Experimental results show that the intermetallic compound, Ag3 Sn, locates randomly but always Experimental results show that the intermetallic compound, Ag3Sn, locates randomly but always appears on the top of the nanowire. The composition of the nanowire, except Ag3 Sn crystal, is mainly appears on the top of the nanowire. The composition of the nanowire, except Ag3Sn crystal, is mainly composed of Sn and Cu (~0.5 wt%). DSC results indicate that SAC305 alloy nanowire has a higher composed of Sn and Cu (~0.5 wt%). DSC results indicate that SAC305 alloy nanowire has a higher melting temperature than SAC305 bulk alloy. The melting point increase from 219.4 ◦ C to 227.7 ◦ C melting temperature than SAC305 bulk alloy. The melting point increase from 219.4 °C to 227.7 °C might be caused by the non-uniform phase distribution and composition difference. Finally, we propose might be caused by the non-uniform phase distribution and composition difference. Finally, we a possible joint structure in which the solder alloy nanowires could provide an electrical connection propose a possible joint structure in which the solder alloy nanowires could provide an electrical between chip and substrate or chip and chip to achieve the high-density joint interconnection. connection between chip and substrate or chip and chip to achieve the high-density joint interconnection. Author Contributions: J.-H.C. and C.-Y.H. conceived and designed the experiments; J.-H.C. performed the experiments; S.-C.L. and S.-C.H. analyzed the data; S.-C.L. contributed analysis tools; J.-H.C. wrote the paper.

Author Contributions: C.Y.H.acknowledge conceived and J.H.C. and performed the Acknowledgments: The J.H.C. authorsand gratefully the designed support ofthe the experiments; Ministry of Science Technology experiments; S.C.L. and S.C.H. analyzed the MOST106-2218-E-003-002. data; S.C.L. contributed analysis tools; J.H.C. wrote the paper. of the Republic of China, through Grant No. Conflicts of Interest:The Theauthors authorsgratefully declare noacknowledge conflicts of interest. Acknowledgments: the support of the Ministry of Science and Technology of the Republic of China, through Grant No. MOST106-2218-E-003-002.

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