Multiple Reflow Research Articles

Dissolution and precipitation kinetics of Cu6Sn5 intermetallics in Cu/Sn/Cu micro interconnects under temperature gradient

Multiple reflows are often required in 3D packaging. To elucidate the effect of temperature gradient during subsequent reflow on existing intermetallic compounds (IMCs), Cu6Sn5 IMC layers were initially formed in Cu/Sn/Cu micro interconnects. Upon subsequent reflow, synchrotron radiation real-time imaging technology was used to in situ study the dissolution and precipitation behavior of the pre-formed Cu6Sn5 under different temperature gradients. The pre-formed Cu6Sn5 IMC at the cold end continued to grow linearly with increasing aspect ratio, whereas that at the hot end dissolved linearly and then maintained a critical thin layer. The thick pre-formed Cu6Sn5 IMC at the hot end significantly hindered the dissolution of the neighboring Cu substrate until a dynamic equilibrium between chemical potential gradient and temperature gradient was satisfied. The thermomigration of Cu atoms from the hot end towards the cold end was responsible for the asymmetrical evolution of the interfacial Cu6Sn5 between the cold and hot ends. A theoretical model was proposed based on Cu diffusion flux to calculate the IMC thickness at the both ends as a function of reflow time and the equilibrium IMC thickness at the hot end under temperature gradient.

Effect of Plasma Surface Finish on Wettability and Mechanical Properties of SAC305 Solder Joints

The wetting behavior, interfacial reactions, and mechanical reliability of Sn-Ag-Cu solder on a plasma-coated printed circuit board (PCB) substrate were evaluated under multiple heat-treatments. Conventional organic solderability preservative (OSP) finished PCBs were used as a reference. The plasma process created a dense and highly cross-linked polymer coating on the Cu substrates. The plasma finished samples had higher wetting forces and shorter zero-cross times than those with OSP surface finish. The OSP sample was degraded after sequential multiple heat treatments and reflow processes, whereas the solderability of the plasma finished sample was retained after multiple heat treatments. After the soldering process, similar microstructures were observed at the interfaces of the two solder joints, where the development of intermetallic compounds was observed. From ball shear tests, it was found that the shear force for the plasma substrate was consistently higher than that for the OSP substrate. Deterioration of the OSP surface finish was observed after multiple heat treatments. Overall, the plasma surface finish was superior to the conventional OSP finish with respect to wettability and joint reliability, indicating that it is a suitable material for the fabrication of complex electronic devices.

Influence of Second Reflow on the Intermetallic Compound Growth with Different Surface Finish

The formation and growth of the intermetallic were frequently discussed since lead free solder took place replacing the lead solder. However, the effect of multiple reflow process on the intermetallic morphology that was subjected to aging still needs further investigation. Thus, this study aimed to investigate the effect of second reflow towards the intermetallic compound formation and growth. Two types of surface finishes were used such as Immersion Tin (ImSn) and Electroless Nickel Immersion Gold (ENIG). Both test boards were reflowed once with Sn-3Ag-0.5Cu at the temperature of 225 °C and soaking for 8 seconds. Then, they were reflowed again at the same temperature for 25 minutes prior to an isothermal aging process for 250, 500, 1000 and 2000 hours at the temperature of 150 °C. The ProgRes C3 IM7200 Optical Microscope and ImageJ were used for the microstructural study, which includes morphology and thickness. Results indicated that IMC thickness formed between solder and ImSn surface finish increased significantly with 1.28 µm incremental when exposed to the second reflow. Whereas the IMC thickness of ENIG surface finish was increased for up to 0.15 µm. In addition, ENIG showed higher activation energy as compared to ImSn.

Suppression of Cu6Sn5 in TiO2 reinforced solder joints after multiple reflow cycles

In the current generation of 3D electronic packaging, multiple reflows are often required during soldering. In addition, electronic packages may be subjected to additional solder rework or other heating processes. This paper investigates the effects of multiple reflow cycles on TiO2 reinforced Sn–0.7Cu solder fabricated by a powder metallurgy microwave sintering technique. Compared to TiO2-free equivalents, a relative suppression of the Cu6Sn5 phase, both as primary crystals and as an interfacial layer was observed. The likely mechanism relates to the TiO2 nanoparticles promoting nucleation and decreasing the amount of time that liquid is in contact with the interfacial layer. The TiO2 particles appear to stabilise the interfacial Cu6Sn5 layer and result in a more planar morphology. The suppression of Cu6Sn5 results in TiO2 reinforced solder joints having a higher shear strength after multiple reflow cycles compared to Sn–0.7Cu solder joints.

Effect of multiple flip-chip assembly on the mechanical reliability of eutectic Au–Sn solder joint

Eutectic Au–Sn solder has widely been used for high temperature bonding since it enables fluxless soldering and provides mechanically stable solder joint. However, effect of multiple bonding process on the mechanical reliability has not been studied in detail. In this study, microstructure evolution of Au–Sn solder joint and its effect on mechanical reliability were systematically investigated. During the multiple reflow process, the eutectic phase gradually transformed into ζ phase, which resulted in loss of solderable area. The phenomenon turned out to be responsible for degradation of bonding strength of the solder joint.

Effect of massive spalling on mechanical strength of solder joints in Pb-free solder reflowed on Co-based surface finishes

This study investigated the effect of massive spalling of intermetallic compounds (IMCs) on the microstructural evolution and mechanical properties of solder joints after multiple reflows. SAC305 solder was reflowed on substrates with four Co-based surface finishes: electroless Co (EC), electroless Co/immersion Au (ECIG), electroless Co/electroless Pd (ECEP), and electroless Co/electroless Pd/immersion Au (ECEPIG). CoSn3 was the main intermetallic compound (IMC) that formed in the systems, and it massively spalled in the systems with Pd and high P content. Systems with massive spalling have a high shear strength and low shear height owing to solid solution strengthening. Due to strain hardening, high shear velocity caused high strength in all systems. Massive spalling of CoSn3 transformed the failure mode from brittle to ductile and enhanced the shear strength of the joints.

Wettability study of lead free solder paste and its effect towards multiple reflow

Nowadays, wafer bumping using solder paste has come into focus as it provides a low cost method. However, since the industries are moving towards lead-free electronic packaging, a new type of no-clean flux was produced specifically for lead-free solder paste. Therefore, this study is used to evaluate the wettability of two different types of no-clean flux onto copper substrate. Besides, its effect towards multiple reflow was also studied. Reflow soldering was conducted for both types of solder paste that contained different type of no-clean flux for up to double reflow. Two different reflow profile was used. The results showed that the Flux A exhibit better soldering performance after first and second reflow soldering. In addition, type of intermetallic compound (IMC) found after first reflow remain the same even after second reflow which was Cu-Sn based. This is shows that Flux A manage to control the diffusion process which will finally leads to a better solder joint performance. Nevertheless, mechanical testing should be carried out in order to evaluate the solder joint strength.

Interfacial microstructure evolution and shear behavior of Au–20Sn/(Sn)Cu solder joints bonded at 250 °C

It is of great significance to realize electronic packaging at low temperature. Au–20Sn was successfully joined with Cu substrates at 250°C (lower than the melting point of Au–20Sn) with the aid of Sn interlayers. Interfacial microstructure evolution and shear behavior of Au–20Sn/(Sn)Cu solder joints during multiple reflows and after isothermal aging were investigated, respectively. During multiple reflows, AuSn/AuSn2/AuSn4/Sn/(Cu,Au)6Sn5/Cu3Sn layers formed initially at the interface of Au–20Sn/(Sn)Cu solder joints. As the reflowing time increased, the Sn layer was gradually consumed and the interface configuration transformed into AuSn/AuSn2/AuSn4/(Cu,Au)6Sn5/Cu3Sn layers, and finally into AuSn/AuSn2/(Cu,Au)6Sn5/Cu3Sn layers. The shear strength of Au–20Sn/(Sn)Cu solder joints first increased, and then decreased as the reflowing time increased. Shear fractures of the joints after multiple reflows always occurred at the (Cu,Au)6Sn5 layer. During the isothermal aging, the AuSn2 and (Cu,Au)6Sn5 layers gradually disappeared and the interface configuration transformed from AuSn/AuSn2/(Cu,Au)6Sn5/Cu3Sn layers to AuSn/Cu3Sn layers. The shear strength of Au–20Sn/(Sn)Cu solder joints decreased continuously as the aging time increased. Shear fracture locations transferred from the AuSn2/(Cu,Au)6Sn5 layers to the AuSn layer, and finally to the Cu3Sn/Cu interface. In addition, Au–20Sn/Cu solder joints after the same treatments and tests were analyzed for comparison.

TEM observation of interfacial compounds of SnAgCu/ENIG solder bump after laser soldering and subsequent hot air reflows

In three-dimensional (3D) packaging and assembly technology, Lead-free SnAgCu solder joints inevitably experience multiple reflows. The interfacial compounds of Sn3.0Ag0.5Cu (SAC305)/ Electroless Ni–P/Immersion Au (ENIG) solder bump after laser soldering and subsequent hot air reflows were observed by transmission electron microscopy (TEM), respectively. The initial laser soldering played a key role in interfacial compound evolution during the subsequent multiple reflows. After laser soldering, a thin P-rich layer and an ultrafine scallop-type (Ni, Cu)3Sn4 layer formed at the interface of SAC305/ENIG. The valleys of ultrafine (Ni, Cu)3Sn4 grains served as rapid channels for Ni diffusion, which contributed to the formation of dendritic η-(Cu, Ni)6Sn5+(Ni, Cu)3Sn2 intermetallic compounds after the first hot air reflow. However, with the number of reflow times increasing, the η-(Cu, Ni)6Sn5 totally transformed to noodle-like (Ni, Cu)3Sn2 owing to their similarly hexagonal lattice structure and the limited Cu supply in solder.

Formation, processing and characterization of Co–Sn intermetallic compounds for potential integration in 3D interconnects

Cobalt exhibits good wetting by lead-free solders (Humpston, 2010) [1], generates Sn-rich intermetallic compounds (IMC’s) and has lower dissolution in solders than Cu and Ni, thus potentially being a more effective solder diffusion barrier with improved electro- and thermo-migration resistance in interconnect applications (Ishida et al., 1991; Okamoto et al., 2006; Wang et al., 2013; Kanatzidis et al., 2005; Liu et al., 2004; Labie, 2007) [5–7]. In this work, we extend prior data (Vakanas et al., 2013; M.O. et al., 2012; Vakanas et al., 2014) on formation, processing, characterization and quantification of Co–Sn IMC phases generated upon deposition, reflow and aging of electroplated and solder paste screen printed samples. Cobalt oxides growth (vs. Cu) was characterized by X-ray Photoelectron Spectroscopy (XPS) as a function of multiple reflows. Cobalt oxides cleaning by microwave Ar/H2 plasma was also demonstrated. Co–Sn IMC growth rate (vs. Cu–Sn and Ni–Sn) was quantified after one reflow cycle and aging in solid-state temperature range of 180–220°C. Relevant kinetic parameters were calculated assuming a diffusion-driven, parabolic growth model. IMC phases were identified by Energy-Dispersive X-ray Spectroscopy (EDS) verifying Sn-rich Co IMC’s. Young’s modulus (E) and hardness (H) were measured by nanoindentation indicating that Co–Sn IMC’s are more compliant and softer than Cu–Sn IMC’s. Implications of the findings for material selection in 3D micro-bumping and stacking integration are discussed with a future outlook and recommendations to improve interconnect reliability.

Effect of thermomigration on the growth kinetics of Cu6Sn5 at liquid-solid interfaces in Cu/Sn/Cu solder joints

With the continuous miniaturization of electronic packaging, micro bumps for chip interconnects are smaller in size, and thus the reliability of interconnects becomes more and more sensitive to the formation and growth of intermetallic compounds (IMCs) at liquid-solid interface during soldering. Thermomigration (TM) is one of the simultaneous heat and mass transfer phenomena, and occurs in a mixture under certain external temperature gradient. In the process of interconnection, micro bumps usually undergo multiple reflows during which nonuniform temperature distribution may occur, resulting in TM of metal atoms. Since the interdiffusion of atoms between solders and under bump metallization (UBM) dominates the formation of interfacial IMCs, TM which enhances the directional diffusion of metal atoms and induces the redistribution of elements, will markedly influence the growth behaviors of interfacial IMCs and consequently the reliability of solder joints. The diffusivity of atoms in liquid solder is significantly larger than that in solid solder and in consequence a small temperature gradient may induce the mass migration of atoms. As a result, the growth of interfacial IMCs becomes more sensitive to temperature difference between solder joints in soldering process. So far, however, few studies have focused on liquid state TM in solder joints, and the growth kinetics of interfacial IMCs under TM during soldering is still unknown to us. In this study, Cu/Sn/Cu solder joints are used to investigate the migration behavior of Cu atoms and its effect on the growth kinetics of interfacial Cu6Sn5 under temperature gradients of 35.33℃/cm at 250℃ and 40.0℃/cm at 280℃, respectively. TM experiments are carried out by reflowing the Cu/Sn/Cu interconnects on a hot plate at 250℃ and 280℃ for different durations. For comparison, isothermal aging experiments are conducted in a high temperature chamber under the same temperatures and reaction durations. During isothermal aging, the growth of interfacial Cu6Sn5 follows a parabolic law and is controlled by bulk diffusion. Under the temperature gradient, asymmetrical growth of interfacial Cu6Sn5 is observed between cold and hot ends. At the cold end, the growth of the interfacial Cu6Sn5 is significantly enhanced and follows a linear law, indicating a reaction-controlled growth mechanism; while at the hot end, the growth of the interfacial Cu6Sn5 is inhibited and follows a parabolic law, indicating a diffusion-controlled growth mechanism. The dissolved Cu atoms from the Cu substrate at the hot end are driven to migration toward the cold end by temperature gradient, providing the Cu atomic flux for the fast growth of the interfacial Cu6Sn5 at the cold end. With the variation of the measured thickness of Cu6Sn5 IMC at the cold end and the simulated temperature gradients, the molar heat of transport Q^* of Cu atoms in molten Sn is calculated to +14.11 kJ/mol at 250℃ and +14.44 kJ/mol at 280℃. Accordingly, the driving forces of thermomigration in molten solder FL are estimated to be 1.62×10-19 N and 1.70×10-19 N, respectively.

Interfacial reaction of Bi–Ag and Bi–Sb solders on copper substrate with multiple reflow number

Owing to the toxicity of lead, much effort has been put in studies on lead-free solders after environmental legislations prohibited the use of lead in these parts worldwide. Here, Bi–Ag and Bi–Sb alternative solder alloys containing 1·5–5 wt-% Ag and Sb were investigated. The effect of the reflow number and weight percentage of Ag and Sb on the surface properties of solders and interfacial reactions between the solder bulk and the Cu substrate were analysed by optical microscopy and scanning electron microscopy–energy dispersive X-ray. The results show that by increasing the reflow number and wt-% of Ag and Sb in the solder bulk, the thickness of the mechanical grain boundary grooving is increased. However, the thickness of the Cu3Sb intermetallic compound layer at the interface of Bi–5Sb decreased by increasing the reflow number. Moreover, our findings show that the amount of Ag and Sb in solder alloys and the reflow number have different behaviours on the wettability properties. By increasing the reflow number, the wetting angle decreased in Bi–Ag solder alloys, whereas it increased in Bi–Sb solder alloys.

A Study on the Thermal Oxidation and Wettability of Lead-free Solders of Sn-Ag-Cu and Sn-Ag-Cu-In

The surface oxidation mechanism of lead-free solder alloys has been investigated with multiple reflow using X-ray photoelectron spectroscopy. It was found that the solder surface of Sn-Ag-Cu-In solder alloy is surrounded by a thin InOx layer after reflow process; this coating protects the metallic surface from thermal oxidation. Based on this result, we have performed a wetting balance test at various temperatures. The Sn-Ag-Cu-In solder alloy shows characteristics of both thermal oxidation and wetting balance better than those of Sn-Ag-Cu solder alloy. Therefore, Sn-Ag-Cu-In solder alloy is a good candidate to solve the two problems of easy oxidation and low wettability, which are the most critical problems of Pb-free solders.

Comparison of Cu-Sn Compound Formation Between Snag and Pure Sn Solder Bumps by EBSD

Anisotropic behavior of Sn based solder during electromigration has been studied by many researchers, and it is very important to control the microstructure for more reliable bump joints[1]. Here, we prepared various SnAg bumps with different Ag concentrations including pure Sn[2]. Unary electroplating has various advantages such as easy handling bath, negligible bump composition and maintaining equipment with securing about environmental health and safety issue.Figure 1 shows our preliminary experiments for the cross-sectional EBSD and EDS measurement of Cu pillar with pure Sn bump, and it is reflowed at 235°C for 6 min. In this study, we will report comparison of Sn microstructures according to various Ag inclusions by using cross-sectional EBSD(electron backscattered diffraction) analysis and EDS(energy dispersive spectroscopy) measurement. We carried out multiple reflows at various atmosphere how this impact IMC(intermetallic compound) growth along with microstructures. Through this crystallographic aspect approach, we will present optimum condition for resistance of electro migration and void formation.[1] Yiwei Wang, Kuan H. Lu, Vikas Gupta, Leon Stiborek, Dwayne Shirley, Seung-Hyun Chae, Jay Im, Paul S. Ho, J. Mater. Res., 27, 1131 (2008)[2] Ui-Hyoung Lee, Moongi Cho, Woojin Choi, Ha-Young You, Jinho Choi and Jaihyung Won, 224th ECS meeting, San Francisco, Oct. (2013)

Microstructural Evaluation of Bi-Ag and Bi-Sb Lead-Free High-Temperature Solder Candidates on Copper Substrate with Multiple Reflow Number

An impetus has been provided towards the development of lead-free solders by worldwide environmental legislation that banned the use of lead in solders due to the lead toxicity.This study focus on Bi-Ag and Bi-Sb solder alloys, in compositions from 1.5 to 5 wt % Ag and Sb. The effects of Ag and Sb amount, and reflow number on the microstructure and morphology of solder bulk were analysed by optical microscope and scanning electron microscope-energy dispersive X-ray. Based on the results, the grain boundary grooving was observed in all samples except Bi-5Sb in all three reflows. Metallurgical and chemical reaction between interface and solders were found in Bi-5Sb solder alloys in different reflow numbers which lead to appearance of Cu3Sb intermetallic compound layer at the interface. Reflow numbers had a significant effect on the size of Cu-rich phase. Also it was observed that, with increasing reflow number Bi-Cu phase found in Bi-2.5Sb solder dissolves into the solder bulk.

Evaluation of mechanical properties of low-Ag ball grid array solder joints using a high-speed ball shear test

Solder joints of Sn–1.0Ag and Sn–1.0Ag–0.1Ce on electroless nickel/electroless palladium/immersion gold (ENEPIG) were studied and mechanically evaluated using a high-speed shear test. The effect of Pd thickness in ENEPIG on the reliability of solder joints was evaluated after multiple reflows. Microstructural observations of the solder joint interfaces and fractures after shear tests are shown. For comparison, conventional SAC305 Pb-free solder was also mechanically evaluated under the same conditions. By adding a thin Pd layer in ENEPIG surface finish, the intermetallic compound (IMC) layer’s morphology of SAC305 change from needle type (as Pd thickness=0μm) to scallop type (as Pd thickness=0.1μm). While the intermetallic compounds (IMCs) layer’s morphology of 1.0Ag solders change from facet type (as Pd thickness=0μm) to needle type (as Pd thickness=0.1μm). ENEPIG with thicker Pd layers was suitable for 1.0Ag solders during high speed shear test whereas conventional SAC305 showed the highest shear strength with an ENIG surface finish.

Optimization of the Ni(P) Thickness for an Ultrathin Ni(P)-Based Surface Finish in Soldering Applications

The effects of the Ni(P) thickness δNi(P) on the interfacial reaction between an Sn-3Ag-0.5Cu solder and an Au/Pd(P)/Ni(P)/Cu pad (thickness: 0.05/0.05/0.1–0.3/20 μm) and the resulting mechanical properties were investigated using scanning electron microscopy equipped with an electron backscatter diffraction system, a focused ion beam system, electron probe microanalysis, and high-speed ball shear (HSBS) testing. Regardless of δNi(P), all of the Au/Pd(P)/Ni(P) surface finishes examined were completely exhausted in one reflow, exposing the Cu pad underneath the solder. Cu6Sn5 dissolved with various Ni contents, termed (Cu,Ni)6Sn5, was the dominant intermetallic compound (IMC) species at the solder/Cu interface. Additionally, Ni2SnP and Ni3P IMCs might form with the (Cu,Ni)6Sn5 in the thick Ni(P) case, i.e., δNi(P) = 0.3 μm, and the two IMCs (Ni2SnP and Ni3P) were gradually eliminated from the interface after multiple reflows. A mass balance analysis indicated that the growth of the Ni-containing IMCs, rather than the dissolution of the metallization pad, played a key role in the Ni(P) exhaustion. The HSBS test results indicated that the mechanical strength of the solder joints was also δNi(P) dependent. The combined results of the interfacial reaction and the mechanical evaluation provided the optimal δNi(P) value for soldering applications.

Metallurgical challenges in microelectronic 3D IC packaging technology for future consumer electronic products

Metallurgical challenges in controlling the microstructural stability of Cu and solder microbumps in 3D IC packaging technology are discussed. Using uni-directional 〈111〉 oriented nanotwinned Cu, the controlled growth of oriented Cu66n5 on the nanotwinned Cu and its transformation to Cu3Sn without Kirkendall voids have been achieved. In order to join a stack of Si chips into a 3D device, multiple reflows of solder microbumps may be required; we consider localized heating to do so by the use of self-sustained explosive reaction in multi-layered Al/Ni thin films of nano thickness. It avoids re-melting of those solder joints which have been formed already in the 3D stacking structure.

Interfacial reactions between electroplated Ni–Zn alloy films and lead-free solders

Because of the miniaturization trend in electronic devices in recent years, the issue of reliability of solder joints in these miniaturized devices becomes very critical. Studies have shown that a thin layer of Ni is effective in reducing the interfacial IMC growth between Cu and Sn-based solder. This is because the reaction kinetics of Ni–Sn compounds are much slower than that of Cu–Sn IMCs. In this study, zinc is incorporated into the nickel barrier film in the form of Ni–Zn alloy by electrodeposition. The effects of the presence of Zn on the interfacial reactions between nickel barrier film and Sn–3.8Ag–0.7Cu (SAC) and Sn–3.5Ag (SA) solders are investigated. Ni–Zn alloy films with 1.73 wt% Zn were prepared from ammoniacal diphosphate baths. Elemental composition of the alloy film was determined by energy dispersive X-ray spectroscopy while X-ray diffraction method was used to determine the phases present in the alloy film. Solders’ spreading rate was characterized with the use of optical microscope. Reflows were done for 1 and 12 cycles to investigate the effect of multiple reflows on the IMC growth and morphology. Results have shown that the IMC formed at the interface of SA/Ni and SAC/Ni was Ni3Sn4 and (Cu,Ni)5Sn6, respectively. (Ni,Cu)3Sn4 IMC was formed at the interface of SA/Ni–Zn alloy film. No spalling was detected at the SA/Ni–Zn solder joint. On the other hand, it has been observed that (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5 layer with continuous non-uniform morphology were formed on the SAC/Ni–Zn alloy film after 1× reflow. As the number of reflow increased, (Cu,Ni)6Sn5 layer spalled from the interface leaving only (Ni,Cu)3Sn4 IMC at the interfacial region.

Interfacial reaction between Sn–Ag–Cu solder and Co–P films with various microstructures

We electroplated Co–P films with nanocrystalline, amorphous and nanocrystalline/amorphous mixed structures and used them as under-bump metallization (UBM) joined with Sn–Ag–Cu lead-free solder. We systematically investigated the interfacial reaction between the Sn–Ag–Cu solder and the Co–P UBM and analyzed the growth mechanisms of the intermetallic compounds formed at the interfaces of the Sn–Ag–Cu/Co–P joints through multiple reflows. Among the three kinds of Co–P films, the film with a mixed structure shows the best diffusion-barrier properties and is a good candidate for the UBM joined with the Sn–Ag–Cu solder. For the nanocrystalline Co–P UBM, Co diffuses quickly toward the solder and Sn does not diffuse into the UBM, whereas for the amorphous Co–P film not only does Co diffuse into the solder, but also Sn diffuses into the Co–P film with a large diffusion rate. In addition, the first-principles calculation shows that the exchange coupling between Co(3d74s2) and Sn(5s25p2) electrons and between Sn(5s25p2) and P(3s23p3) electrons results in the formation of CoSn and SnP3, which originate from the diffusion and reaction of Co and Sn atoms, respectively; this is consistent with the experimental data of transmission electron microscopy characterization.