Sn-Ag-Cu Research Articles

접합 소재에 따른 고출력 플립칩 LED 패키지 특성 연구

고출력 LED 패키지의 열적 경로(thermal path)를 줄이기 위해 플립칩 본딩법에 대한 연구가 활발히 진행되고 있다. 본 연구에서는 Au-Sn 열압착 본딩 및 Sn-Ag-Cu(SAC) 리플로우 본딩을 이용하여 본딩 특성 및 열적특성을 비교 평가 하였다. Au-Sn 열압착 본딩은 50 N에서 <TEX>$300^{\circ}C$</TEX>의 접합온도로 본딩하였고, SAC 솔더는 솔더페이스트를 인쇄한 후 리플로우법으로 피크온도 <TEX>$255^{\circ}C$</TEX>에서 30 sec에서 본딩하였다. SAC 솔더를 사용한 LED 패키지의 전단강도는 <TEX>$5798.5gf/mm^2$</TEX>로 Au-Sn 열압착 본딩의 <TEX>$3508.5gf/mm^2$</TEX>에 비해 1.6배 높았다. 파단면과 단면분석 결과 Au-Sn, SAC 솔더 모두 LED 칩 내부에서 파단이 일어나는 것을 관찰하였다. 반면 Au-Sn 열압착 본딩 샘플의 열저항은 SAC솔더 접합 샘플에 비해 낮았으며, SAC 솔더 접합부 내부의 기공에 의해 열저항이 커짐을 알 수 있었다. Flip chip bonded LED packages possess lower thermal resistance than wire bonded LED packages because of short thermal path. In this study, thermal and bonding properties of flip chip bonded high brightness LED were evaluated for Au-Sn thermo-compression bonded LEDs and Sn-Ag-Cu reflow bonded LEDs. For the Au-Sn thermo-compression bonding, bonding pressure and bonding temperature were 50 N and 300oC, respectively. For the SAC solder reflow bonding, peak temperature was <TEX>$255^{\circ}C$</TEX> for 30 sec. The shear strength of the Au-Sn thermo-compression joint was <TEX>$3508.5gf/mm^2$</TEX> and that of the SAC reflow joint was 5798.5 gf/mm. After the shear test, the fracture occurred at the isolation layer in the LED chip for both Au-Sn and SAC joints. Thermal resistance of Au-Sn sample was lower than that of SAC bonded sample due to the void formation in the SAC solder.

Isothermal Ageing of SnAgCu Solder Alloys: Three-Dimensional Morphometry Analysis of Microstructural Evolution and Its Effects on Mechanical Response

Due to the high homologous temperature and fast cooling rates, the microstructures of SnAgCu (SAC) solders are in a meta-stable state in most applications, which is the cause of significant microstructural evolution and continuous variation in the mechanical behavior of the joints during service. The link between microstructures evolution and deformation behavior of Sn-4.0Ag-0.5Cu solder during isothermal ageing is investigated. The evolution of the microstructures in SAC solders are visualized at different scales in 3D by using a combination of synchrotron x-ray and focused ion beam/scanning electron microscopy tomography techniques at different states of ageing. The results show that, although the grain structure, morphology of dendrites, and overall volume fraction of intermetallics remain almost constant during ageing, considerable coarsening occurs in the Ag3Sn and Cu6Sn5 phases to lower the interfacial energy. The change in the morphometrics of sub-micron intermetallics is quantified by 3D statistical analyses and the kinetic of coarsening is discussed. The mechanical behavior of SAC solders is experimentally measured and shows a continuous reduction in the yield resistance of solder during ageing. For comparison, the mechanical properties and grain structure of β-tin are evaluated at different annealing conditions. Finally, the strengthening effect due to the intermetallics at different ageing states is evaluated by comparing the deformation behaviors of SAC solder and β-tin with similar grain size and composition. The relationship between the morphology and the strengthening effect due to intermetallics particles is discussed and the causes for the strength degradation in SAC solder during ageing are identified.

Interfacial intermetallic growth and mechanical properties of carbon nanotubes reinforced Sn3.5Ag0.5Cu solder joint under current stressing

In this work, multi-walled carbon nanotubes (MWCNTs) with various weight percentages of 0.01wt%, 0.05wt%, 0.1wt% were incorporated into Sn3.5Ag0.5Cu (SAC) solder matrix by mechanical blending followed by reflow process. The microstructure with interfacial intermetallics (IMCs) growth were investigated after aging at 100°C and a current density of 1.2×104A/cm2 for 336h. CNTs were found to be homogeneously dispersed into the solder matrix by EDX analysis. Experimental results revealed that microporosities existed in some areas of doped solders, which retarded the formation of Ag3Sn IMCs. The growth of IMCs for CNTs reinforced solders was less significantly than that of the plain solder, because CNTs affected the reaction mechanisms of both Cu5Sn6 and Cu3Sn at the anode side and alleviated IMCs growth and solder dissolution into Cu substrate at the cathode side. A slight decrease in melting temperature for CNT reinforced solders was found compared to that of the plain solder. The CNT doped solders demonstrated promising improved shear strength and hardness.

Shear strengths of CBGA/PBGA solder ball joints with lead-free solder pastes

The present study is carried out to characterise the ball shear strengths of ceramic ball grid array (CBGA) and plastic ball grid array (PBGA) packages with lead-free solder pastes. Three commercially available Sn/Ag/Cu (SAC) solder pastes such as Sn 96.5/Ag3.0/Cu0.5 and 95.5/Ag3.8/Cu0.7 with a no-clean flux are employed. The effects of temperature profiles during reflow on the strength of solder joints are studied. Their printing abilities are also observed. Of particular concern for ceramic ball grid array packages is the interaction between 90/10 Pb/Sn compound solder balls and SAC solder pastes. The composition of the solder paste region between the I/O pad and the solder ball is analysed using the energy dispersive X-ray (EDX) machine.

J0410405 室温で得たSACはんだのクリープひずみと疲労寿命の関係の高温域への適用性([J041-04]工業材料の変形特性・強度およびそのモデル化 損傷・疲労・破壊,機械材料・材料加工部門)

The fatigue life of solder alloys for the slow-fast cyclic loading condition is much shorter than that for the fast-slow cyclic loading condition, despite the two loading conditions having almost equal inelastic strain amplitudes. This suggests that the inelastic strain amplitude is not the best parameter used for evaluating the fatigue life which shows the strain rate dependency. In the previous study, we conducted the fatigue tests using the Sn-3.0Ag-0.5Cu (SAC) solder at RT. The tests employed the cyclic tension-compression loading using the stepped ramp wave (SW) to analyze the stress-creep strain relations during the tests. The analysis result showed that the creep strain amplitude obtained from the stress-creep strain relation had a higher correlation with the fatigue life than the inelastic strain amplitude. In addition, the equation indicating the relationship between the creep strain amplitude and the fatigue life at RT was likely to be useful to predict the fatigue lives at the other temperature. In this study, we investigated the relationships between the creep strain amplitude and the fatigue life for the SAC solder at 343 K and 393 K by conducting the fatigue tests with cyclic tension-compression loading using the SW. Then we attempted to describe the investigated relations at these two temperatures by the above-mentioned equation. As a result, the equation almost expressed the relationships between the creep strain and the fatigue life at 343 K and 393K even though it had been derived from the test results at RT.

Optimization of Lead Free Plating for Flip Chip Applications

Traditional flip chip processes have consolidated to a SnAgCu (SAC) solder system. Each company based upon their own needs and application space has come to their own method to achieve the desired final composition of the interconnect. These have included different solder compositions for both the pre-solder and the C4. The various interconnect solutions can range from no solder on one side such as pure Cu or Ni to an interconnect that has identical solder composition for both the substrate and the C4. Decisions for the optimized solution include the need for reliability, cost and yield. Picking the right solution also enables the elimination of defects such as solder voids, interfacial voids, white bumps, micro-solder bumps and non-wets. The optimized solutions are dependent upon many factors that include the fragility of the silicon dielectric, the size of the die, type of flux used at assembly, the assembly process used, method by which SnAg is plated such as various layering techniques, final processes steps in C4, test probe concepts, DSP methods and many more. In order to pick the appropriate scheme for each product and for each industry, it is imperative to know the interaction of all of these factors. This paper provides concepts and data about how to optimize assembly and lead free plating for a particular process. In the plating process, this includes the importance of various layering steps and analysis of incoming chemicals, especially the acids, and in the assembly process, the knowledge and matching of solder hierarchy, warpage, flux characteristics and preparation / cleaning steps prior to underfill. In particular, we will provide the data and the scheme by which it is possible to produce void free solder processes without bleed and feed on SnAg baths that are over 100 amp-hr per liter and over 1 year old.

Impact of Ni concentration on the intermetallic compound formation and brittle fracture strength of Sn–Cu–Ni (SCN) lead-free solder joints

Cu6Sn5 and Cu3Sn are common intermetallic compounds (IMCs) found in Sn–Ag–Cu (SAC) lead-free solder joints with OSP pad finish. People typically attributed the brittle failure to excessive growth of IMCs at the interface between the solder joint and the copper pad. However, the respective role of Cu6Sn5 and Cu3Sn played in the interfacial fracture still remains unclear. In the present study, various amounts of Ni were doped in the Sn–Cu based solder. The different effects of Ni concentration on the growth rate of (Cu, Ni)6Sn5/Cu6Sn5 and Cu3Sn were characterized and compared. The results of characterization were used to evaluate different growth rates of (Cu, Ni)6Sn5 and Cu3Sn under thermal aging. The thicknesses of (Cu, Ni)6Sn5/Cu6Sn5 and Cu3Sn after different thermal aging periods were measured. High speed ball pull/shear tests were also performed. The correlation between interfacial fracture strength and IMC layer thicknesses was established.

Recrystallization and ${\rm Ag}_{3}{\rm Sn}$ Particle Redistribution During Thermomechanical Treatment of Bulk Sn–Ag–Cu Solder Alloys

Sn-Ag-Cu (SAC) solders are susceptible to appreciable microstructural coarsening during storage or service. This results in evolution of joint properties over time and thereby influences the long-term reliability of microelectronic packages. Accurate reliability prediction of SAC solders requires prediction of microstructural evolution during service. Microstructure evolution in two SAC solder alloys, such as, Sn-3.0Ag-0.5Cu (SAC 305) and Sn-1.0Ag-0.5 Cu (SAC 105), under different thermomechanical excursions, including isothermal aging at 150°C and thermomechanical cycling (TMC) was studied. In general, between 200 and 600 cycles during TMC, recrystallization of the Sn matrix was observed, along with redistribution of Ag3Sn particles because of dissolution and reprecipitation. These latter effects have not been reported before. It was also observed that the Sn grains recrystallized near precipitate clusters in eutectic channels during extended isothermal aging. The relative orientation of Sn grains in proeutectic colonies did not change during isothermal aging.

The Influence of Sn Orientation on Intermetallic Compound Evolution in Idealized Sn-Ag-Cu 305 Interconnects: an Electron Backscatter Diffraction Study of Electromigration

Previous research showed the relationship between Sn grain orientation and the intermetallic growth rate in Sn-Ag-Cu (SAC)305 interconnects. Samples with the Sn c-axis aligned parallel to the current flow have an intermetallic compound growth rate significantly faster than samples with the c-axis perpendicular to the current flow. This study continues the previous research by investigating intermetallic growth in polygranular joints and in joints that have a thin Ni layer at the cathodic or anodic interface of the interconnect. Planar SAC305 interconnects were sandwiched between two Cu pads (sometimes incorporating a thin Ni layer at the interface) and subjected to uniaxial current. The crystallographic orientation of Sn in these samples was characterized with electron backscatter diffraction before and after electromigration testing. The results show that polycrystalline joints have relatively slow intermetallic growth rates, close to those found in single-crystal joints with the c-axis perpendicular to the current. When a Ni layer was present on the anode side, the intermetallic grew at a rate comparable to that in samples without a Ni layer. However, when the Ni layer was on the cathode side, the intermetallic growth was significantly retarded. The measured growth rates of the intermetallic, combined with literature values for the diffusion of Cu in Sn, were used to calculate values for the effective charge, z*, which is significantly smaller for samples with current parallel to the c-axis than for either polycrystalline samples or samples with the c-axis perpendicular to the electron flow.

Effect of Silver Content on Bulk Properties and Interfacial Morphology of Sn-xAg-Cu Solders

The bulk microstructure, melting behavior, mechanical property, and interfacial Intermetallic compound (IMC) morphology were investigated on Sn-Ag-Cu (SAC) lead free solders with different Ag content (0.3,1.0,2.0,3.0,3.8wt%). The result indicates that SAC solders with higher Ag content present finer and denser intermetallic particles in the bulk solder, as a result that the tensile strength of SAC solders increased with the increasing of Ag content, while the ductility decreased. The melting temperature of SAC305 and SAC387 solders are close to eutectic point from the narrow melting range. It was found that the interfacial IMC morphology didnt appear obvious difference regardless of Ag content for as-soldered. Furthermore, the higher Ag contained solders present smaller IMC grain at the interface of aged joints and all aged joints have a tendency of polyhedron morphology.

Mechanical properties of FeCo magnetic particles-based Sn-Ag-Cu solder composites

We demonstrate magnetic nanoparticles (MNPs) in enabling lead-free solder reflow in RF fields and improved mechanical properties that impact solder joint reliability. Here, we report on Sn-Ag-Cu (SAC) alloys. SAC solder-FeCo MNP composites with 0, 1, 2, 3, and 4 wt. % FeCo MNP and the use of AC magnetic fields to achieve localized reflow. Electron microscopy of the as-reflowed samples show a decrease in the volume of Sn dendrite regions as well as smaller and more homogeneously dispersed Ag3Sn intermetallic compounds (IMCs) with increasing MNP concentrations. Mechanical properties of the composites were measured by nanoindentation. In pure solder samples and solder composites with 4 wt. % MNP, hardness values increased from 0.18 GPa to 0.20 GPa and the modulus increased from 39.22 GPa to 71.22 GPa. The stress exponent, reflecting creep resistance, increased from 12.85 of pure solder to 16.47 for solder composites with 4 wt. % MNP. Enhanced mechanical properties as compared with the as-prepared solder joints are explained in terms of grain boundary and dispersion strengthening resulting from the microstructural refinement.

The ‘IMC rings’ growth mechanism in Sn–Ag–Cu solder ball and NiAu-plated BGA ball pad

Due to environmental pollution concerns, the law says the lead (Pb) inside electronics devices must be eliminated. Lots of lead-free materials have been introduced and been used for electronic products and Sn–Ag–Cu (SAC) is one of most popular lead-free representatives and has been used in high-volume production. The most popular IC packages, BGA packages which have higher I/O counts, and better thermal and electrical performance than lead-frame type packages, use solder balls of SAC for lead-free applications to connect with printed circuit boards. A particular phenomenon, so-called ‘IMC rings’, is only observed on BGA solder ball pad surfaces after the SAC solder balls are mounted on BGA ball pads which are plated with NiAu. It has not been found in either eutectic solder or Sn–Ag solder welding on plated NiAu pads. No significant evidence exists to show that ‘IMC rings’ degrade the strength of solder joints or cause earlier failures in mechanical tests. ‘IMC rings’ appear to be an inevitable outcome after the SAC is soldered onto a plated NiAu ball pad. This study is to find the growth mechanism of ‘IMC rings’ on the ball pad which is created between SAC ball and plated NiAu pad during solder ball temperature reflow. The design of the experiment and data have been discussed.

Role of reactant concentration in size control of SnAgCu nanoparticles

Attributing to the melting temperature depressing resulted from the size effect of nanoparticles, the SnAgCu alloy system can be a promising candidate to replace the traditional toxic SnPb solder in the field of electronic packaging. Chemical reduction method was used to fabricate the Sn3.0Ag0.5Cu (SAC) (mass fraction, %) alloy nanoparticles. Sodium borohydride and 1,10-phenanthroline were chosen as the reducing agent and surfactant respectively. In addition, the morphology of the synthesized nanoparticles was investigated by field emission scanning electron microscopy (FE-SEM), and the size distribution of the as-prepared particles was obtained from the image analysis. It was found that the particle size increased with increasing the reactant concentration. Finally, theoretical analysis was employed to illustrate the influence of reactant concentration on the particle size.

Effect of ZnO nanoparticles addition on thermal, microstructure and tensile properties of Sn–3.5 Ag–0.5 Cu (SAC355) solder alloy

Regarding to the development of Sn–Ag–Cu (SAC) lead-free solders for advance electronic components, the effect of 0.5 wt% nano-sized ZnO particles on the thermal, microstructure and tensile properties of Sn–3.5 wt% Ag–0.5 wt% Cu (SAC355) lead-free solder alloy is investigated. The results showed that addition of 0.5 wt% nano-sized ZnO particles into the conventional lead-free SAC355 solder caused a slight increase of its liquidus temperature by about 1.1 K. Metallographic observations of SAC355–0.5 wt% ZnO (composite solder) revealed an obvious refinement in the microstructure compared with the SAC355 (non-composite) solder. Consequently, addition of nano sized-ZnO particles could improve the stress–strain characteristics proof stress (σy0.2) and ultimate strength (σUTS). This was rendered to suppressing effect of ZnO on the coarsening of the intemetallic compounds (IMCs) Ag3Sn and Cu6Sn5 during the solidification process in the composite solder and subsequently dispersion strengthening is considered to be the dominating mechanism. This will allow the use of SAC355 composite lead-free solder alloy, to be consistent with the conditions of usage for conventional SAC solder alloys and to overcome the serious problem of the excessive growth of IMCs and the formation of microvoids in the SAC lead-free solder alloys.

Effects of reflow on the interfacial characteristics between Zn nanoparticles containing Sn‐3.8Ag‐0.7Cu solder and copper substrate

PurposeThe purpose of this paper is to investigate the effects of zinc (Zn) nanoparticles on the interfacial intermetallic compounds (IMCs) between Sn‐3.8Ag‐0.7Cu (SAC) solder and Cu substrate during multiple reflow.Design/methodology/approachThe nanocomposite solders were prepared by manually mixing of SAC solder paste with varying amounts of Zn nanoparticles. The solder pastes were reflowed on a hotplate at 250°C for 45 s for up to six times. The actual Zn content after reflow was analyzed by inductively coupled plasma‐optical emission spectroscopy (ICP‐OES). The wetting behavior of the solders was characterized by analyzing the contact angles and spreading rates according to the Japanese Industrial Standard (JIS 23198‐3, 2003). The interfacial microstructure of the solder joints were investigated by field emission scanning electron microscope (FESEM) and energy dispersive X‐ray spectroscopy (EDAX).FindingsIt was found that upon the addition of 0.3 wt% Zn nanoparticles to the SAC solder, the growth of interfacial intermetallic compound (IMC) layers was retarded to a minimum value. Excessive amount of Zn nanoparticles (0.8 wt%) induced an additional IMC layer (Cu5Zn8) which increased the total IMC thickness and raising the reliability issue.Originality/valueIt is concluded that Zn nanoparticles undergo melting/reaction during reflow and impart their effect on the IMCs through alloying effects.

Effects of nano-Al2O3 particles on microstructure and mechanical properties of Sn3.5Ag0.5Cu composite solder ball grid array joints on Sn/Cu pads

A Sn3.5Ag0.5Cu (SAC)–XAl2O3 nano-composite solder was prepared by adding 100nm Al2O3 to SAC (wt.%) solder. The interfacial microstructures and mechanical properties of SAC–XAl2O3 nano-composite solder balls on immersion Sn surface finished BGA joints after multiple reflows was investigated. As a whole, adding Al2O3 nanoparticles to SAC solders significantly changed in the interfacial microstructure, and both scallop-type and prism-type modes were observed in the plain SAC solder and SAC–XAl2O3 nano-composite solder after reflowing, respectively. The nanoparticles suppressed the growth of the Cu6Sn5 layer, significantly improving the shear strength. The fracture surfaces of the plain SAC solder showed a semi-brittle fracture mode, but those of the SAC–XAl2O3 nano-composite solder exhibited typical ductile failures.

The Role of Pb Impurity in SAC Solder Alloy

The European Union and Japan initiated the issue of RoHS, the directive about the restriction of hazardous substances, which prohibits certain hazardous substances in electronic equipment - including lead - application. Due to the directive the use of lead free solder alloys is spreaded, however the Pb in the form of contamination may be appear under technological process. The lead impurity has significant effect on microstrucutre and lifetime so it is necessary to carry out detailed examinations. In this paper the study of intermetallic compounds in six-element, Pb impured, thermal cycles test-subjected, Sn-Ag-Cu (SAC) solder alloy is demonstrated

Effects of FeCo magnetic nanoparticles on microstructure of Sn-Ag-Cu alloys

Sn-Ag-Cu (SAC) alloys have been regarded as the most promising candidates for lead-free solders in the electronic packaging industry. We prepared SAC solder-FeCo magnetic nanoparticles (MNPs) composite paste with different MNP concentration and used AC magnetic fields localized heating to cause their reflow. Differential scanning calorimetry results show a reduced undercooling of the composite paste with the addition of MNPs. Transmission electron microscope prove that the FeCo MNPs are distributed in Sn matrix of the reflowed solder composites. Optical micrographs show a decrease in the amount of primary Ag3Sn and β-Sn dendrites, and an increase in the amount of eutectic microconstituents with increasing MNPs. The addition of FeCo MNPs is considered to promote the solidification of β-Sn by providing more heterogeneous nucleation sites at a relatively low undercooling, which results in the microstructural refinement in the as-prepared solder joints.

The Effect of Sn Orientation on Intermetallic Compound Growth in Idealized Sn-Cu-Ag Interconnects

The work reported here explores the influence of crystal orientation on the growth of the interfacial intermetallic layer during electromigration in Cu||Sn||Cu solder joints. The samples were thin, planar Sn-Ag-Cu (SAC) solder layers between Cu bars subject to a uniaxial current. Electron backscatter diffraction (EBSD) was used to characterize the microstructure before and after testing. The most useful representation of the EBSD data identifies the Sn grain orientation by the angle between the Sn c-axis and the current direction. The tested samples included single-crystal joints with c-axis nearly parallel to the current (“green” samples) and with c-axis perpendicular to the current (“red” samples). At current density of 104 A/cm2 (steady-state temperature of ~150°C), an intermetallic layer grew at an observable rate in the “green” samples, but not in the “red” ones. A current density of 1.15 × 104 A/cm2 (temperature ~160°C) led to measurable intermetallic growth in both samples. The growth fronts were nearly planar and the growth rates constant (after an initial incubation period); the growth rates in the “green” samples were about 10× those in the “red” samples. The Cu concentrations were constant within the joints, showing that the intermetallic growth is dominated by the electromigration flux. The measured growth rates and literature values for the diffusion of Cu in Sn were used to extract values for the effective charge, z*, that governs the electromigration of Cu. The calculated value of z* is significantly larger for current perpendicular to the c-axis than along it.

Manufacturing and Microstructure of Cu Coated Diamonds/SnAgCu Composite Solder Bumps

Diamonds/SnAgCu composite solder bumps were prepared on Cu pad by mechanically incorporating diamond particles into Sn3.0Ag0.5Cu (SAC) solder paste and then reflowed at 260°C for 60s. Chemical copper plating method was proved to be an efficient way to achieve high addition amount of diamond particles (&gt; 1 wt.%, which is much better than former studies) in solder, and to get solder bumps with better morphology. The spreading rate of this novel solder was turned out to be acceptable in operating range. Shear strength and microstructure of composite solder bumps were studied, showing that they could be influenced by the changes in morphology of the Ag3Sn net work caused by the various additions of Cu coated diamond particles.