Investigation of Sn Whisker Growth in Electroplated Sn and Sn-Ag as a Function of Plating Variables and Storage Conditions

Sn whiskers are becoming a serious reliability issue in Pb-free electronic packaging applications. Among the numerous Sn whisker mitigation strategies, minor alloying additions to Sn have been proven effective. In this study, several commercial Sn and Sn-Ag baths of low-whisker formulations are evaluated to develop optimum mitigation strategies for electroplated Sn and Sn-Ag. The effects of plating variables and storage conditions, including plating thickness and current density, on Sn whisker growth are investigated for matte Sn, matte Sn-Ag, and bright Sn-Ag electroplated on a Si substrate. Two different storage conditions are applied: an ambient condition (30°C, dry air) and a high-temperature/high-humidity condition (55°C, 85% relative humidity). Scanning electron microscopy is employed to record the Sn whisker growth history of each sample up to 4000 h. Transmission electron microscopy, x-ray diffraction, and focused ion beam techniques are used to understand the microstructure, the formation of intermetallic compounds (IMCs), oxidation, the Sn whisker growth mechanism, and other features. In this study, it is found that whiskers are observed only under ambient conditions for both thin and thick samples regardless of the current density variations for matte Sn. However, whiskers are not observed on Sn-Ag-plated surfaces due to the equiaxed grains and fine Ag₃Sn IMCs located at grain boundaries. In addition, Sn whiskers can be suppressed under the high-temperature/high-humidity conditions due to the random growth of IMCs and the formation of thick oxide layers.     

A Statistical Study of Sn Whisker Population and Growth During Elevated Temperature and Humidity Tests

Storage tests at elevated temperature and humidity conditions have been widely adopted as one of the major acceleration tests for Sn whisker growth. However, the driving force associated and the nucleation and growth process of whiskers are yet to be fully understood. In this paper, Sn whisker growth on Cu leadframe material at two different test conditions is compared. Both loose and board-mounted components were used. At each read point, the length and location of every whisker observed was recorded. Statistical characteristics and growth rate of the whisker population will be presented for each of the tests conditions. On loose components, corrosion of the Sn finish was observed near the tip and the dam bar cut area of the leads with backscatter scanning electron microscopy (SEM) and optical microscopy. The entire population of whiskers was located in these corroded areas, and there were zero whiskers located in the noncorroded areas on the same leads. On board-mounted components, the corrosion level of the Sn finish, as well as the whisker population and length was greatly reduced compared to those on the loose components. These results suggest that the corrosion of Sn finish in high-temperature and high-humidity conditions is the major driving force for whisker growth. The cause for the difference between the loose and board-mounted components is also analyzed     

Whisker growth on surface treatment in the pure tin plating

Whisker behavior at various surface treatment conditions of pure Sn plating are presented. The temperature cycling test for 600 cycles and the ambient storage for 1 year was performed, respectively. From the temperature cycling test, bent-shaped whiskers were observed on matte and semibright Sn plating, and flower-shaped whisker on bright Sn plating. The bright Sn plating has smaller whiskers than the other types of Sn plating, and the whisker growth density per unit area is also lower than the others. After 1 year under ambient storage, nodule growth of FeNi42 lead frame (LF) was observed in some parts. The Cu LF showed about a 9.0 μm hillock-shaped whisker. This result demonstrated that the main determinant of whisker growth was the number of temperature cycling (TC) in the FeNi42 LF, whereas it was the time and temperature in the Cu LF. Also, whisker growth and shape varied with the type of surface treatment and grain size of plating.     

Effect of Pulse-Plated Nickel Barriers on Tin Whisker Growth for Pure Tin Solder Joints

We investigate the influence of pulse-plated Ni barriers, compared to direct current (DC)-plated Ni barriers, on the growth of Sn whiskers in laminated Cu/Ni/Sn samples. The results indicate that the pulse-plated Ni barriers exhibit much better resistance to Sn whisker growth than the DC-plated Ni barriers, i.e., when exposed to ambient of 60°C and 93% relative humidity (RH) for 40 days only a few small hillocks were observed as opposed to the long whiskers and large nodules of Sn for the DC-plated Ni barriers. The underlying mechanisms are addressed based on the texture characteristics of the plated Ni and Sn layers and the formation of intermetallic compounds.     

Whisker mitigation measures for Sn-plated Cu for different stress tests

The effectiveness of the widely-used whisker mitigation measures for Sn-plated Cu base material (annealing at 150 °C for 1 h or a Ni interlayer) were investigated after temperature cycling and after storage at room temperature. It was found that these measures prevent whisker growth during isothermal storage, but not during temperature cycling. These mitigation measures do apparently not reduce the compressive stress that builds up during temperature cycling due to different coefficients of thermal expansion of Sn and Cu. A change of the Sn microstructure to globular grains is proposed and investigated as potential whisker mitigation measure for temperature cycling.     

Mitigation of Sn Whisker Growth by Small Bi Additions

In this study, the morphological development of electroplated matte Sn and Sn-xBi (x = 0.5 wt.%, 1.0 wt.%, 2.0 wt.%) film surfaces was investigated under diverse testing conditions: 1-year room-temperature storage, high temperature and humidity (HTH), mechanical loading by indentation, and thermal cycling. These small Bi additions prevented Sn whisker formation; no whisker growth was observed on any Sn-xBi surface during either the room-temperature storage or HTH testing. In the indentation loading and thermal cycling tests, short (<5 μm) surface extrusions were occasionally observed, but only on x = 0.5 wt.% and 1.0 wt.% plated samples. In all test cases, Sn-2Bi plated samples exhibited excellent whisker mitigation, while pure Sn samples always generated many whiskers on the surface. We confirmed that the addition of Bi into Sn refined the grain size of the as-plated films and altered the columnar structure to form equiaxed grains. The storage conditions allowed the formation of intermetallic compounds between the plated layer and the substrate regardless of the Bi addition. However, the growth patterns became more uniform with increasing amounts of Bi. These microstructural improvements with Bi addition effectively released the internal stress from Sn plating, thus mitigating whisker formation on the surface under various environments.     

Microstructure-Based Stress Modeling of Tin Whisker Growth

A 3-D finite element method (FEM) model considering the elasticity anisotropy, thermal expansion anisotropy, and plasticity of beta-Sn is established. The Voronoi diagrams are used to generate the geometric patterns of grains of the Sn coating on Cu leadframes. The crystal orientations are assigned to the Sn grains in the model using the X-ray diffraction (XRD) measurement data of the samples. The model is applied to the Sn-plated package leads under thermal cycling tests. The strain energy density (SED) is calculated for each grain. It is observed that the samples with higher calculated SED are more likely to have longer Sn whiskers and higher whisker density. The FEM model, combined with the XRD measurement of the Sn finish, can be used as an effective indicator of the Sn whisker propensity. This may expedite the qualification process significantly     

Tin Whisker Test Method Development

The International Electronics Manufacturing Initiative (iNEMI) proposed a set of tests that were subsequently standardized by the Joint Electron Device Engineering Council (JEDEC) Solid State Technology Association as JESD22A121. However, further optimization was needed to validate and verify proposed iNEMI tests to determine whether these test methods could differentiate between component surface finishes, to investigate the connection between short-term and long-term tests, and to determine optimal inspection intervals and test durations. In the current work, 15 surface finishes were evaluated using iNEMI's three proposed environmental test conditions. These finishes were evaluated for whisker presence and length using a scanning electron microscope (SEM) every 1000 h or 500 accelerated thermal cycles for a total of at least 9000 h of isothermal testing or 3000 accelerated thermal cycles. Test results showed that thermal cycling and high-temperature/humidity testing could grow whiskers for all Pb-free Sn-based finishes, while only a few finishes exhibited whiskers after 10 000 h in an air-conditioned office environment. The results described in this paper were used to change the JEDEC tin whisker test method from the original iNEMI proposal by decreasing the humidity of the high-temperature humidity test, controlling the ambient test condition to 30 degC and 60% relative humidity (RH), and increasing the number of leads per sample and number of samples (sample size) for inspection     

The effect of electroplating parameters and substrate material on tin whisker formation

Electroplated tin finishes are widely used in the electronics industry due to their excellent solderability, electrical conductivity and corrosion resistance. However, the spontaneous growth of tin whiskers during service can result in localised electrical shorting or other harmful effects. Until recently, the growth of tin whiskers was successfully mitigated by alloying the tin with lead. However, restriction in the use of lead in electronics as a result of EU legislation (RoHS) has led to renewed interest in finding a successful alternative mitigation strategy.Whisker formation has been investigated for a bright tin electrodeposit to determine whether whisker growth can, at least partially, be mitigated by control of electroplating parameters such as deposition current density and deposit thickness. The influence of substrate material and storage at 55 °C/85% humidity on whisker growth have also been investigated.Whisker growth studies indicate that deposition parameters have a significant effect on both whisker density and whisker morphology. As deposition current density is increased there is a reduction in whisker density and a transition towards the formation of large eruptions rather than potentially more harmful filament whiskers. Increasing the tin coating thickness also results in a reduction in whisker density. Results demonstrate that whisker growth is most prolific from tin deposits on brass, whilst that from tin deposits on rolled silver is greater than that observed for tin deposits on copper.     

外部应力型晶须抑制方法的研究

当电子工业中完全实现无铅化时,晶须问题成为新的悬念。关于Sn晶须的成长机理和抑制方法的研究已经遍及全世界。晶须发生和成长被认为是Sn镀层上的压缩应力引起的。晶须分为内部和外部应力型。文章通过压缩负荷试验研究了外部应力型晶须的抑制方法。这种方法是Sn表面镀层和Cu上的Ni基底镀层之间介入薄Au镀层。结果发现镀Sn以后不久就形成了金属间化合物AuSn4,Au镀层有效地减轻了Sn晶须的数量,缩短了Sn晶须的长度。     

Mitigative Tin Whisker Growth Under Mechanically Applied Tensile Stress

Sn whisker/hillock growth is a result of the release of compressive stress in a Sn thin film. Filamentary Sn whiskers were formed on an electrodeposited Sn thin film aged at room temperature, while Sn hillocks were formed as the aging temperature was raised to 80°C and 150°C. By mechanically applying a tensile stress on the Sn thin film, the growth of the Sn whisker/hillock was significantly mitigated. This mitigation growth suggests that part of the compressive stress in the Sn thin film was neutralized by the mechanically applied tensile stress.     

Investigation of relation between intermetallic and tin whisker growths under ambient condition

The relation between the whisker growth and intermetallic on various lead-free finish materials that have been stored at ambient condition for 2 yrs (6.3 × 10⁷ s) is investigated. The matte Sn plated leadframe (LF) had the needle-shaped whisker and the nodule-shaped whisker was observed on the semi-bright Sn plated LF. Both the Sn plated LFs had a same columnar grain structure and both whiskers were grown in connection with the scalloped intermetallic compound (IMC) layer. The morphology of the IMC layer is similar, regardless of the area which has whisker or not. On the Sn–Bi finish and bright Sn plated LF, hillock-shaped and sparsely grown branch-shaped whiskers were observed, respectively. The IMC grew irregularly under both the areas with or without whisker. The IMC growth along the Sn grain boundaries generated inner compressive stress at the plating layer. Atomic force microscopy (AFM) profiling analysis is useful for characterization the IMC growth on the Sn and Cu interface. The measured root mean square (RMS) values IMC roughness on semi-bright Sn, matte Sn, and bright Sn plated LF were 1.82 μm, 1.46 μm, and 0.63 μm, respectively. However, there is no direct relation between whisker growth and the RMS value. Two layers of η′-Cu₆Sn₅ were observed using field emission transmission electron microscopy (FE-TEM): fine grains and coarse grains existed over the fine grains.     

无铅Sn-Cu电镀层锡须的形成与长大

通过扫描电镜观察,研究不同处理工艺中电子元件引线框架表面Sn-Cu电镀层上锡须的形成与长大,结果显示,高温高湿处理易于促使锡须的形成与长大,经过一定的时间后,锡须生长速度减缓;循环热处理或室温处理对锡须的形成影响较小;当施加恒定外应力后进行室温处理,锡须的形成完全受到抑制.锡须的形成与长大是由于电镀层中存在压应力,压应力促使镀层中锡发生再结晶并长大成锡须.     

挠性电路板引脚嵌合部无铅镀层的锡须生长

以民用挠性印制电路板(FPC)引脚和连接器嵌合部无铅镀层为对象,通过研究引脚上的Ni/Sn无铅镀层的显微形貌和锡须尺寸,探讨了Ni/Sn无铅镀层的长期可靠性。结果表明,在25℃,RH为45%～55%的条件下,挠性印制电路板引脚和连接器嵌合部无铅镀层上生长的锡须呈现针状、柱状等多种不同的显微形貌,其中大部分是针状锡须,少量针状锡须的长度已超过了50μm临界值,很可能因锡须桥接引起电流泄漏和短路,对FPC互连可靠性产生威胁。抑制少量超长的针状晶须的生长,是防止风险的关键。     

Internal Microstructure Investigation of Tin Whisker Growth Using FIB Technology

The problem of tin (Sn) whiskers has been a significant reliability issue in electronics for the past several decades. Despite the large amount of research conducted on this issue, a solution for mitigating the growth of whiskers remains a challenge for the research community. Whiskers have unpredictable growth and morphology, and a study of a whisker??s internal structure may provide further insights into the reason behind their complex growth. This study reports on the internal microstructure and morphology of complex-shaped Sn whiskers grown from an electroplated bright Sn layer on brass substrates exposed to ambient and 95% humid environment. The variables analyzed include surface and microstructure conditions of the film, and morphology and internal microstructure of the Sn whiskers using scanning electron microscopy with focused ion beam technology. Experimental results demonstrated that the whiskers with more complex morphology grow primarily from surfaces exposed to a controlled environment, and some of them have traits of polycrystalline growth rather than only single crystalline, as usually known.     

Tin whisker formation of lead-free plated leadframes

This paper presents the evaluation results of whiskers on two kinds of lead-free finish materials at the plating temperature and under the reliability test. The rising plating temperature caused increasing the size of plating grain and shorting the growth of whisker. The whisker was grown under the temperature cycling the bent shaped in matte pure Sn finish and hillock shape in matte Sn–Bi. The whisker growth in Sn–Bi finish was shorter than that in Sn finish. In FeNi42 leadframe, the 8.0–10.0 μm diameter and the 25.0–45.0 μm long whisker was grown under 300 cycles. In the 300 cycles of Cu leadframe, only the nodule-shaped grew on the surface, and in the 600 cycles, a 3.0–4.0 μm short whisker grew. After 600 cycles, the 0.25 μm thin Ni₃Sn₄ formed on the Sn-plated FeNi42. However, we observed the amount of 0.76–1.14 μm thick Cu₆Sn₅ and 0.27 μm thin Cu₃Sn intermetallics were observed between the Sn and Cu interfaces. Therefore, the main growth factor of a whisker is the intermetallic compound in the Cu leadframe, and the coefficient of thermal expansion mismatch in FeNi42.     

Humidity Effects on Sn Whisker Formation

Due to legislative issues, Pb-containing metallizations on semiconductor components are rapidly converted to Pb-free alternatives. One of the most popular alternatives is Sn electroplating. The major problem of these platings is the formation of Sn whiskers. In earlier publications, two mechanisms were uncovered that are responsible for whisker growth. However, these mechanisms do not explain whisker growth in high humidity. Therefore, Freescale, Infineon, Philips, and STMicrolectronics (E4) joined forces and started a design of experiment (DoE) in order to resolve this mechanism. It is shown that in high humidities, whiskers grow due to oxidation and corrosion of the Sn plating, irrespective of the base material. It is also shown that board assembly mitigates the whisker growth by this mechanism but does not completely prevent it     

A physical model and analysis for whisker growth caused by chemical intermetallic reaction

The formation of intermetallic compound Cu₆Sn₅ gives rise to the internal stress in the lead-free coating, which causes the growth of Sn whiskers. This phenomenon is characterized with the expansion of inclusion in a plate perfectly bonded between two infinite solids. Based on the grain boundary diffusion mechanism, a model is established to evaluate the growth rate of Sn whiskers. The results show that the growth rate of whisker varies with the relative site between whisker and inclusion. When the distance between the whisker and inclusion exceeds a critical value, negative growth rate will appear, and it approaches zero as the distance increases. They explain some phenomena observed in experiments.     

Optimum Thickness of Sn Film for Whisker Growth

By depositing different thicknesses of Sn films over a silicon wafer precoated with Cr and Ni adhesion layers and then by bending the tinned wafer using a dead load applied at the center to introduce the same compressive stresses in the Sn films, the growth rate of whiskers appeared to have a maximum for a certain thickness. This is explained by assuming the Sn atoms to flow along the vertical grain boundaries (perpendicular to the interface) into the interface between Sn and Ni and then along the interface to the root of the whisker through some more vertical grain boundaries. The resistance along the vertical grain boundaries appeared to control the rate of whisker growth for thick films.     

Tin Whisker Test Development—Temperature and Humidity Effects Part II: Acceleration Model Development

The incubation time for both whisker growth and corrosion in thin Sn platings (3–10 $mu{hbox {m}}$ thick) on Cu-based alloys have been found to be well represented by an exponential function of humidity and an Arrhenius function of temperature for both as-deposited and reflowed tin platings. Furthermore, whisker growth was found to follow the same functionality in both corroded and non-corroded regions of the plating. The effective activation energies and humidity coefficients were found to depend upon plating thickness, exposure to reflow, and presence of corrosion. The effective activation energies ranged from 0.23 eV to 0.41 eV and the humidity coefficients ranged from $-$0.012% to $-$0.031%. Corrosion enhanced whisker growth occurred by lowering the effective activation energy for whisker growth. A theory based on excess, non-creep relaxed, oxidation induced strain was developed to explain the corrosion induced energy barrier lowering. The data showed that 60 $^{circ}{hbox {C}}/{hbox {87}}%{hbox {RH}}$ appears to be the optimal high temperature/high humidity test condition at this time for Sn over Cu substrates. Within the limits of the whisker and corrosion (incubation) acceleration functions developed in this study, it is concluded that the JEDEC tests can be used to indicate behavior at other temperature/humidity points that could be relevant storage or service conditions.