Least lead addition to mitigate tin whisker for ambient storage

Long term surface evolution of matte tin electroplating has been investigated under room temperature to understand the tin whisker mitigation by a trace amount of lead addition. No whisker growth has been observed on all the Sn–xPb samples (1 ≤ x ≤ 10 wt%), while at least 3 wt% of Pb addition is required to alter the columnar grain structure of pure Sn plating to equiaxed grains. The mitigation mechanism by such a trace amount of Pb is not caused by the grain texture control, but is due to the less inter-metallic composite (IMC) growth; the segregated Pb at the columnar grain boundaries disrupts the IMC growth, and releases Sn grain boundary migrations to relax the internal stress. This mechanism of stress relaxation and whisker growth suppression suggests that lead-free Sn plating without whisker growth can be realized by co-plating Sn with a Pb-like metal element that precipitates at the grain boundary to interfere with the IMC growth.     

Protrusion and whisker growth on tin coated copper substrate under stresses

Abstract

Tin thin films were coated on copper substrates using chemical method. Tin coated copper specimens under continuously applied compressive and tensile stresses were oxidised at 100°C for 8 days. Protrusion and whisker growth behaviour of tin coated copper specimens was characterised by scanning electron microscope, X-ray diffractometer and transmission electron microscope. The results show that continuously applied compressive stress on tin coated copper specimens would enhance the growth of whisker while tensile stress would reduce whisker growth. A SnO₂ layer was formed during oxidation and intermetallics of Cu₃Sn and Cu₆Sn₅ were formed between SnO₂ layer and copper substrate, which built up the residual compressive stresses in the coated tin layer. Continuously applied stress on the specimens can change the magnitude of residual compressive stress in coated tin layer, which affects the driving force for protrusion and whisker formation and growth.     

管脚无铅覆层的Sn晶须问题

Sn镀层表面在某些情况下会长出长达数百微米的晶须,在电子器件服役过程中会导致电路短路等严重的可靠性问题。目前普遍认为内部压应力是导致Sn晶须生长的主要动力之一;晶须生长所需的Sn原子主要以扩散方式或位错运动方式提供,而温度因素既影响原子扩散速度,又影响镀层的应力松弛。预镀Ni或者预先热处理以形成扩散阻挡层来抑制晶须生长的方法较为常用。温度循环是加速晶须生长的一种有效手段。     

Effect of intermetallic growth rate on spontaneous whisker growth from a tin coating on copper

Intermetallic compound (IMC) growth at the interface between a Sn coating and a Cu substrate with or without a Ni underlayer and its related stress state was evaluated by real-time measurements using the flexure beam method. For the Sn coating without a Ni underlayer, pyramid-shaped IMC grains of Cu₆Sn₅ grew along the grain boundaries of Sn from the Cu substrate, especially at the triple grain boundary junctions and the IMC grains rapidly increased their volume during the initial 3 days, During this time, the IMC growth rate was 2.3 ??m³/day and the compressive stress in the Sn coating rapidly developed and became saturated at about ?11 MPa. In contrast, platelet IMC grains of Ni₃Sn₄ formed at the surface of the Ni underlayer and the IMCs grew at a rate of 1.0 ??m³/day for the Sn/Ni coating. In addition, the stress in the Sn coating with a Ni underlayer remained low, slightly tensile and no whiskers were formed on the Sn plating. Therefore, the IMC growth rate and shape are key factors that affect the mechanism of Sn whisker growth during room temperature storage.     

Inhibiting the growth of Sn whisker in Sn-9Zn lead-free solder by Nd and Ga

The effect of Nd and Ga additions on Sn whisker growth inhibition in Sn–9Zn solder is newly reported in this work. After ageing treatment at room temperature and 125 °C for over 4 months, no Sn whisker growth was observed in the joints soldered with Sn–9Zn–0.5 Ga–0.08Nd and Sn–9Zn–0.5 Ga–1Nd solders. XRD analysis shows that the formation of GaNd and GaNd₃ compounds instead of SnNd compound is the key factor. Because the Gibbs formation free energies of GaNd and GaNd₃ are more negative than that of SnNd, the GaNd and GaNd₃ compounds are relatively more stable, no free Sn atoms released during exposure and oxidizing reaction to feed the Sn whisker growth. EDX analysis was also identified that the formation of Ga–Nd compounds substituted for the Sn–Nd IMC is the main factor that inhibited the spontaneous growth of Sn whisker.     

Growth orientation of the tin whiskers on an electrodeposited Sn thin film under three-point bending

To release the compressive stress in an as-electrodeposited tin (Sn) layer, filamentary Sn whiskers were formed on the layer aged at room temperature. A three-point bending test was performed on an electrodeposited Sn layer to investigate the Sn whisker growth under mechanically applied tensile stress. Sn whisker growth was mitigated on the Sn layer subjected to a tensile stress in bending. The growth orientation of the Sn whiskers formed on the high tensile stress region was random but directional on the low tensile stress region.     

Whisker growth on SnAgCu–xPr solders in electronic packaging

The addition of rare earth Pr into Sn3.8Ag0.7Cu solder results in the formation of PrSn₃ phase, which can induce the whiskers growth. After several hours’ exposure at room temperature in air, different morphologies of whiskers appear in the regions of PrSn₃ intermetallic compounds. The Pr content and storage time are the main parameters for affecting the whiskers growth at ambient temperature. The oxidation mechanism of PrSn₃ phase was used to explain the whiskers growth, the compressive stress is proposed as the driving force for whisker growth.     

Compressive stress-strain response of directionally aligned SiC<Subscript>w</Subscript>/Al composite

A SiC_w/6061Al composite was fabricated through a squeeze-casting route and hot extruded to obtain a composite with directionally aligned whiskers. Based on observed changes in whisker orientation and length before and after deformation, compressive deformation behaviour of the directionally aligned SiC_w/Al composite was investigated. It is found that when the compressive temperature is much lower than the solidus of the matrix alloy, the compressive flow stress of the directionally aligned composite is increased with compressive strain first and then decreased. When the compressive temperature equals the solidus of the matrix, however, the compressive flow stress of the directionally aligned composite is increased monotonously with compression strain. During compression, whisker rotation and breakage occurred, and the higher the compressive temperature, the easier the whisker rotation and hence the smaller the degree of whisker breakage. When the compressive strain was quite high, the degree of whisker breakage was serious even at the temperature as high as the solidus of the matrix. Analyzing changes in whisker orientation and breakage before and after compression indicates that the decreased compressive flow stress with compressive strain is the result of the decreased load carrying ability of whiskers caused by whisker rotation and breakage. Compared with whisker rotation, whisker breakage has a bigger contribution to the decreased compressive flow stress. No strain softening in the composite compressed at 580°C can be thought to be a result of the very low strengthening effect of whiskers at such a high temperature. From the point of view of whisker breakage, to get higher properties of SiC_w/Al composite parts made by means of plastic forming, too high plastic strain should not be suffered by SiC_w/Al composites during the plastic forming.     

An assessment of Sn whiskers and depleted area formation in thin Sn films using quantitative image analysis

An experimental technique was developed to determine the extent of Sn whisker growth and depleted area formation on evaporated 1 μm tin (Sn) films. Deformation of the Si substrate placed a controlled magnitude of compressive or tensile stress across the films. Quantitative image analysis was used to monitor whisker growth and size of the depleted areas. The test conditions were: stresses 10–40 MPa; temperature, 180 °C; and time durations, 1–8 weeks. The whisker length increased with compressive stress. The whiskers appeared within the first week, but then did not grow significantly with additional time. Some whiskers were located in the centers of depleted areas. The depleted areas size was not sensitive to the applied stress, but did increase with annealing time. Both Sn whiskers and depleted areas were the result of potentially similar rapid, long-range diffusion processes. However, differing trends suggested that separate driving forces and/or rate kinetics controlled the two phenomena.     

Effects of reflow atmosphere and flux on Sn whisker growth of Sn–Ag–Cu solders

We evaluated the Sn whisker growth behavior of Sn–Ag–Cu solder fillets on lead frames of quad flat packages (QFPs) upon OSP printed circuit boards that were exposed to 85 °C/85% relative humidity (RH) exposure. Three different concentrations of halogen flux for activated Sn-3.0wt%Ag–0.5wt%Cu were used to solder in air and in an inert N₂ reflow atmosphere. The lead frames of the QFPs consisted of Sn plated Cu and Fe-42wt%Ni (alloy 42). Sn whiskers were observed on the surface of the QFP solder fillet joints that were reflowed with halogen containing flux in an air atmosphere. A substantial amount of Sn oxides were formed in those solder fillets while whisker growth and the amount of Sn oxides increased with the halogen content. Sn oxide formation apparently enhanced whisker formation. The combination of air reflow atmosphere and high halogen flux was the worst combination for solder fillet oxidation resulting in Sn whisker formation regardless of the electrode’s lead frame composition of Cu or alloy 42. In contrast, an inert N₂ reflow atmosphere obviously prevented Sn whisker formation on Sn–Ag–Cu solder fillets under all conditions used in this work.     

Spontaneous Sn whisker formation on Ti<Subscript>2</Subscript>SnC

Spontaneous Sn whisker growth on Sn-containing substrates has resisted interpretation for several decades. In this paper, Ti₂SnC-Sn and Ti₂SnC samples were prepared and then cultivated in the same conditions. Many Sn whiskers had appeared on the ball milled Ti₂SnC-Sn sample after being stored in air for 40 days, while no whisker grew on the Ti₂SnC-Sn samples without ball milling although the cultivation time had been extended to more than half a year. Moreover, even though it was ball milled, the Sn-free Ti₂SnC sample did not grow any whisker. The whiskers formed on this system share typical features with the ones grown on Sn platings and solders, while they do not follow any of the existing growth models. A catalysis-based whisker formation model, in which the cleavage planes of Ti₂SnC grains act as heterogeneous nucleation sites, is proposed, which well interprets Sn whisker formation in the ball milled Ti₂SnC-Sn system.     

Electromigration-induced Bi-rich whisker growth in Cu/Sn–58Bi/Cu solder joints

Effect of current stressing on whisker growth in Cu/Sn–58Bi/Cu solder joints was investigated with current densities of 5 × 10³ and 10⁴ A/cm² in oven at different temperatures. Two types of whiskers, columnar-type and filament-type, were observed on the solder film propagating along the surface of the Cu substrate and at the cathode interface, respectively, accompanied with many hillocks formation. Typically, these whiskers were 5–15 μm in length and 0.06–2 μm in diameter. EDX revealed that these whiskers and hillocks were mixtures of Sn and Bi rather than single crystal. It should be noted that the sprouted whiskers would not grow any more even if the current-stressing time increased again when the solder joint was stressed under lower current density. Nevertheless, when the current density was up to 10⁴ A/cm², the whiskers would melt along with the increasing current-stressing time. Results indicated that the compressive stress generated by precipitation of Cu₆Sn₅ intermetallics provides a driving force for whisker growth on the solder film, and the Joule heating accumulation should be responsible for whisker growth at the cathode interface.     

Tin whiskers prefer to grow from the [001] grains in a tin coating on aluminum substrate

This work aims at understanding the features of the Sn grains from which whiskers preferentially grow.The growth behavior of Sn whiskers on a 50 μm thick hypereutectic Sn-Al alloy coating was observed in situ by mapping the grain orientations before and after aging using the electron backscatter diffraction(EBSD) technique.Sn whiskers were found to grow preferentially from the (001) or near-(001) grains sur-rounded by the grains having perpendicular orientations,such as (100),(110) and (210).The compressive stress in the coating was heterogeneous,and the (001) grains exhibited the higher compressive stress close to the grain boundaries.The orientation relationship between α-Al phase and β-Sn phase was con-firmed as (200)α-Al || (200)β-Sn,[01(1)]α-Al ||[001]β-Sn.The plane matching resulted in approximately 0.7 ％ misfit strain in β-Sn,which had little impact on the growth of whiskers.Dislocations pile-ups were found in the (001) grains and repulsed by the Sn oxide layer,giving the probability of cracking the oxide.Grain boundaries were found between the whisker and underneath grain.The dominant diffusion mode for early whisker growth was grain boundary diffusion aided by pipe diffusion.     

Correlation between surface whisker growth and interfacial precipitation in aluminum thin films on silicon substrates

When subjected to thermal excursions, aluminum thin films on silicon substrates often show whisker or hillock growth on the film surface, along with formation of Si precipitates at the interface. This study demonstrates that the two effects are related, and that interfacial Si precipitation directly influences the growth of Al whiskers on the film surface during isothermal annealing at 300–550 °C. The density of whiskers and hillocks not only increases with increasing annealing temperatures where the film is under greater compressive stress, but also during longer hold times which should relieve the stress. At high temperatures and long annealing times, extensive Si precipitation, eventually leading to a bi-modal precipitate size distribution, occurs continuously at the interface. The total amount of Si precipitates far exceeds the solubility limit of Si in the Al thin film, and can generate enough compressive stress in the film to drive surface whisker growth. By continuously augmenting film stress, interfacial Si precipitation supplies the driving force for whisker/hillock formation on the Al-film surface.     

镀锡银钎料扩散过渡区的物相和形成机制

采用温度梯度法对镀锡银钎料进行热扩散处理,形成了扩散过渡区。为了揭示镀锡银钎料扩散过渡区的形成机制和主要物相的形成过程,借助金相显微镜、扫描电镜(SEM)、能谱分析仪(EDS)、X射线衍射分析仪(XRD)对扩散过渡区的显微组织、Sn元素的面扫描分布、物相组成及形貌进行分析。研究表明,Sn元素在镀锡银钎料中分布均匀、无偏析,在扩散过渡区主要以棒状Ag_3Sn相和块状Cu_3Sn相存在。随着热扩散温度升高,Ag_3Sn相和Cu_3Sn相的相对衍射强度逐渐增大。Ag_3Sn相的形成过程分为三个阶段:Ag_3Sn颗粒相弥散分布、Ag_3Sn颗粒相互相接触合并、生成大块棒状化合物相。Cu_3Sn相主要是锡晶须生长冲破镀层的氧化膜,在张应力和压应力协同作用下形成。镀锡银钎料扩散过渡区的形成机制为"钎接、互扩散、亚稳态、合金化"。     

Influence of surface oxidation on thermal shock resistance and flexural strength of SiC/Al2O3 composites

It is known that SiC whisker/Al₂O₃ matrix composites can oxidize in air at high temperature and then form oxidation layers on their surfaces. Oxidation treatment has been experimentally performed in air at 1450 °C for a pre-determined time. The results show that the surface layer is in a state of compressive residual stress. The oxidized specimens have better resistance to thermal shock damage than the non-oxidized specimens. However, the surface oxidation can degrade the room-temperature flexural strength.     

Stress generation and relaxation in (Al,Ga)N/6<Emphasis Type="Italic">H</Emphasis>-SiC heterostructure grown by plasma-assisted molecular-beam epitaxy

In situ stress generation and relaxation in Al_0.25Ga_0.75N/GaN/AlN heterostructure with an overall thickness exceeding 3 μm in the process of its growth on a 6H-SiC substrate by low-temperature plasma-assisted molecular-beam epitaxy at substrate temperatures ranging from 690 to 740°C was studied. At room temperature, AlN and GaN layers revealed residual compressive stresses of–2.3 and–0.1 GPa, respectively. This made it possible to avoid cracking during postgrowth cooling of the structure.     

Fracture toughness and fatigue crack growth behaviour of an Al2O3-SiC composite

The fracture toughness and fatigue crack growth characteristics of an Al₂O₃-SiC whisker composite were investigated. Quasi static fracture experiments were conducted on double edge-notched tension specimens and on four-point bend specimens containing a through-thickness Mode I crack which was introduced under uniaxial cyclic compression. The toughness results obtained using this procedure are more reproducible than those derived from the indentation technique and the notched bend bar method. The fracture toughness of the composite is about 60% higher than that of the unreinforced matrix material. Crack growth characteristics at room temperature were also investigated in notched plates of Al₂O₃-SiC subjected to fully compressive far-field cyclic loads. In the presence of a stress concentrator, this composite is found to be highly susceptible to fatigue crack growth under cyclic compressive loads.     

Rheology,fiber distribution and mechanical properties of calcium carbonate (CaCO3) whisker reinforced cement mortar

Calcium carbonate (CaCO₃) whiskers are a new kind of microfiber used in cementitious composites and have proved to provide excellent effect on strengthening and toughening. In order to further improve the mechanical properties of CaCO₃ whisker-reinforced cementitious composites, rheological properties of fresh mixtures and the CaCO₃ whisker distribution in the hardened matrix were investigated. The yield stress and plastic viscosity increased with an increasing content of CaCO₃ whisker and a decreasing water-cement ratio. Also, the rheological properties were affected by the distribution of CaCO₃ whisker in the matrix. The largest increments in flexural and compressive strength were 27.59% and 12.60% for the mortars with CaCO₃ whisker contents of 2.0% and 1.5%, respectively. The properties responsible for the mechanical response were explained in terms of the effects of CaCO₃ whisker reinforcement, the distribution of CaCO₃ whiskers, and the porosity as well as pore size distribution.     

Confining effect of oxide film on tin whisker growth

Spontaneous tin whisker growth has been mysterious and catastrophic for more than half century. The difficulty in the research on this topic consists of the randomness of the whisker growth, the slow growth rate and many other tricky factors. Herein, with Ti₂SnC-Sn as a new platform, fast tin whisker growth is realized to facilitate the research. The whisker morphology is found to be modulated by oxide film. A striated whisker morphology forms as growing in air, whereas a faceted morphology forms in vacuum. Furthermore, the evolution to the faceted morphology is attributed to the reconstruction of the whisker surface driven by surface energy reduction. The findings might open a new avenue to uncover the myths of this long-standing issue, and thus develop a long-awaited lead-free tin whisker mitigation strategy.