Evolution of tin whiskers and subsiding grains in thermal cycling

The evolution of surface morphology, including whisker formation, grain boundary cracking, and subsiding grains, was studied in Sn thin films on Si substrates with a Cu interlayer during thermal cycling from ?40 to 85 °C in air for up to 250 cycles and was compared with surface morphologies resulting from room temperature aging. Multiple areas were tracked, and the areal density of whiskers and the grain morphologies within these areas were monitored over time for room temperature aging and with increasing number of thermal cycles. During room temperature aging, the whisker density increased with time until saturation ~3 weeks after plating. As for thermal cycling, the whisker density was observed first to increase but then to decrease as a result of a whisker pinch-off phenomenon. The characteristic features of whiskers formed during thermal cycling included the formation of deep grooves along the in-plane grain boundaries of whiskers (“whisker root”), a decrease in whisker radii as they grew, striation rings on whiskers perpendicular to the whisker growth direction, corresponding striations along grooved surfaces in the film, albeit at different spatial periodicities than those on their corresponding whiskers, and whisker pinch-off as whiskers became prone to fracture as their radii decreased. Whiskers formed during room temperature aging did not display such grooving or pinch-off. A whisker pinch-off model was proposed to explain the observed morphological changes and the resulting decrease in whisker density during thermal cycling, with a calculated whisker growth rate that agrees with the experimental observation.     

Corrosion behavior of pure tin deposit under 55 °C/85 % RH reliability test

With recent moves to implement lead-free manufacturing in the electronics industry, more attention is being paid to the use of tin. One of greatest limitations regarding the reliability of tin is the undesirable development of tin whiskers. Many factors have been identified as being the cause of these whiskers, though tin corrosion has not. Three types of tin deposit grain structures have been developed in an effort to reduce the growth of whisker formations as type (A) with vertical and regular grain boundary; type (B) with horizontal and irregular grain boundary; and type (C) as a combination of both vertical (the top layer) and horizontal (the bottom layer) grain structures. We kept a series of samples in an oven for 1 year at 55 °C/85 % RH (relative humidity), and then used focused ion beam to examine the corrosion microstructure that occurred three types of deposit structures. We found the preferred path of corrosion to occur along the grain boundary and that tin oxide enlarges the volume of the deposit. In order to release the compressive stress that developed during corrosion, whiskers were formed as the grain structure being vertical. Corrosion developed in different locations of various deposits made of the three grain structures. We studied resulting in whisker growth, cracking and the development of internal oxide.     

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.     

Tin pest issues in lead-free electronic solders

Tin pest is the product of the β → α allotropic transition at 13.2°C in pure tin. It is a brittle crumbly material, often responsible for the total disintegration of the sample. The transformation involves nucleation and growth, with an incubation period requiring months or years for completion. Experimental observations reveal a substantial inconsistency and an incomplete understanding of the process. Some alloy additions promote tin pest by reducing the incubation time, whereas others retard or inhibit its formation. Traditional solder alloys have generally been immune to tin pest in service due to the presence of lead, and bismuth and antimony as common impurities. However, the new generation of lead-free solders are more dilute—closely resembling tin. A much debated question is the susceptibility of these alloys to tin pest. Bulk samples of tin-0.5 copper solder undergo the transition at −18°C although not at −40°C after five years exposure. Other lead-free alloys (Sn–3.5Ag, Sn–3.8Cu–0.7Cu and Sn–Zn–Bi) are immune from tin pest after a similar period. Large scale model joints exhibit tin pest but it appears that actual joints may be resistant due to the limited free solder surface available and the constraint of intermetallic compounds and components. It seems likely that impurities are essential protection against tin pest, but for long term applications there is no certainty that tin pest and joint deterioration will never occur.     

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.     

Processing and material issues related to lead-free soldering

The European requirement for lead-free electronics has resulted in higher soldering temperature and some material and process changes. Traditional tin–lead solder melts at 183°C, where as the most common lead-free alternatives have a much higher melting temperature—tin–copper (227°C), tin–silver (221°C) and tin–silver–copper (217°C). These have challenged the ingenuity of the materials and process engineers. This chapter will explore some of the issues that have come up in this transition, and which these engineers have understood and addressed. As we enter the lead-free era, we see changes as printed wiring board (PWB) substrates which were designed for lower soldering temperatures are being replaced by newer materials. Factors such as glass transition temperature (T _g), decomposition temperature (T _d) and coefficient of thermal expansion must be considered. Many electronic components are made for lower peak temperatures than those required by the new solders. Solder flux chemistries are changing to meet the needs of the new metal systems, and cleaning of flux residues is becoming more of a challenge. Finally, there is a potential reliability problem—an increased potential for the growth of conductive anodic filament (CAF), an electrochemical failure mechanism that occurs in the use environment.     

Characterization of metal-matrix composites fabricated by vacuum infiltration of a liquid metal under an inert gas pressure

Silicon carbide whisker-reinforced aluminium was fabricated by vacuum infiltration of liquid aluminium into a porous whisker preform under an argon gas pressure, using an infiltration temperature of 665 °C. The volume fraction of whiskers ranged from 11% to 37%. No whisker pull-out was observed on the fracture surface for an infiltration temperature of 665 °C, but some whisker pull-out was observed for an infiltration temperature of 720 °C. Both the tensile strength and ductility decreased with increasing infiltration temperature above 665 °C. Tensile test results from room temperature to 300 °C are reported. They showed that the quality of these composites was comparable to that of composites made by powder metallurgy or squeeze casting. The coefficient of thermal expansion at 100–150 °C was decreased by 45% by the addition of 37 vol % whiskers.     

Copper-oxide whisker growth on tin–copper alloy coatings caused by the corrosion of Cu6Sn5 intermetallics

This paper reports the effect of corrosion caused by high temperature and humidity on pure tin and tin–copper alloy coatings. A new phenomenon was observed; the development of copper-oxide whiskers on tin–copper alloys plated on copper substrates (1–5 % copper content stored at 105 °C/100 % relative humidity). The copper-oxide whiskers showed similar growth properties to tin whiskers. We have made a model to understand the development of copper-oxide whiskers. Localized corrosion of the tin coating reaches the Cu₆Sn₅ intermetallic layer, and copper oxide accumulates after the corrosion of Cu₆Sn₅. The dilating SnO _x compresses and extrudes out the copper oxide in a whisker form.     

Sn晶须形态的研究

本文研究了稀土相表面Sn晶须的生长行为.研究结果表明,如果将稀土相暴露于空气中,在稀土相的表面会出现Sn晶须的快速生长现象,且Sn晶须在其快速生长过程中会表现出一些特殊的形态特征,如片状Sn晶须的形成、Sn晶须的多次连续转折现象、Sn晶须的变截面生长现象、Sn晶须的分枝与合并以及Sn晶须的搭接现象等.     

Thermal stress driven Sn whisker growth: in air and in vacuum

Whisker growths from matte tin electroplating have been observed during thermal cycling up to 1,000 cycles either in air or in vacuum. The density, length, and width of thermal stress whiskers depend on the plating thickness of 2 and 5 μm in the present study. Whiskers grown on the 2 μm plating are longer and thinner than those on 5 μm plating. In both cases, whiskers grow thinner and faster in vacuum than in air. These apparent variations come from the grain sizes and the thermal stress distributions in the electroplating, intrinsically different in 2 and 5 μm thick films. The grain structure of whisker root, particularly grain boundary cracks oxidized in air, determines the stress concentration to drive the whisker growth. Cracking caused by oxidation was rarely observed in vacuum hence causes thin and straight whiskers even from thick plating. Our results indicate that the stress concentration at whisker root grain is essential for controlling whisker growth morphology, and has a critical impact on various electronic applications.     

Detrimental Effects of Excessive Gold Plating on Lead-Free Solder Joints

While most of the industries are striving very hard to produce totally lead-free electronic products, many concerns remain regarding lead-free solder joint reliability. One major concern is the robustness of gold metallization of the electronic components for lead-free soldering. Increasing gold content has been known to result in embrittlement and early failure in electronic assemblies. Therefore, information about the lead-free solder/gold metallization interdiffusion at high-temperature applications is very important for controlling the technological processes for the reliability of the electronic interconnects. The challenges of solder/gold metallization interdiffusion during high-temperature application/test are gold embrittlement, intermetallics growth, void formation, and also tin-whisker formation. This paper illustrates few case histories of such challenges. Importance of the thickness of the gold termination has been discussed and some parameters to optimizing the thickness of the gold termination have been suggested. Some remedial measures are suggested to control the lead-free solder/gold metallization diffusion in the electronics interconnects.     

Dimensional stability and thermal stress of 6061 Al/SiCw composite during thermal cycles

Abstract

The variations of dimensions and thermal stress of 6061Al/20 vol.-%SiC_w composites were investigated during thermal cycles between 25 and 300°C. Testing results showed that the dimensional stability and thermal stress are mainly related to the whisker distribution in the composite. For the as cast composite, owing to the whiskers being oriented randomly, the temperature cycles hardly affect the dimensions of the specimen, so this material has good dimensional stability. However, for the as extruded composite, owing to whisker alignment, the thermal cycles can evidently affect the dimensions of the specimen, i.e. the dimension is elongated in the alignment direction of whiskers and shortened in another direction. The dimensional stability of the as extruded composite can be improved somewhat by thermal precycles.     

Influence of laminate type on tin whisker growth in tin-rich lead-free solder alloys

This paper describes variations in whisker growth on the surface of tin-rich, lead-free alloys soldered on a Cu layer depending on the laminate used for the printed circuit board, which can be either glass-epoxy or paper-phenol. The structure of the glass-epoxy laminate surface is characterized by spatial nonuniformity caused by the regular structure formed by regions of glass fibers and resin in the top layer of the laminate. The higher value of thermal expansion of the resin than of the glass fiber means that the area of the resin expands more than that of the glass fiber. This causes local compressive stress in the solder layer and as a result promotes whisker growth in the area of the alloy soldered on the Cu layer over the glass fiber. This effect does not occur on the surface of an alloy soldered on Cu layer over a paper-phenol laminate.     

Tin Pest: Elusive Threat in Lead-Free Soldering?

Tin pest is the result of an allotropic transformation of tin from its β phase to its α phase at temperatures below 13 °C. This transformation is accompanied by a change in density from 7.31 to 5.77 g/cm³. The resulting expansion usually results in degradation of the affected part, as the α phase material is mechanically weak and over time becomes little more than a gray powder. The tin pest effect is cumulative unless the sample in question is exposed to temperatures above 70 °C or so, where some reversion back to white tin has been observed. However, by this point a tin sample’s physical integrity may have been destroyed by the partial β to α transformation. The transformation to α tin is inhibited strongly by soluble alloying elements such as bismuth and antimony at concentrations of about 0.5%, or lead at concentrations of about 5%. Hence, tin pest is generally observed only in quite pure tin. However, nearly pure tin alloys may become common. For example, the advent of the European Union’s RoHS law, which essentially eliminates lead from solder, has spawned lead-free alloys that can be almost pure tin and may be at risk of tin pest. This risk has caused some concern over the long-term stability of emergent, lead-free solders during low temperature service. This overview article is written in light of this concern.     

Microscopy Study of Tin Whiskers

Tin whiskers are electrically conductive crystalline structures that may spontaneously grow from pure tin-plated surfaces. Since pure tin-plating is used as an electronic component lead finish, there is an inherent possibility that a short circuit may be formed between the component’s leads by tin whisker growth. The focus of this article is to share tin whisker observations that the authors have encountered over years of tin whisker research.     

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。     

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.     

Observations of ribbon-like whiskers on tin finish surface

The morphology of tin whiskers on a matte tin finish on a phosphor bronze substrate was observed by SEM. The samples were prepared by a sulfate-based bath and held at 328 K/100% RH for several different time increments. The results show that, besides the fluted whiskers, there are tin whiskers with new morphologies, ribbon-like whiskers. The ribbon-like whisker has a section size of about 5 microns in width and 2.5 microns in thickness and it may be straight or bent with kinks to form some irregular shapes. Whiskers with different morphologies can co-exist on the tin finish with the same aging condition. It is suggested that the formation of ribbon-like whiskers takes place when a new recrystallization grain nucleates and abnormally grows in the finish prior to whisker growth.     

Whisker nucleation in indentation residual stress field on tin plated component leads

Tin whisker formation has been a serious concern for application of pure tin as a Pb-free component lead finish. It has been long believed that residual stress is the root cause of whisker formation. A fundamental question is if stress produced by other than the plating processing and post-plating metallurgical reactions can induce whisker formation. In this study, micro indents were made on pure tin plated component leads to introduce a stress field and a scanning electron microscope was used to monitor the nucleation of whiskers in-situ. Nanoindentation was also performed to measure hardness and elastic modulus of the tin coating and substrate. The stress/strain field around the micro-indent was calculated using finite element method. Experimental and theoretical calculation results show that stress gradient plays an important role in whiskers nucleation.     

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.