An Investigation into Zinc Diffusion and Tin Whisker Growth for Electroplated Tin Deposits on Brass

It is widely documented that whisker growth is more rapid for tin deposits on brass compared with deposits produced on other substrate materials, such as copper. As a result, studies investigating the effect of process variables on tin whisker formation are often conducted on brass substrates to take advantage of the increased whisker growth rates. Although it has been understood since the 1960s that the increased whisker growth results from zinc diffusion, to date there has not been any detailed analysis of the zinc/zinc oxide distribution at the surface of the tin deposit. Using a commercial bright tin electroplating bath, the formation of zinc oxide at the surface of tin deposits on brass has been investigated. Analyses show that zinc oxide is present on the surface of the deposit within 1 day of electroplating. During storage at room temperature, a network of zinc oxide is formed at the surface grain boundaries, the extent of which increases with time. The critical role that zinc surface diffusion plays in whisker growth for tin deposits on brass has been demonstrated by electrochemical oxidation of the tin shortly after electroplating. This develops a tin oxide film that is thicker than the native air-formed oxide and subsequently serves as a diffusion barrier to zinc surface diffusion, thereby mitigating whisker growth.     

Studies on various chip-on-film (COF) packages using ultra fine pitch two-metal layer flexible printed circuits (two-metal layer FPCs)

Various fine pitch chip-on-film (COF) packages assembled by (1) anisotropic conductive film (ACF), (2) nonconductive film (NCF), and (3) AuSn metallurgical bonding methods using fine pitch flexible printed circuits (FPCs) with two-metal layers were investigated in terms of electrical characteristics, flip chip joint properties, peel adhesion strength, heat dissipation capability, and reliability. Two-metal layer FPCs and display driver IC (DDI) chips with 35 μm, 25 μm, and 20 μm pitch were prepared. All the COF packages using two-metal layer FPCs assembled by three bonding methods showed stable flip chip joint shapes, stable bump contact resistances below 5 mΩ, good adhesion strength of more than 600 gf/cm, and enhanced heat dissipation capability compared to a conventional COF package using one-metal layer FPCs. A high temperature/humidity test (85 °C/85% RH, 1000 h) and thermal cycling test (T/C test, ?40 °C to + 125 °C, 1000 cycles) were conducted to verify the reliability of the various COF packages using two-metal layer FPCs. All the COF packages showed excellent high temperature/humidity and T/C reliability, however, electrically shorted joints were observed during reliability tests only at the ACF joints with 20 μm pitch. Therefore, for less than 20 μm pitch COF packages, NCF adhesive bonding and AuSn metallurgical bonding methods are recommended, while all the ACF and NCF adhesives bonding and AuSn metallurgical bonding methods can be applied for over 25 μm pitch COF applications. Furthermore, we were also able to demonstrate double-side COF using two-metal layer FPCs.     

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.     

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.     

Length Distribution Analysis for Tin Whisker Growth

The global movement to lead-free electronics has led semiconductor device assemblers to switch terminals and finishes from lead-based to pure tin or high tin lead-free alloys. This transition has resulted in a reliability concern associated with the formation of conductive tin whiskers, which can grow from a device terminal or lead and cause current leakage or short circuits. This paper presents the results of an experimental study on tin whisker growth. Test specimens consisted of matte and bright tin finishes on copper, Alloy-42, and brass substrate materials. The heat treatments included annealing and two types of simulated reflow. Maximum whisker length and whisker density were measured on 24 different types of tin-plated specimens, after three, eight, and 16 months of room ambient storage after various heat treatments     

The Mechanism of Residual Stress Relief for Various Tin Grain Structures

A pure tin deposition process was developed with various tin grain structures to study tin whisker formation. Samples were tested for 4000 h to examine whisker formation, grain structures, and intermetallic formation using a focused ion beam (FIB). The lateral side of the FIB-cut cavity displayed tin protrusions after 6 days. These phenomena, along with the growth of tin whiskers and/or hillocks, could illustrate the residual stress relief behavior of various tin grain structures. In full columnar structures, whiskers formed normal to the deposition surface and relieved most of the stress. In contrast, stress relaxation in semicolumnar and random structures is highly likely to occur, and proceeds rapidly in the direction parallel to the deposition surface after only a few days. In comparing mixed grain structures, it is apparent that stress is more likely to be rapidly relieved within structures with fewer grain boundaries.     

Role of intrinsic stresses in the phenomena of tin whiskers in electrical connectors

The connector industry is at a crossroads with respect to selecting lead-free coatings in response to legislation and customer demands. Component manufacturers have long used tin-lead coatings because of lead's ability to inhibit tin whisker formation. Several lead-free alternate finishes such as pure tin, tin-bismuth, tin-copper, tin-zinc, Pd, and Pd-Ni-Au have been proposed. One simple, economical, and easy drop-in replacement is pure matte tin. Unfortunately, pure tin films have a fairly high risk of whisker formation. Factors, such as substrate material, plating process variables, and assembly operations also play a role in whisker generation. The stresses imposed at each process step pose a serious threat to whiskering. We have critically evaluated three matte tin chemistries by changing bath composition and deposition parameters. We also characterized stresses in these matte tin coatings by using flexure beam and X-ray diffraction techniques. This paper identifies stress types encountered in various connector applications, provides an insight to the users about practical situations, and suggests whisker mitigation strategies. The impact of environmental (e.g., temperature and humidity) stresses on tin coatings is also discussed.     

Tin Whisker Nucleation and Growth on Sn-Pb Eutectic Coating Layer Inside Plated Through Holes With Press-Fit Pins

It has been long believed that residual stress is the root cause for tin whisker formation on pure tin-plated component leads. However, tin whisker formation could be observed on the surface of other tin-based alloys under certain conditions. In this study, the whisker formation was reported on a coating layer of Sn-Pb eutectic hot air solder leveling (HASL), which was under compression stress conditions due to the inserted compliant pins. In-Situ scanning electron microscopy was used to monitor the nucleation and growth of whiskers. In addition, a mechanical experiment and non-linear contact finite element analysis were used to estimate the magnitude of the stress in the HASL coating layer. It was found that the tin whisker formation with whisker size of more than 10 mum could occur on the surface of 60Sn-60Pb plating within less than 30 min at an ambient temperature under compressing stress conditions. The tin whisker initiation and growth were further studied at an elevated temperature of 70 degC to check if a higher temperature effects Tin whisker formation. It is believed that establishment of a quantitative relationship of whisker formation/growth under compressive stress and elevated temperature conditions could lead to better scientific methods for risk and reliability assessment and smooth transitions to lead-free assemblies.     

锡晶须问题的现状与课题

针对锡晶须的形成原因,文章执应力学一说,以电子连接器的触点为叙述点,把机械地从外部到镀层表面形成的应力所产生的锡晶须定义为外部应力型,并主要叙述以连接器为中心的外部应力型晶须;另一方面,把从基底材料的扩散和表面氧化等自然现象所产生的锡晶须定义为内部应力型晶须。文中推荐了锡晶须的测试方法,阐述了锡晶须的形成机理,并简要介绍了对锡晶须研究的现状及今后的研究课题。     

连接器锡晶须形成机理及研究

针对锡晶须的形成原因,把机械地从外部到镀层表面形成的应力所产生的锡晶须定义为外部应力型,并主要叙述以连接器为主的外部应力型晶须;另一方面,把从基底材料的扩散和表面氧化等自然现象所产生的锡晶须定义为内部应力型晶须.推荐了锡晶须的测试方法,阐述了锡晶须的形成机理,并简要介绍了对锡晶须研究的现状及今后的研究课题.     

Tin whisker growth on bulk Sn-Pb eutectic doping with Nd

Tin whisker growth is a serious reliability concern for electronics using high-density packaging. Whisker growth behavior of tin lead eutectic solders doped with different neodymium concentration (5-0.1wt.%) was studied in present work. Results indicate that the Nd in the all of the Sn-Pb-Nd alloys mainly exists in NdSn₃ intermetallic compound, and that the distribution of the NdSn₃ phase is very non-uniform in the alloy with low Nd concentration. Tin whiskers growth was observed on all samples doped with Nd. All of these whiskers were seen to originate from the NdSn₃ phase, it appears that the Pb does not suppress whisker growth in these alloy because it does not influence the formation of the tin-rare earth intermetallic compound. These results suggest that the addition of rare earth elements into tin alloy systems renders whisker formation nearly inevitable. Therefore, any suggestion to dope tin-based solders with rare earth elements for application in high-density electronic packaging should be carefully reconsidered.     

Driving force for the formation of Sn whiskers: compressive stress-pathways for its generation and remedies for its elimination and minimization

Compressive stress is widely accepted as the driving force for tin whisker formation. There are several pathways for compressive stress buildup in Sn coatings, which include the following: residual stress generated during plating; stress formation due to interfacial reactions between tin and copper substrate; mechanical stress; and thermal mechanical stress due to coefficient of thermal expansion (CTE) mismatch between the tin layer and substrate during thermal cycling. In order to prevent or reduce whisker growth in tin deposits, compressive stress has to be eliminated or minimized. This paper discusses the pathways for compressive stress formation and various remedies for its elimination and minimization. Particularly, a novel approach for dealing with thermal mechanical stress due to CTE mismatch is discussed.     

A Model for Rapid Tin Whisker Growth on the Surface of ErSn3 Phase

Spontaneous growth of tin whiskers on the finish of leadframes is an extremely slow process under moderate temperature conditions. It therefore becomes difficult to track the continuous growth of tin whiskers and to vary the experimental conditions to determine their root causes. Accordingly, the fundamental growth behaviors of tin whiskers are still not fully understood. In this study, rapid tin whisker growth was achieved by adding 1 wt.% Er to Sn-3.8Ag-0.7Cu solder alloy. The results showed unique tin whisker morphology with nonconstant cross-section. An explanation is proposed by adding kinetic energy to the conventional energy balance equation. In addition, a double compressive stress zone is proposed to demonstrate the driving force for tin whisker growth in rare-earth-bearing phases.     

Pressure-induced tin whisker formation

This paper presents a fundamental assessment of pressure-induced tin whisker formation. A creep-based tin whisker model was developed to explain experimental data as well as data from industrial studies. A grain boundary diffusion model was then developed to predict a maximum whisker length. It is shown that creep properties obtained from samples can be used to screen plating finishes susceptible to tin whiskering. This enables a manufacturer to assess tin whisker risks prior to volume production.     

An integrated theory of whisker formation: the physical metallurgy of whisker formation and the role of internal stresses

Over 50 years of whisker research (cadmium, zinc, and tin) has not resulted in consensus about whisker formation fundamentals for metal films. New analytical tools have recently provided new insights into microstructural changes that occur during whisker formation. Integration of these newer observations with historical data led the authors to propose an Integrated Theory of Whisker Formation. The Integrated Theory incorporates physical attributes such as microstructure and internal stress states. Particular emphasis is placed on recrystallization, grain boundary diffusion, film-substrate interdiffusion (i.e., the Kirkendall effect), and stress gradients. The Integrated Theory does not require dislocation mechanisms for material transport to the whisker location. Material is transported to a whisker grain by the surrounding grain boundary network under the driving impetus of positive stress gradients. Transported atoms then move from the grain boundary network into the whisker grain. This movement into the whisker grain pushes the free surface of the whisker grain upward and, thereby, grows the whisker structure.     

Reflow discoloration formation on pure tin (Sn) surface finish

Pure tin finish is popularly used as the end finish for semiconductor leadframes and electrical connectors. However, the problem of reflow discoloration formation on pure tin surface finish degrades the packaging reliability of the integrated circuits (ICs), which greatly limits the application of pure tin plating. This paper reported on the aspect of the reflow discoloration formation on pure tin finish, and the effects of impurities of the substrate and the electroplating conditions on the tin finish discoloration after reflow were discussed. A significant improvement in the tin finish reflow discoloration is obtained by taking suitable electroplating process.     

Effects of continuously applied stress on tin whisker growth

A simple four-point bending technique in conjunction with scanning electron microscopy (SEM) was employed to investigate the relationship between continuously applied mechanical stress and tin whisker growth on bright tin-plated copper specimens at room temperature. Measurements of whisker length and density were periodically taken for each specimen during a 7-month exposure to applied stress. Tin whiskers were found on the tin-plated specimens with or without the presence of mechanically applied stresses. A continuously applied compressive stress resulted in formation of longer and more dense tin whiskers, in comparison with the cases of no applied stress and applied tensile stress. However, an increase in the applied compressive stress level caused an increase in the whisker density but a decrease in the whisker length. On the other hand, a continuously applied tensile stress was found to reduce both the whisker density and length, compared to the case of no applied stress. Apparently, application of a continuous tensile stress could provide an effective means in retarding tin whisker growth.     

Stress-Induced Tin Whisker Initiation Under Contact Loading

As electronic components become smaller, tin whiskers have an increasingly adverse effect on the assessment of the reliability of the components. In particular, the contact between components brings about whisker initiation near the contact area. Although a number of spontaneous tin whisker models have been proposed, a model of tin whisker initiation by contact loading has not yet been established. In this paper, the behavior of stress-induced tin whiskering by contact load was examined in a tin plating based on multiaxial creep theory. Creep properties were measured using a nanoindentation creep technique. The creep rate of Sn-10Pb is higher than that of Sn and affects the generation of compressive stresses after stress relaxation. Nonlinear finite-element analysis (FEA) reveals that the multiaxial compressive stress state appears after stress relaxation. The axial compressive stress increases even though the contact stress decreases. When multiaxial stresses are applied, not only axial stress but also the difference between stresses, affects the behavior of whisker formation. The axial compressive stress depends on the structures of the contact bodies, and the contact stress depends on the creep properties of the plating     

A Novel Zero-Insertion-Force (ZIF) Micro $(mu)$ -Connector: Design, Fabrication, and Measurements

This paper presents the design, fabrication, and measured properties of a novel zero-insertion-force (ZIF) micro (mu)-connector. The proposed ZIF mu-connector is shown to remedy a number of problems in the existing microelectromechanical-system-based connectors, such as the wearing effect, the poor signal integrity for high-speed signal transmission, and the lack of latch design. The three-mask and silicon-on-insulator wafers are designed for the simultaneous fabrication of terminals and latches. Prototype connectors are demonstrated with five 1800-mum-long, 100-mum-wide, and 2-mum -high terminals on a 150-mum pitch. The terminals and latches are actuated by electrostatic force to avoid the wearing and kinking during the mating process. The terminal is a multimorph cantilever to form a hooklike out-of-plane shape. The controlled shape of the terminal provides a reliable contact at the interface. The properties of the proposed ZIF mu-connector are measured and analyzed, including the out-of-plane shape of the terminal, driving voltage, dc contact resistance, and the RF characteristics. The potential applications of the ZIF mu-connector include the fine-pitch high-speed interconnection, 3-D reworkable packaging, and the performance enhancement of many existing mu-connectors.     

Theory of Tin Whisker Growth: “The End Game”

Previous theories of the tin whisker growth proposing various dislocation mechanisms have been largely disproved. This paper presents a new and different theory for the mechanism of tin whisker growth. The theory addresses the fundamental requirements for Sn to diffuse to the base of the whisker grain and proposes a mechanism for whisker initiation and growth. Part of this is a theoretical proposal as to what makes the whisker grain different. The role of oxidation in whisker growth is also briefly addressed. Using this theory, attempts are made to illustrate how different whisker shapes are created. Finally, a proposed potential solution to tin whisker growth is presented, assuming that this theory is correct