在无铅组装中控制锡须

改变锡镀层的化学成分和进行烘烤,可以减轻锡须的生长采用无铅焊料使电子制造商更加明白了一种不寻常的金属现象,即通常所说的“锡须”,这个问题可能会影响电路板组装的可靠性。锡须通常发生在使用锡焊料的电路中,也会出现在印制板和元器件的涂层上。研究表明,在适当的湿度和温度条件下,只有在显微镜下才能看见的金属单晶细丝会从金属表面“长”出来,使PCB印制板的导线或器件引脚之间发生短路。可以通过向镀层中添加少量的铅(约为3%)来防止这种现象的出现,但现在欧盟的RoHS指令已经禁止铅的使用。由于RoHS指令要求在几乎所有的电子产品…     

避免锡晶须的可靠性问题-G T Galyon，Ron Gedney．Circuits Assembly，2004，15（8）：26～31（英文）

电子工业正开始转向无铅工艺和产品，以遵守欧盟减少危险物品的指令。减少危险物品指令要求电子工业的各个领域开发用于印制电路板锡铅(Sn-Pn)镀涂层的无铅替代品。对很多引脚铅框架供货商来说，一种简单的制造解决方案就是使用纯锡涂镀层。但是，已知纯锡容易形成针状凸起或针锡晶须，在间距紧密的电子线路中可能引起电气短路。本文回顾探讨关于锡晶须的文献、资料并向系统设计人员提供一些可能的解决方案。     

元件焊端镀层对无铅焊点可靠性的影响

电子产品无铅化的转变加速了对元件焊端镀层的无铅化研究，目前候选无铅镀层有纯锡、锡铋或锡铜合金。当然,Sn—Ag—Cu焊料和以上这些镀层结合而构成的焊点的可靠性是大家关注的重点。Sn—Ag—Cu焊料和元件焊端镀层若不相容会导致焊点变脆、强度降低、缺乏热疲劳抵抗力，特别在产品生命周期的后期。对焊点的可靠性影响尤为明显。在本中我们研究了由Sn-3．8Ag-0．7Cu焊料和不同的元件焊端镀层：纯锡、Sn-3Cu、锡铋合金(铋的重量百分比分别为1％、3％、6％)组成的SMT焊点可靠性。中给出了焊点金相分析、焊点老化前和老化后的引脚拉伸测试的试验结果，首次发表了SMT焊点加速热循环试验的结果，也提到了Sn—Bi和Sn—Pb镀层对波峰焊焊点可靠性的少量研究结果。通过我们的研究发现，所有试验元件的无铅镀层和Sn—Ag—Cu焊料构成的焊点性能至少和常规的锡铅焊点一样好。     

新产品与新技术(34)

全透明的挠性印制电路板通常的挠性印制电路板(FPC)是用透明或半透明薄膜基材,而导体用铜箔或银膏类不透明无机材料。现日本Mektron公司用聚酯(PET)基材和透明的有机导电材料,以及覆盖膜,制作出全透明FPC,光透射率达到80%。这     

新产品与新技术(52)

光学用途的白色挠性电路板日本Mektron公司开发出了白色FPC,计划今春开始在其国内和海外工厂投入大量生产。该白色FPC取得了UL认证,是以其新开发的白色聚酰亚胺为基材的挠性印制电路板,被用于LED照明和LCD背光单元。聚酰亚胺为基材的挠性电路板一般是棕色的,它们     

国内外覆铜板文献摘录（10）

应用于FPC的高弯曲性压延铜箔的开发;由2007JPCA Show看电路板材料技术发展趋势;废弃电路板回收处理工艺与设备;无铅焊接技术对印制电路板可靠性的影响;DOPO与邻甲酚醛环氧树脂反应特性的研究.     

无铅转移与过渡技术

阐述了在无铅转移过程中涉及的可制造性与可靠性问题,包括无铅转移对元器件、印制电路板与焊点的影响以及它们相互之间的兼容问题.重点论述了前向兼容与后向兼容、锡须、空洞与微空洞、可焊性涂层以及如何避免无铅转移中出现的问题.     

双面贴装电路板上BGA焊点的潜在失效机理

采用双面贴装回流焊工艺在FR4基板表面贴装Sn3.0Ag0.5Cu(SnAgCn)无铅焊点BGA器件,通过对热应力加速实验中失效的SnAgCu无铅BGA焊点的显微结构分析和力学性能检测,研究双面贴装BGA器件的电路板出现互连焊点单面失效问题的原因,单面互连焊点失效主要是由于回流焊热处理工艺引起的.多次热处理过程中,NiSnP层中形成的大量空洞是导致焊点沿(Cu,Ni)6Sn5金属间化合物层和Ni(P)镀层产生断裂失效的主要因素.改变回流焊工艺是抑制双面贴装BGA器件的印制电路板出现互连焊点单面失效问题的关键.     

FPC垂直连续电镀镀铜均匀性探究

挠性电路板（FPC）镀铜厚度的均匀性好坏,影响着产品质量,对多层挠性板产品可靠性和稳定性,以及对特性阻抗要求高的产品,镀铜厚度均匀性的重要性更加突出。本文研究了垂直连续电镀对挠性板镀铜厚度均匀性的影响因素,对改善镀层均匀性及制程进行讨论。     

微连接技术在3D-WLP中可靠性的研究现状

为了研究新型技术对微连接可靠性的影响,阐述了微连接技术在倒装焊、热压力焊、硅通孔技术中的应用情况。对微连接技术在晶圆级封装(3D-WLP)中影响其可靠性的常见问题及主要因素进行了综述。分析表明,在常见的Sn基无铅焊料与Cu基板的反应过程中,金属间化合物层主要由Cu6Sn5和少量Cu3Sn组成。柯肯达尔孔洞、电迁移、金属间化合物的机械性能、锡须及尺寸效应是影响微连接可靠性的常见问题。在此基础上,归纳并探讨了焊料成分、微量合金、稀土元素、镀层及粉末精度等影响微连接可靠性主要因素的研究情况和发展趋势。     

Key reliability concerns with lead-free connectors

This paper presents the key reliability issues of lead-free connectors. The paper first discusses electrical contact resistance of lead-free and lead-based solder-dipped contacts under various aging conditions. Then, the fretting corrosion of lead-free contacts is discussed. Finally, the reliability due to tin whiskers induced by mating pressure between connector contact elements is presented.     

Statistical analysis of tin whisker growth

As the result of the global movement to lead-free electronics, companies which assemble semiconductor devices are switching from finishes incorporating lead to pure tin or high tin lead-free alloys. This transition has resulted in a reliability issue, concerning the formation of conductive tin whiskers which can grow across leads of a package and cause current leakage or short circuits. This paper presents the results of an experimental tin whisker growth study of bright tin on brass substrates. A probabilistic model is applied to describe the phenomenon of whisker growth in terms of whisker density, length and growth rate.     

钎料稀土相表面Sn晶须生长的研究

为了解对钎料可靠性影响极大的Sn晶须的生长机理,系统研究了钎料稀土相CeSn3、LaSn3及ErSn3表面Sn晶须生长的影响因素.结果表明:稀土相的氧化倾向与时效温度共同影响其表面Sn晶须的生长.室温时效条件下,在稀土相CeSn3与LaSn3表面易出现小尺寸的线状Sn晶须,直径为0.1～0.2 μ.m,而在稀土相ErS...     

Modelling methodology for thermal analysis of hot solder dip process

The shift of electronics industry towards the use of lead-free solders in components manufacturing brought also the challenge of addressing the problem of tin whiskers. Manufacturers of high reliability and safety critical equipment in sectors such as defence and aerospace rely increasingly on the use of commercial-of-the-shelf (COTS) electronic components for their products and systems. The use of COTS components with lead-free solder plated terminations comes with the risks for their long term reliability associated with tin whisker growth related failures. In the case of leaded type electronic components such as Quad Flat Package (QFP) and Small Outline Package (SOP), one of the promising solutions to this problem is to “re-finish” the package terminations by replacing the lead-free solder coatings on the leads with conventional tin–lead solder. This involves subjecting the electronic components to a post-manufacturing process known as Hot Solder Dip (HSD). One of the main concerns for adopting HSD (refinishing) as a strategy to the tin whisker problem is the potential risk for thermally induced damage in the components when subjected to this process.     

Whisker test methods of JEITA whisker growth mechanism for test methods

As the industry moves toward lead-free electronics, a replacement for the traditional tin-lead plating is required. Pure tin is the coating of choice for many suppliers, but there is a well-known problem with the growth of tin whiskers. A whisker is comprised of a tin crystal which, after an incubation period, grows with time. Whiskers may grow long enough to cause shorts in electronic circuits. A test method is required for evaluating the reliability risk inherent in tin whiskers. The Japanese government requested the Japan Electronics and Information Technology Industries Association (JEITA) to establish test methods for solderability, reliability, whisker, and migration for lead-free electronics. The goal of this subcommittee was to propose the test methods for tin whiskers by March 2004. The committee carried out a literature survey, and developed hypotheses for whisker growth, which was then applied to test methods. Two types of studies were undertaken. One is a fundamental study to verify the hypothesis. The other looked at accelerated tests to develop recommended test conditions. As a result, we found that the diffusion of copper, oxidation, and thermal cycling all influence tin whisker growth. This paper describes the results of the fundamental studies and reports on the JEITA whisker growth mechanism.     

锡基无铅钎料的性能研究与新进展

介绍了现阶段锡基无铅钎料的使用和生产情况,归纳了钎料合金的特点,综述了不同系列无铅钎料的熔化特性、焊后的剪切强度及可焊性等,总结了目前无铅钎料研究所取得的新成果、新进展以及存在的问题。从锡晶须,虚焊等方面指出了无铅钎料可靠性的不足,并提出部分解决方案,指出了无铅钎料的发展趋势和应用前景。     

无铅电子元器件的锡须风险评估与对策分析

针对无铅器件的锡须问题,重点分析锡须的形成机理、诱发因素以及使用风险,探讨抑制锡须生长的有效措施和风险规避对策,为无铅电子元器件在军工、航空航天领域的高可靠使用提供借鉴依据和指导。     

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     

Tin Whisker Electrical Short Circuit Characteristics—Part I

Existing risk simulations make the assumption that when a free tin whisker has bridged two adjacent exposed electrical conductors, the result is an electrical short circuit. This conservative assumption is made because shorting is a random event that has a currently unknown probability associated with it. Due to contact resistance, electrical shorts may not occur at lower voltage levels. In this experiment, we study the effect of varying voltage on the breakdown of the contact resistance which leads to a short circuit. From this data, we can estimate the probability of an electrical short, as a function of voltage, given that a free tin whisker has bridged two adjacent exposed electrical conductors. In addition, three tin whiskers grown from the same Space Shuttle Orbiter card guide used in the aforementioned experiment were cross sectioned and studied using a focused ion beam (FIB).     

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.