无铅焊料的发展动态

本文综述了国外无铅焊料的研究开发动态，论述了无铅焊料应用的迫切性和必要性以及无铅焊料所具备的性能要求。     

电子封装无铅化趋势及瓶颈

欧盟通过法令限制电子用品中铅等有害物质的使用,含铅电子产品不久将退出市场,电子封装的无铅化已成必然趋势。焊料的无铅化是无铅封装的关键,目前无铅焊料的研究集中在Sn基焊料和导电胶粘剂两个方向文中介绍了三种典型无铅焊料体系的不足之处以及银导电胶研究中的三个难题,另外,关于无铅焊料还没有一个统一的标准     

无铅再流焊装置新的使用方法

1、引言 在电子领域,限制使用无铅焊料己成为一种义务,任何Sn—Pb焊料均由无铅焊料所代替。然而,在替代过程中,确立对替代焊料与器件的评价方法,整合支撑制造过程稳定可靠的制造技术,要付出相当大的努力,这其中涉及到很多问题,如工艺条件的改变方法、怎样选定装置与焊料、如何进行组装等。这里,将根据先进的研究成果和各种SMT研究交流所获得的有关无铅焊接的内容作一介绍。     

无铅焊料

无铅焊料合金成分的标准化目前还没有明确的规定。无铅化的核心和首要任务是无铅焊料。据统计全球范围内共研制出焊膏、焊丝、波峰焊棒材等100多种无铅焊料，但真正公认能用的只有几种。     

无铅条件下PCBA可制造性初探

在无铅条件下提高PCBA可制造性涉及的因素包括:PCB和元器件的制造、焊料(焊膏)的选用、设备的加工能力以及工艺技术等.本文通过分析PCBA可制造性的内涵,提出装联无铅化的关键所在,重点论述了在采用Sn-Ag-Cu和Sn-Cu钎料的条件下,实现无铅再流焊和无铅波峰焊的技术途径和对策.     

Sn—Bi无铅焊料的研究

通过总结传统Sn-Pb焊料的特点和研制无铅焊料的条件,综述Sn-Bi无铅焊料低熔点等优点以及脆性大、易偏析的缺点,分析添加In,Ag,AI,Sb等微量合金元素对Sn-Bi无铅焊料微观组织及性能的影响,提出了此合金系焊料的抗蠕变性、导电性、润湿性、拉伸性能等尚待完善的性能及可采用合金化、开发新型焊剂等新方法使Sn-Bi焊料成为理想的无铅焊料,为无铅焊料的进一步研究提供一定的参考.     

锡铜无铅焊料在电路板组装中的应用

在无铅组装工艺中,大多数电路板组装厂优先采用低成本焊料合金。没有添加剂的锡铜无铅焊料本身存在局限性,可是添加某种成份后,就能克服锡铜焊料通常所遇到的不足之处。文章分析了几种锡铜焊料相对于SAC焊料的特点,并叙述了它们在波峰焊和手工焊接工艺中的应用情况。     

无铅再流焊对塑料封装湿度敏感性的影响

环保方面的原因不断推动无铅电子产品的市场需求。目前印刷电路板的组装都是采用低熔点的锡铅合金对元件进行焊接。已确定的典型的锡铅焊料的替代物--无铅锡合金的熔点比传统锡铅焊料的熔点要高出30-40℃。相应的再流焊峰值温度必须提高,以保证可靠的连接。无铅焊料所要求的再流温度的提高,势必对于湿度敏感的塑封表面安装器件（SDMs)产生影响。本文研究了无铅焊料模拟组装传引脚器件（160MQFP和144LQFP）和层压结构封装的器件（27mm和35mmPBGA和low profile细间距BGA）的湿度/再流试验效果。用C模式扫描显微镜和横断面失效分析确认失效位置,评估湿度/再流试验。结果表明再流温度升至260℃时,湿度敏感性至少降低一个IPC/JEDEC等级。对于一种封装（160MQFP）的试验分析表明,即使对“干燥”封装在260℃下再流后,也发现了芯片表面裂纹。无铅焊料提高了温度要求,这就要求封装材料和封装设计的改进,以适应广泛的湿度/再流试验条件。     

无铅焊料

无铅焊接合金成分的标准化目前还没有明确的规定。无铅化的核心和首要任务是无铅焊料。据统计全球范围内共研制出焊膏、焊丝、波峰焊棒材等100多种无铅焊料，但真正公认能用的只有几科。     

无铅锡焊技术与无铅免清洗锡膏

本文主要论述无铅锡焊技术基础,无铅焊料的必要条件与流动焊工艺技术,无铅锡膏技术与再流焊工艺技术、无铅免清洗锡膏的性能特点、低银无铅锡膏与低温锡饿铜无铅锡膏等。     

有铅焊料焊接无铅BGA回流参数探索

航天电子产品尚未允许采用无铅焊接,而航天电子产品中采用的进口元器件多为无铅器件,因此需要对有铅焊料焊接无铅元器件进行研究。通过对无铅焊球和有铅焊料的焊接特性分析,设计数种回流参数,进行回流焊接试验,并对试验结果进行焊接分析,得出回流峰值温度为228℃~232℃,液相线(217℃)以上时间为50s~60s的回流曲线能较好完成有铅焊料对无铅BGA的焊接。     

Interfacial reactions in the pb-free composite solders with indium layers

A Pb-free composite solder is prepared with a Pb-free solder substrate and a plated-indium layer. The indium layer melts during the soldering process, wets the substrates, and forms a sound solder joint. Since the melting temperature of indium is 156.6°C, lower than that of the eutectic Sn-Pb, which is at 183°C, the soldering process can be carried out at a temperature lower than that of the conventional soldering process. Composite solder joints with three different Pb-free solders, Sn, Sn-3.5 wt.% Ag, and Sn-3.5 wt.% Ag-0.5 wt.% Cu, and two substrates, Ni and Cu, are prepared. The interfaces between the indium layer, Pb-free solder, and Ni and Cu substrate are examined. A good solder joint is formed after a 2-min reflow at 170°C. A very thick reaction zone at the indium/Pb-free solder interface and a thin reaction layer at the indium/substrate interface are observed.     

Failure Mechanisms and Crack Propagation Paths in Thermally Aged Pb-Free Solder Interconnects

Studies were conducted on Pb-free solder interconnects after aging for up to 1,000 h at temperatures of up to 150°C and submitting to drop testing. Cracks were shown to propagate along interphase boundaries (bulk solder/Cu₆Sn₅, Cu₆Sn₅/Cu₃Sn, Cu₃Sn/Cu, and bulk solder/Ag₃Sn) and along the bulk solder. Intergranular propagation in the solder was rarely observed, although cross sectioning showed evidence of that particular path. There is a transition in preferred crack propagation with aging time and temperature, from the intermetallic layer to the bulk solder. Sulfur was found in the fracture surfaces and may be a weakening agent in these solders.     

SAC305无铅钎料在Au／Ni镀层上的铺展性能研究

文章阐述了钎料润湿的基本原理,分析了关键工艺参数对钎料铺展性能的影响规律。针对温度、时间和表面状态等关键参数／状态,对无铅钎料SAC305在Au／Ni镀层上的铺展性能开展试验研究。结果表明,随着焊接温度和保温时间的增加,无铅钎料SAC305的铺展性能明显改善,铺展系数逐渐趋于100％。同时,通过RF的Ar等离子对钎料进行表面预处理也能够使其铺展性能有所改善。试验结果与理论分析具有较好的一致性。     

Board-level reliability of Pb-free solder joints of TSOP and various CSPs

To evaluate various Pb-free solder systems for leaded package, thin small outline packages (TSOPs) and chip scale packages (CSPs) including leadframe CSP (LFCSP), fine pitch BGA (FBGA), and wafer level CSP (WLCSP) were characterized in terms of board level and mechanical solder joint reliability. For board level solder joint reliability test of TSOPs, daisy chain samples having pure-Sn were prepared and placed on daisy chain printed circuit board (PCB) with Pb-free solder pastes. For CSPs, the same composition of Pb-free solder balls and solder pastes were used for assembly of daisy chain PCB. The samples were subjected to temperature cycle (T/C) tests (-65/spl deg/C/spl sim/150/spl deg/C, -55/spl deg/C/spl sim/125/spl deg/C, 2 cycles/h). Solder joint lifetime was electrically monitored by resistance measurement and the metallurgical characteristics of solder joint were analyzed by microstructural observation on a cross-section sample. In addition, mechanical tests including shock test, variable frequency vibration test, and four point twisting test were carried out with daisy chain packages too. In order to compare the effect of Pb-free solders with those of Sn-Pb solder, Sn-Pb solder balls and solder paste were included. According to this paper, most Pb-free solder systems were compatible with the conventional Sn-Pb solder with respect to board level and mechanical solder joint reliability. For application of Pb-free solder to WLCSP, Cu diffusion barrier layer is required to block the excessive Cu diffusion, which induced Cu trace failure.     

Studies of the mechanical and electrical properties of lead-free solder joints

The mechanical and electrical properties of several Pb-free solder joints have been investigated including the interfacial reactions, namely, the thickness and morphology of the intermetallic layers, which are correlated with the shear strength of the solder joint as well as its electrical resistance. A model joint was made by joining two “L-shaped” copper coupons with three Pb-free solders, Sn-3.5Ag (SA), Sn-3.8Ag-0.7Cu (SAC), and Sn-3.5Ag-3Bi (SAB) (all in wt.%), and combined with two surface finishes, Cu and Ni(P)/Au. The thickness and morphology of the intermetallic compounds (IMCs) formed at the interface were affected by solder composition, solder volume, and surface finish. The mechanical and electrical properties of Pb-free solder joints were evaluated and correlated with their interfacial reactions. The microstructure of the solder joints was also investigated to understand the electrical and mechanical characteristics of the Pb-free solder joints.     

Characteristics extraction of Pb free solder fillet profile forexternal feature inspection

First, the light intensity (luminance) distributions reflected from quad flat package (QFP) gull wing lead solder fillets were calculated by a simple geometrical model, using a modified diffusion equation. The dependence of the microscope angle &thetas; (= incident angle of light) on the highlight distribution from specular surfaces was obtained. Luminance distributions from top to toe fillets were experimentally observed, using Sn-Pb eutectic solder and two kinds of Pb-free solder (Sn-Ag-Cu and Sn-Zn-Bi systems) and the characteristics were compared with the simulation results. As the Sn-Pb solder fillet has a specular (or smooth) surface, it is not so difficult to identify the profiles of Sn-Pb fillets by changing &thetas;. On the other hand, the luminance distributions for Pb-free solder were strongly affected by the surface roughness. However, it is not impossible to extract the characteristics of the fillet profile if wetting properties of Pb-free solder are taken into consideration. It was found for Pb-free solder that the fillet formation characteristics extraction becomes easy if elements of Bi and In are doped because the wetting properties were improved. In addition, some problems with characteristics extraction of fillet formation for Pb free solder are discussed from the viewpoint of an external feature inspection system     

Effects of Phase Change of Pb-Free Flip-Chip Solders During Board-Level Interconnect Reflow

The impact of phase change (from solid to liquid) on the reliability of Pb-free flip-chip solders during board-level interconnect reflow is investigated. Most of the current candidates for Pb-free solder are tin-based with similar melting temperatures near 230 degC. Thus, Pb-free flip-chip solders melt again during the subsequent board-level interconnect reflow cycle. Solder volume expands more than 4% during the phase change from solid to liquid. The volumetric expansion of solder in a volume constrained by chip, substrate, and underfill creates serious reliability issues. The issues include underfill fracture and delamination from chip or substrate. Besides decreasing flip-chip interconnect reliability in fatigue, bridging through underfill cracks or delamination between neighboring flip-chip interconnects by the interjected solder leads to failures. In this paper, the volume expansion ratio of tin is experimentally measured, and a Pb-free flip-chip chip-scale package (FC-CSP) is used to observe delamination and solder bridging after solder reflow. It is demonstrated that the presence of molten solder and the interfacial failure of underfill can occur during solder reflow. Accordingly, Pb-free flip-chip packages have an additional reliability issue that has not been a concern for Pb solder packages. To quantify the effect of phase change, a flip-chip chip-scale plastic ball grid array package is modeled for nonlinear finite-element analysis. A unit-cell model is used to quantify the elongation strain of underfill and stresses at the interfaces between underfill and chip or underfill and substrate generated by volume expansion of solder. In addition, the strain energy release rate of interfacial crack between chip and underfill is also calculated     

Use of thermodynamic data to calculate surface tension and viscosity of Sn-based soldering alloy systems

A thermodynamic database for the Pb-free soldering alloy systems, which include Sn, Ag, Cu, Bi, and In, has been made using the CALPHAD method. The resulting thermodynamic properties of the Sn-based binary alloy systems were used to determine the surface tensions and viscosities. The surface tensions were calculated using Butler’s monolayer model and the viscosities by Hirai’s and Seetharaman’s models. Butler’s model was also used to determine the surface active element. The results for binary systems were extended to the Sn-based ternary systems (Sn-Ag-Cu, Sn-Ag-Bi). The surface tensions of commercial eutectic Sn-Pb and Sn-Pb-Ag solder alloys were measured by the sessile drop method. The measured values and other researchers’ results were compared with the calculated data.     

Effects of mechanical deformation and annealing on the microstructure and hardness of Pb-free solders

The microstructure property relations of several Pb-free solders are investigated to understand the microstructural changes during thermal and mechanical processes of Pb-free solders. The Pb-free solder alloys investigated include pure Sn, Sn-0.7% Cu, Sn-3.5% Ag, and Sn-3.8% Ag-0.7% Cu (in weight percent). To reproduce a typical microstructure observed in solder joints, the cooling rate, ingot size, and reflow conditions of cast alloys were carefully controlled. The cast-alloy pellets are subjected to compressive deformation up to 50% and annealing at 150°C for 48 h. The microstructure of Pb-free solders is evaluated as a function of alloy composition, plastic deformation, and annealing. The changes in mechanical property are measured by a microhardness test. The work hardening in Sn-based alloys is found to increase as the amount of alloying elements and/or deformation increases. The changes in microhardness upon deformation and annealing are correlated with the microstructural changes, such as recrystallization or grain growth, in Pb-free solder alloys.