Evaluation of Electrochemical Migration on Flexible Printed Circuit Boards with Different Surface Finishes

This study investigated the influence of three parameters, the distance between electrodes, bias voltage, and surface finish material, on the electrochemical migration (ECM) in flexible printed circuit boards (FPCBs) during water drop testing. The ECM rate increased when the distance between electrodes of opposite polarity was decreased and the bias voltage was increased. Irrespective of the distance between the electrodes and the bias voltage, the growth rate of dendrites from the cathode to the anode decreased with the following order of surface finish material: Cu, electroless Ni, and electroless nickel-immersion gold. The order of the corrosion rate in distilled water with pH 6.5 was the same. Therefore, the ECM rate on the FPCB increased with increasing corrosion rate of the FPCB surface finish material.     

Impact of board and component metallizations on microstructure and reliability of lead-free solder joints

Aging and accelerated thermal cycling (ATC) have been performed on 2512 chip resistors assembled with Sn3.8Ag0.7Cu (wt.%) solder. The boards were finished with immersion Ag (IAg), electroless nickel/immersion gold (ENIG), and hot air solder leveling Sn–Pb eutectic solder (HASL), and the components’ terminations were finished with 100% Sn and Sn8.0Pb (wt.%). The boards were reflowed with an average cooling rate of 1.6 °C/s. It was found that the microstructure and reliability of the solder joints depended on the board surface finish. The boards containing small amounts of Pb (from board/component terminations) were the most reliable. Solder joints to copper showed a significantly higher number of cycles to first failure than the joints on nickel. Better reliability of the Sn3.8Ag0.7Cu/Cu joints was attributed to an increased copper content in the bulk due to substrate dissolution.     

Effect of Interfacial Reactions on the Reliability of Lead-Free Assemblies after Board Level Drop Tests

The reliability of lead-free electronic assemblies after board level drop tests was investigated. Thin small outline package (TSOP) components with 42 FeNi alloy leads were reflow soldered on FR4 printed circuit boards (PCBs) with Sn3.0Ag0.5Cu (wt%) solder. The effects of different PCB finishes [organic solderability preservative (OSP) and electroless nickel immersion gold (ENIG)], multiple reflow (once and three times), and isothermal aging (500 h at 125°C after one time reflow) were studied. The ENIG finish showed better performance than its OSP counterparts. With the OSP finish, solder joints reflowed three times showed obvious improvement compared to those of the sample reflowed once, while aging led to apparent degradation. The results showed that intermetallic compound (IMC) types, IMC microstructure and solder microstructure compete with each other, all playing very important roles in the solder joint lifetime. The results also showed that it is important to specify adequate conditions for a given reliability assessment program, to allow meaningful comparison between results of different investigators.     

浅析化镍金焊接后黑垫产生原因

化学沉镍金是一种能满足大多数的组装要求的可行的表面涂层,不仅具备抗氧化功能,并有平整的PAD表面,在电子\通讯领域有十分广泛之用途,但化镍金焊接后存在黑垫问题一直困扰PCB制造商、药水供应商以及下游SMT客户,目前PCB业界对化镍金焊接后黑垫产生原因比较模糊未有明确定义,本文将通过试验对比对化镍金板焊接后出现黑垫产生原因进行分析及探讨。     

Drop impact reliability testing for lead-free and lead-based soldered IC packages

Board-level drop impact testing is a useful way to characterize the drop durability of the different soldered assemblies onto the printed circuit board (PCB). The characterization process is critical to the lead-free (Pb-free) solders that are replacing lead-based (Pb-based) solders. In this study, drop impact solder joint reliability for plastic ball grid array (PBGA), very-thin quad flat no-lead (VQFN) and plastic quad flat pack (PQFP) packages was investigated for Pb-based (62Sn–36Pb–2Ag) and Pb-free (Sn–4Ag–0.5Cu) soldered assemblies onto different PCB surface finishes of OSP (organic solderability preservative) and ENIG (electroless nickel immersion gold). The Pb-free solder joints on ENIG finish revealed weaker drop reliability performance than the OSP finish. The formation of the brittle intermetallic compound (IMC) Cu–Ni–Sn has led to detrimental interfacial fracture of the PBGA solder joints. For both Pb-based and Pb-free solders onto OSP coated copper pad, the formation of Cu₆Sn₅ IMC resulted in different failure sites and modes. The failures migrated to the PCB copper traces and resin layers instead. The VQFN package is the most resistant to drop impact failures due to its small size and weight. The compliant leads of the PQFP are more resistant to drop failures compared to the PBGA solder joints.     

Major factors to the solder joint strength of ENIG layer in FC BGA package

Since electroless nickel and immersion gold (ENIG) process was implemented as the surface finish of printed circuit board (PCB) substrate, there have been lots of reports on the brittle fracture between the Ni–P (phosphorous) layer and solder which results in the poor solder joint strength performance. Galvanic corrosion during immersion Au plating process and P-content in Ni–P layer were considered as major factors in the solder joint strength of ENIG layer in this investigation. The attempt to reduce the galvanic corrosion attack in Ni–P layer was made by changing immersion Au plating process to partial electroless Au plating process. Reducing the galvanic corrosion attack was proved to be effective to improve the solder joint strength of ENIG layer. Evaluation of the solder joint performances in variation with the thickness of the Ni layer leads to the conclusion that the thicker Ni layer has the better solder joint strength performances. The result also showed that higher P-content in Ni layer is more favorable to the solder joint strength.     

镍层耐硝酸腐蚀性测试——一种简单的预先探测ENIG镍层“黑盘”现象的测试方法

随着更加精细的SMT、BGA等表面贴装技术的运用,化学沉镍金(ENIG)作为线路板最终表面处理得到了越来越广泛的应用,同时可怕的“黑盘”现象也随之更广泛地“流行”起来,直接导致贴装后元器件焊接点不规则接触不良。为了贯彻执行最好的流程控制和采取有效的预防措施,了解这种焊接失败的产生机理是非常重要的,及早的观测到可能发生“黑盘”现象的迹象变得同样关键。本文介绍了一种简单的预先探测ENIG镍层“黑盘”现象的测试方法-镍层耐硝酸腐蚀性测试,这种测试可以用于作为一种常规的测试方法监测一般化学沉镍溶液在有效使用寿命范围内新鲜沉积的镍层的质量。利用Weibull概率统计分析在不同的金属置换周期(MTO)下镍层的可靠性能表现。结合试验结果得出了一个镍层耐硝酸腐蚀性的判定标准。     

Influence of ENIG defects on shear strength of pressureless Ag nanoparticle sintered joint under isothermal aging

An Ag nanoparticle sintering is a promising die attach method for use in high-temperature electronics and could potentially be substituted for conventional high Pb-containing solders. An electroless Ni/immersion Au (ENIG) is a common surface finish for solder, wire bonding, and Ag nanoparticle sintering. However, there is no report on the influence of ENIG defects on the shear strength and bonding quality of Ag nanoparticle sintered joint. In this study, the relationship between the presence of ENIG defects and shear strength of Ag nanoparticle sintered joint after bonding and isothermal aging test using two types of ENIG surface finished Cu substrates (with ENIG defects and without defects) was investigated. The initial shear strength of Ag sintered joint showed similar values of approximately 36 MPa despite the presence of ENIG defects. However, the ENIG defects could affect the shear strength degradation of Ag sintered joint after thermal aging at 250 °C. This degradation came from the Ni oxidation by the presence of ENIG defects and subsequent brittle fracture of joint alone the Ni oxidation layer.     

关于沉金镍腐蚀问题的研究

近年来,在无铅化的大背景下,PCB产业随之发生重大变化,表面处理工艺亦然如此。沉金工艺在诸多选择中可谓异军突起,因其固有的优点,其所占比重迅速提高,当前已经占据PCB总量的半壁江山。但沉金工艺又有其难以克服的顽疾,镍腐蚀问题就是一直困扰沉金的难题。关于镍腐蚀问题,国内外同行都做了许多方面的研究,尽管对镍腐蚀控制取得长足的进步,然而对此问题的认识、理解依旧不尽而一。实际生产中镍腐蚀问题仍是难以根除,沉金焊接性投诉还是时有发生。本章由生产实际出发结合相关实验测试,对镍腐蚀问题的关键影响因素进行了独特的探讨,分析其内在机理,以期能够帮助生产实际,为业内同行改善沉金工艺品质提供切实、有效的参考。     

Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.     

尘土颗粒的介电特性对电路板电化学迁移的影响

随着印制电路板上相邻导线之间间距不断减小,潮湿环境下的尘土颗粒改变了诱发电化学迁移失效的电场分布.本文采用有限元法对尘土污染的带电电路板上平行导线间的电场分布进行仿真分析,得出尘土介电常数和带电量对电场分布的作用机理,预测电场分布的改变对电化学迁移晶枝生长路径的影响,最后模拟实验验证晶枝生长的路径与电场分布的关系,并讨论尘土对电化学迁移失效时间的影响.     

Effects of Surface Finishes and Current Stressing on Interfacial Reaction Characteristics of Sn-3.0Ag-0.5Cu Solder Bumps

The effects of surface finishes on the in situ interfacial reaction characteristics of ball grid array (BGA) Sn-3.0Ag-0.5Cu lead-free solder bumps were investigated under annealing and electromigration (EM) test conditions of 130°C to 175°C with 5.0 × 10³ A/cm². During reflow and annealing, (Cu,Ni)₆Sn₅ intermetallic compound (IMC) formed at the interface of electroless nickel immersion gold (ENIG) finish. In the case of both immersion Sn and organic solderability preservative (OSP) finishes, Cu₆Sn₅ and Cu₃Sn IMCs formed. Overall, the IMC growth velocity of ENIG was much lower than that of the other finishes. The activation energies of total IMCs were found to be 0.52 eV for ENIG, 0.78 eV for immersion Sn, and 0.72 eV for OSP. The ENIG finish appeared to present an effective diffusion barrier between the Cu substrate and the solder, which leads to better EM reliability in comparison with Cu-based pad systems. The failure mechanisms were explored in detail via in situ EM tests.     

Additive Effect of Kirkendall Void Formation in Sn-3.5Ag Solder Joints on Common Substrates

The Sn-3.5Ag and Sn-3.5Ag-0.2Co-0.1Ni lead-free solders were investigated on common electronics substrates, namely, organic solderability preservative (OSP) and electroless Ni/immersion Au (ENIG) surface finishes. The formation of Kirkendall voids at the interfacial region during isothermal solid aging was explored. For the Sn-3.5Ag-0.2Co-0.1Ni/OSP solder joint, the Kirkendall voids were present after isothermal solid-state aging at higher temperature (e.g., 150°C); however, the size of voids did not change remarkably with prolonged aging time due to the depressed Cu₃Sn layer growth. For ENIG surface finishes, the 0.2Co-0.1Ni additions seemed to enhance the longitudinal groove-shaped voids at the Ni₃P layer; however, void formation at the solder/Ni₃Sn₄ interface was effectively reduced. This might be attributed to the reduced Sn activity in the solder matrix and the suppressed Ni-P-Sn layer formation.     

PTCR元件化学镀镍电极的最优工艺条件

采用正交试验法研究了络合剂浓度、镀液 pH值、施镀温度与时间对BaTiO3 陶瓷PTCR元件化学镀镍电极的影响 ,得到制备化学镀镍电极的最优工艺条件。重复试验证明 ,在该工艺条件下获得的以Ni镀层为底层电极的PTCR元件 ,其耐电压和耐工频电流冲击性能良好。     

化学镍/化学钯/浸金的表面涂覆层的可焊性和可靠性

概述了化学镍/化学钯/浸金(ENEPIG)表面涂(镀)覆层的优点。它比化学镍/浸金(ENIG)有更好的可焊接性和焊接可靠性。化学镍/化学钯/浸金表面涂(镀)覆层应该是有发展前景的。     

SnAgCu钎料焊点电化学迁移的原位观察和研究

通过恒定电压条件下的水滴实验,对Sn-4Ag-0.5Cu钎料焊点的电化学迁移(ECM)行为进行了原位观察和研究。结果表明,树枝状的金属沉积物总是在阴极上生成,并向着阳极方向生长,在接触阳极的瞬间,发生短路失效。外加电压不超过2 V时,形成的沉积物数目往往比较少并且粗大。焊点间距的减少和外加电压的增加都会使得ECM造成的短路失效时间显著缩短。当钎料不能完全包裹焊盘或者焊盘局部位置上钎料的厚度很薄时,发生ECM的金属除了来自钎料焊点,还来自Cu焊盘;钎料中的Ag不发生迁移。     

Interfacial reaction and failure mode analysis of the solder joints for flip-chip LED on ENIG and Cu-OSP surface finishes

The interfacial reactions and failure modes of the solder joints for flip-chip light emitting diode (LED) on electroless nickel/immersion gold (ENIG) and Cu with organic solderability preservatives (Cu-OSP) surface finishes were investigated in this study. The experimental results demonstrate that the interfacial reactions in the Au/Sn–Ag–Cu(SAC)/ENIG and Au/SAC/Cu systems are different but the failure mechanisms of the two types of solder joints are similar during the shear test. For the Au/SAC/ENIG system, the Au layer on the surface finish of the diodes dissolved into the molten solder and transformed into a continuous (Au, Ni)Sn₄ IMC layer at the diode/solder interface during reflow and the interfacial IMC at the solder/ENIG interface is dendritic Ni₃Sn₄ IMC grains which are surrounded by (Au, Ni)Sn₄. For the Au/SAC/Cu system, however, no IMC layers can be observed at the diode/solder interface. The interfacial IMC at the solder/Cu interface is (Cu, Au)₆Sn₅ and a Cu₃Sn IMC layer at the (Cu, Au)₆Sn₅/Cu interface. Tiny (Au, Cu)Sn₄ IMC grains distribute in the solder layer and surround the (Cu, Au)₆Sn₅ grains. For the two types of systems, the primary failure mode for the cathode is due to the broken of the Si-based insulation layer which led to a high residue stress and poor connection between the Si-based layer and the solder layer. Meanwhile, the failure of the solder joint for the anode is mainly because of the failure of the solder layer under the conductive via. The crack generally forms at this area and then propagated along the diode or the diode/solder interface.     

Development of lead-free wave soldering process

Lead-free wave soldering was studied in this work using a 95.5Sn/3.8Ag/0.7Cu alloy. A process DOE was developed, with three variables (solder bath temperature, conveyor speed, and soldering atmosphere), using a dual wave system. Four no-clean flux systems, including alcohol- and water-based types, were included in the evaluation. A specially designed "Lead-Free Solder Test Vehicle", which has various types of components, was used in the experiments. Both organic solderability preservative (OSP) and electroless nickel/immersion gold (Ni/Au, or ENIG) surface finishes were studied. Soldering performance (bridging, wetting and hole filling) was used as the responses for the DOE. In addition, dross formation was measured at different solder bath temperatures and atmospheres. Dross formation with Sn/Ag/Cu bath was compared to that with eutectic Sn/Pb bath. Regarding the connector-type component, a pad design giving the best soldering performance was evaluated based on the DOE results. Finally, a confirmation run with the optimum flux and process parameters was carried out using the Sn/Ag/Cu solder, and a comparative run was made with the Sn/Pb solder alloy and a no-clean flux used in production. The soldering results between the two runs indicate that with optimum flux and process parameters, it is possible to achieve acceptable process performance with the Sn/Ag/Cu alloy.     

Effect of solder resist dissolution on the joint reliability of ENIG surface and Sn–Ag–Cu solder

The electroless nickel immersion gold (ENIG) process results in surface defects, such as pinholes and black pads, which weaken the solder joint and eventually degrade the reliability of the PCB. Contamination of the plating solutions, including dissolution of the solder resist (SR), can be a cause of the pinholes and black pads. This study examined the effects of SR dissolution on the solder joint reliability and electroless Ni plating properties. Electroless Ni plating was performed by adding 1 to 10 ppm hardener (melamine) to the fresh Ni solution. Many black pads were observed in the 7 and 10 ppm hardener-added surfaces. In addition, the content of P was highest when 7 and 10 ppm hardener was added. The ball shear tests were carried out to confirm the joint reliability between the ENIG surface with hardener-added and the Sn-3.0Ag-0.5Cu solder (SAC 305). The ball shear strength decreased with increasing dissolution of the hardener. In particular, the shear strength was the lowest at 7 and 10 ppm hardener addition. In addition, the failure mode of the solder joint was changed from ductile to brittle mode with increasing hardener addition. That is, as the hardener additive increases, intermetallic compound (IMC) phases were changed from (Cu,Ni)₆Sn₅ to (Cu,Ni)₃Sn₄ and Cu₆Sn₅ (brittle structure).     

Galvanically Replaced,Single‐Bodied Lithium‐Ion Battery Fabric Electrodes

Despite extensive research on flexible/wearable power sources, their structural stability and electrochemical reliability upon mechanical deformation and charge/discharge cycling have not yet been completely achieved. A new class of galvanically replaced single‐bodied lithium‐ion battery (LIB) fabric electrodes is demonstrated. As a proof of concept, metallic tin (Sn) is chosen as an electrode active material. Mechanically compliable polyethyleneterephthalate (PET) fabrics are conformally coated with thin metallic nickel (Ni) layers via electroless plating to develop flexible current collectors. Driven by the electrochemical potential difference between Ni and Sn, the thin Ni layers are galvanically replaced with Sn, resulting in the fabrication of a single‐bodied Sn@Ni fabric electrode (Sn is monolithically embedded in the Ni matrix on the PET fabric). Benefiting from the chemical/structural uniqueness and rationally designed bicontinuous ion/electron transport pathways, the single‐bodied Sn@Ni fabric electrode provides exceptional redox reaction kinetics and omnidirectional deformability (notably, origami‐folding boats), which lie far beyond those attainable with conventional LIB electrode technologies.