A Neurofuzzy Method for the Evaluation of Soldering Global Quality Index

Recently, surface mount technology is extensively used in the production of printed circuit boards due to the high level of miniaturization and to the increase of density in the electronic device integration. In such production process several defects could occur on the final electronic components, compromising its correct working. In this paper a neurofuzzy solution to process information deriving from an automatic optical system is proposed. The designed system provides a global quality index of a solder joint, starting from the assessment of a human inspector. This target is achieved by reproducing the modus operandi of the expert, evaluating the area, the shape and the barycentre position of a solder joint. The proposed architecture is constituted by three supervised neural networks and two fuzzy rule-based modules which automate expert's work and provide a refined evaluation of the quality. The considered solution presents some attractive advantages: a complex acquisition system is not needed, equipment costs could be reduced by shifting the assessment of a solder joint on the fuzzy parts. Moreover, intermediate variables used in the method could be employed as control parameters in the production process under analysis.      

临时阻焊膜在高频电路板表面组装中的应用

高频电路板不能涂覆永久性阻焊膜,因为阻焊膜作为一种电介质将严重影响电路板的高频性能.但是,没有阻焊膜会给高频电路板上表面组装元件的焊接带来一系列的问题,出现诸如:焊点焊料缺乏、元件歪斜、元件脱离焊盘、元件立碑等现象,给电路板带来严重的外观和可靠性的问题.现在,我们可以用临时阻焊膜来解决这一问题.     

Solder connections for high frequency applications between flexible and rigid printed circuit boards

This article examines the backplane connector technology for high frequency use and presents a novel concept for shielded enclosures without air vents. The experimental soldering of connections between a flexible printed circuit (flex, for short) and a FR4-board as part of a backplane connector is presented. A very high contact density is reached by soldering vias on the flex to pads on the FR4; this soldering was performed both with ordinary Sn/Pb solder material and as well with lead-free solder alloy. The solder joints were examined using a scanning electron microscope (SEM) and the electrical properties of the flex/solder joint structure were measured.     

SMT/THT混装回流焊工艺技术研究

含有大量表面贴装元件和少量通孔插装元件的高密度混装型印制板是电子装联技术的主要类型,这种SMT/THT混装型印制板采用传统的回流焊接无法一次完成印制板的组装。采用波峰焊或手工焊接会增加工序而且可能使印制板翘曲变形,采用SMT/THT混装回流焊则可以较好的解决问题。通过选用可用于回流焊的表面贴装和通孔元件制作样件,对通孔元件的焊膏印刷模板的合理设计,导入SMT/THT混装回流焊工艺,完成试验板的组装和焊接,焊点质量满足要求。     

波峰焊焊接空洞问题的分析与解决方法

无铅波峰焊接工艺是常见组装焊接工艺之一,其中焊接空洞是较严重的问题。焊接空洞的存在是电子学产品的潜在隐患,影响产线补板效率,增加生产成本。以实际生产过程中混装电路板过波峰焊时遇到的通孔焊接空洞现象进行分析,主要从印制板制程工艺与波峰焊工艺参数这两方面提出解决方案。     

Interfacial reaction and mechanical reliability of PTH solder joints with different solder/surface finish combinations

The effects of minor Ni addition (0.05 wt.%) on the microstructures and mechanical reliability of the lead-free solder joints used in the pin through hole (PTH) components were carefully investigated using a scanning electron microscope (SEM), a field-emission electron probe x-ray microanalyzer, and a pull tester. The PTH walls (i.e., Cu) of printed circuit boards (PCBs) were coated with organic solderability preservative (OSP) or electroless nickel/immersion gold (ENIG) surface finish before soldering. During soldering, the pins of the electronic components were first inserted into the PTHs deposited with OSP or ENIG, and then joined using a Sn–3Ag–0.5Cu (SAC) solder bath through a typical wave-soldering process. After wave soldering, a rework (the second wave soldering) was performed, where an SAC or Sn–0.7Cu–0.05Ni (SCN) solder bath was employed. The SCN joints were found to possess a higher tensile strength than the SAC ones in the OSP case. The sluggish growth of Cu₃Sn, along with few Kirkendall voids at the solder/Cu interface caused by minor Ni addition into the solder alloy (i.e., SCN), was believed to be the root cause responsible for the increase in the strength value. However, the mechanical strength of the PTH components was revealed to be insensitive to the solder composition in the alternative case where an ENIG was deposited over the PTH walls. The implication of this study revealed that minor addition of Ni into the solder is beneficial for the solder/Cu joints, but for the solder/Ni(P) joints.     

Study on Cu particles-enhanced SnPb composite solder

The Sn-Pb solder is widely used in the electronics industry. With the development of surface mount technology and miniaturization of elements, mechanical properties of the solder are critical. Creep resistance and size stability of soldered joints are important for optical electronics. In the present work, Cu particles with a size of about 8 μm were added to the eutectic 63Sn-37Pb solder to improve the creep property of the soldering alloy. The contents of the Cu particles are 5 vol.% and 10 vol.% separately. The solder matrix is 63Sn37Pb particles with a normal size of 43 μm. The composite solder pastes are manufactured from a mixture of these particles with no-clean flux. Under reflow soldering, the metal particles were uniformly dispersed in the Sn-Pb alloy, and very thin intermetallic compounds were formed between the particles and matrix. To simulate practical soldering of printed circuit boards, a specially designed mini specimen with lap joint is used for the creep-rupture test. For the condition of ambient temperature, the creep-rupture lifetime of the soldered joint can be increased by one order quantitatively using the composite solder compared to the 63Sn37Pb eutectic solder. Other mechanical properties are measured also. In addition, the wetting property of the enhanced solder is good through the wettability test.     

几种SMT焊接缺陷及其解决措施

主要针对采用了表面组装技术 (SMT)生产的印制电路组件中出现的焊料球、立片、桥接等几种焊接缺陷现象进行分析 ,并将有效的解决措施进行了经验性总结。     

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.     

波峰焊过程产生板脏问题的分析和解决方法

实施无铅化工艺后,导入新的无铅波峰焊设备.在生产过程中,有时会发现被焊接板底面焊点之间粘有脏物,导致电路漏电,造成电气故障.经分析,主要原因为部件板在过波峰焊时,因设备相关问题,锡炉锡渣粘到印制板上造成.根据一年多跟踪与分析,提出相应解决办法.     

Simplified Measuring Method of Copper Concentration in Solder Bath Utilizing Copper Dissolution

In lead-free soldering on printed circuit boards (PCBs), the dissolution and disappearance of PCB copper electrodes in solder bath has been a problem for the dip soldering method, such as wave soldering, in which PCBs are dipped in molten solder. In the former report, it has been estimated the influence of solder composition, temperature, and flowing velocity on the dissolution of copper electrodes by the experiment of the dip soldering process and clarified the dissolution rate for Sn–3.0Ag–xCu alloys, which is defined by solder temperature and copper concentration. Based on above study, the simplified measuring method of copper concentration in the solder bath has been developed, which uses the dissolution of wedge-shaped copper pattern formed on FR-4 PCB. As a result of the experiment on dipping the PCB in solder of Sn–3.0Ag–xCu (${rm x}=0.5, 0.9, 1.2 ~{hbox {and}}~ 1.5$) alloys, it has been confirmed that the method enables the measuring copper concentration in the solder bath easily.      

Origin of Surface Defects in PCB Final Finishes by the Electroless Nickel Immersion Gold Process

We suggest a unique mechanism for surface defect generation causing solder joint or bonding failures in printed circuit boards (PCBs). Surface defects can be defined as corroded holes or spikes of the Ni-P layers on the soldering or wire bonding pads of PCBs. The typical defects are the black pad or pinhole pad defects generated after final finishing by the electroless nickel immersion gold (ENIG) process. Once corroded voids or spikes are plentifully created in nickel/gold interfaces, the bonding strength of solder or wire bonding joints is reduced. Therefore, it is important to characterize the details of these surface defects. In this paper, the defect microstructures and the P content variation with the ENIG processes are investigated. The surface defect selectivity with pad size and pad connectivity is suggested based on the key findings of P content variation. An overall mechanism is proposed based on a mixed mode of concentration cell corrosion and galvanic cell corrosion. Based on these results, more reasonable root causes are suggested.     

Sn–Ag–Cu Soldering Reliability as Influenced by Process Atmosphere

To develop an optimal surface mount reflow soldering process with Sn–Ag–Cu, the influences of atmosphere and cooling speed on soldering reliability have been examined by using Sn plated chip components and of Pd plated small outline packages (SOPs) on a printed circuit board (PCB). Typical three Sn–Ag–Cu alloy pastes, i.e., Sn–3.0wt%Ag–0.5wt%Cu, Sn–3.8wt%Ag–0.75wt%Cu, and Sn–4.0wt%Ag–0.9wt%Cu, were used for reflow soldering in air or ${hbox {N}} _{2}$ atmospheres. In the case of chip component joints, the solder compositions, cooling speed, and atmospheres during reflow treatment slightly affect the dendritic microstructure of the solder fillets. In contrast, these parameters rarely affect the solder wettability both on boards/components and shear strengths of the solder joints. In the case of the SOP joints, however, the atmospheres in reflow treatment and the fluxes strongly affect the appearances of solder fillet surfaces structure. Despite the types of solder fluxes, ${hbox {N}} _{2}$ process atmosphere obviously improved wettability of the solders on the lead-frames of the SOP. Moreover, the scatter in shear strengths becomes smaller and the wetting of solders on the lead-frames becomes stabler in ${hbox {N}} _{2}$ atmosphere than in air atmosphere.      

Drop impact reliability of Sn–1.0Ag–0.5Cu BGA interconnects with different mounting methods

The poor drop-shock resistance of near-eutectic Sn–Ag–Cu (SAC) solder interconnects drives the research and application low-Ag SAC solder alloys, especially for Sn–1.0Ag–0.5Cu (SAC105). In this work, by dynamic four-point bend testing, we investigate the drop impact reliability of SAC105 alloy ball grid array (BGA) interconnects with two different surface mounting methods: near-eutectic solder paste printing and flux dipping. The results indicate that the flux dipping method improves the interconnects failure strain by 44.7% over paste printing. Further mechanism studies show the fine interfacial intermetallic compounds (IMCs) at the printed circuit board side and a reduced Ag content inside solder bulk are the main beneficial factors overcoming other negative factors. The flux dipping SAC105 BGA solder joints possess fine Cu₆Sn₅ IMCs at the interface of solder/Cu pads, which increases the bonding strength between the solder/IMCs and the fracture resistance of the IMC grains themselves. Short soldering time of flux dipping joints above the solder alloy liquidus mitigates the growth of interfacial IMCs in size. In addition, a reduced Ag content in flux dipping joint bulk causes a low hardness and high compliance, thus increasing fracture resistance under higher-strain rate conditions.     

Solder joint crack propagation analysis of wafer-level chip scalepackage on printed circuit board assemblies

The solder-joint reliability of a low-cost wafer-level chip scale package (WLCSP) on printed circuit board (PCB) under thermal fatigue is studied. The solder joints are subjected to thermal cycling and their crack lengths at different thermal cycles are measured. Also, the stress intensity factors at the crack tip of different crack lengths in the corner solder joint are determined by fracture mechanics with finite element method. Furthermore, an empirical equation for predicting the thermal-fatigue life of flip chip solder joints is proposed     

基于深度学习的电路板焊接异常检测算法研究

焊点的焊接质量决定了电路板的可靠性,而电路板焊接异常的快速检测是大批量生产的先决条件。为了快速地实现焊接异常的精确检测,提出了一种基于深度学习的焊点图像识别算法。该算法通过自适应矩估计配合加速卷积神经网络实现,可对大量焊接图片进行快速分类识别检测。实验选取5 000幅焊接图像训练集测试,并与传统的K-means聚类算法和Canny边缘检测算法对比。实验结果显示,在小球和连桥缺陷中3种方法效果相近,而在虚焊、少锡缺陷中,本算法具有明显优势。在1 000组测试集实验中,其综合检出率及召回率分别达97.92%和98.21%,明显优于传统方法,验证了本算法具有更好的应用前景。     

埋头焊接的可靠性

埋头焊接是针对通孔插装元器件的一种特殊焊接方法.介绍了埋头焊接的优点,通过UG软件建立了电路板和焊点的三维有限元模型,利用焊点在近似试验载荷条件下的应力应变三维有限元数值模拟,就"先剪后焊"埋头焊接方法形成的焊点和普通焊点强度做了对比,给出了焊点的应力变化,并结合振动分析试验、温度循环试验和基于2D图像的X射线检测和分析,证明了埋头焊接方法应用的可行性.     

胶粘剂及其涂覆工艺

在波峰焊及回流焊过程中使用胶粘剂,以保持SMD（表面贴装器件）在PCB上的位置及方向。胶粘剂必须能适应使用高温焊接料及助焊接、清洗剂时的环境,还不能影响电路的性能。因此,选择合适的胶粘剂与涂敷工艺是非常重要的。     

Microstructure evolution of eutectic Sn-Ag solder joints

Laser and infrared reflow soldering methods were used to make Sn-Ag eutectic solder joints for surface-mount components on printed wiring boards. The microstructures of the joints were evaluated and related to process parameters. Aging tests were conducted on these joints for times up to 300 days and at temperature up to 190°C. The evolution of microstructure during aging was examined. The results showed that Sn-Ag solder microstructure is unstable at high temperature, and microstructural evolution can cause solder joint failure. Cu-Sn intermetallics in the solder and at copper-solder interfaces played an important role in both the microstructure evolution and failure of solder joints. Void and crack formation in the aged joints was also observed.     

复杂表面散射对干涉图对比度的影响及对干涉图的处理

以带有焊点的印刷电路板形成的干涉图为例,讨论了复杂物体表面的散射对干涉图对比度的影响,提出了处理复杂干涉图的简单方法.