Conductive filament formation in printed circuit boards: effects of reflow conditions and flame retardants

Conductive filament formation, a major cause of failures in printed circuit boards, is an electrochemical process that involves the transport of a metal through or across a nonmetallic medium under the influence of an applied electric field. With an increasing potential to market “green” electronics, environmental and health legislations, and the advent of lead-free and halogen-free initiatives, newer types of printed circuit board materials are being exposed to ever higher temperatures during solder assembly. The higher temperatures can weaken the glass-fiber bonding, thus enhancing conductive filament formation. The effects of the inclusion of halogen-free flame retardants on conductive filament formation in printed circuit boards are not completely understood. Previous studies, along with analysis and examinations conducted on printed circuit boards with failure sites that were due to conductive filament formation, have shown that the conductive path is typically formed along the delaminated fiber glass and epoxy resin interfaces. This paper is a result of a year-long study on the effects of reflow temperatures, halogen-free flame retardants, glass reinforcement weave style, and conductor spacing on times to failure due to conductive filament formation.     

Interface Stress-Induced Degradation of the Performance of Electrically Conductive Adhesives

Electrically conductive adhesives are of increasing importance as a powerful alternative to eutectic bonding. In the use of electrically conductive adhesives in soldering applications, a lack of conductivity of the adhesive bond between surface mounted devices (SMD) and the printed circuit board was frequently observed. This lack of conductivity caused failure of the unit because of an unacceptable series resistivity in the circuit. A failure analysis proved that the resistance of the contact between the adhesive and device depends on the stress situation in the bonded interface induced by the shrinkage of the adhesive during curing. Under stress, the formation of a resin segregation layer of low conductivity results. The outer shape of the electrical component as well as the SMD assembly process has strong influence on the electrical performance.     

A review of the influencing factors on anisotropic conductive adhesives joining technology in electrical applications

New interconnect materials are always necessary as a result of evolving packaging technologies and increasing performance and environmental demands on electronic systems. Polymer-based conductive-adhesive materials have become widely used in many electronic packaging interconnect applications. Among all the conductive-adhesive materials, the anisotropic conductive adhesives (ACA) (or anisotropic conductive adhesive films, ACF) have gained popularity as a potential replacement for solder interconnects. The interest in using ACA instead of solder comes partly from the fact that the use of ACA for the direct interconnection of flipped silicon chips to printed circuits (flip chip packaging) offers numerous advantages such as reduced thickness, improved environmental compatibility, lowered assembly process temperature, increased metallization options, reduced cost, and decreased equipment needs. In this review, a summary of our understanding of the electrical, physical, thermal, chemical, environmental, and cost behaviors of ACA in conjunction with various packaging applications is elaborated. First, the formulation and curing kinetics of ACA materials, as well as the conduction mechanisms of ACA joints, are introduced; second, the influencing factors, including the boding process (boding temperature, boding pressure, curing conditions, reflow and misalignment processes, etc), the environmental factors (temperature, humidity, impact load, etc), and the properties of the components (the properties of the ACA, substrates, conductive particles, the bump height, etc), on the reliability of ACA joining technology are presented. Finally, future research areas and remaining issues are pointed out. The purpose is simply to pinpoint the most important papers that have played significant role for the advancement of the ACA bonding technology.     

Bending strength of adhesive joints in microelectronic components: Comparison of edge-bonding and underfilling

The bending strength of underfilled and edge-bonded ball grid array (BGA) microelectronic packages assembled on printed circuit boards (PCBs) was compared using double cantilever beam (DCB) specimens. All specimens with fillets of the same size and shape failed at the same load, with cracks initiating and propagating within the PCB. This was consistent with measurements of the crack initiation strain energy release rate for PCB interfacial failure, which was significantly smaller than that of cohesive failure within the adhesives. Finite element analysis (FEA) indicated that the stress state in the PCB near the PCB-fillet interface in both underfilled and edge-bonded specimens was only a function of the adhesive fillet size and shape, and independent of the extent of the adhesive layer between the PCB and the BGA, and independent of the adhesive mechanical and thermal properties over the broad range of properties of the tested adhesives. This explained why decreasing the fillet curvature in edge-bonded specimens produced a significant increase in the joint strength. The crack path in the PCB of the edge-bonded specimens was found to change with the adhesive cure temperature; however, this had a negligible effect on the failure load.     

导电胶的研究进展

目的 综述电子封装中用于代替锡铅焊料的导电胶的研究进展,对导电胶未来研究方向进行展望,为导电胶的应用提供参考。方法 从导电胶的组成、导电机理、类型入手,重点介绍导电胶应用时的关键性能要求与测试方法,并总结近几年在提高导电性、稳定性及降低固化温度、成本等方面的研究进展。结果 对导电胶中基体树脂进行改性并选择合适的导电填料(形状、组成),可改善导电胶的固化条件,并提高导电胶的导电性能、黏结性能、耐久性,满足苛刻应用环境下对器件连接高可靠性的要求。结论 相比传统铅锡焊料焊接的方式,导电胶具有绿色环保、连接温度低、分辨率高等特点。因此导电胶适用于电子封装与智能包装领域。目前导电胶的研究方向主要为提高导电性、黏结强度以及黏结稳定性。但是在面对固化时间长、耐湿热性弱、成本较高等缺点时,仍需不断优化组成,以满足实际应用要求。     

The inspection of solder joints on printed circuit boards by phase-shift holographic interferometry

The principle of inspecting solder joints on printed circuit boards (PCBs) by phase-shift holographic interferometry is described. By using this technique, defective solder joints can be easily distinguished from good ones without analysing complicated holographic interferograms. Experiment shows that most defects in solder joints, such as breaks and voids, can be detected successfully.     

基于粗糙集的印刷锡膏自动识别和定位

为了准确获取贴装后印刷电路板(PCB)上锡膏的印刷位置,提出了一种综合了锡膏颜色和纹理特征的粗糙集方法.以红、绿、蓝三种颜色分量直方图特征为等价关系求得锡膏印刷区域的上近似集;以均值,方差和一致性等纹理特征为等价关系获得锡膏的下近似集;分别计算上、下近似集的相对权重,获得分割后的锡膏印刷区域的重心.实验结果表明,综合了颜色和纹理信息的粗糙集方法,定位精度为1个像素,定位速度约为10 ms,精度高,速度快.     

Inkjet printing and adhesion characterisation of conductive tracks on a commercial printed circuit board material

Silver nanoparticle-based conductive tracks were inkjet printed using a piezoelectric drop-on-demand inkjet printer on a commercially available electronics grade fibre glass (E-glass) reinforced substrate material, and the experimental results have been summarised. Ink jetting was done on two variants of this substrate material, viz. etched and unetched, to determine the influence of substrate surface topography on adhesion and accuracy of the printed tracks. The pull-off adhesion test method was used to quantify adhesive strength. The dependence of the pull-off test results on local geometry of the test area are illustrated with the aid of scanning electron microscope images and interferometer studies. Based on the outcomes of the experiments, conclusions concerning the suitable surface topography for inkjet printing have been arrived at.     

秸秆人造板用胶黏剂研究进展

农作物秸秆人造板的使用填补了我国因木质资源短缺造成木基人造板供不应求的缺口,胶黏剂的性能则成为了制约秸秆人造板性能的关键因素。综述了秸秆人造板用脲醛树脂、酚醛树脂、异氰酸酯、三聚氰胺树脂、橡胶等胶黏剂的性能和使用现状,分析了这些胶黏剂在制作秸秆板时存在的突出问题。并且针对这些问题,介绍了无机胶黏剂、生物胶黏剂等新型胶黏剂在秸秆人造板中的应用,最后指出了传统的有机胶黏剂和新型的无机、生物胶黏剂的发展方向,为秸秆人造板用胶黏剂的进一步研究提供参考。     

Direct flip-chip bonding of bare dies to polypropylene-coated paper substrates without adhesives or solders

Paper-based electronics is an emerging concept with the prospect of developing recyclable, low cost, flexible, and green products such as paper displays, smart labels, RFID tags, smart packages, electronic magazines, biological and medical devices. Compared to conventional printed circuit board (PCB) materials, utilizing paper as an electronics substrate has both physical and chemical challenges. Nowadays, the integration of components on papers are mainly conducted using adhesives [such as anisotropic conductive paste (ACP), isotropic conductive paste (ICP), and non-conductive pastes (NCP)] or low-temperature solders. The application of adhesives and solders in a roll-to-roll fabrication line of papers requires an additional dispensing or printing unit, which has its own drawbacks. Therefore, alternative approaches such as pre-applied adhesive films either on bare dies or papers can gain significant attention. In this study, by exploiting the unique properties of a paper coating material (i.e., polypropylene) as a non-conductive adhesive, it was shown that direct flip-chip bonding of the bare dies and devices could be successfully performed on paper without using any additional adhesives or solders. The electrical and mechanical performance of the flip chip-bonded dies on the polypropylene-coated paper substrate were assessed utilizing daisy-chain contact resistance measurement and die-shear analysis, respectively. Moreover, for an RFID tag application, RFID chips were flip chip bonded to the coated papers and functional tests via NFC communication were also successfully exerted. It was concluded that the polypropylene film on the paper can be considered as an intrinsic NCP layer for flip-chip integration of bare dies.

     

Sn-Ag-Cu系无铅焊料的显微组织与性能

用无铅焊料取代现有的含铅焊料已经成为历史发展的必然趋势.Sn-Ag-Cu系合金具有优异的可靠性和可焊性,受到了电子工业界的广泛关注.阐述了近年来该系焊料合金的微观组织和性能的一些研究成果,并对该系无铅焊料的特性进行了比较.结果表明,低银焊料的组织和性能比高银焊料好,而且成本低,为确定综合性能最佳的该系焊料合金提供依据.     

Reliability of printed circuit boards containing lead-free solder in aggressive environments

Lead-free solders were never an industry choice until government legislation, their wide spread use is still in its infancy due to long term reliability issues. A specific SAC (Tin-Silver-Copper) family of solder alloys has emerged as the favourite to offer technical advantages as well as meeting those legislative requirements. This paper investigates accelerated life behaviour of lead-free solder joints and printed circuit boards using thermal and electrical stress cycling. The aim is to understand the degradation of these materials in a practical operating environment. Whilst corrosion and debris deposits have been found, no significant evidence has been obtained for tin whiskering. EDX analysis has shown the presence of high concentrations of elements considered to arise from the packaging material. Thermal cycling tests have presented an aggressive environment to the samples and the effect on them has been supported by microscopic and macroscopic observations of debris and corrosion. The electrical behaviour, i.e., the joint resistance, has not however, significantly degraded.     

导热胶粘剂研究

导热胶粘剂因良好的导热及力学性能广泛应用于微电子封装以及热界面材料,对于电子元器件散热具有重要意义.介绍了导热胶粘剂导热原理、导热模型,分析了影响导热率的因素,以及提高导热率的途径;综述了导热非绝缘及导热绝缘胶粘剂的研究进展,最后展望了其应用前景.     

用新型包装方法实现某电子设备印制板的长期储存

对于某系统电子设备来说,产品的设计已逐步模块化,印制板作为模块化设备中的重要元素被大量使用和备份.简述了某系统电子设备的特点和防护包装要求,重点介绍了应用综合储存包装技术,对某电子设备中的印制板进行包装并确保其长期储存的方法.实际应用证明此方法是切实可行的,解决了此类设备中印制板长期储存的问题.     

Effect of different types of silver and epoxy systems on the properties of silver/epoxy conductive adhesives

In this study, the effect of amine adduct powder (AAP) on the electrical conductivity, thermal expansion, and flexural modulus of one-part system conductive adhesives are investigated. One-part system conductive adhesives are prepared using two types of silver (silver A and silver B) and different percentage of filler loadings, 10 vol.% ?40 vol.%. Silver A adhesive systems exhibit higher electrical conductivity with lower percolation thresholds, high flexural moduli, and low coefficients of thermal expansion (CTE) compared silver B adhesive systems. A comparison of the electrical conductivity and thermal expansion properties of the one part and two-part of silver A adhesives system is undertaken. The one-part silver A adhesives system shows high electrical conductivity and low CTE values compared to the two-part system. This is due to the higher cross linking density of the one-part system compared to that of the two-part system.     

The use of silane coupling agents in the design of electrically stable interfaces of 6061 T6 aluminum alloy surfaces and epoxy-based electrically conductive adhesives

Electrically conductive interfaces between metal surfaces, including aluminum, are commonly formed by means of bonding with an electrically conductive adhesive. Humid environments induce an increase in electrical resistance between aluminum objects bonded in this manner. However, joints that are electrically stable through stress testing in environments of elevated temperature and humidity (85°C and 80% RH for up to 137.5 h) have been obtained by treating the aluminum surface with a thin layer (less than 50 Å) of an organo-silane coupling agent, that is, a material capable of bonding chemically with the aluminum oxide surface layer and potentially bonding with the polymer binder in the adhesive. The present study shows that besides their traditional use as surface modifiers for adhesion improvement, organo-silanes can act as corrosion inhibitors of aluminum surfaces to stabilize electrical performance. Whereas other treatments used for preparation of metal surfaces for bonding, using electrically-insulating structural adhesives, typically do not ensure reliable electrical performance, minimal and reproducible increase in resistance has been observed for joints prepared using the procedure described here. Application of this method does not require any significant investment in equipment and the surface pretreatment of the metal surface is quite simple.     

还原氧化石墨烯基纳米银线透明导电薄膜研究进展

透明导电薄膜已广泛应用于印刷电子领域,传统的透明导电薄膜氧化铟锡（ITO）因其高脆性低柔韧性而不能满足高速发展的柔性电子行业;纳米银线（AgNWs）和石墨烯均具有良好光学性能、导电性能以及机械性能,使其能成为制备透明导电薄膜的理想材料。综述了近年来还原氧化石墨烯（rGO）基AgNWs透明导电薄膜的研究进展。介绍了柔性导电薄膜的关键参数及rGO/AgNWs透明导电薄膜的成膜工艺;归纳了影响rGO/AgNWs透明导电薄膜光电性能的主要因素和相关研究;阐述了rGO/AgNWs透明导电薄膜在印刷电子领域的应用现状,并展望了rGO/AgNWs透明导电薄膜的未来发展趋势。     

纳米导电胶粘剂的研究

采用化学镀法在碳纳米管表面包覆金属银,从而获得了导电性极好的纳米银-碳复合管,并以该复合管为导电功能体制备导电胶。研究表明,所得的纳米导电胶导电性和理化性能均较好,且比传统的银粉导电胶节省银30%～55%。     

各向异性导电胶用新型导电复合粒子的制备

采用无钯活化的化学镀的方法成功地制备出外镀银/铜/环氧树脂新型导电复合粒子,并对导电复合粒子进行了光学显微镜、SEM和EDS分析.结果表明:制备的导电微粒密度小,具有良好的导电性能,可以满足各向异性导电胶应用需求.     

A Cu/ZnO Nanowire/Cu Resistive Switching Device

Nano-Micro Letters - A new device has been realized using flip-chip joining two printed circuit boards (PCBs) on which zinc oxide (ZnO) nanowires were synthesized. Energy dispersive X-ray...