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.     

银含量对随机振动条件下无铅焊点可靠性的影响

为了探究银含量对无铅焊点在随机振动条件下的可靠性的影响,对Sn-3.0Ag-0.5Cu、Sn-1.0Ag-0.5Cu和Sn-0.3Ag-0.7Cu三种不同Ag含量材料的焊点做窄带范围内的随机振动疲劳实验,并对失效焊点进行分析。结果表明:三种材料焊点的失效位置基本都在靠近PCB侧,最外围焊点最容易失效,失效模式均为脆性断裂,并且随着Ag含量的降低,金属间化合物的厚度逐渐减小,焊点的疲劳寿命逐渐延长。     

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。     

QFP焊点可靠性研究

随着人们对健康和环境的要求越来越高,无铅焊料的研究倍受封装业的重视。塑性应变是影响电子封装焊点可靠性的主要因素,文章采用在多次温度循环条件下进行有限元数值模拟的方法,针对由不同元素(Sn,Pb,Ag,Cu)及配比构成的焊料,计算QFP焊点的塑性应变,定量评估其可靠性。给出焊料各参数对焊点可靠性的影响程度,仿真表明焊料激活能与气体常数的比值的变化对焊点可靠性影响最大,相应的焊点Y向塑性应变均值仅为优化前的11%。所得的结果可为今后QFP封装时的焊料选择提供新的理论依据。     

热循环与随机振动加载下电路板的寿命预测

随着电子封装业的快速发展,产品可靠性成为重要研究课题之一,寿命作为可靠性的衡量指标具有参考价值。利用ANSYS,结合有限元理论与寿命预测模型,在热循环与随机振动加载下对装配有典型封装结构的电路板进行响应分析和寿命预测。采用Anand统一本构模型,根据体积加权平均法,选择基于能量的Darveaux模型预测热疲劳寿命。根据Manson经验高周疲劳关系式,结合Miner准则,选择基于高斯分布的Steinberg模型预测振动疲劳寿命。采用线性损伤叠加法得到热与振动同时加载下结构的寿命。结果表明:焊点与Cu引线的连接层易发生破坏。焊点易出现热疲劳失效,Cu引线易出现振动疲劳失效。     

Cu6Sn5纳米颗粒对SAC3005/Cu焊点形貌和性能的影响

研究了添加Cu₆Sn₅纳米颗粒对SAC3005焊料焊点金属间化合物的形貌和性能的影响。采用湿化学法制备Cu₆Sn₅纳米颗粒,将Cu₆Sn₅纳米颗粒添加到SAC3005焊料中,经回流焊后,制备SAC3005-xCu₆Sn₅(x=0%,0.12%,0.18%,质量分数)复合焊点。采用金相显微镜对焊点的横断面进行观察,对焊点的横断面金属间化合物(IMCs)进行测量。采用ANSYS有限元软件对界面IMCs模型进行模拟,分析印刷电路板(PCB板)焊点失效机理。结果表明：添加Cu₆Sn₅纳米颗粒改性SAC3005/Cu焊点后的IMCs层厚度变薄。Cu₆Sn₅纳米颗粒的加入抑制了回流焊接过程中IMCs的生长,提高了焊点的可靠性。Cu₆Sn₅纳米颗粒的添加阻碍了Sn原子和Cu原子在界面处的扩散,抑制...     

电子封装微焊点中的柯肯达尔孔洞问题

简述目前在微焊点内部产生的六种孔洞缺陷。微焊点中的孔洞会导致焊点强度快速下降,一直危害着电子产品的可靠性,软钎料／Cu基盘界面在高温时效老化后会出现了大量的柯肯达尔孔洞,柯肯达尔孔洞的存在将会对应用于高温环境和高应力的封装焊点可靠性产生不好的影响。结合目前的研究状况分析了时效老化时Kirkendall（柯肯达尔）孔洞的形成机理和Kirkendall孔洞对焊点可靠性的影响,重点研究了焊盘材料、材质、焊料掺杂元素和UBM预处理等因素影响柯肯达尔孔洞形成的机理。     

电子组装钎料研究的新进展

随着微电子技术的发展,印制电路板的组装密度不断提高,人们越恶霸虎重视焊点工作的可靠性。在分析焊点失效和原因的基础上,论述了改善焊点可靠性的途径,同时,随着人们对环保要求的提高,积极开发和推广使用无铅钎料及免清洗钎剂也是当务之急,结合我们的研究成果,详细介绍了无铅钎料开发的现状及前景。     

随机振动下板级组件焊点可靠性研究进展

针对板级组件焊点在随机振动下的可靠性问题,对与之相关的实验研究成果和仿真结果进行了综述。首先介绍了球栅阵列(BGA)封装芯片焊点失效的位置,BGA焊点的材料以及BGA参数和印制电路板(PCB)参数对焊点可靠性的分析,并介绍了疲劳寿命预测方法和子模型法、特殊结构的随机振动分析。最后,提出了随机振动情况下的BGA结构的优化方法。     

电子组装钎料研究的新进展（刊中刊）

随着微电子技术的发展,印制电路板的组装密度不断提高,人们越来越重视焊点工作的可靠性。在分析焊点失效原因的基础上,论述了改善焊点可靠性的途径,同时,随着人们对环保要求的提高,积极开发和推广使用无铅钎料及免清洗钎剂也是当务之急,结合我们的研究成果,详细介绍了无铅钎料开发的现状及前景。     

Intermetallics Characterization of Lead-Free Solder Joints under Isothermal Aging

Solder interconnect reliability is influenced by environmentally imposed loads, solder material properties, and the intermetallics formed within the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminal plating, board pad plating, and solder materials. These metallurgies react together and form intermetallic compounds (IMCs) that affect the metallurgical bond strength and the reliability of solder joint connections. This study evaluates the composition and extent of intermetallic growth in solder joints of ball grid array components for several printed circuit board pad finishes and solder materials. Intermetallic growth during solid state aging at 100°C and 125°C up to 1000 h for two solder alloys, Sn-3.5Ag and Sn-3.0Ag-0.5Cu, was investigated. For Sn-3.5Ag solder, the electroless nickel immersion gold (ENIG) pad finish was found to result in the lowest IMC thickness compared to immersion tin (ImSn), immersion silver (ImAg), and organic solderability preservative (OSP). Due to the brittle nature of the IMC, a lower IMC thickness is generally preferred for optimal solder joint reliability. A lower IMC thickness may make ENIG a desirable finish for long-life applications. Activation energies of IMC growth in solid-state aging were found to be 0.54 ± 0.1 eV for ENIG, 0.91 ± 0.12 eV for ImSn, and 1.03 ± 0.1 eV for ImAg. Cu₃Sn and Cu₆Sn₅ IMCs were found between the solder and the copper pad on boards with the ImSn and ImAg pad finishes. Ternary (Cu,Ni)₆Sn₅ intermetallics were found for the ENIG pad finish on the board side. On the component side, a ternary IMC layer composed of Ni-Cu-Sn was found. Along with intermetallics, microvoids were observed at the interface between the copper pad and solder, which presents some concern if devices are subject to shock and vibration loading.     

Design of Experiments for Board-Level Solder Joint Reliability of PBGA Package Under Various Manufacturing and Multiple Environmental Loading Conditions

A design of experiments was conducted to determine the reliability of plastic ball grid array packages under various manufacturing and multiple environmental loading conditions. Parameters included conformal coating methods, underfill, solder mask defined, and non-solder mask defined pads. Board-level temperature cycling, vibration, and combined temperature cycling and vibration testing were performed to quantify the reliability and identify preferred design parameters. Through the main effects and interaction analysis, test results show underfill is the key parameter related to the solder joint reliability improvement. Conformal coat method and printed circuit board pad design are not main effects on solder joint reliability. No interactive relationship exists among these three factors under temperature cycling loading, but some interactive relationship between printed circuit board pad type and the conformal coating method exists under vibration and combined loading conditions.     

Computational parametric analyzes on the solder joint reliability of bottom leaded plastic (BLP) package

The bottom-leaded plastic (BLP) package is a lead-on-chip type of chip scale package (CSP) developed mainly for memory devices. Because the BLP package is one of the smallest plastic packages available, solder joint reliability becomes a critical issue. In this study, a 28-pin BLP package is modeled to investigate the effects of molding compound and leadframe material properties, the thickness of printed circuit board (PCB), the shape of solder joint and the solder pad size on the board level solder joint reliability. A viscoplastic constitutive relation is adopted for the modeling of solder in order to account for its time and temperature dependence on thermal cycling. A three-dimensional nonlinear finite element analysis based on the above constitutive relation is conducted to model the response of a BLP assembly subjected to thermal cycling. The fatigue life of the solder joint is estimated by the modified Coffin-Manson equation. The two coefficients in the modified Coffin-Manson equation are also determined. Parametric studies are performed to investigate the dependence of solder joint fatigue life on various design factors.     

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.     

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。     

Solder Joint Reliability Assessment for Flip Chip Ball Grid Array Components with Various Designs in Lead-Free Solder Materials and Solder Mask Dimensions

This study investigates the reliability of flip chip ball grid array (FCBGA) components with three types of solder materials: eutectic solder with a composition Sn63Pb37 and the lead-free solders SnAg3.0Cu0.5 and SnAg4.0Cu0.5. Two substrate-side solder mask (S/M) opening sizes, 0.4 mm and 0.525 mm, were used. Both the monotonic and cyclic mechanical four-point bend tests are conducted for the reliability assessment. It is found that the FCBGA components with SnAg3.0Cu0.5 solder have the best durability during the cyclic bend test, yet the eutectic solder is the strongest during the monotonic bend test. Besides, the FCBGA components with 0.525-mm S/M opening have around 3 times more life cycles than those with the 0.4-mm S/M opening in the cyclic bend test. It is also noteworthy that the lead-free solder materials have much variation in the failed cycles during the cyclic test. Moreover, the failure locations for those components with 0.4-mm S/M openings are found to be at the interface between the package side metal pad and the solder ball, and those with an S/M opening of 0.525 mm are observed to be failed mostly at the interface between the printed circuit board (PCB) side metal pad and the solder ball.     

Effect of Metal Bond-Pad Configurations on the Solder Microstructure Development of Flip-Chip Solder Joints

Various microstructural zones were observed in the solidified solder of flip-chip solder joints with three metal bond-pad configurations (Cu/Sn/Cu, Ni/Sn/Cu, and Cu/Sn/Ni). The developed microstructures of the solidified flip-chip solder joints were strongly related to the associated metal bond pad. A hypoeutectic microstructure always developed near the Ni bond pad, and a eutectic or hypereutectic microstructure formed near the Cu pad. The effect of the metal bond pads on the solder microstructure alters the Cu solubility in the molten solder. The Cu content (solubility) in the molten Sn(Cu) solder eventually leads to the development of particular microstructures. In addition to the effect of the associated metal bond pads, the developed microstructure of the flip-chip solder joint depends on the configuration of the metal bond pads. A hypereutectic microstructure formed near the bottom Cu pad, and a eutectic microstructure formed near the top Cu pad. Directional cooling in the flip-chip solder joint during the solidification process causes the effects of the metal bond-pad configuration. Directional cooling causes the Cu content to vary in the liquid Sn(Cu) phase, resulting in the formation of distinct microstructural zones in the developed microstructure of the flip-chip solder joint.     

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.     

Interfacial reaction and solder joint reliability of Pb-free solders in lead frame chip scale packages (LF-CSP)

Chip scale packages (CSP) have essential solder joint quality problems, and a board level reliability is a key issue in design and development of the CSP type packages. There has been an effort to eliminate Pb from solder due to its toxicology. To evaluate the various solder balls in CSP package applications, Pb-free Sn-Ag-X (X=In, Cu, Bi) and Sn-9Zn-1Bi-5In solder balls were characterized by melting behavior, phases, interfacial reaction, and solder joint reliability. For studying joint strength between solders and under bump metallurgy (UBM) systems, various UBMs were prepared by electroplating and electroless plating. After T/C (temperature cycle) test, Sn3.5Ag8.5In solder was partially corroded and its shape was distorted. This phenomenon was observed in a Sn3Ag10In 1Cu solder system, too. Their fractured surface, microstructure of solder joint interface, and of bulk solder ball were examined and analyzed by optical microscopy, SEM and EDX. To simulate the real surface mounting condition and evaluate the solder joint reliability on board level, Daisy chain test samples using LF-CSP packages were prepared with various Pb-free solders, then a temperature cycle test (−65∼ 150°C) was performed. All tested Pb-free solders showed better board level solder joint reliability than Sn-36Pb-2Ag. Sn-3.5Ag-0.7Cu and Sn-9Zn-1Bi-5In solders showed 35%, 100% superior solder joint reliability than Sn-36Pb-2Ag solder ball, respectively.     

Evaluation on Influencing Factors of Board-Level Drop Reliability for Chip Scale Packages (Fine-Pitch Ball Grid Array)

Board-level drop testing is an effective method to characterize the solder joint reliability performance of miniature handheld products. In this study, drop test of printed circuit boards (PCBs) with a four-screw support condition was conducted for a 15 mm times 15 mm fine-pitch ball grid array (FBGA) package assembly with solder ball compositions of 36Pb-62Sn-2Ag and Sn-4Ag-0.5Cu on printed circuit board (PCB) surface finishes of organic solderability preservative, electroless nickel immersion gold, and immersion tin. Finite element modeling of the FBGA assembly was performed to study the stress-strain behavior of the solder joints during drop test. The drop test results revealed a strong influence of different intermetallic compound formation on soldered assemblies drop durability. The lead-based solder supersedes the lead-free composition regardless of the types of surface finish. Joints on organic solderability preservative were found to be strongest for each solder type. Other factors affecting drop reliability such as component location on the board and thermal cycling aging effects are reported. Finite element modeling results showed that a solder joint is more prone to failure on the PCB side, and the predicted solder joint stresses are location dependent. Predicted failure sites based on simulation results are consistent with experimental observations.