Pb/Sn凸剪切性能研究

在实际使用条件下,Pb/Sn凸点会由于承受温度循环而产生剪切应力,剪切应力导致的主要失效方式是开裂.通过对倒装焊后Pb/Sn凸点剪切强度的测量及对剪切后断口的分析,发现破坏主要发生在凸点下金属(UBM)层内部或UBM与Al焊盘之间,平均剪切强度受凸点尺寸影响很小,范围在21～24MPa。     

Pb/Sn凸点的制备

介绍了用电镀法制备Pb / Sn凸点的工艺,包括:凸点的设计,圆片的准备,溅射UBM(凸点下金属层),厚膜光刻,电镀Cu柱, Pb / Sn凸点,回流凸点等。结果表明:采用电镀法可以得到球形Pb / Sn凸点,50μm的厚胶工艺是可行的。     

凸点下金属层湿法刻蚀与凸点底切的控制

介绍了电镀凸点封装工艺流程和其中有关金属层湿化学刻蚀的问题.通过槽式批量和单片机刻蚀相应的UBM(凸点下金属层)的均匀度、刻蚀速率和凸点底切的对比,结果显示单片湿法刻蚀机刻蚀均匀度小于5%且片与片之间刻蚀速率差异小于2%,以及凸点底切小于2μm.槽式刻蚀均匀度较差,一般会大于20%,同时同一批次片与片之间刻蚀速率差异也较大,实验显示超过10%,且刻蚀后凸点底切较深.因此采用单片机进行UBM金属刻蚀可以较好的控制刻蚀过程和提高产品的良率.     

焊料凸点倒装焊技术

介绍了倒芯片面阵式凸点制作、多层陶瓷基板焊盘制作及倒装焊各关键技术 ,并成功地获得了芯片与基板的互连。     

板级立体组装凸点互联技术

通过对板级立体组装凸点互联焊盘设计技术的研究,找到了凸点焊盘设计的一种可靠的理论途径,很好地解决了凸点互联中对位精度控制的问题,实现了板级立体组装的凸点互联工艺技术。     

用于倒装芯片的金球凸点制作技术

倒装芯片中凸点用于实现芯片和基板的电路互连,芯片凸点的制作是倒装芯片技术的关键技术之一.对金球凸点制作进行了介绍.金球凸点直接粘附于芯片上,同时又可具有电路互连的作用,可以完成倒装芯片与基板的电气连接.金球凸点的优势是简单、灵活、便捷、低成本,最大特点是无需凸点下金属层(UBM),可对任意大小的单个芯片进行凸点制作,平整度可达到±4μm.     

化学镀镍、模版印刷法制备倒扣芯片焊料凸点

回顾了低成本制备芯片上焊料凸点的方法,即化学镀镍制备凸点下金属层、模版印刷焊料,最后回流形成焊料凸点,并综述了该方法的最新研究进展.     

高可靠硅基MCM芯片凸点技术研究

介绍了采用芯片凸点和倒装焊技术(FCB)的MCM-Si工艺,重点研究了凸点下金属化(UBM)结构设计对电路可靠性的影响;通过再流焊和可靠性试验,以及凸点拉脱力测试,进行了可靠性评价.采用该工艺制作的电路已达到<混合集成电路通用规范>(GJB2438A-2002)靠性等级的H级.     

柔性凸点热-结构耦合应力应变有限元分析

建立了芯片尺寸封装焊点的柔性凸点三维有限元分析(FEA)模型,对该模型进行了热-结构耦合有限元分析,研究了热-结构耦合条件下柔性凸点温度场和应力应变的分布规律,对比了有无柔性层结构的凸点内应力应变的大小,分析了柔性层厚度、上下焊盘直径对柔性凸点应力应变的影响。结果表明:柔性层结构有效降低了凸点内的应力应变;随着柔性层厚度的增加,凸点内最大应力应变减小;随上焊盘和下焊盘直径的增加,凸点内最大应力应变的变化无明显规律。     

多金球凸点倒装片技术

文章提出了一种在IC芯片的一个I/O端制作多个金球凸点的倒装片IC焊盘设计和金球凸点芯片制作的方法。在一个I/O端制作多个金球凸点,在芯片安装时可以提高芯片安装的机械强度,进而提高芯片安装的可靠性。多焊盘芯片也可用于制作KGD。     

Comparison of electroplated eutectic Bi/Sn and Pb/Sn solder bumpson various UBM systems

The effect of a reflow process and under bump metallurgy (UBM) systems on the growth of intermetallic compounds (IMC) of the 57Bi/43Sn and 37Pb/63Sn solder bump/UBM interfaces was investigated. The selected UBM systems were sputtered Al/Ti/Cu, sputtered Al/NiV/Cu, Al/electroless Ni/immersion Au, and Al/Ti/electroless Cu. An alloy electroplating method was used for the solder bumping process. The microstructure and composition of intermetallic compound (IMC) phases and their morphologies were examined using scanning electron microscopy and X-ray diffraction. The Cu₆Sn₅ η'-phase IMC appeared on all Cu containing UBM cases with Pb/Sn and Bi/Sn solders and the Cu ₃Sn ϵ-phase was detected only with Pb/Sn solder bumps. The Ni₃Sn₄ IMC was found to be the main IMC phase between Ni and solder. The Ni₃Sn secondary IMC was also detected on the electroless Ni UBM with PbSn solder after ten times reflow. Through the bump shear test, Al/NiV/Cu, Al/elNi/Au, and Al/Ti/elCu UBMs showed good stability with Bi/Sn and Pb/Sn solder in terms of metallurgical aspects     

Manufacturing of Cu/electroless nickel/Sn-Pb flip chip solder bumps

A process for manufacturing Cu/electroless Ni/Sn-Pb solder bump is discussed in this paper. An attempt to replace zincation with a Cu film as an active layer for the electroless Ni (EN) deposition on Al electrode on Si wafer is presented. Cu/electroless Ni is applied as under bump metallurgy (UBM) for solder bump. The Cu film required repeated etches with nitric acid along with activation to achieve a satisfactory EN deposit. Fluxes incorporating rosin and succinic acid were investigated for wetting kinetics and reflow effectiveness of the electroplated solder bump. The solder plating current density and the reflow condition for achieving solder bumps with uniform bump height were described. The Cu/EN/Sn-Pb solder system was found to be successfully produced on Al terminal in this study that avoids using zincating process     

Plasma reflow bumping of Sn-3.5 Ag solder for flux-free flip chip package application

A new flux-free reflow process using Ar+10%H/sub 2/ plasma was investigated for application to solder bump flip chip packaging. The 100-/spl mu/m diameter Sn-3.5wt%Ag solder balls were bonded to 250-/spl mu/m pitch Cu/Ni under bump metallurgy (UBM) pattern by laser solder ball bonding method. Then, the Sn-Ag solder balls were reflowed in Ar+H/sub 2/ plasma. Without flux, the wetting between solder and UBM occurred in Ar+H/sub 2/ plasma. During plasma reflow, the solder bump reshaped and the crater on the top of bump disappeared. The bump shear strength increased as the Ni/sub 3/Sn/sub 4/ intermetallic compounds formed in the initial reflow stage but began to decrease as coarse (Cu,Ni)/sub 6/Sn/sub 5/ grew at the solder/UBM interface. As the plasma reflow time increased, the fracture mode changed from ductile fracture within the solder to brittle fracture at the solder/UBM interface. The off-centered bumps self-aligned to patterned UBM pad during plasma reflow. The micro-solder ball defects occurred at high power prolonged plasma reflow.     

Pb-free Sn/3.5Ag electroplating bumping process and under bump metallization (UBM)

Pb-free solder is one of the biggest issues in today's electronic packaging industry. This paper introduces a newly developed Sn/3.5Ag alloy plating process for wafer level bumping. The effects of Under Bump Metallization (UBM) on the process, interfacial reaction, and mechanical strength have been investigated. Four different types of sputtering-based UBM layers-TiW/Cu/electroplated Cu, Cr/CrCu/Cu, NiV/Cu, and TiW/NiV-were fabricated with eutectic Pb/63Sn and Sn/3.5Ag solder. The result shows that the Sn/Ag solder gains Cu or Ni from UBM's and becomes Sn/Ag/Cu or Sn/Ag/Ni during reflow process. Sn/Ag solder has higher reactivity with Cu and Ni than Pb/63Sn. The Intermetallic Compound (IMC) spalling from the interface between UBM/solder has been observed on Cr/CrCu/Cu and TiW/NiV UBM's. However, the IMC spalling phenomena did not decrease the bump shear strength with a bump size of 110 /spl mu/m, whereas a size of 60 /spl mu/m brought a decrease in shear value and failure mode change.     

Influence of Cu content on compound formation near the chip side for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump during aging

Sn-Ag-Cu solder is a promising candidate to replace conventional Sn-Pb solder. Interfacial reactions for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder joints were investigated after aging at 150°C. The under bump metallization (UBM) for the Sn-3.0Ag-(0.5 or 1.5)Cu solders on the chip side was an Al/Ni(V)/Cu thin film, while the bond pad for the Sn-3.0Ag-0.5Cu solder on the plastic substrate side was Cu/electroless Ni/immersion Au. In the Sn-3.0Ag-0.5Cu joint, the Cu layer at the chip side dissolved completely into the solder, and the Ni(V) layer dissolved and reacted with the solder to form a (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). For the Sn-3.0Ag-1.5Cu joint, only a portion of the Cu layer dissolved, and the remaining Cu layer reacted with solder to form Cu₆Sn₅ IMC. The Ni in Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. At the plastic substrate side, three interfacial products, (Cu_1−y,Ni_y)₆Sn₅, (Ni_1−x,Cu_x)₃Sn₄, and a P-rich layer, were observed between the solder and the EN layer in both Sn-Ag-Cu joints. The interfacial reaction near the chip side could be related to the Cu concentration in the solder joint. In addition, evolution of the diffusion path near the chip side in Sn-Ag-Cu joints during aging is also discussed herein.     

A reliability comparison of electroplated and stencil printed flip-chip solder bumps based on UBM related intermetallic compound growth properties

The effects of under bump metallurgy (UBM) microstructures on the intermetallic compound (IMC) growth of electroplated and stencil printed eutectic Sn-Pb solder bumps were investigated. The process parameters and their effects on UBM surface morphology and UBM shear strength were studied. For the electroplating process, the plating current density was the dominant factor to control the Cu UBM microstructure. For the stencil printing process, the zincation process has the most significant effect on the Ni UBM surface roughness and Ni grain sizes. In both processes, the good adhesion of UBM to aluminum can be obtained under suitable UBM processing conditions. Samples with different UBM microstructures were prepared using the two processes. The resulting samples were thermal aged at 85/spl deg/C, 120/spl deg/C, and 150/spl deg/C. It was observed that the Cu UBM surface roughness had larger effect on the IMC growth and solder ball shear strength than the Ni UBM surface roughness. The thickness of Cu/sub 3/Sn and Cu/sub 6/Sn/sub 5/ IMC depended strongly on the UBM microstructure. However, for Ni/Au UBM, no significant dependence was observed. More likely, the thickness of Au-Ni-Sn IMC near the IMC/solder interface was controlled by the amount of gold and the gold diffusion rate in the solder. Shear tests were performed after thermal aging tests and thermal/humidity tests. Different failure modes of different sample groups were analyzed. Electroless Ni UBM has been developed because it is a mask-less, low-cost process compared to electroplated Cu UBM. This study demonstrated that the process control was much easier for Ni UBM due to its lower reactivity with Sn material. These properties made Ni UBM a promising candidate for the lead-free solder applications.     

The microstructure characterization of ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallization

The microstructure of the ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallizations (UBMs) was investigated as a function of cooling rate. The ultrasmall eutectic Bi-Sn solder bump, about 50 μm in diameter, was fabricated by using the lift-off method and reflowed at various cooling rates using the rapid thermal annealing system. The microstructure of the solder bump was observed using a backscattered electron (BSE) image and the intermetallic compound was identified using energy dispersive spectroscopy (EDS) and an x-ray diffractometer (XRD). The Bi facet was found at the surface of the ultrasmall Bi-Sn solder bumps on the Au/Cu/Ti UBM in almost all specimens, and the interior microstructure of the bumps was changed with the solidification rate. The faceted and polygonal intermetallic compound was found in the case of the Bi-Sn solder bump on the Au (0.1 μm)/Ni/Ti UBM, and it was confirmed to be the (Au_1−x−yBi_xNi_y)Sn₂ phase by XRD. The intermetallic compounds grown form the Au (0.1 μm)/Ni/Ti UBM interface, and they interrupted the growth of Bi and Sn phases throughout the solder bump. The ultrasmall eutectic Bi-Sn solder bumps on the Au (0.025 μm)/Ni/Ti UBM showed similar microstructures to those on the Au/Cu/Ti UBM.     

Experimental characterization and mechanical behavior analysis of intermetallic compounds of Sn–3.5Ag lead-free solder bump with Ti/Cu/Ni UBM on copper chip

Due to today’s trend towards ‘green’ products, the environmentally conscious manufacturers are moving toward lead-free schemes for electronic devices and components. Nowadays the bumping process has become a branch of the infrastructure of flip chip bonding technology. However, the formation of excessively brittle intermetallic compound (IMC) between under bump metallurgy (UBM)/solder bump interface influences the strength of solder bumps within flip chips, and may create a package reliability issue. Based on the above reason, this study investigated the mechanical behavior of lead-free solder bumps affected by the solder/UBM IMC formation in the duration of isothermal aging. To attain the objective, the test vehicles of Sn–Ag (lead-free) and Sn–Pb solder bump systems designed in different solder volumes as well as UBM diameters were used to experimentally characterize their mechanical behavior. It is worth to mention that, to study the IMC growth mechanism and the mechanical behavior of a electroplated solder bump on a Ti/Cu/Ni UBM layer fabricated on a copper chip, the test vehicles are composed of, from bottom to top, a copper metal pad on silicon substrate, a Ti/Cu/Ni UBM layer and electroplated solder bumps. By way of metallurgical microscope and scanning-electron-microscope (SEM) observation, the interfacial microstructure of test vehicles was measured and analyzed. In addition, a bump shear test was utilized to determine the strength of solder bumps. Different shear displacement rates were selected to study the time-dependent failure mechanism of the solder bumps. The results indicated that after isothermal aging treatment at 150 °C for over 1000 h, the Sn–Ag solder revealed a better maintenance of bump strength than that of the Sn–Pb solder, and the Sn–Pb solder showed a higher IMC growth rate than that of Sn–Ag solder. In addition, it was concluded that the test vehicles of copper chip with the selected Ti/Cu/Ni UBMs showed good bump strength in both the Sn–Ag and Sn–Pb systems as the IMC grows. Furthermore, the study of shear displacement rate effect on the solder bump strength indicates that the analysis of bump strength versus thermal aging time should be identified as a qualitative analysis for solder bump strength determination rather than a quantitative one. In terms of the solder bump volume and the UBM size effects, neither the Sn–Ag nor the Sn–Pb solders showed any significant effect on the IMC growth rate.     

A study in flip-chip UBM/bump reliability with effects of SnPb solder composition

This paper aims to investigate the electromigration phenomenon of under-bump-metallization (UBM) and solder bumps of a flip-chip package under high temperature operation life test (HTOL). UBM is a thin film Al/Ni(V)/Cu metal stack of 1.5 μm; while bump material consists of Sn/37Pb, Sn/90Pb, and Sn/95Pb solder. Current densities of 2500 and 5000 A/cm² and ambient temperatures of 150–160 °C are applied to study their impact on electromigration. It is observed that bump temperature has more significant influence than current density does to bump failures. Owing to its higher melting point characteristics and less content of Sn phase, Sn/95Pb solder bumps are observed to have 13-fold improvement in Mean-Time-To-Failure (MTTF) than that of eutectic Sn/37Pb. Individual bump resistance history is calculated to evaluate UBM/bump degradation. The measured resistance increase is from bumps with electrical current flowing upward into UBM/bump interface (cathode), while bumps having opposite current polarity cause only minor resistance change. The identified failure sites and modes from aforementioned high resistance bumps reveal structural damages at the region of UBM and UBM/bump interface in forms of solder cracking or delamination. Effects of current polarity and crowding are key factors to observed electromigration behavior of flip-chip packages.     

Solder joint reliability and characteristics of deformation and crack growth of Sn-Ag-Cu versus eutectic Sn-Pb on a WLP in a thermal cycling test

Sn-Ag-Cu (SAC) is now recognized as the standard lead free solder alloy for packaging interconnect in the electronics industry. This paper analyzes the performance of both SAC and eutectic Sn-Pb solder alloys on Kulicke & Soffa's (K&S') Ultra CSP/spl reg/ wafer level package (WLP) at a thermal cycling (TC) test. The Ultra CSP standard Al/Ni-V/Cu under bump metallurgy (UBM) system was used to analyze if this UBM system with SAC solder would produce acceptable reliability in the TC test. In this study, two TC tests were performed. In the first test, two parts were removed from the TC chamber about every 200 cycles to investigate the characteristics of deformation and crack growth in the SAC and eutectic Sn-Pb Ultra CSP solder joints during TC testing. These TC test results showed that both the SAC and eutectic Sn-Pb Ultra CSPs exhibited normal solder joint fatigue failures during the testing. The SAC Ultra CSP had an equal or 18% higher Weibull life than the eutectic Sn-Pb one. Based on these results it was concluded that the SAC Ultra CSP with the Al/Ni-V/Cu UBM system produces acceptable solder joint reliability in a TC test. The results also revealed that the deformation and crack growth characteristics of the SAC and eutectic Sn-Pb Ultra CSP solder joints were significantly different. The eutectic Sn-Pb solder joints showed significant inelastic shear deformation during the TC testing while the SAC solder joints did not display significant inelastic deformation even at the high temperature regime of the TC test.