Aging study on interfacial microstructure and solder-ball shear strength of a wafer-level chip-size package with Au/Ni metallization on a Cu pad

Eutectic solder balls (63Sn-37Pb) joined to Cu pads with an Au/Ni metallization have been widely used in wafer-level chip-size package (WLCSP) technology for providing electrical and mechanical interconnections between components. However, some reliability issues must be addressed regarding the intermetallic compounds (IMCs). The formation of a brittle IMC layer between the solder/Cu pad interface impacts considerably upon the solder-ball shear strength. In addition, it will degrade the long-term operating reliability of the WLCSP. This study investigates, by means of experiments, the growth of the IMC layer under isothermal aging for the eutectic Sn-Pb solder reflowed on a Cu pad with an Au/Ni metallization. Forming the Cu pad with an Au/Ni metallization was achieved by a simple semiconductor-manufacturing process. The effects of the intermetallic layer on solder-ball shear strength were examined for various parameters, including the thickness of the Au layer, solder-ball size, and the diameter of the Cu pad. Experimental results indicate that two IMC layers, Au_0.5Ni_0.5Sn₄ and Ni₃Sn₄, form at the solder/Cu pad interface after aging. The Au_0.5Ni_0.5Sn₄ intermetallic layer dominates the total thickness of the IMC layer and grows with aging time while the solder-ball shear strength decreases after aging. The degradation of the solder-ball shear strength was found to be caused mainly by the formation of the Au_0.5Ni_0.5Sn₄ layer. The experimental results established that a thinner Au layer on Cu pad can effectively control the degradation of solder-ball shear strength, and this is especially true for smaller ball sizes.     

Mechanical strength of Sn-3.5Ag-based solders and related bondings

The tensile strengths of bulk solders and joint couples of Sn-3.5Ag-0.5Cu, Sn-3.5Ag-0.07Ni, and Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge solders and the shear strengths of ball grid array (BGA) specimens, solder-ball-attached Cu/Ni/Au metallized substrates were investigated. The tensile strength of the bulk is degraded by thermal aging. The Ni-containing solder exhibits lower tensile strength than Sn-3.5Ag-0.5Cu after thermal aging. However, the Ni-containing solder joints show greater tensile strength than the Cu/Sn-3.5Ag-0.5Cu/Cu joint. Fracture of the solder joint occurs between the intermetallic compound (IMC) and the solder. The shear strength and fracture mechanism of BGA specimens are the same regardless of solder composition.     

冲击载荷下航天用PBGA焊点的优化设计

从动力学角度考虑PBGA焊点承受不同加速度情况下冲击载荷的情况,对PBGA焊点尺寸建立了正交设计表。运用有限元分析技术对该表中各种因素和水平分别进行有限元分析,运用数理统计对正交结果进行分析,在现有尺寸结构上提出了最优化设计。对各种因素进行了显著性分析及比较,得出在动力载荷下,焊点直径、高度等参数对焊点可靠性的影响比焊点位置参数对焊点可靠性的影响要低得多。在动力载荷下,焊点位置、形状变为可靠性设计的主要考虑因素。并通过扩大参数取值范围得到二次正交设计表验证了其正确性。     

Sn-Ag-Cu表面贴装元件的水清洗技术

采用SMT450-LD全自动SMT水清洗系统对模拟试件和贴装有片式电阻的PCB板表面残留物采用水基清洗剂清洗。结果表明，用Sn—Ag-Cu免清洗焊膏贴装在FR4基板上的片式电阻，焊后采用水基清洗剂清洗，表面残留物离子浓度为1．9μg/cm^2，达到美国军用标准MIL—STD-2000小于5．7μg／cm^2的规定。清洗过程中参数的设置，如清洗温度，清洗时间会对清洗效果产生较大影响。     

Processing and properties of CVD diamond for thermal management

One of the many remarkable properties of diamond is its thermal conductivity, about five times that of copper and the highest of all known materials. The high thermal conductivity in combination with the relative ease of diamond film growth by chemical vapor deposition process makes the material suitable for many applications such as thermal management in high power electronic circuits. For thermal managements applications, various processing steps are needed for the diamond films, such as the metallization for reliable solder bonding, metallurgical processes for planarizing of the faceted growth surface and removal of fine-grained diamond regions with poor thermal conductivity. This paper will review the properties and processing of diamond films for thermal management applications.     

印制板液态感光阻焊油墨的工艺控制

本文介绍印制板液态感光阻焊油墨的工艺流程、工艺控制，并对生产中较常出现的几种质量问题进行讨论，进而提出解决措施。     

电迁移致SnAgCu微焊点强度退化及尺寸效应研究

在100℃温度下,对直径为300μm、高度为100,200和300μm的Sn-3.0Ag-0.5Cu钎料微焊点施加密度为1×104A/cm2的直流电流。通电48h后,利用DMA对电迁移作用后微焊点拉伸强度的变化进行了研究。结果表明,电迁移作用导致微焊点拉伸强度明显退化,且退化程度随微焊点高度的减小而减弱。其中,高度为300μm的微焊点的平均拉伸强度由初始态的44.4MPa降至27.8MPa,下降了37.4%。电迁移还导致微焊点的断裂由延性变为延性与脆性相结合的模式。     

ENIG焊点的失效机理及镀层重工方法研究

采用显微形貌、微观结构和元素成分分析等物理分析方法,以不同类型的化镍浸金(electroless nickel/immersion gold,ENIG)基板为对象,分析了其焊点在不同情形下的失效模式和失效机理,阐述了黑盘缺陷的主要失效特征,研究了具有黑盘缺陷的化镍浸金基板的重工工艺.研究结果显示,Ni层断裂表面单一的高P含量或轻微Ni层腐蚀不能作为黑盘缺陷的唯一依据,已形成良好金属间化合物(intermetallic compound,IMC)层的Ni层腐蚀位置的焊接界面仍具有良好的机械结合强度,采用喷锡工艺(hot air solder level,HASL)对具有黑盘缺陷的化镍浸金基板进行重新处理切实可行.     

A comparative study of the kinetics of interfacial reaction between eutectic solders and Cu/Ni/Pd metallization

A comparative study of the kinetics of interfacial reaction between the eutectic solders (Sn-3.5Ag, Sn-57Bi, and Sn-38Pb) and electroplated Ni/Pd on Cu substrate (Cu/Ni/NiPd/Ni/Pd) was performed. The interfacial microstructure was characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope (SEM). For a Pd-layer thickness of less than 75 nm, the presence or the absence of Pd-bearing intermetallic was found to be dependent on the reaction temperature. In the case of Sn-3.5Ag solder, we did not observe any Pd-bearing intermetallic after reaction even at 230°C. In the case of Sn-57Bi solder the PdSn₄ intermetallic was observed after reaction at 150°C and 180°C, while in the case of Sn-38Pb solder the PdSn₄ intermetallic was observed after reaction only at 200°C. The PdSn₄ grains were always dispersed in the bulk solder within about 10 μm from the solder/substrate interface. At higher reaction temperatures, there was no Pd-bearing intermetallic due to increased solubility in the liquid solder. The presence or absence of Pd-bearing intermetallic was correlated with the diffusion path in the calculated Pd-Sn-X (X=Ag, Bi, Pb) isothermal sections. In the presence of unconsumed Ni, only Ni₃Sn₄ intermetallic was observed at the solder-substrate interface by SEM. The presence of Ni₃Sn₄ intermetallic was consistent with the expected diffusion path based on the calculated Ni-Sn-X (X=Ag, Bi, Pb) isothermal sections. Selective etching of solders revealed that Ni₃Sn₄ had a faceted scallop morphology. Both the radial growth and the thickening kinetics of Ni₃Sn₄ intermetallic were studied. In the thickness regime of 0.14 μm to 1.2 μm, the growth kinetics always yielded a time exponent n >3 for liquid-state reaction. The temporal law for coarsening also yielded time exponent m >3. The apparent activation energies for thickening were: 16936J/mol for the Sn-3.5Ag solder, 17804 J/mol for the Sn-57Bi solder, and 25749 J/mol for the Sn-38Pb solder during liquid-state reaction. The corresponding activation energies for coarsening were very similar. However, an apparent activation energy of 37599 J/mol was obtained for the growth of Ni₃Sn₄ intermetallic layer during solid-state aging of the Sn-57Bi/substrate diffusion couples. The kinetic parameters associated with thickening and radial growth were discussed in terms of current theories.     

弯曲应力状况下微型BGA封装可靠性比较

文中采用传统的表面贴装技术进行焊接,研讨μBGA的PCB装配及可靠性。弯曲循环试验（1000με～-1000με）,用不同的热因数（Qη）回流,研究μBGA、PBGA和CBGA封装的焊点疲劳失效问题。确定液相线上时间,测定温度,μBGA封装的疲劳寿命首先增大,接着随加热因数的增加而下降。当Qη接近500s·℃时,出现寿命最大值。最佳Qη范围在300s·℃～750s·℃之间,此范围如果装配是在氮气氛中回流,μBGA封装的寿命大于4500个循环。采用扫描电子显微镜（SEM）,来检查μBGA和PBGA封装在所有加热因数状况下焊点的失效。每个断裂接近并平行于PCB焊盘,在μBGA封装中裂纹总是出现在焊接点与PCB焊盘连接的尖角点,接着在Ni3Sn4金属间化合物（IMC）层和焊料之间延伸。CBGA封装可靠性试验中,失效为剥离现象,发生于陶瓷基体和金属化焊盘之间的界面处。