Effects of moisture and elevated temperature on reliability of interfacial adhesion in plastic packages

The interfacial-adhesion performance between the lead frame and molding compound was studied after temperature cycles and hygrothermal aging, simulating a typical package-assembly process. The hygrothermal aging involved a treatment at 85°C and 85% relative humidity (RH) for 168 h and three cycles of infrared (IR) solder-reflow condition. The interfacial-bond strengths were measured using shear and lead-pull tests. The lead-frame surface finishes studied include a bare Cu, microetched Cu, spot Ag coating, Ni, Pd/Ni, and Au/Ni coatings. Special emphasis was placed on the study of the changes in surface characteristics and the corresponding interfacial adhesion after various manufacturing processes. It was found that moderate thermal cycles enhanced the interfacial adhesion for all coated lead frames, except the Ni coating. Hygrothermal aging was detrimental to the interfacial-bond strength, especially for hydrophilic or polar surfaces, such as bare Cu, Ag, Pd/Ni, and Au/Ni coated lead frames. The introduction of tiny dimples etched on the lead frame was effective in mitigating the reduction in interfacial-bond strength arising from hygrothermal aging. This result confirms the important role of the mechanical-interlocking mechanism provided by dimples in retaining the interfacial adhesion in a humid environment.     

Optimization of black oxide coating thickness as an adhesion promoter for copper substrate in plastic integrated-circuit packages

Copper-oxide coating applied onto the copper substrate has emerged as an alternative to metallic coatings to improve adhesion with polymeric adhesives and molding compounds. The interfacial-bond strengths between the black oxide-coated Cu substrate and epoxy-based, glob-top resin were measured in button-shear tests, and the failure mechanisms were identified from the fracture-surface examination. The emphasis was to establish the correlation between the coating thickness, the surface roughness, and the interfacial adhesion with respect to treatment time. It was found that at the initial stage of treatment a thin layer of flat, cuprous oxide developed, above which fibrillar-cupric oxide was formed with further treatment until saturation with densified fibrils at about 150 sec. The interfacial-bond strength between the oxide-coated copper substrate and glob-top resin increased gradually with increasing treatment time, until the bond strength reached a plateau constant after a treatment for about 150 sec. There was a functional similarity between the oxide thickness, the surface roughness, and the interface-bond strength with respect to treatment time. A treatment time of 150 sec is considered an optimal condition that can impart the highest interface adhesion.     

Failure analysis of leakage current in plastic encapsulated packages

Plastic encapsulated packages exhibit high leakage currents after a few hundred hours in the steam pressure pot test (SPP). The present study investigates two possible causes of leakage current. These are: (a) mold compound, (b) the polyimide tape used for co-planarity of lead frame fingers. The results of this study indicate that the leakage cur-rent is independent of the frame and is not caused by the mold compound. The data indicates that it is the ionic content and acrylic-based adhesive layer of the polyimide tape which cause the leakage current. To eliminate the leakage current, polyimide tape with low ionic content and non acrylic-based adhesive should be used. *Permanent address: School of Physics, Universiti Sains Ma-laysia, 11800 USM Penang, Malaysia.     

Prediction of interfacial adhesion strength of nanoscale Al/TiN films passed through patterned BEOL interconnects

The patterned layout arrangement of back-end of line (BEOL) is an important factor among reliability issues of advanced interconnects technology. BEOL determines the likelihood of interfacial fracture when adhesion between dielectric materials and conductive metals becomes poor, especially when ultra low-k dielectric is introduced into the process of semiconductor devices. Interfacial delamination of dissimilar thin films comprising multi-level interconnects increases under external loading or thermal cycling stress during device manufacturing. An Al/TiN coating underneath an ordinary pattern of BEOL interconnects is selected in this research to estimate adhesive energy by using interfacial fracture theory based on finite element analysis. By adopting the framework of four-point bending test, the proposed simulated approach was validated before comparison of the adhesion of Al/TiN stacked coatings with experimental data. Analysis indicate that increasing dielectric thickness between interfaces of Al/TiN films and Al metal line patterns from 100 nm to 600 nm induces fracture growth with crack lengths exceeding 3.5 mm. In addition, the foregoing driving force of fracture could be achieved because of Young’s modulus of dielectrics, which are larger than 70 GPa.     

CO2激光液相诱导沉积金属层附着力的研究

在水溶液中,用CO2激光诱导沉积的方法在环氧树脂基材上沉积金属银,并以Ag作为活化金属层进行化学镀铜。利用电子探针和原子力显微镜对所得到的金属层进行了微观分析,结果表明,Cu金属层的附着力与Ag金属层的表面形貌、厚度及基材表面粗糙度等因素有直接的关系。     

Oxidation of Lead Frame Copper Alloys with Different Compositions and Its Effect on Oxide Film Adhesion

Oxidation of four typical lead frame copper alloys was investigated. The oxidation rate and adhesion strength of oxide films to copper alloy substrates were studied by measuring the thickness and carrying out peel tests. The results show that, although copper alloys C5191 and C7025 have thinner oxide films, a lower adhesion strength and a higher proportion of CuO were obtained than in the other copper alloys EFTEC64T and C194. The adhesion strength is mainly influenced by the structure of the oxide film of the copper alloys, especially the CuO/Cu₂O ratio in the film. The highest adhesion strength is obtained for the copper oxide film with a basic structure of CuO/Cu₂O/Cu and a CuO/Cu₂O ratio of about 0.1. The segregation of additional elements in the copper alloy plays an important role in the oxide film structure.     

激光冲击成形技术的研究

介绍了一种利用激光冲击波来进行金属板料塑性加工的新技术———激光冲击诱导的超快塑性成形技术,分析了其成形机理、特点以及影响板料激光冲击成形的主要因素。指出了激光冲击成形技术在工业应用中所必须研究的一些主要内容。     

微机械构件的表面去粘着技术

由于尺寸的减小和需要产生弹性形变,微机械构件的表面力(表面张力、粘着力)成为影响系统性能的关键因素。本文介绍了造成微构件表面粘着的因素,从材料、工艺和设计等方面综述了减弱或去除微机械构件表面粘着力的七种技术,并对相关工作进行了总结和展望。     

Effect of fluorine contamination on barrier metal oxidation

We clarified that interfacial barrier metal oxidation with inter layer dielectric (ILD) could be revealed by X-ray photoelectron spectroscopy (XPS) on the peeled-side of the barrier metal, and the barrier metal oxidation was promoted by fluorine contamination which adsorb to the ILD surface during etching. To consider the effect of fluorine contamination on barrier metal oxidation, hydrolysable property of fluorine contamination was evaluated by measuring the change of F 1s spectrum after dipping in boiling water. Moreover, fluoride ions and the acidity of water in which fluorine contamination was dipped were measured by Ion chromatography and pH measurement, respectively. According to our experiments, it was suggested that hydrofluoric acid (HF) acted as an oxidizing catalyst to promote barrier metal oxidation at the interface of barrier metal and ILD.     

A step forward in the transient thermal characterization of chips and packages

Generating compact dynamic thermal models is a key issue in the thermal characterization of packages. A further but related problem is the modeling of the thermal coupling between chip locations, for the use in electro-thermal circuit simulators. The paper presents a measurement based method which provides a way to solve both problems. A thermal benchmark chip has been designed and realized, to facilitate thermal transient measurements. The developed evaluation method provides the compact thermal multiport model of the IC chip including package effects, for the accurate electro-thermal simulation of the ICs. The evaluation method is also suitable to generate the compact thermal model of the package.     

Fabrication and characterization of bilayer metal wire-grid polarizer using nanoimprint lithography on flexible plastic substrate

In this work, we demonstrate the fabrication of bilayer metal wire-grid polarizers and the characterization of their performance. The polarizers with 200 nm period were fabricated on flexible plastic substrates by nanoimprint lithography (NIL), followed by aluminum deposition. Transmission efficiency over 0.51 and extinction ratio higher than 950 can be achieved in the visible range when the aluminum thickness of the polarizer is 100 nm. The fabrication process only involves direct imprinting on flexible plastic substrates and aluminum deposition, without any resist spin-coating, lift-off, and etching processes, which is much simpler, less costly, and applicable to large volume production.     

Morphology‐Control Strategy of the Superhydrophobic Poly(Methyl Methacrylate) Surface for Efficient Bubble Adhesion and Wastewater Remediation

As one common form of gas existing in aqueous environment, gas bubbles have attracted considerable worldwide attention, owing to their promising applications in industrial production and daily life, such as pressure sensors, the recovery of valuable minerals from ores, aeration process, and water treatment. Usually, the behaviors of gas bubbles in aqueous environment are mainly dominated by buoyancy force. It drives the gas bubbles out of aqueous medium rapidly, which is unfavorable in various processes, especially in wastewater treatment. In this paper, various types of superhydrophobic poly(methyl methacrylate) (PMMA) sheets are facilely fabricated, such as five‐pointed star, triangle, circular, and ellipse. Compared with other shapes of superhydrophobic PMMA sheets, the prepared superhydrophobic PMMA circular sheet is capable of efficiently adhering gas bubbles and subsequently elongating their retention time in an aqueous environment. Furthermore, superhydrophobic PMMA circular sheet arrays are prepared, which can greatly improve the degradation efficiency of methyl blue by ozone (O₃). The investigations indicate that the present approach will find wild applications in bubble‐related fields and provide people with inspirations to develop efficient methods to manipulate gas bubbles in aqueous environment.     

激光冲击波技术用于材料加工的研究进展

随着高功率短脉冲激光器技术的日益成熟,高能激光和材料相互作用产生高幅冲击波技术的应用研究日趋广泛。本文介绍利用激光诱导冲击波技术在金属材料表面改性和成形方面的最新研究成果,综述了激光冲击强化、激光冲击成形和激光喷丸成形技术的机理、特点、工业应用及今后的发展趋势。     

Effects of strain rates and biaxial stress conditions on plastic yielding and flow stress of solder alloys

The rate-dependent mechanical properties of Sn3.8Ag0.7Cu (SAC387) Pb-free alloy and Sn-Pb eutectic alloy were investigated in this study under pure shearing and biaxial stress conditions with thin-walled specimens using a servo-controlled tension-torsion material testing system. The pure shearing tests were conducted at strain rates between 6.7 × 10⁻⁷ and 1.3 × 10⁻¹/sec. In addition, axial tensile stresses were superimposed onto the shearing samples to examine the effects of biaxial stress conditions on the yielding and on post-yielding plastic flow of the solder alloys. Strain hardening is observed for the Pb-free alloy at all the tested strain rates, while strain softening happens with the Sn-Pb eutectic solder at low strain rates. Special tests were also conducted for sudden strain-rates changes and stress relaxation for the purpose to develop a viscoplastic model to simulate time-dependent multiaxial deformation and to assess damage and fatigue life of general solder interconnections.     

Effects of substrate metallization of solder/under-bump metallization interfacial reactions in flip-chip packages during multiple reflow cycles

The effects of various elements of substrate metallization, namely, Au, Ni, and P, on the solder/under-bump metallization (UBM), (Al/Ni(V)/Cu) interfacial reactions in flip-chip packages during multiple reflow processes were systematically investigated. It was found that Au and P had negligible effects on the liquid-solid interfacial reactions. However, Ni in the substrate metallization greatly accelerated the interfacial reactions at chip side and degraded the thermal stability of the UBM through formation of a (Cu,Ni)₆Sn₅ ternary compound at the solder/UBM interface. This phenomenon can be explained in terms of enhanced grain-boundary grooving on (Cu,Ni)₆Sn₅ in the molten solder during the reflow process. This could eventually cause the rapid spalling of an intermetallic compound (IMC) from the solder/UBM interface and early failure of the packages. Our results showed that formation of multicomponent intermetallics, such as (Cu,Ni)₆Sn₅ or (Ni,Cu)₃Sn₄, at the solder/UBM interface is detrimental to the solder-joint reliability.     

Effect of thin film coating of Au on joint strength in invar-invar packages

The effect of a thin film coating of Au on the joint strength, weld width, and penetration depth in laser welding techniques for Invar-Invar packages is investigated experimentally. It is found that the joint strength, weld width, and penetration depth are strongly dependent on the coating of Au thickness on the Invar material. The welded joints with thick Au coating show narrower weld width, shallower penetration, and hence less joint strength than those of the packages with thin Au coating. The increase in both the thermal conductivity and the vapor volume in the welded joints as the coating of Au thickness increases are the possible mechanisms for the reduction. Detailed knowledge of the effect of thin film coatings of Au on the welded materials, which gives both the highest joint strength and good adhesion, is essential for practical design and fabrication of reliable optoelectronic packaging.     

稀土金属氧化物涂层对铸铁激光强化区组织和性能的影响

系统研究了稀土金属氧化物涂层对铸铁激光强化过程的影响。研究表明,稀土金属氧化物能够显著改善激光强化区的组织和耐磨性能。     

晶片表面几何特性对键合的影响

由最小能量原理导出的键合条件出发,利用线性薄板理论,在同一理论模型框架下,通过量度键合过程能否进行的弹性应变能累积率,分析了晶片表面的宏观尺度的弯曲和微观尺度的起伏对晶片键合的影响,并对所得结果进行了详细讨论．     

金属离子掺杂的TiO2-SiO2增透膜的制备及性能

为了得到高增透的TiO2-SiO2薄膜,采用sol-gel法制备了金属离子掺杂的TiO2-SiO2薄膜,并对其透射光谱和薄膜厚度进行了表征,研究了退火温度及不同金属离子掺杂(Al3+、Fe3+和Zn2+)对TiO2-SiO2薄膜增透性能的影响。结果表明:Al3+、Fe3+可提高薄膜在可见光波段的增透性,其中,掺杂Al3+的摩尔分数为0.4%的薄膜,未经退火处理的增透性能最佳(透射率可达98%)。掺杂Fe3+使薄膜的截止波长红移量最大,约为14nm。     

封装用铜框架表面化学组成和粘接强度

电子封装用铜框架表面采用不同温度处理,与电子封装用环氧材料复合制备用于拉伸测试的Tap pull样品。XPS分析表明,一般铜框架表面成分包括一价的氧化亚铜和二价的氢氧化铜等。175℃热处理120 min可使表面的二价氧化铜质量分数达到73%,氧化铜和表面约20%的氢氧化铜一起显著提高界面的粘接性能。