板上芯片固化及热处理过程中表面残余应力的演变

利用硅压阻传感器实时原位地记录粘接剂固化过程中的应力变化和残余应力的分布状况,以及在热处理过程中应力的演化过程.研究表明,若粘合剂固化后在空气中储存20天,应力将在后续热处理过程中急剧增加;而固化后接着经历峰值为150℃左右的热处理过程,则可以使残余应力稳定在一个相对低的值.     

固化残余应力对倒装焊器件热–机械可靠性的影响

采用底充胶的与固化过程相关的粘弹性力学模型,通过有限元仿真的方法,模拟了底充胶的整个固化过程,计算出了热循环加载下硅片、底充胶/硅片界面、底充胶内部的应力分布及其幅值大小.结果表明:底充胶的固化过程及由此产生的固化残余应力不但使得硅片中的最大垂直开裂应力位置偏离中心线达1.6 mm之多,而且还劣化了热循环加载下底充胶中应力幅值约6.25%.最后得出了固化残余应力对倒装焊器件热–机械可靠性的影响是不可忽略的结论.     

固化残余应力对倒装焊器件热-机械可靠性的影响

采用底充胶的与固化过程相关的粘弹性力学模型,通过有限元仿真的方法,模拟了底充胶的整个固化过程,计算出了热循环加载下硅片、底充胶/硅片界面、底充胶内部的应力分布及其幅值大小。结果表明:底充胶的固化过程及由此产生的固化残余应力不但使得硅片中的最大垂直开裂应力位置偏离中心线达1.6 mm之多,而且还劣化了热循环加载下底充胶中应力幅值约6.25%。最后得出了固化残余应力对倒装焊器件热–机械可靠性的影响是不可忽略的结论。     

Evolution of Contact Resistance during the Bonding Process of NCA Flip-Chip Interconnections

Non-conductive adhesive (NCA) flip-chip interconnects are emerging as an attractive alternative to lead or lead-free solder interconnects due to their environmental friendliness, lower processing temperatures, and extendability to fine-pitch applications. The electrical connectivity of an NCA interconnect relies solely on the pure mechanical contact between the integrated circuit (IC) bump and the substrate pad; the electrical conductivity of the contact depends on the mechanical contact pressure, which in turns depends to a large extent on the cure shrinkage characteristics of the NCA. Therefore, it is necessary to monitor the evolution of the electrical conductivity which could reflect the impact of cure- and thermal-induced stresses during the curing and cooling process, respectively. In this article, in situ measurement of the development of contact resistance during the bonding process of test chips was developed by using a mechanical tester combined with a four-wire resistance measurement system. A drop of resistance induced by the cure stress during the bonding process is clearly observed. With decreasing bonding temperature, the drop of contact resistance induced by cure shrinkage becomes larger, while the cooling-induced drop of resistance becomes smaller. The evolution of contact resistance agrees well with experimental observations of cure stress build-up. It is found that vitrification transformation during the curing of the adhesive could lead to a large cure stress and result in the reduction of the␣contact resistance. Furthermore, no obvious changes were observed when the applied load was removed at the end of bonding.     

Numerical investigation and experimental validation of residual stresses building up in microelectronics packaging

This paper comprises the numerical approach and the experimental validation technique developed to obtain the residual stresses building up during encapsulation process of integrated circuits. Residual stresses can be divided into cure and cooling induced parts. The curing originated stress had been mostly neglected in the literature and a special attention had always been given to detection of the thermal induced stress. In this study, both of the residual stresses, evolving during packaging, were investigated independently. The material behavior of the epoxy molding compound, EMC, was determined by the series of characterization experiments. The volumetric behavior of the EMC was investigated using PVT analysis, in which the total cure shrinkage of an initially uncured sample and the coefficient of thermal expansion of the same sample after full conversion were determined. The cure kinetics was studied using differential scanning calorimetry, DSC. The dynamic mechanical behavior was examined by dynamic mechanical analysis, DMA, at a fixed frequency. Besides, the time dependent behavior of the EMC was also determined by implementing the time–temperature superposition, TTS, test set-up in DMA. The shift factor was modeled using the combination of the WLF equation and the polynomial of second degree. The constitutive equations were developed based on the applied boundary conditions and the epoxy compound's mechanical behavior in the respective stage. A two dimensional numerical model was constructed using a commercially available finite element software package. For the experimental verification of the numerically obtained residual stresses a flexible board with the stress measuring chip was encapsulated. The real-time stress data were measured during the encapsulation. Using this technique, the in-plane stresses and the temperature changes during the die encapsulation were measured successfully. Furthermore, the measured stress data was compared with the predicted numerical results of the cure and the thermal stages, independently.     

Active alignment of optical fibers to planar waveguides using a thermal-curing adhesive

This paper described a process that was developed for the active alignment of optical fibers to planar waveguides using a thermal-curing adhesive, in order to improve the performance and reliability of waveguide-based components. The process combines alignment at a temperature selected for adhesive curing with offline postthermal treatment of the aligned device that is suitable for volume manufacturing. A specially designed fiber-array holder and a waveguide-device holder were used to overcome the effects of temperature and stress on the alignment, simplifying the alignment from high temperature to room temperature. By studying the adhesive curing kinetics and the buildup of adhesive bonding strength, the alignment was tuned to balance performance and efficiency. Using this alignment process, fiber-to-waveguide couplings that provide low optical loss, strong bonding, and excellent reliability were achieved.     

激光冲击强化AZ31镁合金表面残余应力分析

为了优选激光冲击工艺参量以获得最大的表面残余压应力,利用激光冲击和塑性变形理论推导出了激光冲击AZ31镁合金表面最大残余压应力公式,并采用ABAQUS有限元软件分析了其激光冲击后的残余应力场。结果表明,获得较大残余压应力场的激光冲击波载荷范围为1.2GPa~1.7GPa,随着载荷的增加,残余应力增加,当载荷在1.4GPa~1.6GPa时,最大残余压应力为125MPa左右;冲击载荷在1.8GPa时,出现轻微的残余应力洞现象;而在大于1.9GPa时,均出现明显的残余应力洞现象;载荷p=1.474GPa时最大残余应力为-128.5MPa。理论推导和有限元分析结果基本一致。     

Investigation on bondability and reliability of UV-curable adhesive joints for stable mechanical properties in photonic device packaging

The demand for volume deployment of photonic components has increased, and with it the need to effectively manufacture in a reproducible and cost effective way. Therefore, it is important to keep the assembly process consistent and stable. A simple shear test setup was used in this study to determine the shear force that can be used as an instant indicator to the process stability of fiber arrays assembly. The shear test was then used to study the effect of curing conditions, surface roughness, and reliability test on the adhesion of the adhesive joints of single channel fiber arrays. In general, shear force increases with the curing conditions. However, thermal-induced residue stress during improper controlled curing process might lead to loss in ductility and reliability of the joints. Results showed that rough bonding surface for the coverlid can provide better wettability and enhances the shear strength. However, it might result in incomplete polymerization due to transmission loss of UV radiation during assembly process that was caused by the uneven surface. Furthermore, rough surface can affect the flow of adhesive that could result in incomplete-fills. Adhesives at the surrounding of V-grooves and edges of the joints were attacked by moisture at elevated temperature. This paper also discussed in details the possible failure mechanism of adhesive joints after temperature and humidity test.     

Vertical die crack stresses of Flip Chip induced in major package assembly processes

This paper focuses on the FEM prediction of vertical die crack stresses in a Flip Chip configuration, induced in the major package assembly processes and subsequent thermo-mechanical loading. An extended Maxwell model is used to describe the time dependent inelastic behavior of the solder bumps. Two types of viscoelastic models, describing the mechanical properties of underfill resin during and after the curing process, are used. The die stresses caused by both the soldering and the underfill curing processes are obtained. These stresses are used as initial stress-state for the further modeling of subsequent thermal cycling. Using this methodology, the complete die stress evolution in a selected Flip Chip can be obtained, the physics of thermal stress induced vertical die cracks can be better understood and the possible die cracks can be reliably predicted.     

Prestressed Ceramic Coatings for Enhanced Reliability of Silicon Wafer Fracture Strength

The objective of this paper is to investigate thin, solid, prestressed ceramic films as a means of enhancing the reliability of silicon semiconductor wafers stressed in bending. To characterize the effect of thin films on strength, one-micrometer ceramic films were deposited on wafers using plasma-enhanced chemical-vapor deposition. The modulus of rupture (MOR) of the coated wafers was determined from four-point bend testing of coated samples. Adhesion testing of the coated wafers primarily showed cohesive rather than adhesive failure. A series of residual stresses was introduced into the coating-silicon interface and the MOR was determined. The results showed that for a thin brittle coating (1 mum) on a silicon wafer (635 mum), the minimal shear stress at the surface led to dominance of the residual stress over intrinsic coating strength as the critical parameter affecting failure. A correlation between MOR and residual stress was established.     

激光选区熔化/干式铣削复合加工实验研究

为研究激光选区熔化增材与干式铣削减材复合加工中的工艺交互作用对零件成形质量的影响,以316L奥氏体不锈钢粉末为研究对象,分别采用单一增材工艺和“增材-减材”交替复合加工工艺制备试样,分析成形区的致密度、残余应力。实验结果表明,在制件致密度和残余应力方面,以上两种加工方法均受到激光能量密度的影响且规律相似。此外,复合加工中的铣削减材工艺会对成形样件的致密度和残余应力造成影响,在激光能量密度设置为100J·mm-3和125J·mm-3时,铣削减材工艺可以明显提高致密度并降低残余应力水平。     

激光热处理对化学沉积Ni-P合金薄膜性能的影响

用扫描电镜(SEM)观察了化学沉积Ni-P合金薄膜/单晶硅基体的结构与颗粒度,利用X射线衍射(XRD)技术测试了其化学沉积后的残余应力,测量了激光热处理后残余应力的变化规律,分析了残余应力对磨损性能及界面结合强度的影响。实验结果表明,化学沉积Ni-P合金薄膜/硅基体的残余应力均表现为拉应力,经过激光热处理后残余应力发生了变化,由高值的拉应力变为低值的拉应力或压应力;薄膜残余应力对其磨损性能有明显的影响,其磨损量随着残余应力的减小而减小;薄膜与基体结合强度随着残余应力的增大而减小,合理地选择激光热处理参数可以精确地控制薄膜残余应力,提高其结合强度。     

PBGA器件固化残余应力的有限元分析

采用有限元分析软件模拟研究了PBGA器件在先固化再进行温度加载和直接从温度循环开始加载两种情况下,固化过程对 PBGA 器件的影响,发现固化过程不仅改变了器件中芯片的应力分布,而且劣化了 DA(环氧树脂)材料与芯片接触面的应力值,也同时使得 BT 材料基底中的应力分布产生较大的改变。并且对 DA 材料选用几种不同的固化时间,发现固化时间的不同对器件封装残余应力的大小和最大应力位置有重要的影响,这为预测芯片的垂直开裂的位置提供了很好的依据。     

激光冲击圆杆曲面诱导的残余应力数值模拟

采用有限元分析方法对激光冲击2024铝合金圆杆圆周曲面诱导的动态应力波及其残余应力进行了数值模拟。首先,在ABAQUS/Explicit显式分析模块中模拟了峰值压力为2 GPa激光冲击波在16 mm杆中诱导的应力波的传播过程。随后,在ABAQUS/Standard隐式分析模块中进一步计算在圆杆曲面上诱导的残余应力。在此基础上,分析了圆杆直径的大小对应力波峰值衰减和残余应力分布的影响,并进行了相关的试验验证。研究结果表明,峰值压力为2 GPa冲击波在16 mm的杆中诱导的弹塑性的应力波,应力波的峰值压力在400 ns时间内迅速衰减至250 MPa。冲击后,在距光斑中心小于0.5 mm冲击区域内分布不均匀残余应力,在光斑中心处形成了残余拉应力,轴向S11值为42 MPa;在半径为0.5~1.5 mm的冲击区域分布着残余压应力,S11值大约在250 MPa。应力波在传播过程中,其压力峰值衰减的速率随着杆径的增大而减慢,表面形成的残余压应力均随杆径的增大而增加。     

Micro-deformation of flexible substrate for electronic devices during handling prior to lithography patterning

The dimensional stability of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) foils during the handling and production process, is crucial for the quality of the final multilayer flexible electronic devices. The foil-on-carrier (FOC) - foil/adhesive/Si-wafer sandwich - is a handling method, which is currently used to temporary attach the foil to a Si-wafer carrier during the lithography processing. This handling method ensures good surface flatness of the foils during the processing. However this technique has two drawbacks: (1) it creates an in-plane micro-deformation of the PEN foil and (2) bending of FOC due to the lamination procedure and lithography processing. In this article the temperature dependence of in-plane foil deformation and bending of FOC was reported and the final curing step of lamination procedure was identified as the main source of in-plane deformation and bending. Finally the resultant stresses introduced to FOC by the curing step derived from the experimental measurements were compared to stress values calculated from the Stoney’s and Ohring’s equations.     

选区激光熔化激光功率对316L不锈钢熔池形貌及残余应力的影响

针对选区激光熔化(SLM)工艺参数的匹配性对成形质量的影响,选取三种激光功率在不同的扫描速度和扫描方式下进行实验,研究了激光功率对熔池形貌及残余应力的影响。结果表明:随着激光功率增大,熔池的几何尺寸和成形件中的残余应力均变大。这主要是因为在上述参数序列下,随着激光功率增大,热流密度增大,相同层厚与截面下的温度梯度增大,熔池温度升高,熔池尺寸变大,从而导致成形件熔融时的晶面夹角及晶界间距较大,进而产生了较大的热应力,成形件冷却凝固后的残余应力过大。在实际应用中,通过合理设计匹配的工艺参数,可以得到较适合的熔池几何尺寸(即较合理的温度梯度分布),从而减小热应力,进而减小残余应力,得到成形质量较高的SLM工件。     

金属板料激光喷丸成形理论研究与数值模拟

在对激光喷丸成形(LPF)机制分析的基础上,采用ABAQUS软件对激光喷丸成形过程进行了有限元数值模拟,分析了激光喷丸后板料的变形和残余应力场的分布情况。结果表明激光喷丸在板料表层的塑性变形层中诱导出压应力,在塑性变形层以下部位出现拉应力,这种应力分布形式打破了板料内部原有力系的平衡,促使板料发生弯曲变形,从而使板料内部应力重新分布以达到新的平衡,最终在板料厚度方向形成上下两面为压应力,而中部为拉应力的新的残余应力场。研究结果对理解激光喷丸成形过程及其本质,进行激光喷丸工艺参数的合理优化、板料变形过程的有效控制和进一步的实验研究具有指导意义。     

高芯片键合质量与高生产率的新型银浆体系的研究

高键合强度与高生产率的银浆体系是芯片实现小型化、轻薄化的基础,本文研发了一种高芯片键合强度的新型银浆体系(银浆B),通过五元素三水平(53)正交实验,探讨了银浆量、点胶高度、芯片键合力、银浆固化时间、固化温度等五因素对芯片键合强度及结构的影响;以及基于实验设计(DOE)和响应曲面分析(RSM)等统计方法,分析了芯片键合的过程,优化了芯片键合过程的固化时间、固化温度和银浆量等参数。采用银浆B体系以及优化的制程参数,使得芯片键合强度制程能力指数(Cpk)从0.56提高到2.8。     

Measurement of nonlinear mechanical properties of PDMS elastomer

This paper presents the measurement of the nonlinear mechanical properties of polydimethylsiloxane (PDMS) elastomer based on the mixing ratio of base polymer to curing agent. Strip-type PDMS samples with different mixing ratios were prepared using a simple coating, curing, and cutting process. A cyclic uniaxial tension test with a fixed magnitude of applied strain and a single-pull-to-failure tension test were performed with a micro-tensile tester at room temperature.Our new finding is that when the PDMS is mixed with excessive curing agent, stress softening occurs and residual strain exists in cyclic tension tests when the magnitude of the applied strain increases. For the PDMS-05 samples, in which the mixing ratio of base polymer to curing agent was 5 to 1, there were large differences in the stresses for the same strain level under loading and unloading during the first cycle with a 100% fixed strain amplitude, but the softening effect of the stress in the PDMS dropped rapidly starting from the second cycle.Nonlinear mechanical Neo-Hookean, third-order Mooney, and second-order Ogden models of three different PDMS films were computed from the stress-strain data. The results showed that all models were preferable for the small strain region of PDMS compared with other models. In the nonlinear, large strain region, only the second-order Ogden model properly described the mechanical behavior of the PDMS, while the Neo-Hookean and third-order Mooney-Rivlin models were too stiff or flexible in the measurement range. The bulk modulus of PDMS increased with the amount of curing agent in it. Therefore, the second-order Ogden model is preferable for analyzing the PDMS structure over the entire measurement range. This could provide reasonable mechanical models of PDMS for rapid computational prototyping and for designing active and passive components from PDMS.     

Effect of spin coating on the curing rate of epoxy adhesive for the fabrication of a polymer optical waveguide

Spin coating is a simple process for rapidly depositing thin, solid polymeric films onto relatively flat substrates. Evaporation occurs during spinning of the relatively volatile species in any solution. The curing behavior of spin-coated polymeric film is influenced by the evaporation of any reactive component. An investigation was carried out on a silicon substrate to study the effects of spin coating on the curing behavior of the epoxy adhesive. The degree of curing for both spin and without spin-coated epoxy adhesive was measured by Fourier-transform infrared spectroscopy (FTIR). A slower curing reaction rate was observed for the spin-coated epoxy adhesive. The composition gradient established by solvent evaporation during spinning is responsible for the slower curing reaction rate of the spin-coated epoxy adhesive. From this study, it is proposed to use solvents that are less volatile and allow a greater part of the thinning behavior to occur without significant changes in the fluid properties during the spinning process.