A study of delamination growth in the die-attach layer of plastic IC packages under hygrothermal loading during solder reflow

This paper describes a comprehensive numerical study of the effects of many parameters on the propagation of delaminations in the die-attach layer using a finite element linear elastic fracture mechanics approach. An 80-pin plastic quad flat package of dimensions 20 mm/spl times/14 mm/spl times/2.7 mm was considered. The parameters investigated include the coefficient of thermal expansion (CTE) of the die-attach material, the thickness of the die-attach layer, the CTE of the pad material, and the size of the initial delamination (defect). The effect of the location of the initial delamination was studied by considering delaminations at the center, edge, and quarter-point locations along the pad/die-attach interface. The effect of swelling of the encapsulant due to moisture absorption during preconditioning at 85/spl deg/C/85%RH was also considered. The combined effect of the hygrostress and thermal stress developed during solder reflow was obtained by suitably superimposing the respective stress intensity factors. The effect of the pressure of water vapor at the delamination was also investigated and found to be negligible for small delaminations. This study has shown that the propagation of delamination from an initial defect located at the edge of the pad/die-attach interface is unstable. Once delamination propagates, it will continue until the entire interface is completely delaminated.     

Improved thermal, structural and electrical properties of epoxy for use at high voltages

In this paper, the epoxy elasticity factors were investigated by thermomechanical analysis (TMA), dynamic mechanical thermal analysis (DMTA), thermogravimetric analysis (TGA) and field emission scanning electron microscope (FESEM) to improve toughness and reduce brittleness of existing epoxy resin. Dumbbell shaped specimens were made and tested at rates of 0, 20 and 35 parts per hundred resins (phr). TMA and DMTA temperatures ranged from -20 to 200 /spl deg/C and TGA ranged from 0 to 600 /spl deg/C. The glass transition temperature (Tg) of elastic epoxy was measured by thermal analysis. Also investigated were the thermal expansion coefficient (/spl alpha/), the high-temperature characteristics, modulus and the loss factor (Tan /spl delta/). We analyzed the structure using FESEM and have found elastomer particles (elastic-factors) in the elastic epoxy matrix. We have made elastic epoxy by adding elastomer particles to existing epoxy. Generally, the toughness of elastic epoxy can be improved by changing the structure of existing epoxy of poor impact-strength. In addition, we measured the permittivity and Tan /spl delta/ for investigation of the electrical properties of elastic epoxy. The permittivity and Tan /spl delta/ depend on the elastomer composition. Namely, the permittivity and Tan /spl delta/ increase according to the elastomer contents. For experimental analysis results, 20 phr was considered an excellent specimen.     

Filter optimization for X-ray inspection of surface-mounted ICs

A thin Zn filter (/spl sim/300 /spl mu/m) and relatively low X-ray tube voltage (/spl sim/45 kV) is recommended for X-ray inspection of surface-mounted device solder joints on printed wiring boards (PWBs). An optimal filter minimizes the Si dose that could result in cumulative damage to sensitive integrated circuit (IC) nodes, yet provides good contrast for metals such as Cu traces on PWBs and device solder balls. While we expect orders of magnitude Si dose reductions when effective filters are inserted, a properly chosen filter should not attenuate the portion of the white X-ray spectrum required to image Cu, Sn, and Pb (solder balls). Some X-ray inspection suppliers can achieve a Si dose of as little as 0.060 rads, while other X-ray inspection suppliers, not yet optimized for minimum dose, may use as much as four orders of magnitude more dose. We used thermo luminescent detectors (TLDs) to measure the X-ray dose that IC product shipments would encounter during a shipping process, for example, as personal baggage or cargo, as /spl les/0.050 rads.     

Numerical analysis on thermal characteristics for chip scale package by integrating 2D/3D models

The objective of this paper is to investigate stress and strain of a special scale package‐substrate on chip for reliability evaluation or manufacture strategy in deep‐seated situation. A two‐dimensional model with one‐half of cross‐section (2D model) and a three‐dimensional model with one‐fourth of whole package (3D model) were built, respectively, to simulate the thermal stress and strain of CSP‐SOC under the condition of the standard industry thermal cycling temperature −40 to125°C. The different locations can be processed by using the two models, respectively, based on different modeling simplified modes. By using 2D model, the numerical simulation shows that the maximum deformation of the prototype occurs in printed circuit board (PCB), the maximum stress and strain occurs in the outer solder balls. In the meantime, by the results of 3D model, the simulation shows that the maximum elastic strain occurs in the interface between the solder balls and PCB, the minimum strain occurs in the underfill tape, the maximum packaging stress occurs in the edge area of the chip. The result from 3D model maybe more impersonal to reflect the stress and strain characteristics because the third direction is considered in modeling. The analysis by integrating the 2D model and 3D model can get a more comprehensive profile for the thermal investigation of chip scale package (CSP) than by using any single model. The investigation built a basis for improving reliability in engineering design of CSP product. Copyright © 2008 John Wiley & Sons, Ltd.     

Rapid die heating for low-stress die attach

During traditional isothermal die attach assembly, significant thermomechanical stress develops in the solder joints between the die and board. The coefficient of thermal expansion (CTE) of the silicon die and the woven composite circuit board materials are widely different. Under isothermal die attach, there is, hence, a mismatch between the thermal expansion displacements of the die and substrate, thereby leading to stress in the solder joints and die interconnect layers. One avenue to alleviate these stresses is to use alternate die attach processes that rely on localized heating of the die and solder joints so as to minimize the thermal expansion displacement mismatch. Die attach stress can be reduced significantly through rapid die heating (RDH), which results in the die being hotter than the board at the solder solidification point. Analytical modeling shows that RDH can reduce residual stress by up to 80% compared to traditional, isothermal die attach processing. Limited experimental results demonstrate 40% stress reduction to date. This paper will detail these results and physical analysis of the resulting solder joints.     

Thermomechanical analysis of gold-based SiC die-attach assembly

The thermomechanical stresses due to mismatch of the coefficients of thermal expansion (CTE) of the base material (SiC) and the packaging has a significant impact on the stresses in MEMS pressure sensors used in high-temperature applications, to 600/spl deg/C. The pressure sensor studied essentially consists of a SiC die attached to an AlN substrate using a gold die attach. Characterization of the stress distribution within the die attach, die and substrate along with the fatigue resistance of the die attach at 600/spl deg/C is essential to estimating the reliability of the packaging structure. A parametric study has been performed using nonlinear finite element analysis to optimize the die-attach thermomechanical performance at high temperatures. This study includes the effects of varying porosity levels and varying reference temperatures (stress-free temperature). This study also provides information about the mechanical deformations of the pressure sensor due to the thermomechanical load, which must be compensated, for the effective performance of the pressure sensor. The outcome of the study provides guidelines to optimize the design of the pressure sensor.     

Automated X‐ray inspection method for fillet‐less mounted chip components

Chip components mounted on the printed circuit board are rapidly being miniaturized. Furthermore, the fillet‐less chip soldering technique, which does not use a solder fillet, is widely used in portable products such as mobile phones. However, there is no method to inspect the soldering of fillet‐less chip mounting. In this paper, we propose an automated X‐ray inspection technique for fillet‐less chip mounting. It extracts three inspection parameters from the X‐ray image. In the experiments, we evaluate the repeatability and inspecting ability of the technique and confirm that sufficient information for failure detection is obtained. An automated X‐ray inspection system using this technique is now in operation at some factories, so in conclusion our automated method would be useful in practice. Copyright © 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Acoustic response of cartilage during laser-mediated stress relaxation

In this study, laser reshaping of porcine septal cartilage was performed using an Nd:YAG laser (/spl lambda/=1.32 /spl mu/m), while changes in acoustic waves were observed, in an attempt to develop technique to monitor the reshaping process. Concurrent measurements of strain (during tensile compression and tension, as well as flexure), temperature, and a 5-MHz ultrasonic signal were recorded during laser irradiation (/spl lambda/=1.32 /spl mu/m, 4 W, 13 s). The sample was set up in a water bath to enhance acoustic coupling. From the ultrasonic signal, both time of flight (TOF) and signal amplitude as reflected from the back wall of the cartilage were extracted and correlated with temperature and strain measurements. The onset of stress relaxation of the cartilage generally occurred between 50/spl deg/C and 60/spl deg/C. While TOF measurements indicated a generally constant increase in the speed of sound of the cartilage during the irradiation period, the amplitude of the reflected acoustic signal correlated directly with the stress relaxation of the cartilage. At the point of stress relaxation, the amplitude of the acoustic signal consistently attenuated to roughly 50% of its original magnitude.     

On the Way to Zero Defect of Plastic-Encapsulated Electronic Power Devices—Part III: Chip Coating, Passivation, and Design

Concerning thermomechanically induced failures such as metal-line deformation and passivation cracks, there is a practicable way to achieve the zero-defect limit of plastic-encapsulated power devices. This limit can be reached by, first, evaluating the influence of the major components involved and, consequently, by selecting the appropriate materials and measures, and, second, by always keping in mind the interdependence between all components, i.e., chip and package have to be regarded as an entity. An important finding was that applying simply one improvement step will not necessarily lead to the desired goal. Only the implementation of all improvement steps considering their interdependence is the key for the perfect overall system chip and package. In Part III of this series of papers, the influence of passivation and die coating materials on thermomechanical damage is investigated. Finally, it is shown that an intelligent chip design, in combination with a stiff Al multilayer, a low-stress molding compound (low coefficient of thermal expansion and high Young's modulus), a new passivation material, and an appropriate polyimide layer, may reduce the thermomechanical damage to zero, even for electronic power devices.      

ITO-Schottky photodiodes for high-performance detection in the UV-IR spectrum

High-performance vertically illuminated Schottky photodiodes with indium-tin-oxide (ITO) Schottky layers were designed, fabricated, and tested. Ternary and quarternary III-V material systems (AlGaN-GaN, AlGaAs-GaAs, InAlGaAs-InP, and InGaAsP-InP) were utilized for detection in the ultraviolet (UV) (/spl lambda/<400 nm), near-IR (/spl lambda//spl sim/850 nm), and IR (/spl lambda//spl sim/1550 nm) spectrum. The material properties of thin ITO films were characterized. Using resonant-cavity-enhanced (RCE) detector structures, improved efficiency performance was achieved. Current-voltage, spectral responsivity, and high-speed measurements were carried out on the fabricated ITO-Schottky devices. The device performances obtained with different material systems are compared.     

Promising fluorescent probes from phycobiliproteins

Phycobiliproteins are brilliant-colored and water-soluble pigment-protein complexes existing widely in cyanobacteria, red algae, and cryptophytes. They function as predominant light-harvesting complexes to absorb the sunlight from 480 to 660 nm, and efficiently transfer the energy to chlorophyll a. The phycobiliproteins are classified into three types based on their absorption spectra: phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP). They are all composed of hetero-subunits /spl alpha/ and /spl beta/ and commonly exist in trimer (/spl alpha//spl beta/)3 or hexamer (/spl alpha//spl beta/)6 made up of equimolar monomer (/spl alpha//spl beta/). One monomer contains two to five chromophore phycobilins. In cyanobacteria and red algae, phycobiliproteins assemble a macromolecular particle, phycobilisome. Efficient excitation energy coupling among the chromophores in the phycobiliprotein trimer/hexamer and among the phycobiliproteins in the phycobilisome gives them some special spectroscopic properties superior to organic fluorescent dyes. These properties make the phycobiliproteins become promising fluorescent probes used in various fields of biological investigation.     

Low-temperature deposition of hafnium silicate gate dielectrics

The physical and electrical properties of hafnium silicate (HfSi/sub x/O/sub y/) films produced by low-temperature processing conditions (/spl les/150/spl deg/C) suitable for flexible display applications were studied using sputter deposition and ultra-violet generated ozone treatments. Films with no detectable low-/spl kappa/ interfacial layer were produced. Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy were used to determine the composition, chemical bonding environment, thickness, and film interface. The electrical behavior of the as-deposited and annealed hafnium silicate films were determined by current-voltage (I--V) and capacitance-voltage (C--V) measurements.     

Temperature stability of ZnO thin film SAW device on fused quartz

Surface acoustic wave (SAW) filters for low-frequency (38-65 MHz) applications have been developed using a radio frequency (RF)-magnetron-sputtered ZnO film on fused-quartz substrates. SAW propagation characteristics such as electromechanical coupling coefficient (K/sup 2/), SAW phase velocity (v), insertion loss, and temperature coefficient of delay (TCD) have been measured. The intergidital transducer (IDT)/ZnO/fused-quartz device structure yields almost zero TCD (1 ppm/spl middot//spl deg/C/sup -1/) with 0.316 /spl lambda/ thick ZnO layer (for the device operating at 60 MHz). Alternately, an overlayer of positive TCD material (ZnO itself) has also been deposited on the IDT/ZnO(<0.316 /spl lambda/)/fused-quartz device at a low substrate temperature to reduce the TCD. A modified layered structure consisting of ZnO/IDT/ZnO/fused quartz yields almost zero TCD (-3 ppm/spl middot//spl deg/C/sup -1/) with a 5.3-/spl mu/m-thick ZnO overlayer and a 8.1-/spl mu/m-thick (0.183 /spl lambda/) ZnO bottom layer. Experimentally obtained SAW propagation characteristics have been compared with the theoretical results.     

Fundamental Limits of Organic Packages and Boards and the Need for Novel Ceramic Boards for Next Generation Electronic Packaging

The system-on-a-package (SOP) paradigm proposes a package level integration of digital, RF/analog and opto-electronic functions to address future convergent microsystems. Two major components of SOP fabrication are sequential build-up of multiple layers (4–8) of conducting copper patterns with interlayer dielectrics on a board and multiple ICs flip-chip bonded on the top layer. A wide range of passives, wave-guides and other RF and opto-electronic components buried within the dielectric layers provide the multiple functions on a single microminiaturized platform.The routing of future nanoscale ICs with 10,000+ I/Os require multiple build-up layers of ultra fine board feature sizes of 10 m lines/space widths and 40 m pad diameters. Current FR4 boards cannot achieve this build-up technology because of dimensional instability during processing. These boards also undergo high warpage during the sequential build-up process which limits the fine-line lithography and also causes misalignment between the vias and their corresponding landing pads. In addition, the CTE mismatch between the silicon die and the board leads to IC-package interconnect reliability concerns, particularly in future fine-pitch assemblies where underfilling becomes complicated and expensive.This work reports experimental and analytical work comparing the performance of organic and novel ceramic boards for SOP requirements. The property requirements as deduced from these results indicate that a high stiffness and tailorable CTE from 2–4 ppm/C is required to enable SOP microminiaturized board fabrication and assembly without underfill. A novel ceramic board technology is proposed to address these requirements.     

Measurement of /spl lambda/-i characteristics of asymmetric three-phase transformers and their applications

Modeling of three-phase transformers with three-limb cores is important, such units are very ubiquitous and are asymmetric due to the different lengths of the three limbs of the core. It is, therefore, important to devise a measuring procedure, where the (/spl lambda/-i) characteristics of the three limbs and the zero-sequence (/spl lambda//sub 0/-i/sub 0/) characteristic due to the tank can be determined. A simulation program is developed so that currents at no-load and symmetric or asymmetric loads can be computed. Measured currents are compared with computed results for 2.3-kVA and 45-kVA units.     

Parametric investigation of a graphite foam evaporator in a thermosyphon with fluorinert and a silicon CMOS chip

High thermal conductivity graphitic foam was utilized as the evaporator in a modified thermosyphon. The foam was soldered directly to the back of a silicon CMOS die and mounted in a standard PGA. Fluorinert FC-87 and FC-72 were evaluated as the working fluids of choice and a variety of variables on the foams were explored. It was found that the density of the foam evaporators affected the thermal performance of the system. However, the fluid level and fluid type had very little effect on the overall performance in the system, making fabrication of a commercial device less challenging. The most significant effect on performance was the modifications to the foam structure. Slotted patterns were found to enhance the rate of return of fluid to the foam closest to the die, thus improving performance. With a slotted foam evaporator, a heat flux of 150W/cm/sup 2/ resulted in wall superheats of only 11/spl deg/C. The experimental setup used in this research gives accurate measurements of the actual active layer in the chip and temperatures less than 71/spl deg/C have been achieved at heat fluxes of 150 W/cm/sup 2/. This performance is significantly better than any prior literature data. In fact, the graphite foam thermosyphons were shown to outperform spray cooling. In addition, it was found that critical heat flux was not reached in these experiments with graphite foam evaporators at heat fluxes as high as 150 W/cm/sup 2/.     

Analysis of new anisotropic conductive film (ACF)

This paper designs a new multilayered particle anisotropic conductive film (ACF) compound. Using the particle-reinforced composite model and probability theory, the novel ACF compound is compared with three traditional ACFs having the same particle volume fraction. The particle-reinforced model applies the concept of bonded and debonded structures in the interfaces between the adhesive resin and the particles. The elastic modulus of the particle-reinforced ACF is a function of the particle volume fraction and the bonded condition. In the failure model, probability theory is used to calculate the probability of opening and bridging. The volume fraction of the conductive particles plays an important role in determining the optimal ACF design. The current results indicate that the flip chip packaging performed using the novel multilayered particle ACF compound (particles distributed in the top/bottom surface layers) exhibits superior particle-reinforcement properties and a lower failure probability than traditional ACFs. The improved understanding of reinforcement mechanisms and failure probability developed by this study facilitates the enhanced design of novel ACF compounds.     

Laser-activated shape memory polymer microactuator for thrombus removal following ischemic stroke: preliminary in vitro analysis

Due to the narrow (3-h) treatment window for effective use of the thrombolytic drug recombinant tissue-type plasminogen activator (rt-PA), there is a need to develop alternative treatments for ischemic stroke. We are developing an intravascular device for mechanical thrombus removal using shape memory polymer (SMP). We propose to deliver the SMP microactuator in its secondary straight rod form (length=4 cm, diameter=350 /spl mu/m) through a catheter distal to the vascular occlusion. The microactuator, which is mounted on the end of an optical fiber, is then transformed into its primary corkscrew shape by laser heating (diode laser, /spl lambda/=800 nm) above its soft-phase glass transition temperature (T/sub gs/=55/spl deg/C). Once deployed, the microactuator is retracted, and the captured thrombus is removed to restore blood flow. The SMP is doped with indocyanine green (ICG) dye to increase absorption of the laser light. Successful deployment of the microactuator depends on the optical properties of the ICG-doped SMP, as well as the optical coupling efficiency of the interface between the optical fiber and the SMP. Spectrophotometry, thermal imaging, and computer simulation aided the initial design effort and continue to be useful tools for optimization of the dye concentration and laser power. Thermomechanical testing was performed to characterize the elastic modulus of the SMP. We have demonstrated laser activation of the SMP microactuator in air at room temperature, suggesting this concept is a promising therapeutic alternative to rt-PA.     

基于2-D彩色图像模型的PCB焊点检测方法研究

提出了一种基于2-D彩色图像模型的PCB焊点检测方法.首先,基于3色LED环形结构光源和彩色数字相机的图像采集子系统获取的彩色PCB图像,分析了片式晶片元器件焊点表面信息与其焊点图像色彩分布规律,建立了片式晶片良品焊点类型的焊点检测模型,其次,给出了反映可接受焊点图像中心感兴趣区域(ROI)色彩分布规律的灰度曲线图.在...     

Performance of high-speed PWM control chips at cryogenic temperatures

Planetary exploration missions and deep space probes require electronics capable of low-temperature operation. Such electronics will not only improve circuit performance and reliability, but also increase system efficiency, and reduce development and launch costs. DC/DC converters are an essential part of most aerospace power management distribution systems. Therefore, DC/DC converters that can operate at cryogenic temperatures are crucial for space missions where low temperatures are encountered. An important component of a DC/DC converter is the pulsewidth modulation (PWM) chip that provides the control to the converter main switches. In the process of designing low-temperature DC/DC converters, experimental investigations were carried out to evaluate the performance of a number of high-speed PWM chips as a function of temperature in the range of 25/spl deg/C to -190/spl deg/C. These IC chips ranged in their electrical characteristics, modes of control, packaging options, and applications. This paper presents and discusses the experimental procedures along with the experimental data obtained on the investigated chips.