Interfacial Delamination Mechanisms During Soldering Reflow With Moisture Preconditioning

This paper first examines the commonly-used thermal-moisture analogy approach in thermal-moisture analogy approach. We conclude that such an analogy using a normalized concentration approach does not exist in the case of soldering reflow, when the solubility of each diffusing material varies with temperature or the saturated moisture concentration is not a constant over an entire range of reflow temperatures. The whole field vapor pressure distribution of a flip chip BGA package at reflow is obtained based on a multiscale vapor pressure model. Results reveal that moisture diffusion and vapor pressure have different distributions and are not proportional. The vapor pressure in the package saturates much faster than the moisture diffusion during reflow. This implies that the vapor pressure reaches the saturated pressure level in an early stage of moisture absorption, even the package is far from moisture saturated. However, the interfacial adhesion degrades continuously with moisture absorption. Therefore, the package moisture sensitivity performance will largely reply on the adhesion strength at elevated temperature with moisture. A specially designed experiment with a selection of six different underfills for flip chip packages was conducted. Results confirm that there is no correlation between moisture absorption and the subsequent interface delamination at reflow. The adhesion at high temperature with moisture is the only key modulator that correlates well with test data. Such a parameter is a comprehensive indicator, which includes the effects of thermal mismatch, vapor pressure, temperature and moisture. In this paper, a micromechanics based mechanism analysis on interfacial delamination is also presented. With the implementation of interface properties into the model study, it shows that the critical stress, which results in the unstable void growth and delamination at interface, is significantly reduced when the effect of moisture on debonding is considered.     

The effect of hygro-mechanical and thermo-mechanical stress on delamination of gold bump

The reliability of the FC–CSP (flip chip–chip scaled package) package with gold bump at the MRT (moisture resistance test) reflow temperature, was evaluated by using the finite element method. The moisture properties of EMC (epoxy molding compound) obtained from the test described in JEDEC standard, were used to characterize the local moisture concentration analysis by transient moisture diffusion, the hygro-mechanical analysis by CME, the vapor pressure analysis and the thermo-mechanical analysis by CTE mismatch. Also, after precondition, the package reliability under the reflow process was predicted, by comparing and integrating each factors, package swelling and stress due to by vapor pressure, as well as thermo-mechanical stress. Consequently, the result showed that the effects on hygro-mechanical stress and vapor pressure in a package could not be negligible, when it is compared with that of the thermo-mechanical stress by CTE mismatch, which is recognized as the main effect on the package crack under reflow temperature. The stress was concentrated at interface between gold bump and die, where most of delamination occurred.     

Effects of Anisotropic Conductive Film Viscosity on ACF Fillet Formation and Chip-On-Board Packages

In this paper, the effects of anisotropic conductive film (ACF) viscosity on ACF fillet formation and, ultimately, on the pressure cooker test (PCT) reliability of ACF flip chip assemblies were investigated. The ACF viscosity was controlled by varying the molecular weight of the epoxy materials. It was found that the ACF viscosity increased as the increase of molecular weight of the epoxy materials. However, there was little variation of the thermomechanical properties among the evaluated ACFs with different viscosites. Also, the results showed that the ACFs have no differences in moisture absorption rate, die adhesion strength, and degree-of-cure. In scanning electron microscopy images, the lower ACF viscosity resulted in the smoother ACF fillet shape and the higher fillet height. From the results of PCT, the ACF flip chip assembly with the smoother fillet shape showed better reliability in terms of contact resistance changes. After 130 h of PCT, the flip chip assembly with lower ACF viscosity also showed a lesser degree of delamination at the ACF/chip interface.     

Comprehensive treatment of moisture induced failure-recent advances

This paper describes a comprehensive treatment of moisture induced failure in integrated circuit (IC) packaging with emphasis on recent advances. This includes advanced technique for modeling moisture diffusion under dynamic boundary conditions such as experienced by packages during solder reflow, autoclave, and temperature-humidity cycling; advanced characterization technique for moisture sorption and diffusion properties of packaging materials including effect of edge diffusion on transverse diffusivity, anisotropic diffusivity in organic laminates, impact of non-Fickian sorption; advanced techniques for modeling vapor pressure during solder reflow; advanced techniques for modeling dynamic delamination propagation during solder reflow; interfacial fracture strength as a function of temperature and moisture; as well as plastic analysis of popcorn cracking.     

A study of moisture diffusion in plastic packaging

The absorption and desorption processes of moisture in plastic material were studied with experimental measurement and finite element (FE) simulation. The diffusion coefficient and the saturate concentration were determined by experiment and simulation results with Fick’s law. The water molecules inside the plastic material were chemically bonded with polymers by hydrogen bonds in the microholes formed by the polymer molecule chains. On the saturate concentration, the moisture density in the effective volumes was 100 times larger than the vapor density in standard state. However, it is only 8% of liquid water. The water inside the plastic material was in a liquid situation. The delamination and the delamination recovery of flip-chip packaging inspected by C-mode scanning acoustic microscropy (C-SAM) during a high-temperature and high-humidity accelerating test could be explained with the state change of the water in plastic material space. The delamination recovery resulted from the increase in content of liquid water. The bonding of water molecules and polymers reduced the adhesive strength at the interface between epoxy material and die, and the delamination on the interface was initiated. A comparison of three cases with noncoated film, top-side SiC_x coated, and both-sides SiC_x coated indicated that the delamination would occur when the moisture concentration was between 50% and 95% of the saturate concentration. During the reflow process, the low interface-adhesive strength and the high vapor pressure of the wet sample might cause popcorning of the plastic packaging. Popcorning could be predicted by simulation of the moisture concentration at the interface.     

热–机械应力和湿气引起QFN器件层间开裂分析

采用通用有限元软件分析和计算器件在潮湿环境下的潮湿扩散,并计算由于吸潮使器件在无铅回流的高温下产生蒸汽压力,并对层间开裂现象进行分析。结果表明,气压是层间开裂的主要原因,在气压和热机械应力的作用下,层合面上的微孔洞扩张并结合起来,导致器件最后失效。     

Material properties of anisotropic conductive films (ACFs) and their flip chip assembly reliability in NAND flash memory applications

In this paper, the material properties of anisotropic conductive films (ACFs) and ACF flip chip assembly reliability for a NAND flash memory application were investigated. Measurements were taken on the curing behaviors, the coefficient of thermal expansion (CTE), the modulus, the glass transition temperature (T_g), and the die adhesion strength of six types of ACF. Furthermore, the bonding processes of the ACFs were optimized. After the ACF flip chip assemblies were fabricated with optimized bonding processes, reliability tests were then carried out. In the pressure cooker test, the ACF with the highest adhesion strength showed the best reliability and the ACF flip chip assembly revealed no delamination at the chip-ACF interface, even after 96 h. In the high temperature storage test and the thermal cycling test, the reliability of the ACF flip chip assembly strongly depends on the T_g value of the ACF. In the thermal cycling test, in particular, which gives ACF flip chip assemblies repetitive shear stress, high value of CTE above T_g accelerates the failure rate of the ACF flip chip assembly. From the reliability test results, ACFs with a high T_g and a low CTE are preferable for enhancing the thermal and thermo-mechanical reliability. In addition, a new double-sided chip package with a thickness of 570 μm was demonstrated for NAND flash memory application. In conclusion, this study verifies the ACF feasibility, and recommends the optimum ACF material properties, for NAND flash memory application.     

Investigation of delamination control in plastic package

Interfacial delamination control is important for the mechanical reliability of plastic package, especially facing the challenge of lead-free requirement. Efforts of delamination performance improvement including structure, material and process for plastic package were made. It was demonstrated that the improvement of packaging material mechanical properties should depend on the package and purpose and should be the comprehensive consideration of advantage and disadvantage. The fracture mechanics was introduced to explain the mechanism of bumpy interface as the delamination retardant. The moisture absorption experiments of three kinds of molding compound were carried out to confirm that large difference of delamination performance among different molding compounds for the same device in our project was induced by moisture absorption difference, and the relation between the moisture weight absorbed and weight ratio of resin was obtained, given compounds of the same resin. It was substantiated that the moisture absorption of molding compound observes the Fick’s law at (Moisture Sensitivity Level 3) MSL3. Quantitative analyses of the moisture influence at the risk location were conducted to evaluate the risk of delamination with the consideration of vapor pressure during solder reflow, moisture and thermal expansion.     

Integrated vapor pressure, hygroswelling, and thermo-mechanical stress modeling of QFN package during reflow with interfacial fracture mechanics analysis

In this paper, a comprehensive and integrated package stress model is established for quad flat non-lead package with detailed considerations of effects of moisture diffusion, heat transfer, thermo-mechanical stress, hygro-mechanical stress and vapor pressure induced during reflow. The critical plastic materials, i.e., moldcompound and die attach are characterized for hygroswelling and moisture properties, which are not easily available from material suppliers. The moisture absorption during preconditioning at JEDEC Level 1, and moisture desorption at various high temperatures are characterized. The moisture diffusivity is a few orders higher at reflow temperature than moisture preconditioning temperature. Due to coefficient of moisture expansion mismatch among various materials, hygro-mechanical stress is induced. The concept is analogous to coefficient of thermal expansion mismatch which results in thermo-mechanical stress. Thermal diffusivity is much faster than the moisture diffusivity. During reflow, the internal package reaches uniform temperature within a few seconds. The vapor pressure can be calculated based on the local moisture concentration after preconditioning. Results show that the vapor pressure saturates much faster than the moisture diffusion, and a near uniform vapor pressure is reached in the package. The vapor pressure introduces additional strain of the same order as the thermal strain and hygrostrain to the package. Subsequently, the interfacial fracture mechanics model is applied to study the effect of crack length on die/mold compound and die/die attach delamination.     

A novel flip chip technology using nonconductive resin sheet

The process flow of this new packaging system is as follows. First, epoxy base resin sheet is laminated onto substrate to cover the substrate surface including land electrodes. Bumped chip alignment and attachment was done through the resin sheet, in the second stage with pressure and temperature. The bumps under the chip penetrate with removal of resin sheet material eventually reaching to the metal land of the substrate in this process. Metal connection and curing of the interface resin have been completed in the third stage. This new process has the potential to make flip chip packages simple compared with the current process using liquid resin with dispensing system. The throughput time can be reduced to less than 10 s/unit in actual model case even for large flip chip package which has over 15×15 (mm) square area IC chips. The other advantages are thermal stability of material in the process, moisture related performance, and warpage control performance. For current underfill process the only choice is to use anhydride type resin system which has many disadvantages. This new process made it possible to introduce moisture and thermally stable epoxy resin with phenol curing system for flip chip packaging. Drastic process ability improvement can be achieved by the new process and material. As a typical improvement of thermal shock performance, it was confirmed that the life of chip damage is over 10 times longer by flip chip bonding parameters which can be controlled only by this new flip chip packaging process     

Moisture-induced failures of adhesive flip chip interconnects

Adhesive flip chip interconnect has been recognized as a promising substitute for solder interconnection due to its fine-pitch, lead-free, and low-temperature processing capabilities. As adhesives are made of polymers, moisture absorption by the polymeric resin remains as one of the principal contributors to adhesive joint failure mechanisms. In this research, the reliability performance of the adhesive flip chip in the pressure cooker test and moisture sensitivity test conditions was investigated. The failure modes were found to be interfacial delamination and bump/pad opening which may eventually lead to total loss of electrical contact. Different sizes of bump/pad opening in the interconnections were discussed in the context of the significance of mismatch in coefficient of moisture expansion (CME) between adhesive and other components in the package, which induces a hygroscopic swelling stress. The effect of moisture diffusion in the package and the CME mismatch were also evaluated from the standpoint of finite element modeling. In this study, it is concluded that hygroscopic swelling assisted by loss of adhesion strength upon moisture absorption is responsible for the moisture-induced failures in these adhesive flip chip interconnects.     

Reduced thermal strain in flip chip assembly on organic substrateusing low CTE anisotropic conductive film

Flip chip assembly directly on organic boards offers miniaturization of package size as well as reduction in interconnection distances, resulting in a high performance and cost-competitive packaging method. This paper describes the usefulness of low cost flip-chip assembly using electroless Ni/Au bump and anisotropic conductive films on organic boards such as FR-4. As bumps for flip chip, electroless Ni/Au plating was performed as a low cost bumping method. Effect of annealing on Ni bump characteristics informed that the formation of crystalline nickel with Ni₃P precipitation above 300°C causes an increase of hardness and an increase of the intrinsic stress. As interconnection material, modified ACFs composed of nickel conductive fillers for conductive fillers, and nonconductive fillers for modification of film properties, such as coefficient of thermal expansion (CTE), were formulated for improved electrical and mechanical properties of ACF interconnection. Three ACF materials with different CTE values were prepared and bonded between Si chips and FR-4 boards for the thermal strain measurement using moire interferometry. The thermal strain of the ACF interconnection layer, induced by temperature excursion of 80°C, was decreased according to the decreasing CTEs of ACF materials. This result indicates that the thermal fatigue life of ACF flip chip assembly on organic boards, limited by the thermal expansion mismatch between the chip and the board, could be increased by low CTE ACF     

An overview of solder bump shape prediction algorithms withvalidations

The trend to reduce the size of electronic packages and develop increasingly sophisticated electronic devices with more, higher density inputs/outputs (I/Os), leads to the use of area array packages using chip scale packaging (CSP), flip chip (FC), and wafer level packaging (WLP) technologies. Greater attention has been paid to the reliability of solder joints and the assembly yield of the surface mounting process as use of advanced electronic packaging technologies has increased. The solder joint reliability has been observed to be highly dependent on solder joint geometry as well as solder material properties, such that predicting solder reflow shape became a critical issue for the electronic research community. In general, the truncated sphere method, the analytical solution and the energy-based algorithm are the three major methods for solder reflow geometry prediction. This research develops solder joint reliability design guidelines to accurately predict both the solder bump geometry and the standoff height for reflow soldered joints in area array packages. Three simulation methods such as truncated-sphere theory force-balanced analytical solution and energy-based approach for prediction of the solder bump geometry are each examined in detail, and the thermal enhanced BGA (TBGA) and flip chip packages are selected as the benchmark models to compare the simulation and experimental results. The simulation results indicate that all three methods can accurately predict the solder reflow shape in an accurate range     

Influence of underfill materials on the reliability of coreless flip chip package

A flip chip package was assembled by using 6-layer laminated polyimide coreless substrate, eutectic Sn37Pb solder bump, two kinds of underfill materials and Sn_3.0Ag_0.5Cu solder balls. Regarding to the yield, the peripheral solder joints were often found not to connect with the substrate due to the warpage at high temperature, modification of reflow profile was benefit to improve this issue. All the samples passed the moisture sensitive level test with a peak temperature of 260 °C and no delamination at the interface of underfill and substrate was found. In order to know the reliability of coreless flip chip package, five test items including temperature cycle test (TCT), thermal shock test (TST), highly accelerated stress test (HAST), high temperature storage test (HTST) and thermal humidity storage test (THST) were done. Both of the two underfill materials could make the samples pass the HTST and THST, however, in the case of TCT, TST and HAST, the reliability of coreless flip chip package was dominated by underfill material. A higher Young’s modules of underfill, the more die crack failures were found. Choosing a correct underfill material was the key factor for volume production of coreless flip chip package.     

基于内聚力模型脱粘仿真的内埋芯片PI分层研究

芯片埋入式封装技术的难点主要集中在封装制造过程对芯片造成的一系列不利影响,如裂纹、分层、翘曲、静电等.基板埋入芯片的特殊结构,大大增加了封装工艺难度.首先根据开发过程中出现的芯片聚酰亚胺分层现象建立简化模型,其次应用ANSYS工具中的内聚力单元对界面脱粘过程进行了模拟仿真,并分析了模型的等效应力值分布.结果 表明,实验...     

Virtual qualification of moisture induced failures of advanced packages

This paper presents a combined numerical and experimental methodology for predicting and preventing moisture induced failures in encapsulated packages. Prevention of such failures will enable efficient and optimal pre-selection of materials, their interfaces and geometric design with respect to the desired resistance to moisture. This virtual qualification methodology is illustrated for a specific BGA package which showed 50% failures (broken stitch-bonds) during HAST testing due to excessive warpage and/or delamination of different interfaces. For three different material combinations the moisture diffusion during the HAST test is predicted and subsequently thermo-mechanical-moisture simulations are performed where the effects of hygro-swelling, vapor pressure, thermal expansion and delamination on the failure mechanisms are predicted. The comparison of the simulation results of the different molding compounds with the observations of HAST testing indicates that the developed methods and models can predict the observed trends. Application of the presented methodology will result in shorter time-to-market and significant cost reduction due to reduced trial-and-error design cycles and effective material usage.     

Adhesion issues in flip-chip on organic modules

Flip chip attach on organic carriers is a novel electronic packaging assembly method which provides advantages of high input/output (I/O) counts, electrical performance and thermal dissipation. In this structure, the flip chip device is attached to organic laminate with predeposited eutectic solder. Mechanical coupling of the chip and the laminate is done via underfill encapsulant materials. As the chip size increases, the thermal mismatch between silicon and its organic carrier becomes greater. Adhesion becomes an important factor since the C4 joints fail quickly if delamination of the underfill from either chip or the solder mask interface occurs. Newly developed underfills have been studied to examine their properties, including interfacial adhesion strength, flow characteristics, void formation and cure kinetics. This paper will describe basic investigations into the properties of these underfills and also how these properties related to the overall development process. In addition, experiments were performed to determine the effects on adhesion degradation of flip chip assembly processes and materials such as IR reflow profile, flux quantity and residues. Surface treatment of both the chip and the laminate prior to encapsulation were studied to enhance underfill adhesion. Accelerated thermal cycling and highly accelerated stress testing (HAST) were conducted to compare various underfill properties and reliability responses     

Effect of autoclave test on anisotropic conductive joints

This paper reports that the stress-corrosion cracking induced by autoclave test condition reduces the mechanical strength of anisotropic conductive joints and also increases the contact resistance by allowing more moisture to reach the aluminium metallization. The use of anisotropic conductive joints with bumpless chips allows a reduction in the costs of the flip chip bonding process. The epoxy-based anisotropic conductive adhesive film (ACF) absorbs moisture and experiences hygroscopic swelling, hence degrading adhesion strength and elasticity in hazardous environments, e.g. if moisture at high vapour pressure, and test temperature near to its glassy temperature (T_g) are applied. Contact resistances also show an increasing trend that is similar to that typical of a corrosion process. It is most probably due to the formation of an oxidation layer on top of the aluminium metallization and to the hygroscopic swelling of the ACF. The elastic properties of ACF joints reduce by about 50% after 384 h test time. In this study, 336 h of autoclave test is the critical test duration to affect the electrical and mechanical properties of an ACF joint.     

Yield analysis and process modeling of low cost, high throughputflip chip assembly based on no-flow underfill materials

As a concept to achieve low-cost, high-throughput flip chip on board (FCOB) assembly, a new process has been developed implementing next generation flip chip processing based no-flow fluxing underfill materials. The low-cost, high throughput flip chip process implements large area underfill printing, integrated chip placement and underfill flow and simultaneous solder interconnect reflow and underfill cure. The goals of this study are to demonstrate feasibility of no flow underfill materials and the high throughput flip chip process over a range of flip chip configurations, identify the critical process variables affecting yield, analyze the yield of the high throughput flip chip process, and determine the impact of no-flow underfill materials on key process elements. Reported in this work is the assembly of a series of test vehicles to assess process yield and process defects. The test vehicles are assembled by depositing a controlled mass of underfill material on the chip site, aligning chip to the substrate pads, and placing the chip inducing a compression type underfill flow. The assemblies are reflowed in a commercial reflow furnace in an air atmosphere to simultaneously form the solder interconnects and cure the underfill. A series of designed experiments identify the critical process variables including underfill mass, reflow profile, placement velocity, placement force, and underfill material system. Of particular interest is the fact that the no-flow underfill materials studied exhibit an affinity for unique reflow profiles to minimize process defects     

Study of anisotropic conductive adhesive joint behavior under 3-point bending

Flip chip interconnections using anisotropic conductive film (ACF) are now a very attractive technique for electronic packaging assembly. Although ACF is environmentally friendly, many factors may influence the reliability of the final ACF joint. External mechanical loading is one of these factors. Finite element analysis (FEA) was carried out to understand the effect of mechanical loading on the ACF joint. A 3-dimensional model of adhesively bonded flip chip assembly was built and simulations were performed for the 3-point bending test. The results show that the stress at its highest value at the corners, where the chip and ACF were connected together. The ACF thickness was increased at these corner regions. It was found that higher mechanical loading results in higher stress that causes a greater gap between the chip and the substrate at the corner position. Experimental work was also carried out to study the electrical reliability of the ACF joint with the applied bending load. As per the prediction from FEA, it was found that at first the corner joint failed. Successive open joints from the corner towards the middle were also noticed with the increase of the applied load.