Characterization study of time- and temperature-dependent mechanical behavior of polyimide materials in electronic packaging applications

The thermo-mechanical testing of the type HPP ST polyimide films with high performance, supplied by Dupont, was realized under different strain rates and temperature effects. Therefore, the rate-temperature-dependent stress-strain behavior of materials was investigated and the dependence of the Young’s modulus on temperature and strain rate was reported. In view of the uncertainty of the Young’s modulus determination, the specimens were tested with the unloading-reloading technique to verify the test results. The constant strain rate uniaxial tensile test and long-time creep test at various temperatures were performed to characterize the time-temperature-dependent mechanical property precisely. The cyclic loading test was also implemented on the specimen to investigate cyclic stress-strain behavior. In addition, the nanoindentation test was carried out at room temperature to validate the elastic modulus derived from the uniaxial tensile test. This research is expected to investigate the time-temperature-dependent mechanical behavior of the polyimide materials for different service regimes including tensile and cyclic mechanical loading under elevated temperature in a systematic manner.     

Effects of underfill material properties on the reliability ofsolder bumped flip chip on board with imperfect underfill encapsulants

Three different types of underfill imperfections were considered; i.e., (1) interfacial delamination between the underfill encapsulant and the solder mask on the PCB (crack initiated at the tip of underfill fillet), (2) interfacial delamination between the chip and the underfill encapsulant (crack initiated at the chip corner), and (3) the same as (2) but without the underfill fillet. Five different combinations of coefficient of thermal expansion (CTE) and Young's modulus with the aforementioned delaminations were investigated. A fracture mechanics approach was employed for computational analysis. The strain energy release rate at the crack tip and the maximum accumulated equivalent plastic strain in the solder bumps of all cases were evaluated as indices of reliability. Besides, mechanical shear tests were performed to characterize the shear strength at the underfill-solder mask interface and the underfill-chip passivation interface. The main objective of the present study is to achieve a better understanding in the thermo-mechanical behavior of flip chip on board (FCOB) assemblies with imperfect underfill encapsulants     

Time and temperature-dependent mechanical behavior of underfill materials in electronic packaging application

The thermo-mechanical testing of HYSOL FP4549 polymer-filled underfill materials was conducted under different strain rate and temperature environment. A new specimen preparation procedure and further test methodology are developed to characterize the time–temperature mechanical behaviors of underfill materials. The stress–strain behavior of materials is simulated with constitutive framework, and the dependence of Young’s modulus on temperature and strain rate was evaluated. In addition, the specimens were tested with microforce testing system to evaluate the creep curve of underfill materials as a function of temperature and stress level. In view of the uncertainty of the Young’s modulus determination, the specimens were tested with unloading–reloading technique to verify the test results and investigate its cyclic mechanical behaviors. On the other hand, the adhesion strength of underfill materials are tested between different adhesion surface by different deformation rate after some isothermal and hygro-thermal environments attack, which is to simulate the environment that the electronic components may be encountered. The results reveal that the rise of the temperature and moisture cause the apparent reduction of the surface adhesion strength, due to the microstructure transition of materials and the diffusion and concentration of moisture. For all conditions of the experiment after environmental preconditioning, the specimen fracture surfaces occur between solder mask and FR4 substrates, which means the measured strength is the adhesion strength between solder mask and FR4. Comparing different adhesion surface, the adhesion strength of underfill/FR4 is higher than solder mask/FR4. The interface of solder mask/FR4 is more sensitive to the temperature and moisture. In all of the cases, increasing the moisture level has a varying but significant effect on both fracture strength and absorption energy Ψ. The failure mode transfer and the strength degradation are attributed to the moisture uptake between the FR4/solder mask and solder mask/underfill interface.     

Die stress characterization in flip chip on laminate assemblies

Minimizing device side die stresses is especially important when multiple copper/low-k interconnect redistribution layers are present. Mechanical stress distributions in packaged silicon die resulting during assembly or environmental testing can be accurately characterized using test chips incorporating integral piezoresistive sensors. In this paper, measurements of thermally induced stresses in flip chip on laminate assemblies are presented. Transient die stress measurements have been made during underfill cure, and the room temperature die stresses in final cured assemblies have been compared for several different underfill encapsulants. In addition, stress variations have been monitored in the assembled flip chip die as the test boards were subjected to slow temperature changes from -40 to +150/spl deg/C. Using these measurements and ongoing numerical simulations, valuable insight has been gained on the effects of assembly variables and underfill material properties on the reliability of flip chip packages.     

Creep, stress relaxation, and plastic deformation in Sn-Ag and Sn-Zn eutectic solders

Because of the high homologous operation temperature of solders used in electronic devices, time and temperature dependent relaxation and creep processes affect their mechanical behavior. In this paper, two eutectic lead-free solders (96.5Sn-3.5Ag and 91Sn-9Zn) are investigated for their creep and stress relaxation behavior. The creep tests were done in load-control with initial stresses in the range of 10-22 MPa at two temperatures, 25 and 80°C. The stress relaxation tests were performed under constant-strain conditions with strains in the range of 0.3-2.4% and at 25 and 80°C. Since creep/relaxation processes are active even during monotonie tensile tests at ambient temperatures, stress-strain curves at different temperatures and strain rates provide insight into these processes. Activation energies obtained from the monotonic tensile, stress relaxation, and creep tests are compared and discussed in light of the governing mechanisms. These data along with creep exponents, strain rate sensitivities and damage mechanisms are useful for aiding the modeling of solder interconnects for reliability and lifetime prediction. Constitutive modeling for creep and stress relaxation behavior was done using a formulation based on unified creep plasticity theory which has been previously employed in the modeling of high temperature superalloys with satisfactory results.     

Study on effect of coupling agents on underfill material in flipchip packaging

Coupling agents are widely used in order to improve the adhesion property of underfill. In this study, three different silane coupling agents, two titanate coupling agents, and one zirconate coupling agent were added into an epoxy underfill material. Their effects on the flow behavior and curing profile of the epoxy underfill were studied with a rheometer and a differential scanning calorimeter, respectively. The thermal stability of the cured underfill material was studied with a thermogravimetric analyzer. A thermal mechanical analyzer and a dynamic mechanical analyzer were used to measure the coefficient of thermal expansion, the glass transition temperature (T_g), and the storage modulus (E'). In addition, the adhesion of the underfill on benzocyclobutene passivated silicon die and polyimide passivated silicon die was measured through die shear test. The effects of aging in an 85°C/85% relative humidity chamber were also studied through moisture absorption test and die shear test     

Novel thermally reworkable underfill encapsulants for flip-chipapplications

The flip-chip technique of integrated circuit (IC) chip interconnection is the emerging technology for high performance, high input/output (I/O) IC devices. Due to the coefficient of thermal expansion mismatch between the silicon IC (CTE=2.5 ppm/°C) and the low cost organic substrate such as FR-4 printed wiring board (CTE=18-22 ppm/°C), the flip-chip solder joints experience high shear stresses during temperature cycling. Underfill encapsulant is used to couple the bilayer structure and is critical to the reliability of the flip-chip solder interconnects. Current underfill encapsulants are filled epoxy-based materials that are normally not reworkable after curing. This forms an obstacle to flip-chip on board (FCOB) technology development, where unknown bad dies (UBD) are still a concern. Approaches have been taken to develop the thermally reworkable underfill materials in order to address the nonreworkability problem of the commercial underfill encapsulants. These approaches include introduction of thermally cleavable blocks into epoxides and addition of additives to the epoxies. In the first approach, five diepoxides containing thermally cleavable blocks were synthesized and characterized. These diepoxides were mixed with hardener and catalyst. Then the mixture properties of Tg, onset decomposition temperature, storage modulus, CTE, and viscosity were studied and compared with those of the standard formulation based on the commercial epoxy resin ERL-4221E. These mixtures all decomposed at lower temperature than the standard formulation. Moreover, one mixture, Epoxy5, showed acceptable Tg, low viscosity, and fairly good adhesion. In the second approach, two additives were discovered that provide die removal capability to the epoxy formulation without interfering with the epoxy cure or properties of the cured epoxy system. Furthermore, the combination of the two approaches showed positive results     

倒装焊SnPb焊点热循环失效和底充胶的影响

采用实验方法,确定了倒装焊SnPb焊点的热循环寿命．采用粘塑性和粘弹性材料模式描述了SnPb焊料和底充胶的力学行为,用有限元方法模拟了SnPb焊点在热循环条件下的应力应变过程．基于计算的塑性应变范围和实验的热循环寿命,确定了倒装焊SnPb焊点热循环失效Coffin-Manson经验方程的材料参数．研究表明,有底充胶倒装焊SnPb焊点的塑性应变范围比无底充胶时明显减小,热循环寿命可提高约20倍,充胶后的焊点高度对可靠性的影响变得不明显．     

An improved methodology for determining temperature dependentmoduli of underfill encapsulants

Finite element analyses (FEAs) have been widely used to preventively predict the reliability issues of flip-chip (FC) packages. The validity of the simulation results strongly depends on the inputs of the involved material properties. For FC packages Young's modulus-temperature relationship is a critical material property in predicting of the package reliability during -55°C to 125°C thermal cycling. Traditional tensile tests can obtain the modulus at selected temperatures, but are tedious, expensive, and unable to accurately predict the Young's modulus-temperature relationship within a wide temperature range. Thus, this paper is targeted to provide a simple but relatively accurate methodology to obtain the Young's modulus-temperature relationship. In this paper, three commercial silica filled underfill materials were studied. A simple specimen (based on ASTM D638M) preparation method was established using a Teflon mold. A dynamic-mechanical analyzer (DMA) was used to obtain the stress-strain relationship under controlled force mode, storage and loss modulus under multi-frequency mode, and stress relaxation under stress relaxation mode. A simple viscoelastic model was used and an empirical methodology for obtaining Young's modulus-temperature relationship was established     

Underfill constraint effects during thermomechanical cycling of flip-chip solder joints

The presence of an “underfill” encapsulant between a microelectronic device and the underlying substrate is known to substantially improve the thermal fatigue life of flip-chip (FC) solder joints, primarily due to load-transfer from the solder to the encapsulant. In this study, a new single joint-shear (SJS) test, which allows the measurement of the strain response of an individual solder ball during thermomechanical cycling (TMC), has been used to investigate the impact of the constraint imposed by the underfill on a solder joint. Finite element (FE) modeling has been used to demonstrate that the SJS sample geometry captures most of the deformation characteristics of an FC joint and to provide insight into experimental observations. It has been shown that the strain response of a eutectic Pb-Sn solder joint is influenced significantly by in-situ microstructural coarsening during TMC, which in turn is dependent on the underfill properties. In general, underfill properties, which allow the imposition of large compressive-hydrostatic stresses on the solder joint, were the most effective in reducing coarsening. Phase coarsening prevented the stabilization of the stress-strain response of the solder, even in the absence of crack damage, and was found to depend strongly on the local inelastic-strain state within the joint. This necessitates that future solder deformation models account for strain-history-dependent microstructural evolution and that underfill properties be optimized to minimize the extent of coarsening during TMC in order to maximize joint life.     

Underfill selection methodology for fine pitch Cu/low-k FCBGA packages

A systematic underfill selection approach has been presented to characterize and identify suitable underfill encapsulants for large size flip chip ball grid array (FCBGA) packages. In the selection scheme, a total of six evaluation factors such as fracture toughness, coefficient of moisture expansion, flowability, delamination performance and filler settlement were considered. Driving stresses for package failure were also included as a factor of consideration, which clearly depends on the package size and geometry. Based on the approach adopted, underfill material that is suitable for 35 × 35 mm² packages with 15 mm die size and 45 × 45 mm² packages with 21 mm die size was selected. Target value for underfill properties has also been revised.     

倒装焊SnPb焊点热循环失效和底充胶的影响

采用实验方法 ,确定了倒装焊 Sn Pb焊点的热循环寿命 .采用粘塑性和粘弹性材料模式描述了 Sn Pb焊料和底充胶的力学行为 ,用有限元方法模拟了 Sn Pb焊点在热循环条件下的应力应变过程 .基于计算的塑性应变范围和实验的热循环寿命 ,确定了倒装焊 Sn Pb焊点热循环失效 Coffin- Manson经验方程的材料参数 .研究表明 ,有底充胶倒装焊 Sn Pb焊点的塑性应变范围比无底充胶时明显减小 ,热循环寿命可提高约 2 0倍 ,充胶后的焊点高度对可靠性的影响变得不明显     

Effects of Phase Change of Pb-Free Flip-Chip Solders During Board-Level Interconnect Reflow

The impact of phase change (from solid to liquid) on the reliability of Pb-free flip-chip solders during board-level interconnect reflow is investigated. Most of the current candidates for Pb-free solder are tin-based with similar melting temperatures near 230 degC. Thus, Pb-free flip-chip solders melt again during the subsequent board-level interconnect reflow cycle. Solder volume expands more than 4% during the phase change from solid to liquid. The volumetric expansion of solder in a volume constrained by chip, substrate, and underfill creates serious reliability issues. The issues include underfill fracture and delamination from chip or substrate. Besides decreasing flip-chip interconnect reliability in fatigue, bridging through underfill cracks or delamination between neighboring flip-chip interconnects by the interjected solder leads to failures. In this paper, the volume expansion ratio of tin is experimentally measured, and a Pb-free flip-chip chip-scale package (FC-CSP) is used to observe delamination and solder bridging after solder reflow. It is demonstrated that the presence of molten solder and the interfacial failure of underfill can occur during solder reflow. Accordingly, Pb-free flip-chip packages have an additional reliability issue that has not been a concern for Pb solder packages. To quantify the effect of phase change, a flip-chip chip-scale plastic ball grid array package is modeled for nonlinear finite-element analysis. A unit-cell model is used to quantify the elongation strain of underfill and stresses at the interfaces between underfill and chip or underfill and substrate generated by volume expansion of solder. In addition, the strain energy release rate of interfacial crack between chip and underfill is also calculated     

Assembly of lead-free bumped flip-chip with no-flow underfills

Lead-free solder reflow process has presented challenges to no-flow underfill material and assembly. The currently available no-flow underfill materials are mainly designed for eutectic Sn-Pb solders. This paper presents the assembly of lead-free bumped flip-chip with developed no-flow underfill materials. Epoxy resin/HMPA/metal AcAc/Flux G system is developed as no-flow underfills for Sn/Ag/Cu alloy bumped flip-chips. The solder wetting test is conducted to demonstrate the fluxing capability of the underfills for lead-free solders. A 100% solder joint yield has been achieved using Sn/Ag/Cu bumped flip-chips in a no-flow process. A scanning acoustic microscope is used to observe the underfill voiding. The out-gassing of HMPA at high curing temperatures causes severe voiding inside the package. A differential scanning calorimeter (DSC) used to study the curing degree of the underfill after reflow with or without post-cure. The post-curing profiles indicate that the out-gassing of HMPA would destroy the stoichiometric balance between the epoxy and hardener, and result in a need for high temperature post-cure. The material properties of the underfills are characterized and the influence of underfill out-gassing on the assembly and material properties is investigated. The impact of lead-free reflow on the material design and process conditions of no-flow underfill is discussed.     

High-temperature reliability of Flip Chip assemblies

Flip Chip technology has been widely accepted within microelectronics as a technology for maximum miniaturization. Typical applications today are mobile products such as cellular phones or GPS devices. For both widening Flip Chip technology’s application range and for addressing the automotive electronics’ volume market, developing assemblies capable of withstanding high temperatures is crucial. A typical scenario for integrating electronics into a car is a control unit within the engine compartment, where ambient temperatures are around 150 °C, package junction temperatures may range from 175 °C to 200 °C and peak temperatures may exceed these values.If Flip Chip technology is used under harsh environment conditions, it is clear that especially the polymeric materials, i.e., underfiller, solder mask or the organic substrate base material, are challenged. Generally, the developmental goal for encapsulants compatible with high-temperature applications are materials with high T_g and low degradation even at temperatures >200 °C.According to these demands, a test group of advanced underfill encapsulants has been used for assembling Flip Chip devices. These test vehicles were built using lead-free and lead-containing solders such as SnAgCu and eutectic PbSn and standard FR4 substrates, for evaluating the reliability potential of state-of-the-art underfillers. Material analysis is performed for studying both material degradation as well as temperature-dependent thermo-mechanical and adhesive properties. For assessing reliability, temperature cycling is performed with different maximum test temperatures ranging from 150 °C to 175 °C. The device status is intermediately analyzed by using electrical measurement for detecting bond integrity and acoustomicroscopy for determining the occurrence and growth of delaminations. Extensive failure analysis is added to investigate device failure mechanisms, especially related to the respective test temperature.In summary, an empirical status of the high-temperature potential of state-of-the-art underfillers and material combinations is attained and an outlook on future demands and developments is provided.     

Experimental determination of fatigue behavior of lead free solder joints in microelectronic packaging subjected to isothermal aging

The effects of aging on the cyclic shear stress–strain and fatigue behavior of lead-free solders have been explored experimentally and have been presented in this paper. An experimental procedure has been developed for preparing Iosipescu shear specimens of SAC105 (Sn–1.0Ag–0.5Cu) lead-free solder, and the resulting solder joint specimens have been subjected to cyclic shear stress/strain loading at different aging conditions. A combination of four-parameter hyperbolic tangent empirical models has been used for the empirical fit of the entire cyclic stress strain curve. The fatigue life data were then fit using popular empirical failure criteria such as the strain-based Coffin–Manson model and the energy-based Morrow model. Evolution of shear hysteresis loop of SAC 105 with aging has been studied. Degradation of isothermal fatigue life due to aging has also been studied in this paper. A comparison between uniaxial fatigue data and shear fatigue data is shown and a good qualitative agreement has been found. Subsequent microstructure analysis has also been presented in the paper in support of isothermal aging effects.     

Development of no-flow underfill materials for lead-free solderbumped flip-chip applications

No-flow underfill process in flip-chip assembly has become a promising technology toward a smaller, faster and more cost-efficient packaging technology. The current available no-flow underfill materials are mainly designed for eutectic tin-lead solders. With the advance of lead-free interconnection due to the environmental concerns, a new no-flow underfill chemistry needs to be developed for lead-free solder bumped flip-chip applications. Many epoxy resin/hexahydro-4-methyl phthalic anhydride/metal acetylacetonate material systems have been screened in terms of their curing behavior. Some potential base formulations with curing peak temperatures higher than 200°C (based on differential scanning calorimetry at a heating rate of 5°C/min) are selected for further study. The proper fluxing agents are developed and the effects of fluxing agents on the curing behavior and cured material properties of the potential base formulations are studied using differential scanning calorimetry, thermomechanical analysis, dynamic-mechanical analysis, thermogravimetric analysis, and rheometer. Fluxing capability of the developed no-flow formulations is evaluated using the wetting test of lead-free solder balls on a copper board. The developed no-flow underfill formulations show sufficient fluxing capability and good potential for lead-free solder bumped flip-chip applications     

无铅倒装焊封装工艺中的芯片应力及开裂分析

采用粘塑性Garofalo-Arrhenius模型描述无铅焊料的非弹性力学行为,确定了Sn3.5Ag焊料该模型的材料参数。采用与固化过程相关的粘弹性力学模型描述倒装焊底充胶的力学行为。利用有限元仿真的方法,模拟了无铅倒装封装器件封装的工艺及可靠性测试。结果表明:由于无铅技术在封装中的引入,芯片破裂的可能性随之增加,破裂出现时裂纹的尺寸更小。     

Strain evaluation of epoxy-cured fiber-optic connectors

Optical fiber connectors are passive components used to link two fiber links or a fiber link to a photonic device. One widely used type of fiber connector, a design that uses a thermally cured epoxy adhesive, has been evaluated via Bragg grating-based fiber strain sensors. Strain sensors were used to evaluate the strain incurred by the optical fiber as a result of installation and subsequent environmental testing. Preliminary mechanical modeling and a strain analysis using Bragg grating-based strain sensors are discussed. Since the strain sensors were not exposed to uniaxial loading, mechanical modeling was used to determine the optimum placement of the sensors and the expected response. Also discussed are ongoing studies to evaluate the viscoelastic behavior of the epoxy and its effect on the strain state of the connector assembly.     

Studies of latent catalyst systems for pot-life enhancement of flipchip underfills

Underfill encapsulant is the material used in flip-chip devices that fills the gap between the integrated circuit (IC) chip and the organic board, and encapsulates the solder interconnects. This underfill material can dramatically enhance the reliability of flip-chip devices as compared to nonunderfilled devices. Current underfill encapsulants generally consist of epoxy resin, anhydride hardener, catalyst, silica filler, and other additives to enhance the adhesion, flow, etc. Catalyst determines underfill properties including pot-life, cure speed, and cure temperature. However, long pot-life and fast cure at relatively low temperature (~150°C) are desirable, as such, it requires a room temperature latent catalyst which would be able to catalyze the epoxy curing efficiently at desirable temperature. Currently, the pot-life of commercial underfills at room temperature is normally less than one day. The underfills have to be stored in the freezer at -40°C and in the dry ice for shipping. The objective of this work was to test various catalyst systems that have the potential to enhance the pot-life of the underfill without adversely affecting its curing. The pot-lives of the underfill with various catalysts were obtained from their viscosity versus time relationships, which were established by measuring the viscosities of the underfill with these catalysts periodically using a stress-controlled rheometer. The curing of the underfills was studied using a differential scanning calorimetry (DSC). The pot-life and curing data of the underfill pre-mixed with each of these catalysts are presented in this paper