Epoxy Underfill Research Articles

Correlated Noise and Gain in Unfilled and Epoxy‐Underfilled Hybridized HgCdTe Detectors

ABSTRACTSpatially correlated noise in focal plane arrays (FPAs) affects the photometric signal‐to‐noise ratio and can lead to overestimation of conversion gain (detector nodal capacitance). Capacitive coupling leads to charge sharing between neighboring pixels and correlates the measured pixel signal. This correlation can lead to image degradation, in particular for undersampled large survey telescopes. We present new comparative measurements of the interpixel coupling of 1.7 and 2.5 μm cutoff HgCdTe FPAs. Rockwell Scientific FPAs with epoxy underfill exhibit correlations that result in a 16%–20% systematic overestimation of the conversion gain. Results for a Raytheon Vision Systems 1.7 μm cutoff HgCdTe detector tested both before and after epoxy underfill show increased coupling in the presence of the epoxy, but the coupling is not spatially symmetric. We speculate that the difference in the observed correlation for each vendor is related to multiplexer geometry and pixel nodal capacitance.

RF packaging and passives: design, fabrication, measurement, and validation of package embedded inductors

Design, modeling, and characterization of inductors embedded in a package substrate promising higher quality factor (Q) and lower cost than on-chip inductors is described. In addition to the problem of large conductor losses, on-die inductors with or without magnetic materials consume considerable die area and require the removal of the first-level interconnect bumps beneath them to maintain a reasonably high Q value. Moving inductors to the package eliminates the need for bump array depopulation and, thus, mitigates the potential reliability problems caused by voids in the epoxy underfill between the die and the substrate. Competency developed to design, fabricate, and characterize inductors based on standard organic flip-chip packaging technology is described. Physical design details along with measurement procedures and results are discussed. In addition, modeling techniques for achieving good correlation to measured data are included.

Measurements of deformations and strains of a Si-epoxy-FR4 structure during thermal testing

Deformations and strains of a Si-epoxy-FR4 structure were evaluated during thermal testing using high sensitivity, real-time Moiré interferometry. The specimen studied was a sandwich structure consisting of a silicon chip, epoxy underfill and FR4 substrate. The deformations and strains of the FR4–underfill and silicon–underfill interfaces of the specimen under certain thermal loading were examined. The results show that the shear strain increases significantly along the interfaces, with the maximum shear strain occurring at the intersection of the specimen edge and the silicon–underfill interface. The shear strain at the silicon–underfill interface experienced a 2% increase after heated for two hours at 100 °C, but the shear strain at the FR4–underfill interface showed a 12% increase. This is an indication that the creep effect is more dominant in the FR4–underfill interface. The interfaces of the specimen experience partial strain recovery after one hour of the holding time at 20 °C.

Various Adhesives for Flip Chips

Flip chips using various adhesives were studied. The assembly yields using nonconductive adhesive and anisotropic conductive film investigated were 97% and 100%, respectively. A packaging process using anisotropic conductive paste with a 100% packaging yield was developed. All the packages passed various reliability tests such as burn in, artificial sweat and humidity tests, and temperature cycling tests. The reliability of the packages was 100% meeting the requirements for an industrial application. A stud bump bonding process with reduced process steps was proposed. Curing of conductive adhesive and underfill epoxy was not required, resulting in reduced packaging time.

Interfacial behavior of a flip-chip structure under thermal testing

In this paper, the interfacial behavior of a flip-chip structure under thermal testing was investigated using high sensitivity, real-time Moire interferometry. The model package studied was a sandwich structure consisting of a silicon chip, epoxy underfill and FR4 substrate. The behavior of FR4-underfill and silicon-underfill interfaces of the specimen under certain thermal loading was examined. The results show that the shear strain variation increases significantly along the interfaces, with the maximum shear strain concentration occurring at the edge of the specimen. At the edge, the maximum shear strain occurs at the silicon-underfill interface, and the FR4-underfill interface experiences a slightly lower shear strain. The creep effect is more dominant in the FR4-underfill interface when the specimen is heated for 2 h at 100/spl deg/C. Upon cooling to 20/spl deg/C, both the interfaces of the specimen experience partial strain recovery.

Externally guided underfill encapsulation and subsequent filler particle migration during electronics packaging

We present a numerical study of the transient multiphase flow of a liquid encapsulant in a microgap formed between parallel plates with special reference to electronics packaging. The surface tension and polarization forces are assumed to drive the fluid flow. The governing equations are solved using the volume of fluid model within the framework of the finite volume method in order to predict the free surface front of the encapsulant together with the time and spatial evolution of filler particle concentration during the underfill process. The numerical results are found to be in good agreement with available reported experimental data. The influence of the polarization force is found to reduce the encapsulant fill time, increase the flow length and distribute the filler particles to a larger flow length. The new process has practical implications, such as overcoming the partial curing of underfill epoxy before the flow is complete.

Surface property of passivation layer on integrated circuit chip and solder mask layer on printed circuit board

Adhesion of underfill to passivation layer on integrated circuit chip and solder mask layer on printed circuit board is critical to the reliability of an underfilled flip chip package. In this study, the surface properties of solder mask and four passivation materials: benzocyclobutene (BCB), polyimide (PI), silicon dioxide (SiO/sub 2/), and silicon nitride (SiN) were investigated. A combination of both wet and dry cleaning processes was very effective to remove contaminants from the surface. The element oxygen, introduced during O/sub 2/ plasma treatment or UV/O/sub 3/ treatment, led to the increase of the base component of surface tension. X-ray photoelectron spectroscopy (XPS) experiments confirmed the increase of oxygen concentration at the surface after UV/O/sub 3/ treatment. Wetting of underfill on passivation and solder mask was slightly improved at higher temperatures. Although UV/O/sub 3/ cleaning and O/sub 2/ plasma treatment significantly improved the wetting of underfill on passivation materials, they did not improve adhesion strength of epoxy underfill to passivation. Therefore, the wetting was not the controlling factor in adhesion of the system studied.

Investigation on flip chip solder joint fatigue with cure-dependent underfill properties

A cure-dependent viscoelastic constitutive relation is applied to describe the curing process of epoxy underfill in flip chip on board (FCOB). The chemical shrinkage of the epoxy underfill during the curing process is applied via incremental initial strains. Thus, the stress and strain build-up, caused by the simultaneous increase in stiffness and shrinkage during the curing process, are simulated. Accelerated fatigue experiments with thermal cycles from -55/spl deg/C to 80/spl deg/C are carried out for a specially designed flip chip configuration. Based on the obtained curing induced initial stress and strain fields, thermo-mechanical predictions are presented for the test carriers. The solder bumps are modeled with temperature dependent visco-plastic properties. A combination of a Coffin-Manson based fatigue relation and a creep fatigue model is used as fatigue failure criterion. The results show that the finite element method (FEM)-based fatigue life predictions match better with the experimental results, if the curing induced initial stress state is taken into account. The effect of cure-induced hydrostatic stress is qualitatively investigated by using a modified energy partitioning damage model with a correction factor in the creep damage formulation to take into account the effect of the hydrostatic stress.

Thermal management of AlGaN-GaN HFETs on sapphire using flip-chip bonding with epoxy underfill

Self-heating imposes the major limitation on the output power of GaN-based HFETs on sapphire or SiC. SiC substrates allow for a simple device thermal management scheme; however, they are about a factor 20-100 higher in cost than sapphire. Sapphire substrates of diameters exceeding 4 in are easily available but the heat removal through the substrate is inefficient due to its low thermal conductivity. The authors demonstrate that the thermal impedance of GaN based HFETs over sapphire substrates can be significantly reduced by implementing flip-chip bonding with thermal conductive epoxy underfill. They also show that in sapphire-based flip-chip mounted devices the heat spread from the active region under the gate along the GaN buffer and the substrate is the key contributor to the overall thermal impedance.

Effect of Moisture and Graded-Layer Mechanical Properties on Deformation and Interfacial Adhesion

AbstractControlling material properties over nanometer length scales is crucial for current and emerging high-density microelectronic device packages. Miniaturization of devices is increasingly limited by the ability to “connect” to the device, and the required packaging structures must be fabricated where layer thickness and feature sizes approach micron size scales while achieving the required mechanical, thermal and electrical properties. Second phase additions such as sub-micron sized particles are often added to locally adjust the material properties of constituent layers in the complex package structure. This results in significant variation of mechanical properties over sub-micron length scales. Such manipulation of material structure and its effects on mechanical and interfacial fracture behavior are addressed using experimental and modeling studies. Underfill layers consisting of an epoxy matrix with dispersed silica beads are shown to exhibit variations of elastic and flow properties in excess of three-fold across the layer thickness. Mechanical properties are not only affected by the distribution of second-phase fillers, but also by the adhesion properties of the filler/matrix interface. Interfaces are susceptible to stress corrosion cracking associated with moisture which can lead to progressive debond growth at loads much lower than that required to exceed the critical interface fracture energies. Subcritical debonding is affected by temperature, humidity, and the bond chemistry of the interface. The effects of these variations are considered on the adhesive and subcritical debonding behavior of interfaces between model epoxy underfills and SiNxchip passivation. Implications for other constrained complex layered structures are considered.

An Analytical Cure Model for Underfill Epoxies

An analytical model relating the degree of cure of an underfill epoxy to the heat transfer coefficient of the heating process and time is presented. The model was verified experimentally by partially curing samples of a commercial underfill epoxy in two different heating environments for various times and scanning the samples in a differential scanning calorimeter to determine the degree of cure. A simplified equation can be used to relate the time required to achieve a given degree of cure to the heat transfer coefficient and the isothermal cure time.

Stud bump bond packaging with reduced process steps

The stud bump bonding (SBB) process was evaluated for a flip chip in package application. An SBB packaging process, having a reduced number of process steps compared with the typical SBB process, was proposed. The two separate steps of curing of conductive adhesive and underfill epoxy, which for the application targeted in this study needed a total of four hours for curing plus further time for cooling, were not required. This proposed process therefore resulted in a reduced packaging time. Several issues affecting the assembly are also discussed in this paper.

Study on effect of coupling agents on underfill material in flip chip 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/sub 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/spl deg/C/85% relative humidity chamber were also studied through moisture absorption test and die shear test.

Studies of Silane Adhesion Promoters on Silica Filler Particles for use in Microelectronic Packaging

ABSTRACTTo avoid delamination which often occurs in microelectronic packaging as a result of widely varying thermal expansion coefficients, epoxy underfill is often loaded with particulate silica filler. This filler typically complicates the fracture behavior of the epoxy and surrounding layers. However, delamination of interfaces within the composite may contribute to toughening if the fracture path is adhesive between the epoxy and surrounding layers or otherwise removed from filler-epoxy delamination regions. In this work, the interface between silica filler and epoxy is altered with the use of silane coupling agents to elucidate the fracture and toughening mechanisms of filled epoxies. Two model epoxies are investigated, and coupling agents were chosen to encourage chemical reaction with each system. For the aliphatic epoxy system, coupling agents were identified which both augment and degrade the interface between filler and epoxy. In composite structures, it was observed that those coupling agents which degrade the filler-epoxy interface act as toughening agents. SEM indicates that most deformation is not at the filler-epoxy interface, but rather through the epoxy, very close to the filler particle surfaces. In contrast, no coupling agents were identified which act as toughening agents in the bisphenol-f system. Further, no variation in fracture toughness of composite structures was observed with the use of coupling agents, indicating that filler-epoxy delamination does not occur in this system. With few exceptions, SEM supports this interpretation.

Effect of Composition and Bead Settling on Debonding of Underfill Layers

ABSTRACTWith the increased complexity of microelectronic devices continuing to drive chip packages to smaller sizes, the need for reliable high-density flip chip packaging is increasingly important. In these packages, epoxy underfills are used to surround a solder ball grid array in order to support thermomechanical stresses in the system. This research focuses on determining the mechanisms that cause debonding at interfaces in these packages, particularly between the underfill and chip passivation. A fracture mechanics approach is used to study the adhesion of Si3N4to two model underfills, one based on aliphatic epoxy and the other based on bisphenol F epoxy. Interestingly, these two underfills, which are similar mechanically, appear to show very different critical and subcritical behavior. The aliphatic underfill exhibits subcritical debond growth under constant displacements, behavior typically seen in interface systems, while the bisphenol F underfill surprisingly shows little susceptibility to stress corrosion. Although both underfills show evidence of silica bead settling, the addition of filler has opposing effects on their critical adhesion values. The effects of varying filler content in these underfills as well as environmental effects on the critical and subcritical behavior of these systems will be examined.

Water-assisted sub-critical crack growth along an interface between polyimide passivation and epoxy underfill

Direct chip attach (DCA) microelectronic packaging technology is gaining prominence due to its numerous advantages. Delamination (debonding) of the underfill epoxy/ polyimide passivation interface of a DCA during hydro-thermal reliability testing has always been one of the salient problems. We have studied the water-assisted sub-critical crack growth along this interface and our measurement offers important clues as to the origins of the poor hydro-thermal testing results for these interfaces. A modified asymmetric double cantilever beam (ADCB) testing technique has been used to measure the sub-critical crack growth velocity v at various relative humidities and temperatures as a function of the crack driving force (strain energy release rate) G*. The presence of a significant partial pressure of water pH2O produces a marked decrease (by up to a factor of 12) in the threshold G* for crack growth at measurable velocities. Above the threshold log v rises linearly with \(\sqrt {G^ * } \) but then enters a regime where the crack velocity (v=v*) is almost independent of \(\sqrt {G^ * } \). Finally, at the values of G* corresponding to rapid crack propagation in the absence of water, log v increases very rapidly with G*. By analogy to the classic work on water-assisted sub-critical crack growth in silica-based glasses, where very similar features are observed, we believe that the sub-critical crack growth along the polyimide-epoxy interface results from stress-assisted hydrolysis of primary covalent bonds, in our case ester bonds across the interface. The regime of \(\sqrt {G^ * } \) just above the threshold corresponds to a physicochemical situation where the water activity (pH2O) at the crack tip is the same as that of the gaseous environment. In the regime where v=v*≈ constant, the water activity at the crack tip is below that in the environment and the crack growth velocity is limited by the transport of water vapor to the bonds ahead of the crack tip. We develop a model of this crack growth following Wiederhorn 1967 that allows us to predict the sub-critical crack growth as a function of G* for arbitrary relative humidity and temperature conditions.

Adhesion of benzocyclobutene-passivated silicon in epoxy layered structures

Adhesion and subcritical debonding at interfaces between a silica-filled epoxy underfill and a silicon die passivated by silicon nitride and benzocyclobutene (BCB) layers were investigated. Adhesion was measured in terms of a critical value of the applied strain energy release rate, G (J/m2). Subcritical debond-growth rates in the range of 10−9to 10−3m/s were characterized as a function of applied G. Adhesion and subcritical debonding were affected by changes in interfacial chemistry and environment. The surprisingly large effect of adjacent layer elastic properties on interfacial adhesion was demonstrated with simulations of interfacial fracture using a mechanics of materials approach. Interfacial chemistry was modified by using different adhesion promoters, by varying the BCB cure state, and by using different epoxy underfill resins. The effects of environmental variables were studied with temperature- and humidity-controlled environments in order to determine the separate roles of moisture activity and temperature.

Fundamental Study on Adhesion of Epoxy Underfill Materials with Passivation Layer in Flip-chip Packaging

ABSTRACTThe influence of aging in an environment with a high temperature and a high humidity on the adhesion performance of underfill material (epoxy cured with acid anhydride) to the passivation layer in flip chip packaging is discussed. Adhesion of underfill to different passivation materials degrades after aging in a high temperature and high humidity environment. The extent of this degradation is dependent on the hydrophilicity of the passivation material. Hydrophilic passivation such as silicon oxide (SiO2) and silicon nitride (Si3N4) shows much more severe adhesion degradation than hydrophobic passivation such as benzocyclobutene (BCB) and polyimide (PI). The mobility of absorbed water and of polymer chains is studied with solid state nuclear magnetic resonance (NMR) spectroscopy. Higher mobility of absorbed water and of polymer chains in rubbery state polymers contributes to faster adhesion degradation during high temperature and high humidity aging. The adhesion stability of hydrophilic passivation can be successfully improved by use of a silane coupling agent that introduces stable chemical bond at interface. A flow micro-calorimeter was used to study the absorption of silane coupling agent onto glass surface. The difference in adhesion retention improvement between aminosilane and epoxysilane is discussed.

STUDY ON PROPERTY OF UNDERFILL BASED ON EPOXY CURED WITH ACID ANHYDRIDE FOR FLIP CHIP APPLICATION

The adhesion and thermo-mechanical properties of epoxy underfills depend on the epoxy resin, the hardener, and the catalyst. In this study, three different epoxy resins, ERL4221 (cycloaliphatic type), EPON862 (bisphenol F type), and EPON 8281 (bisphenol A type), were cured with 4-methylhexahydrophthalic anhydride (MHHPA) as the hardener using different catalysts: cobalt acetylacetonate (CAA), imidazole derivatives, and tertiary amines. The flow behavior of the epoxy systems was studied with a rheometer. The curing profiles were recorded using a differential scanning calorimeter (DSC), revealing varying catalytic effect for the different catalysts. The curing peak temperature increased in the following order: tertiary amine < imidazole derivatives < cobalt acetylacetonate. The bulk properties of the systems were studied through a thermo-mechanical analyzer (TMA). Epoxy resins cured with different catalysts showed different glass transition temperatures (Tg) and coefficients of thermal expansion (CTE). Among them, the CAA catalyzed systems showed the highest Tg and low CTE. Due to the cycloaliphatic structure of the ERL4221 resin, the cured ERL4221 systems showed the highest Tg compared to the other systems with the same catalysts. EPON8281 systems generally had lower moisture absorption than the other epoxy systems. The surface tension of the underfills was measured. The adhesion strength was evaluated by a die shear test with SiO2 and Si3N4 (SiN) passivated silicon dies as substrates. Although there was no significant difference among the surface tension and contact angles of the underfill materials on the SiO2 or SiN, there were significant differences in the adhesion. For the ERL4221+MHHPA systems, the CAA catalyzed system exhibited excellent adhesion with SiO2 and SiN passivation. However, poor adhesion resulted with the catalysis by the imidazole derivatives and the tertiary amines. The bisphenol A and bisphenol F type epoxy resins showed excellent adhesion. Although the CAA catalyzed systems had higher Tg compared to the other systems, their adhesion was worse after aging in a high humidity environment at high temperatures.

Assembly issues in three flip chip processes

Discusses three simple and low‐cost flip chip assembly processes. First, flip chip on board using non‐conductive adhesive is evaluated. This process can give reasonable reliability and high assembly yield, when the parameters for epoxy placement and bonding are optimised. Second, the flip chip assembly process using reflowable no‐flow underfill is discussed. Because the underfill epoxy is already placed in the gap between the IC chip and the substrate before reflow, it is not easy to control the solder joints’ collapse and obtain the desired solder‐joint shapes and stand‐off distance during reflow. Finally, the stud bump bonding process is also discussed. It is not easy although possible to maintain the optimal dipping of the conductive adhesive, when the average height of the gold bumps is small. Some solutions for overcoming the above‐mentioned difficulties are presented in this paper.