Optimizing maximum equivalent strain on solder balls in flip‐chip packages

Flip‐chip packaging provides a high‐performance low‐cost approach for development of electronic packages. A three‐dimensional (3D) viscoelastic‐plastic finite element analysis using the commercial software ANSYS has been performed to study the thermo‐mechanical behavior in flip‐chips assemblies, i.e., the four components: chip, solder ball, underfill, and substrate. The viscoelastic behavior of underfill is modeled by a Maxwell constitutive equation, while the viscoplastic behavior of solder balls is modeled by an Anand model. Both chip and substrate are assumed to elastic materials modeled by Hooke's law. As in standard industry practice, temperature cycling from 125 to −40°C is used. Thermo‐mechanical behavior of solder balls is presented, and the effects of underfill material properties are investigated. Further, Taguchi methods are used to optimize flip‐chip package performance. The design goal is to minimize the maximum equivalent strain on the solder balls. The eight flip‐chip assembly factors chip‐thickness/substrate‐thickness ratio, underfill modulus (G_i), underfill relaxation time (λ_i), solder height‐to‐diameter ratio, chip coefficient of thermal expansion (CTE), underfill CTE, solder CTE, and substrate CTE are chosen for optimization. POLYM. COMPOS., 2008. © 2007 Society of Plastics Engineers     

Study of additive‐epoxy interaction of thermally reworkable underfills

Underfill is the material used in a flip‐chip device to dramatically enhance its reliability as compared to a nonunderfilled device. Current underfills are mainly epoxy‐based materials that are not reworkable after curing, which is an obstacle in flip‐chip technology developments, where unknown bad die is a concern. Reworkable underfill is the key to address the nonreworkability of the flip‐chip devices. The ultimate goal of this study is to develop epoxy‐based thermally reworkable underfills. Our previous work showed that when incorporated into epoxy matrix, special additives could provide the epoxy formulation with die‐removal capability around solder reflow temperature. The additive‐epoxy interactions were studied and the results show that the additives do not adversely affect the epoxy properties. Moreover, when the additive decomposition temperature is reached, the decomposition of the additive causes a microexplosion within the epoxy matrix. Subsequently, the adhesion of the epoxy matrix is greatly reduced. Among the four additives studied, Additive1 and Additive2 may be used in reworkable underfills that can be reworked around solder reflow temperature, Additive3 cannot be used in underfill because it greatly reduces the shelf life of the underfill, and Additive4 may be used to develop reworkable underfill that withstands multiple reflows. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1868–1880, 2001     

Enhancement of antifillet cracking performance for no‐flow underfill by toughening method

Fillet cracking of no‐flow underfill in a flip‐chip device during a reliability test such as thermal shock or thermal cycling has been a serious reliability problem. The effect of toughening agents and modification of epoxy on fillet cracking of no‐flow underfill was investigated. The best epoxy formulation and the appropriate loading level of toughening agent regarding the antifillet cracking performance were found. In the case where the epoxy was modified with polysiloxanes, the second‐phase particle with a submicron particle size was formed and the size of the particle depended on the kind of toughening agent. The morphology was observed by a scanning electron microscopy and confirmed by a dynamic mechanical analyzer measurement. The physical properties such as the fracture toughness, flexual modulus, coefficient of thermal expansion, and adhesion were measured, and the liquid–liquid thermal shock (LLTS) test under ?55 to 125°C was performed with different formulations. One of the formulations toughened by amine/epoxy‐terminated polysiloxane, which has higher die shear strength, lower modulus, and higher toughness, passed 1000 cycles of the LLTS test. In order to obtain a high reliable no‐flow underfill, the physical properties of the no‐flow underfill should be well controlled and balanced. Finally, a correlation between physical properties of the no‐flow underfill and the fillet cracking capability for those approaches was discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2439–2449, 2003     

Polyurethane‐modified epoxy resin: Solventless preparation and properties

A polyurethane‐modified epoxy resin system with potential as an underfill material in electronic packaging and its preparation procedure were studied. The procedure enabled the practical incorporation of an aliphatic polyurethane precursor, synthesized from poly(ethylene glycol) and hexamethylene diisocyanate without a solvent, as a precrosslinking agent into a conventional epoxy resin. With a stoichiometric quantity of the polyurethane precursor added to the epoxy (ca. 5 phr), the polyurethane‐modified epoxy resin, mixed with methylene dianiline, exhibited a 36% reduction in the contact angle with the epoxy–amine surface, a 31% reduction in the cure onset temperature versus the control epoxy system, and a viscosity within the processable range. The resultant amine‐cured thermosets, meanwhile, exhibited enhanced thermal stability, flexural strength, storage modulus, and adhesion strength at the expense of a 5% increase in the coefficient of thermal expansion. Exceeding the stoichiometric quantity of the polyurethane precursor, however, reduced the thermal stability and modulus but further increased the coefficient of thermal expansion. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Studies on epoxy resins cured by cationic latent thermal catalyst at elevated temperature

In this work, the effect of the catalyst content was investigated in terms of the thermal and mechanical properties of epoxy resins at elevated temperature. A synthesized cationic latent thermal catalyst, N‐benzylpyrazinium hexafluoroantimonate (BPH), was used to cure epoxy resins. The content of catalyst added was 0.5, 1, 2, 3 and 5 wt% and the heating time was varied from 0 to 1024 h. As a result, the mechanical properties, including flexural strength, elastic modulus in flexure and impact strength, as well as thermal‐oxidative resistance, showed a maximum value in the presence of 1 wt% BPH. With increasing elapsed time, up to 4 h, the thermal and mechanical properties of the specimens were improved. These results show that the internal structure of the epoxy system was stabilized, and post‐curing for a longer period of time, resulted in improved thermal and mechanical behaviour of the cast specimens. Copyright © 2004 Society of Chemical Industry     

Study on the Curing Process and the Gelation of Epoxy/Anhydride System for No‐Flow Underfill for Flip‐Chip Applications

No‐flow underfill is used in the assembly of microelectronics to increase the productivity and to decrease the cost of the flip‐chip manufacturing. The curing process, especially the gelation of the no‐flow underfill, is essential for the yield and reliability of the flip‐chip assembly. A differential scanning calorimeter (DSC) and a stress rheometer are used to study the curing process of epoxy/anhydride system at different curing rates and different isothermal temperatures. The gel point is found when the storage modulus and the loss modulus of the resin measured by the rheometer equals to each other. The degree of cure (DOC) at gelation is calculated according to the results from DSC. The results indicate a strong dependence of the DOC at gelation on the heating rates and the curing temperatures. In order to further investigate the difference in the curing process at various heating rates, FTIR spectra of the resin are taken during curing. The change of different functional groups is recorded and compared. The results do not show a significant difference in the chemical structure at different heating rates. The early gelation at high heating rate/ high temperature can be caused by the structure difference in the epoxy network at the early stage of curing due to the chain initiation and propagation of the molecules in the curing process.

Dependence of gelation of the sample on the isothermal temperatures.     

Thermal and mechanical properties of tetra‐functional mesogenic type epoxy resin cured with aromatic amine

A novel tetra‐functional epoxy monomer with mesogenic groups was synthesized and characterized by ¹H‐NMR and FTIR. The synthesized epoxy monomer was cured with aromatic amine to improve the thermal property of epoxy/amine cured system. The glass transition temperature (T_g) and coefficient of thermal expansion (CTE) of the cured system were investigated by dynamic mechanical analysis and thermal mechanical analysis. The properties of the cured system were compared with the conventional bisphenol‐A type epoxy and mesogenic type epoxy system. The storage modulus of the tetra‐functional mesogenic epoxy cured systems showed the value of 0.96 GPa at 250 °C, and T_g‐less behavior was clearly observed. The cured system also showed a low CTE at temperatures above 150 °C without incorporation of inorganic components. These phenomena were achieved by suppression of the thermal motion of network chains by introduction of both mesogenic groups and branched structure to increase the cross linking density. The temperature dependency of the tensile property and thermal conductivity of the cured system was also investigated. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46181.     

Effects of the complexed moisture in metal acetylacetonate on the properties of the no‐flow underfill materials

A potential no‐flow (compression filling of encapsulant) underfill encapsulant for simultaneous solder joint reflow and underfill cure has been reported by the authors. The encapsulant is based on a cycloaliphatic epoxy/organic anhydride/Co(II) acetylacetonate system. The key of this no‐flow encapsulant is the use of a latent metal acetylacetonate catalyst that provides the solder reflow prior to the epoxy gellation and fast cure shortly after the solder reflow. However, most of the metal acetylacetonates can easily absorp moisture as their ligand. Therefore, it is of practical importance to understand the effect of the complexed water on the properties of the no‐flow material before and after cure. In this paper, differential scanning calorimetry, thermal gravimetric analysis, thermal mechanical analysis, dynamic mechanical analysis, and Fourier transform infrared spectrometry were used to validate the existence of complexed moisture in the Co(II) acetylacetonate. The effects of the complexed water on the curing profile, glass transition temperature, and storage modulus of the cured no‐flow underfill material were studied. A possible catalytic mechanism of the metal acetylacetonate in the cycloaliphatic epoxy/anhydride system was subsequently discussed and proposed. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 103–111, 1999     

Adhesion characteristics of underfill resins with flip chip package components

Of fundamental importance to enhance the reliability of flip chip on board (FCOB) packages is to avoid the initiation and propagation of various interfacial failures and, therefore, robust interfacial bonds between the underfill and other components are highly desired. In the present study, the interfacial bond strengths of both conventional and no-flow underfill resins with die passivation, eutectic solder and epoxy solder mask are measured using the button shear test. The surface characteristics of these substrates are analyzed using various techniques, including optical scanning interferometry, scanning electron microscopy and contact angle measurements. It is found that the interfacial bond strength of the underfill with the eutectic solder is far weaker than of other interfaces. The degradation of underfill bond strength with silicon nitride passivation, eutectic solder and polymeric solder mask surfaces is enhanced in the presence of solder flux, and cleaning the fluxed surface with a saponifier is an efficient means to restore the original interfacial adhesion. The necessity of post-solder reflow cleaning is shown by performing thermal cycle tests on FCOB packages with different extents of flux residue. Distinctive solder failure behaviors are observed for the packages with and without post-solder reflow cleaning from the cross-sectional analysis.     

Comparative study of thermally conductive fillers in underfill for the electronic components

General underfill for the flip-chip package had a low thermal conductivity of about 0.2 W/mK. Thermal properties of underfill were measured with various fillers, such as silica, alumina, boron nitride, (BN) and diamond. Coefficient of thermal expansion (CTE) was changed by filler content and CTE of silica 60 wt.% was 28 ppm; BN 30 wt.%, 25 ppm; alumina 60 wt.%, 39 ppm; and diamond 60 wt.%, 24 ppm. The viscosity of underfill was measured with the cone and plate rheometer. Thermal diffusivity was measured with the laser flash method. Diamond filler loaded underfill showed the highest thermal conductivity 60 wt.%; 1.17 W/mK at 55 °C. Thermal conductivity of underfill was changed with a transition of heat capacity by the temperature increment in same filler content. In case of different filler content, thermal conductivity was changed with a transition of the thermal diffusivity.     

Adhesion of underfill and components in flip chip encapsulation

The underfill material is a polymeric adhesive used in flip chip packaging. It encapsulates the solder joints by filling the gap between a silicon die and an organic substrate or board. Within a typical flip chip structure, there are interfaces between the various components, namely, substrate, solder mask, flux residue, underfill encapsulant and die passivation layer, etc. Maintaining a good adhesion condition, both as-made and after temperature/humidity aging, is vital for these interfaces because of the expected performance of the flip chip device, where the underfill material is employed to enhance the reliability of the flip chip interconnect. We have studied the adhesion strength between the various components for different process variables as measured with the lap shear and die shear test configurations. The effects of the assembly factors, i.e. solder mask, flux residue, underfill, and die passivation, etc., were evaluated and the adhesion strength was found to depend greatly on these factors. The die shear strength of a passivated die assembled onto an organic board coated with a solder mask was much higher after using a no-clean flux on the solder mask than for the assembly without such a no-clean flux. The influence of some accelerated aging tests on the adhesion durability was also investigated. A die passivation layer of benzocyclobutene exhibited better capability in retaining the die shear strength than a passivation layer of silicon nitride or polyimide, especially for the initial aging period. The knowledge obtained in this study should provide insights into the interfacial adhesion in the flip chip assembly structure.     

Simultaneously enhanced cryogenic mechanical behaviors of cyanate ester/epoxy resins by poly(ethylene oxide)‐co‐poly(propylene oxide)‐co‐poly(ethylene oxide) (PEO‐PPO‐PEO) block copolymer and clay

Cryogenic mechanical properties are important parameters for thermosetting resins used in cryogenic engineering areas. The hybrid nanocomposites were prepared by modification of a cyanate ester/epoxy/poly(ethylene oxide)‐block‐poly(propylene oxide)‐block‐poly(ethylene oxide) (PEO‐PPO‐PEO) system with clay. It is demonstrated that the cryogenic tensile strength, Young's modulus, ductility (failure strain), and fracture resistance (impact strength) are simultaneously enhanced by the addition of PEO‐PPO‐PEO and clay. The results show that the tensile strength and Young's modulus at 77 K of the hybrid nanocomposite containing 5 wt% PEO‐PPO‐PEO and 3 wt% clay were enhanced by 31.0% and 14.6%, respectively. The ductility and impact resistance at both room temperature and 77K are all improved for the hybrid composites. The fracture surfaces of the neat BCE/EP and its nanocomposites were examined using scanning electron microscopy (SEM). Finally, the dependence of the coefficients of thermal expansion (CTE) on the clay and PEO‐PPO‐PEO contents was examined by thermal dilatometer. POLYM. COMPOS., 38:2237–2247, 2017. © 2015 Society of Plastics Engineers     

Low-stress encapsulants by vinylsiloxane modification

Dispersed silicone rubbers were used to reduce the stress of cresol–formaldehyde novolac epoxy resin cured with phenolic novolac resin for electronic encapsulation application. The effects of structure, molecular weight, and contents of the vinylsiloxane oligomer on reducing the stress of the encapsulant were investigated. Morphology and dynamic mechanical behavior of rubber-modified epoxy resins were also studied. The dispersed silicone rubbers effectively reduce the stress of cured epoxy resins by reducing flexural modulus and the coefficient of thermal expansion (CTE), whereas the glass transition temperature (T_g) was hardly depressed. Electronic devices encapsulated with the dispersed silicone rubber modified epoxy molding compounds have exhibited excellent resistance to the thermal shock cycling test and have resulted in an extended device use life. © 1994 John Wiley & Sons, Inc.     

Effect of Lattice Distortion and Disordering on the Mechanical Properties of Titania‐Doped Yttria‐Stabilized Zirconia

TiO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds with low thermal conductivity have been considered as a promising thermal barrier coating material. In the present research, a series of TiO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds have been synthesized and investigated. Lattice distortion and disordering caused by TiO₂ doping were observed and their effects on mechanical properties, such as fracture toughness, elastic modulus, and coefficient of thermal expansion (CTE), were also investigated. Lattice distortion enhanced the ferroelastic toughening and the fracture toughness, whereas the variation in elastic modulus and CTE is due to the lattice disordering. The combination of thermal and mechanical properties bodes well for the potential application as thermal barrier coating materials.     

Internal stress analysis of epoxy adhesively-boned joints based on their thermomechanical properties at cryogenic temperature

Internal stress analysis is essential to structural design of materials applied in cryogenic engineering. In this contribution, thermomechanical properties including dynamic thermomechanical properties and thermal expansion behavior of four epoxy resins, namely the polyurethane modified epoxy resin (PUE), diglycidyl ether of bisphenol A (DGEBA), tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and triglycidyl-p-aminophenol (TGPAP) were studied by dynamic thermomechanical analysis. Internal stress of the epoxy layer in the bonded joint was calculated based on the thermomechanical properties. Meanwhile, the structure-cryogenic property relationship of epoxy resins were investigated. Results demonstrate that internal stress in the four epoxies bonded joints is 6 ~ 21 MPa at −150°C, and is positively correlated with the average thermal expansion coefficient (CTE) of epoxy resins. TGDDM and TGPAP showed higher retention of lap shear strength both at −196°C and after temperature cycling due to their lower CTE. Morphology of the fractured surface of bonded joints demonstrated that internal stress is responsible for the severe interface failure at −196°C. It reveals that selection of epoxy resins with low CTE is beneficial for designing high-performance epoxy adhesive systems served at cryogenic temperature.     

Rice husk‐derived porous carbon/silica particles as green filler for electronic package application

Bio‐based porous carbon/silica particles (denoted as RH‐carbon/silica) were successfully prepared from agricultural waste rice husk by using acid‐hydrothermal treatment and pyrolysis under nitrogen condition. As green filler, the cure behavior, thermal‐mechanical properties, and thermal conductivity of the epoxy‐carbon/silica biocomposites at different filler contents (5, 9, 17, 29 wt %) were characterized. Because of superior surface properties (surface area, porosity, and silica segment) and high content of carbon component in the RH‐carbon/silica, the characteristics of the biocomposites were significantly improved with the increase of the filler content. At 29 wt % of filler content, the epoxy biocomposites exhibit lower curing temperature (148 °C), lower CTE (42 ppm/°C), higher T_g (123 °C), higher storage modulus (4059 MPa), and higher effective thermal conductivity (0.29 W/mK). In brief, the RH‐carbon/silica particles that can serve not only as reinforcing agent but also as thermal transport medium used in epoxy composite, is a green and high‐performance filler for this purpose. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44699.     

Crosslinkable deoxidizing hybrid adhesive of epoxy–diacid for electrical interconnections in semiconductor packaging

For electrical interconnections in semiconductor packaging, epoxy‐based pastes have recently attracted considerable interest due to their excellent adhesion to various substrates and their reasonable electrical and mechanical properties, especially when combined with deoxidizing agents (to remove metallic oxides). Here, epoxy–diacid‐based hybrid pastes were examined to achieve a deoxidizing capability for eliminating Sn‐based solder oxides and adhesion between microchip and substrate as a one‐step process. Onset, exothermic peak and end temperatures of the reaction between epoxy and diacids were systematically probed using DSC, rheometry and Fourier transform infrared (FTIR) spectroscopy. The last moment of the adhesive reaction during heating substantially enhanced the thermal and mechanical properties of the epoxy–diacid adhesive despite the absence of exothermic enthalpy detected by DSC. The glass transition temperature (T_g) and Young's modulus gradually decreased as a function of aliphatic chain length of diacids except when the length was extremely short and voids were produced. Soldering (wetting) and deoxidizing capabilities of the hybrid adhesive were observed via optical microscopy and FTIR. The correlation between the reaction, T_g, conversion and viscosity was also investigated. Lastly, complete wetting and electrical interconnection with good mechanical robustness were achieved for a commercial chip/substrate set by flip‐chip bonding. © 2018 Society of Chemical Industry     

Property and reliability of liquid underfill material for IC packaging

Thermoplastic nylon powder was added to naphthalene epoxy to serve as a stress release agent to reduce the stress resulting from the shrinkage during the cure of naphthalene epoxy. The purpose of this study was to explore the physical impact and effect on the forming object after adding nylon powder onto naphthalene epoxy. Mechanical properties were explored through the Izod impact test, the three‐point bending test, tensile test, and lap shear adhesion test. Thermal mechanical analysis (TMA) and dynamic mechanical analysis (DMA) were conducted to identify the coefficient of thermal expansion (CTE) and the glass transition temperature (T_g). The rate of water absorption was measured via a test of pressure cook test (PCT), and insulation resistance was assessed through the breakdown voltage experiment. The results indicate that the addition of nylon powder increases the fracture energy of the cured epoxy; however, mechanical properties (lap shear strength, flexural strength, tensile strength) decreased slightly. The TMA and DMA results showed that the CTE (α₁) decreased when nylon was added and the heat resistance decreased a little. The water absorption rate test and PCT showed that the rate of water absorption increased to a small extent, whereas the breakdown voltage decreased slightly. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3504–3509, 2006     

Thermo-mechanical behaviors and moisture absorption of silica nanoparticle reinforcement in epoxy resins

An investigation of the thermo-mechanical behavior of silica nanoparticle reinforcement in two epoxy systems consisting of diglycidyl ether of bisphenol F (DGEBF) and cycloaliphatic epoxy resins was conducted. Silica nanoparticles with an average particle size of 20 nm were used. The mechanical and thermal properties, including coefficient of thermal expansion (CTE), modulus (E), thermal stability, fracture toughness (K_IC), and moisture absorption, were measured and compared against theoretical models. It was revealed that the thermal properties of the epoxy resins improved with silica nanoparticles, indicative of a lower CTE due to the much lower CTE of the fillers, and furthermore, DGEBF achieved even lower CTE than the cycloaliphatic system at the same wt.% filler content. Equally as important, the moduli of the epoxy systems were increased by the addition of the fillers due to the large surface contact created by the silica nanoparticles and the much higher modulus of the filler than the bulk polymer. In general, the measured values of CTE and modulus were in good agreement with the theoretical model predictions. With the Kerner and Halpin-Tsai models, however, a slight deviation was observed at high wt.% of fillers. The addition of silica nanoparticles resulted in an undesirable reduction of glass transition temperature (T_g) of approximately 20 °C for the DGEBF system, however, the T_g was found to increase and improve for the cycloaliphatic system with silica nanoparticles by approximately 16 °C. Furthermore, the thermal stability improved with addition of silica nanoparticles where the decomposition temperature (T_d) increased by 10 °C for the DGEBF system and the char yield significantly improved at 600 °C. The moisture absorption was also reduced for both DGEBF and cycloaliphatic epoxies with filler content. Lastly, the highest fracture toughness was achieved with approximately 20 wt.% and 15 wt.% of silica nanoparticles in DGEBF and cycloaliphatic epoxy resins, respectively.     

Freeze–thaw resistance of unidirectional‐fiber‐reinforced epoxy composites

The freeze–thaw resistance of unidirectional glass‐, carbon‐, and basalt‐fiber‐reinforced polymer (GFRPs, CFRPs, and BFRPs, respectively) epoxy wet layups was investigated from ?30 to 30°C in dry air. Embedded optic‐fiber Bragg grating sensors were applied to monitor the variation of the internal strain during the freeze–thaw cycles, with which the coefficient of thermal expansion (CTE) was estimated. With the CTE values, the stresses developed in the matrix of the FRPs were calculated, and CFRPs were slightly higher than in the BFRP and GFRP cases. The freeze–thaw cycle showed a negligible effect on the tensile properties of both GFRP and BFRP but exhibited an adverse effect on CFRP, causing a reduction of 16% in the strength and 18% in the modulus after 90 freeze–thaw cycles. The susceptibility of the bonding between the carbon fibers and epoxy to the freeze–thaw cycles was assigned to the deterioration of CFRP. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012