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.     

Adhesion between an underfill and the passivation layer in flip-chip packaging

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

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     

Development of a high curing latency no-flow underfill with self-fluxing ability for lead-free solder interconnects

Tin-lead solders have been used as joining materials in the electronics industry for many years. As present there are great concerns about the potential harm of lead-containing solders to human beings and many efforts have been made to remove the lead from the electronics packages. However, there is no no-flow underfill currently on the market that is compatible with the lead-free solder. In this study, a new no-flow underfill for flip chip bumped with lead-free solder has been developed based on a curing agent and catalysts with high curing latency. The curing agent itself has the fluxing ability, so the need of extra flux is eliminated. Compared with the underfill for lead-containing solder, the new underfill has a higher curing temperature, compatible with the melting profile of lead-free solder. The wetting compatibility of the underfill with the lead-free solder has been studied by wetting experiments. The material properties of the underfill have been investigated using DSC, TGA, TMA and DMA. The results show that the newly developed underfill material has the potential to be used in lead-free interconnects.     

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     

Advanced underfill for high thermal reliability

The key to the success of flip‐chip technology lies in the availability of sucessful underfill materials. However, the reliability of flip‐chip technology using current underfill materials is generally found to be lower than that of conventional wire‐bond connection packaging materials such as epoxy molding compound (EMC) because of the high coefficients of thermal expansion (CTE) and moisture absorption of cured underfill material. In this study desbimide (DBMI), which has a low melting point (about 80°C), was used in the underfill materials as a cohardener. As a result, DBMI‐added underfill can show excellent thermal reliability, which is due to the superior properties of the CTE, the elastic modulus, and water resistance. When the properties of a 2 wt % DBMI‐added underfill were compared with those of a typical underfill (epoxy/anhydride), the CTE value was reduced to less than one‐half at the solder reflow temperature (about 200°C), the elastic modulus was reduced to less than one‐half in the temperature region below the glass‐transition temperature, and the water resistance was improved twofold. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2617–2624, 2002     

Joining of sintered SiC ceramics at a lower temperature using borosilicate glass with laser cladding Si modification layer

The silicon carbide whose surface had been modified by laser cladding silicon layer was soldered by borosilicate glass. And the borosilicate glass solder had a thermal expansion coefficient similar to that of the silicon carbide substrate. The laser cladding silicon layer significantly improved the wettability between molten glass solder and silicon carbide and could reduce the soldering temperature. A sandwich-like joining structure (SiC/Si/solder/Si/SiC) was made after the borosilicate glass slurry put on the laser cladding silicon layers. The microstructure, compositions, and interfacial properties were studied. Results showed that good adhesion between silicon carbide and the glass soldering layer was achieved. The flexural strength of the connection structure prepared at 900 °C in the air reached 110 MPa. This research provides an effective technical solution for realizing local heat treatment soldering of large silicon carbide components.     

铝压铸件镀锌军绿色钝化工艺

为了在铝合金压铸件表面获得结合力良好的镀锌层,采用了先化学镀镍-磷合金,再进行电镀锌的工艺,获得了结合力良好的镀锌层;采用低铬军绿色钝化液,克服了常规五酸钝化稳定性差、维护难的缺点,得到了高质量的军绿色钝化膜。     

无铅焊膏在电子封装组装中的应用

随着人们对环境的日益重视和电子封装组装焊接技术的发展，合金焊膏的无铅化和质量的要求越来越高，开发无铅、无毒焊膏成为焊膏开发的重要方向，本文论述了几种无铅焊膏Sn-Ag系，Sn-Bi和Sn-Zn系的特点，同时，也检测和评价了一种Sn-Ag免洗焊膏，其绝缘抗阻性，抗腐蚀性，产品清洁度和产品可靠性等均符合要求。     

Formation and assembly of carbon nanotube bumps for interconnection applications

A simple assembly process of carbon nanotube (CNT) bumps which could be used for flip chip interconnects was proposed and investigated. Firstly, high density aligned CNT bumps were grown on both top and bottom substrates. By employing typical flip chip bonding technique, the CNT bumps on the flipped top substrate were aligned with those on the bottom substrate. Applying a downward force on the die, the CNT bumps on the top substrate were pressed and inserted into the CNT bumps on the bottom substrate. After CNT insertion, the CNTs were held together with van der Waals force and the CNT interconnection bumps were formed. The electrical conductivity of the CNT interconnection bumps was measured and compared to conductive silver adhesive. The conductivity of CNT interconnection bump was found to be of many magnitudes higher than that of silver adhesive. It was demonstrated that the CNT bumps have a much superior electrical properties over the typical metallic bumps.     

Study on properties of a novel photosensitive polysiloxane urethane acrylate for solder mask

The photosensitive and physical and mechanical properties of a novel polysiloxane urethane acrylate (PSUA) for solder mask were investigated using real‐time FTIR, DMTA and TGA. It is noted that PSUA showed a notable photosensitivity and a good compatibility with the acrylic monomers and resins. PSUA cured film exhibited excellent thermal property, tensile strength and toughness, and chemical resistance. The decomposition temperature of PSUA was 402 °C. Thermal weight losses of pure PSUA cured film at 300 °C were only 5%. Elongation percentage of PSUA cured film was up to 59%. PSUA resin can be used for solder mask materials for printed circuit. Technology performances of photosensitive imaging flexible solder mask containing PSUA answers operating requirements of the solder masker for printing circuit board. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

印制板用电解铜箔后处理工艺的研究

研究了一种新的印制板用电解铜箔后处理工艺,依次进行电镀Zn—Ni基三元合金、铬酸盐钝化、浸有机膜等。给出了电镀Zn—M基三元合金和铬酸盐钝化的工艺参数,并对各影响因素进行了分析。对经过上述工艺处理的电解铜箔进行了性能测试,结果表明,经过该工艺处理的电解铜箔的耐蚀性、耐热性和与印制板基体的结合强度等均有明显提高,铜箔的抗剥强度为2．15,劣化率为1．38％。     

Bilayered coatings of BN/diamond grown on Si3N4 ceramic substrates

Cubic boron nitride (c-BN) is a well known material to be used in machining of ferrous metallic alloys, namely as a coating. However, in most practical cases, there is a lack of adhesion to the substrate material. In this work, BN coatings were deposited by magnetron sputtering on silicon nitride (Si₃N₄) ceramic substrates using an intermediate layer of CVD microcrystalline (MCD) or nanocrystalline diamond (NCD). The goal was to improve the c-BN content by using diamond interlayers, and to optimize film adhesion to the substrate by employing such ceramic, which is known to provide very high adhesion strength to CVD diamond. The BN/NCD/Si₃N₄ combination demonstrated to be unique regarding the absence of delamination at both the BN/diamond and diamond/substrate interfaces, also providing the highest c-BN phase content.     

Carbon nanotube bumps for the flip chip packaging system

ABSTRACT: Carbon nanotube [CNT] interconnection bump joining methodology has been successfully demonstrated using flip chip test structures with bump pitches smaller than 150 μm. In this study, plasma-enhanced chemical vapor deposition approach is used to grow the CNT bumps onto the Au metallization lines. The CNT bumps on the die substrate are then 'inserted' into the CNT bumps on the carrier substrate to form the electrical connections (interconnection bumps) between each other. The mechanical strength and the concept of reworkable capabilities of the CNT interconnection bumps are investigated. Preliminary electrical characteristics show a linear relationship between current and voltage, suggesting that ohmic contacts are attained.     

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     

Internal stresses and adhesion of thin silicon oxide coatings on poly(ethylene terephthalate)

The effect of internal stresses on the cohesion and adhesion of a thin silicon oxide (SiO_x) oxygen-barrier coating, evaporated on a poly(ethylene terephthalate) (PET) film substrate was studied. Internal stresses were generated during annealing in the temperature range for recrystallization of the PET,during calendering in a multilayer structure where two SiO_x /PET films were laminated together with a polypropylene film, and during long-term thermal aging below the glass transition temperature of the polymer. The cohesion of the coating and its adhesion to the polymer substrate were derived from fragmentation tests, in which the failure of the oxide coating was analyzed as a function of the applied stress during uniaxial tensile loading of the substrate. The intrinsic coating strength at critical length and the interfacial shear strength were found to be equal to 1350 MPa and 73 MPa, respectively. It was found that none of the thermal treatments investigated altered the interfacial interactions. Rather, these treatments induced shrinkage of the PET substrate, which increased the coating internal compressive stress and the SiO_x /PET interfacial shear strength. A linear relationship between the SiO_x /PET interfacial shear strength and the coating internal stress was determined from a stress transfer analysis. The coefficient of this linear relationship, equal to-1.34 · h _c/l _c, where h _c is coating thickness and l _c is the critical stress transfer length, reproduces the experimental data with good accuracy.     

Study of adhesion failure due to molding compound additives at chip surface in electronic devices

Today the microelectronics market requires devices with failure levels approaching zero. To attain this goal all production processes must be subjected to extreme quality control. Molding is one of the most critical assembly processes in power plastic packages. This is related to the complexity of phenomena which may occur at the interfaces involved in this process. This paper reports an adhesion study of epoxy-phenolic molding compounds to the most relevant surfaces encountered in power devices assembled in plastic packages such as copper oxide-hydroxide, nickel oxide-hydroxide, aluminium oxide-hydroxide, and silicon 'nitride'. The study was carried out by combining delamination (scanning acoustic microscopy) and pull strength data with the interface chemistry studied using ESCA. Different adhesion failure mechanisms were found to be operative in these systems. These mechanisms are related to either the chemical nature and thickness of the inorganic layer or the segregation of various additives such as wax, polyoxyalkylene ethers, and alkylsiloxanes, contained in the molding compound.     

Ultrasonic-assisted soldering of sapphire through metallic transition layers of Al and Zn using Sn-xZn-2Al solder alloys

Ultrasonic-assisted soldering has potential in the electrical industry especially for the joining of ceramics. The Al-activated Sn-based alloys are promising ultrasonic solders due to simple preparation, while the current approaches required long ultrasonic action. In order to increase the efficiency and reduce ultrasonic-induced damage, this study investigated the soldering of sapphire (monocrystalline α-Al₂O₃) performed under an ultrasonic action for 0.5 s by using Sn-xZn-2Al(x = 9, 25, 45) solder alloys. Microstructures of interfacial transition layers between the sapphire and solders were focused on. It has been found that at the interfaces no interfacial reaction phases formed and the interfacial bonding was realized via metallic transition layers. Three kinds of interfacial structures existed, that is, sapphire/Al atomic layer/β-Sn, sapphire/Al atomic layer/(Zn enrichment layer)/β-Sn and sapphire/Al atomic layer/Zn nanocrystalline clusters/β-Sn. The elements of Al and Zn in the solder alloys underwent a selective and asynchronous adsorption process during the ultrasonic action. An Al atomic layer formed on the sapphire surface by the stronger chemical adsorption and acted as a transition layer between sapphire and β-Sn. The Zn enrichment layer was distributed locally along the interface and as the Zn content increased in the solder alloys, more localized Zn nanocrystalline clusters formed. These Zn transition structures strengthened the interfacial bonding by transforming the Al atomic layer/β-Sn interface into the Al atomic layer/Zn transition structures/β-Sn interfaces. The joints possessed a shear strength of up to 28 MPa when soldering with Sn–45Zn–2Al at 350 °C.     

Electrochemical etch-stop control for silicon structures containing electronic components

A predictive analysis of the feasibility of electrochemical etch-stop control in the fabrication of some silicon sensors was validated experimentally. The application is for sensors with thin silicon structures containing electronic components, such as cantilever accelerometers and diaphragm pressure gauges with strain gauges in the surface to detect deflection. Such structures are formed by deep anisotropic etching of silicon. Since the depth of the etch determines the thickness of the cantilever or diaphragm, the ability to stop the etching precisely is critical. Accurate etch-stop control can be accomplished by electrochemical passivation of an n-type epitaxial layer on a p-type silicon wafer, where the epi layer thickness becomes that of the diaphragm or cantilever. The analysis shows that passivation of the epi layer can be maintained even underneath the electronic components for conditions which allow etching of the p-type silicon substrate. Therefore, electrochemical etch-stop control appears feasible in most practical sensor designs.