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 Rubber Compounds Containing Various Adhesion Promoters and Brass-Plated Steel Cords. Part I. Effect of Sulfur Loading in Rubber Compounds

The adhesion property between rubber compounds containing different types of adhesion promoters (resinous adhesion promoter (containing both methylene donor and methylene acceptor), cobalt salt and zinc borate) and different loading amounts of sulfur and brass-plated steel cords was investigated to understand the effect of sulfur loading in the rubber compounds on their adhesion characteristics to the brass-plated steel cords. The adhesion property of the rubber compounds to brass-plated steel cords was largely dependent on both the type of adhesion promoter and the loading amount of sulfur in the rubber compounds. The pull-out force of adhesion samples increased significantly with increasing loading amount of sulfur in the rubber compounds containing resinous adhesion promoter, whereas it decreased slightly with increasing loading amount of sulfur in rubber compounds containing cobalt salt or zinc borate. In humidity aging, the best adhesion retention was observed in the rubber compound containing zinc borate and low loading of sulfur. Regardless of the type of adhesion promoter, adhesion retention after thermal aging treatments improved with increasing loading amount of sulfur in the rubber compounds. The adhesion property was interpreted in terms of the interphases formed between the rubber compounds and the brass-plated steel cords as studied using Auger electron spectroscopy.     

Adhesion Properties and Structure of Ion-Beam Modified Polyimide Films

Polyimide (PI) surface modification was carried out by an ion beam treatment to improve the adhesion between the polyimide film and copper. The PI film surface was treated with an ion-beam source at ion doses ranging from 1.96 × 10¹³ to 2.38 × 10¹³ ions/cm² using a mixture of nitrogen (N₂) and hydrogen (H₂). Contact angle measurement, atomic force microscopy and X-ray photoelectron spectroscopy, respectively revealed an increase in the surface roughness, a decrease in contact angle, and the formation of oxygen complexes and functional groups on the treated PI surfaces. Adhesion between the copper and PI film treated with the beam was superior to that of the untreated PI film. The 90° peel test revealed the highest peel strength of 7.8 N/cm.     

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.     

Thermal and hygroscopic reliability of flip-chip packages with an anisotropic conductive film

Thermal and hygroscopic reliability of anisotropic conductive film (ACF) joints in relation to flip-chip bonding force was evaluated by thermal shock and constant temperature/humidity testing. The failure mode by thermal shock testing varied with increasing bonding force, i.e., (1) formation of a conduction gap between conductive particles and Au bump or Ni/Au plated Cu pad at low bonding force and (2) delamination of adhesive matrix from the plated Cu pad on the flexible substrate at high bonding force. The delamination initiated as a crack under a conductive particle and propagated sideward resulting in a complete delamination of the ACF from the Cu pad. However, delamination was observed between the ACF and Au bump on the chip side after the constant temperature/humidity testing for 500 h. A theoretical calculation was also conducted to predict the connection resistance of the ACF joint before and after reliability tests. The calculation showed the importance of the bonding gap between the electrodes.     

Adhesion analysis of two-level hierarchical morphology in natural attachment systems for 'smart adhesion'

The attachment ability of insects and lizards is well known. The Tokay gecko, in particular, has the most complex adhesion structures. The pads are covered by a large number of small hairs (setae) that contain many branches per seta with spatulae. Seta branch morphology is hierarchical. Hierarchical morphology of setae is responsible for adaptation of a large number of spatulae to rough surfaces. Van der Waals attraction between the large numbers of spatulae in contact with a rough surface is the primary mechanism for high adhesion. In order to investigate the effect of hierarchical structure, for the first time, the two-level hierarchical model has been developed. We consider one- and two-level hierarchically structured spring models for simulation of setae contacting with random rough surfaces and demonstrate the effect of the two-level hierarchical structure on the adhesion force, the number of contacts and the adhesion energy. Tip of spatula in a single contact was assumed as spherical. Rough surfaces with various roughness parameters which cover a common range of most of natural and artificial rough surfaces at the scale of gecko's pad were generated. It was found that significant adhesion enhancements are created with the two-level structure until a certain value of roughness which appears to be related to the maximum spring deformation. We conclude that the hierarchical morphology of a gecko seta is the necessary part for 'smart adhesion' of gecko, the ability to cling on and detach from different smooth, as well as rough surfaces.     

Enhancing the Adhesion Retention by Controlling the Structure of the Adhesion Interphase Between Rubber Compound and Metal. Part I. Effect of Cobalt Salt

The adhesion between cobalt salt-containing rubber compounds and brass-plated steel cords was studied to understand the role of cobalt salt as an adhesion promoter. An improvement in adhesion was shown with the low loading of cobalt salt in the range 0.5–1 phr, while a significant decline in adhesion was observed with high loading at 2 phr and a long aging time of 15 days under humidity aging. The adhesion interphase between the rubber compound and the brass-plated steel cord studied using Auger electron spectroscopy (AES) showed stable depth profile by cobalt salt incorporation at low loading, resulting in the enhancement of the adhesion retention. For the high loading of cobalt salt, adhesion interphase grew excessively, especially ZnO layer formed under humidity aging, which resulted in reduced adhesion.     

Enhancing the Adhesion Retention by Controlling the Structure of the Adhesion Interphase between Rubber Compound and Metal. Part II. Effect of Zinc Borate

The adhesion between zinc borate-containing rubber compounds and brass-plated steel cords was studied to understand the role of zinc borate as an adhesion promoter. No improvement in adhesion after cure was shown with loading of zinc borate in the range 0.5–2 phr, while enhancement of adhesion retention after humidity aging was observed with loading of zinc borate in the range 0.5–2 phr. The adhesion interphase between the brass-plated steel cord and the rubber compound subjected to humidity aging treatment showed, using AES, a stabilized depth profile by a moderate loading of zinc borate, resulting in enhancement of adhesion retention. For the high loading of zinc borate, copper sulfide layer and zinc oxide layer grew excessively in the adhesion interphase under humidity aging.     

Adhesion behavior of blends of polybutadiene and tackifiers

The adhesion behavior of blends of polybutadiene (PB) and tackifiers was investigated. The peel strength was found to be significantly dependent on blend composition, and a maximum strength behavior was observed for all blend systems. The tack mechanisms were proposed to explain the observed adhesion behavior. The position of the glass transition temperature is related to where the highest tack appears for the miscible blend; for the partially miscible blend the dispersed phase plays an important role in determining the maximum adhesion, and the critical size of the dispersed phase and the Tg of the tackifier phase together determine what concentration of tackifier gives the maximum tack. For the immiscible blends, the maximum adhesion is controlled by both the surface glass transition temperature and the critical diameter of the domains of the tackifier phase.     

Adhesion between rubber compounds and ternary-alloy-coated steel cords. Part II: Effects of sulfur and cobalt salt in rubber compounds

The adhesion properties between rubber compounds with different amounts of cobalt salt and sulfur and ternary-alloy-coated steel cord with 2 wt% cobalt plating amount were investigated to understand the effects of cobalt salt and sulfur in rubber compounds on their adhesion characteristics to the ternary-alloy-coated steel cord. The adhesion properties of the rubber compounds to ternary-alloy-coated steel cord were largely dependent on the amounts of both cobalt salt and sulfur in the rubber compounds. The pull-out force of adhesion sample increased significantly with increasing concentration of cobalt salt in the rubber compound with constant sulfur loading, while it decreased slightly with increasing sulfur into the rubber compound with constant cobalt salt loading. Adhesion retention after various hostile aging treatments improved in the rubber compounds incorporating both cobalt salt and high loading of sulfur. The interphases between the rubber compounds and the ternary-alloy-coated steel cord studied using AES showed stable adhesion patterns by incorporating cobalt salt, resulting in enhancement of the adhesion retention.     

Adhesion between rubber compounds and ternary-alloy-coated steel cords,Part I: effect of cobalt plating amount in ternary-alloy-coated steel cords

The adhesion between a rubber compound and ternary-alloy-coated steel cords with different cobalt plating amounts (0, 2 and 4 wt%) was investigated to understand the role of cobalt in stabilizing adhesion to the rubber compound. The adhesion property of ternary alloy coated steel cords to the rubber compound did show significant enhancement after cure for the ternary alloy coated steel cord with 2 wt% cobalt plating. Further increase of cobalt plating in ternary-alloy-coated steel cord was responsible for the poor adhesion to the rubber compound. An improvement in adhesion durability after aging in various hostile environments was shown for the ternary-alloy-coated steel cord with 2 wt% cobalt plating. The interphase between the ternary-alloy-coated steel cords and the rubber compound studied using AES showed a stable adhesion interphase by optimum cobalt plating, resulting in enhancement of adhesion retention.     

Adhesion property of novel polyimides containing fluorine and phosphine oxide moieties

Novel diamine monomers containing fluorine and phosphine oxide - bis(3-aminophenyl)-3,5-bis(trifluoromethyl)phenyl phosphine oxide (mDA6FPPO) and bis(3-aminophenyl)-4-(trifluoromethyl)phenyl phosphine oxide (mDAFPPO) - were utilized to prepare polyimides with dianhydrides such as 6FDA, BTDA or ODPA by the conventional two-step route, i.e. preparation of poly(amic acid) followed by solution imidization. The polyimides were characterized by FT-IR, NMR, DSC, and intrinsic viscosity measurements. The adhesion property of the polyimides was evaluated via a peel test with bare Cu foil, as well as silane/Cr-coated Cu foil, and failure surfaces were analyzed by SEM/EDX to elucidate the failure mechanism. The results were compared with those from the polyimides prepared from bis(3-aminophenyl)phenyl phosphine oxide (mDAPPO) containing only the phosphine oxide moiety, 1,1-bis(4-aminophenyl)-1-phenyl-2,2-trifluoroethane (3FDAm) containing only the fluorine moiety, and a commercial 3,3′-diaminodiphenylsulfone (mDDS). The polyimides with 3FDAm exhibited the highest T _g, followed by the mDAPPO-, mDA3FPPO-, and mDA6FPPO-based polyimides, but the mDAPPO-based polyimides exhibited the highest adhesion properties, followed by mDA3FPPO, mDA6FPPO, mDDS, and 3FDAm, which is attributed to the phosphine oxide and fluorine moieties.     

Application of FFT-Based Signal Analysis to Micro-Scratch Testing for Adhesion Strength Measurement of Thin Films and Measured Results Based on Different Measurement Parameters

An FFT-based signal processing method for the micro-scratch test, proposed by Baba was compared with a conventional micro-scratching method to quantitatively evaluate the suitability of the technique. Original scratch signals from a micro-scratch system were first digitized by a 16-bit analog-to-digital converter with a sampling frequency of 44.1 kHz, which were fed into a personal computer. Then, the FFT-based method and the conventional method were used to process the signals according to their individual algorithms. The output results from the two methods were compared, showing that the FFT-based method effectively reduces unnecessary signals arising from the micro-scratching mechanism, and that the output level of the unnecessary signals for the FFT-based method is about 50% lower than that observed in the conventional method. The FFT-based method was implemented into a micro-scratch tester for actual application and critical loads of various thin films were measured using various measurement parameters. The results showed that the critical load increases with increasing stylus radius, or increasing thickness of the film, whereas it decreases with increasing frequency of forced vibration. It is also shown that the amplitude of the forced vibration has only little effect on the critical load.     

Adhesion and corrosion resistance of epoxy primers used in the automotive industry

One of the most important factors in corrosion prevention by protective coatings is the loss of adhesion of the coating under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In this work, the adhesion of different epoxy primers (pigment-free, zinc-rich and chromate-based) was examined on steel. Both the dry and wet adhesion strengths of organic primers were measured directly by a pull-off standardized procedure, as well as indirectly by the NMP test. The corrosion stability of coated samples was investigated by electrochemical impedance spectroscopy. It was shown that under dry test conditions all the samples showed very good adhesion. However, different trends in adhesion for different primers during exposure to the corrosive agent (3% NaCl solution) were observed. The lowest adhesion values were obtained for chromate-based epoxy primer; however, the change in adhesion of this protective system during immersion in 3% NaCl solution for 25 days was the smallest of all investigated samples. Electrochemical impedance measurements in 3% NaCl solution confirmed good protective properties of pigmented epoxy primers on steel, i.e., greater values of pore resistance and charge-transfer resistance, and smaller values of coating capacitance and double-layer capacitance, were obtained for these protective systems.     

Peculiarities of Adhesion Interaction of Ag,Cu and Au with Diamond

Wettability tests were performed for Cu, Ag, Au liquid drops on diamond substrates as well as measurements of mechanical contact strength (MCS) between solidified metal drops and diamond. The work of adhesion was calculated using data for metal surface tension and observed contact angles. Cu, Ag, Au (metals of the group I_B of the Periodic Table) show non-monotonic changes of wettability and work of adhesion within their group in the sequence Au → Cu → Ag. The work of adhesion was found to be almost two times higher for Ag than for Cu and almost five times higher for Ag than for Au. Measured mechanical contact strength (MCS) was ～4 times higher, on average, for Ag/diamond samples than for Cu/diamond or Au/diamond samples. Maximum value of the MCS reached ～70 MPa for Ag/diamond samples. Non-monotonic changes of the wettability and work of adhesion observed for the group I_B metals (Cu, Ag, Au) are discussed on the basis of classical London equation for the energy of van der Waals (vdW) bond for two atoms.     

Implications of Growth and Starvation Conditions in Bacterial Adhesion and Transport

Biofouling is synonymous with unwanted biofilms and leads to problems ranging from efficiency and resource loss to health risks. While a number of bacterial properties including biomass concentration and hydrophobicity are considered critical to biofilm development and bacterial adhesion, the variations in these properties under growth and starvation conditions are not very well known. Here, we describe the trends in these properties for four Gram-negative bacteria under growth and extended starvation conditions. A convenient and frequently-used laboratory assay, the microbial adhesion to hydrocarbons (MATH) test, was used to determine the microbial hydrophobicity based on the partitioning of cells at an aqueous-hydrocarbon interface. The bacteria tested exhibited a plateau in hydrophobicity values during the stationary growth phase and longer starvation durations (≥10 days). Starved cultures had higher hydrophobicity and lower cell sizes than growth cultures. Interestingly, hydrocarbon exposure led to an increase in cell size for starved cells as compared to control cultures, while cells under growth conditions did not show significant size changes due to hydrocarbon presence. Cells starved for short durations (up to 7–10 days) exhibited significant variations in microbial hydrophobicity, cell size, and biomass concentration (total proteins and optical density). These results show the importance of studying the bacterial properties as a function of growth and starvation phase for cell adhesion in the context of biofilm formation and biofouling.     

Adhesion and corrosion resistance properties of coatings obtained from modified low-molecular-weight polystyrenes

In this study, as a continuation of our previous studies, chemical modification of low-molecular-weight polystyrenes (PSs) was carried out with various functional group modifiers: epichlorohydrin (ECH), maleic anhydride (MA) and acetic anhydride (AA), in a single stage using a cationic catalyst. It was determined that the amounts of the functional groups bound to the structure of the polymer depended on the molecular weight of the polymer used, and more functional groups were bound to the lower-molecular-weight PSs. It was found that the coating properties (adhesion properties and resistance to aggressive conditions) of the functional group containing PS to the metal surface depended on the structure and the amount of the functional groups bound to the aromatic ring of the polymer. In addition, it was observed that the PS modified with MA and ECH having carboxyl- and epoxy-groups in their aromatic rings had higher adhesion, as well as higher corrosion resistance properties. Various functional groups bound to the aromatic ring of the polystyrene and their amounts were determined by spectral and chemical analysis methods.     

Adhesion Map of Spheres: Effects of Curved Contact Interface and Surface Interaction Outside Contact Region

In this work three dimensionless parameters are introduced in the debate concerning hard contact models. These parameters are related to the overall adhesive contact area, curved surface contribution and surface interaction forces outside the contact region. With the variations of these three parameters, the relations and transitions between the different hard contact models such as Hertz, Bradley, Johnson–Kendall–Roberts (JKR), Derjaguin–Muller–Toporov (DMT) and Maugis–Dugdale (MD) models are presented in a systematic way. The combination of the three parameters provides a new hybrid model. The influence of these three parameters on the contact between spheres has been studied. By analyzing the pressure profiles of contact region, two new instability jumps are proposed. The instability jumps together with the three parameters are used to explain some recent experimental and numerical observations which deviate more or less from those predicated by the classical hard contact models.     

Intentional Polymer Particle Contamination and the Simulation of Adhesion Failure due to Transit Scratches in Ultra-thin Solar Control Coatings on Glass

The major in-service failure mechanism of modern solar control coatings for the architectural glass can be mechanical (e.g., scratch damage). Many of these coatings are multilayer structures of less than 100 nm thickness and different coating architectures are possible (i.e., different layer materials, thickness and stacking order). For high-performance solar control coatings deposited by physical vapour deposition processes the active layer is a thin silver coating (approx. 8 nm thick) surrounded by antireflection coatings (e.g., ZnO, SnO₂) and barrier layers (e.g., TiO _x N _y ). Scratches are often found during delivery of the coated glass (called transit scratches) and it has been determined that the cause of the scratches was the polymer balls sprayed onto the glass to separate sheets while in transportation. This study has developed a simulation test for the transit scratches and has determined that the adhesion of layers within the multilayer stack is critical in determining performance. To test the adhesion of the coatings, coated samples have been subjected to scratch tests using a range of indenters and the most visible damage has been characterised. Through-thickness cracks were observed and it was seen that the coating was stripped by the balls at the weakest point in the coating stack. Microanalysis reveals this weakest point to be the silver/zinc oxide interface in the materials analysed in this study.