Adhesion of electroless nickel plating on polished silicon

This work reports a study of the adhesion of electroless nickel plating on polished silicon as a function of several plating variables, namely substrate resistivity, activator composition, plating duration, and plating bath temperature. It is shown that an adhesion strength of ～100 kg/cm² or more can be obtained, irrespective of the silicon doping, for plating used in semiconductor devices. However, for this purpose, an appropriate silicon activation step is essential, except when the silicon is heavily N-type doped.     

Modification of poly(tetrafluoroethylene) and gold surfaces by thermal graft copolymerization for adhesion improvement

The adhesion between a poly(tetrafluoroethylene) (PTFE) film and a gold substrate was achieved by surface graft copolymerization of glycidyl methacrylate (GMA) on an argon plasma-pretreated PTFE film at elevated temperature with simultaneous lamination to a surface-modified gold substrate. The plasma pretreatment introduces peroxides which are thermally degraded into radicals to initiate the graft copolymerization of GMA on the PTFE surface. The gold surface, on the other hand, was first pretreated with 3-mercaptopropionic acid (MPA), 3-mercaptopropionic acid-2-ethylhexyl ester (MPAEE), or (3-mercaptopropyl)trimethoxysilane (MPTMS) to form self-assembled monolayers (SAMs) and then subjected to Ar plasma treatment. The simultaneous graft copolymerization and lamination of the PTFE film to the gold surface was carried out in the presence of GMA and an amine hardener at an elevated temperature under atmospheric conditions. The modified surfaces and interfaces were characterized by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. The gold/GMA/PTFE assembly exhibited a T-peel adhesion strength above 10 N/cm and the joint delaminated by cohesive failure inside the bulk of the PTFE film. The strong adhesion of the Au/PTFE laminate is the result of concurrent graft copolymerization on both the Ar plasma-pretreated PTFE surface and the SAM of the Au surface to form a covalent network. The network is further strengthened by the crosslinking reaction promoted by the presence of the hardener.     

Promotion effect of a chlorotriazine derivative on the adhesion between rubber compounds and a brass-plated steel cord

The adhesion between chlorotriazine derivative (TZ)-loaded rubber compounds and a brass-plated steel cord was studied to understand how TZ acted as an adhesion promoter. With the loading of the rubber compound with TZ, the cure rate became slow, but changes in the physical properties were not significant. An improvement in adhesion was obvious at a low TZ loading ranging from 0.5 to 2.0 phr. An adverse effect was observed with high loadings up to 8 phr and aging treatments for 15 days. Since the concentration of sulfur in the interphase of the rubber compound/ brass film adhesion samples after humidity aging was high for the rubber compound with high loadings of TZ, the acceleration of sulfide formation by TZ loading was confirmed. Lack of oxygen in the interphase indicated the conversion of zinc oxide to zinc sulfide in the rubber compound containing TZ. The control of zinc oxide formation in the adhesion interphase by TZ is suggested to be the reason for the adhesion promotion by TZ.     

Residual stresses and practical adhesion: effect of organo-metallic complex formation and crystallization

Epoxy-amine liquid pre-polymers are often applied onto metallic substrates and cured to obtain painted materials or bonded joint structures. The overall performance of such systems depends on the interphase created between the epoxy-amine polymer and the metallic substrate. When epoxy-amine liquid mixtures are applied onto a metallic oxide layer, concomitant amine chemisorption and oxide dissolution occur leading to organo-metallic complex formation. Depending on the amine nature, as soon as the organo-metallic complex concentration is higher than the solubility product (e.g., isophoronediamine (IPDA)), these organo-metallic complexes crystallize as sharp needles. At the same time, the uncrystallized organo-metallic complexes react with the epoxy monomer to form, after curing cycle, a new network. Moreover, the crystal size increases with the solid/liquid contact time leading to an increase of intrinsic residual stresses and Young's modulus. When aliphatic diethylenetriamine (DETA) was used, no crystallization occurred, but the interphase formation was observed. The aim of this study was to understand and to establish the role of crystallization of organo-metallic complexes formed within the interphase on the practical adhesion performance. As the crystallization of the organo-metallic complex depends on the nature of the amine, two amine hardeners were used (IPDA inducing the formation of crystals and DETA without formation of crystals). For DGEBA-IPDA systems, the ultimate load decreases while residual stresses increase when the liquid/solid contact time increases. When no crystal formation was observed (e.g., DGEBA-DETA system), residual stresses, coating Young's modulus and ultimate load values all remained nearly constant irrespective of the liquid/solid contact time.     

Influence of block molecular weight on the adhesion properties of segmented polyamides

The effects of molecular weight variations in the hard and soft segments on the adhesion strength of segmented polyamides against aluminium, copper, and steel were investigated using 180° peel strength measurements. It was found that the adhesion strength of the segmented polyamides was largely influenced by block molecular weight variations. The nature of the substrate, the rate of peeling, cooling in different environments, and thermal ageing, etc. had significant effects on the adhesion strength of the joints, whereas variation in the moulding conditions used in these experiments did not have much impact on the strength of the joints. The joint strength increased with a decrease in hard block molecular weight at a constant soft block molecular weight of 1000, or with an increase in soft block molecular weight at a constant hard block molecular weight of 1100.     

Cellulose fiber/polymer adhesion: effects of fiber/matrix interfacial chemistry on the micromechanics of the interphase

The objective of this study was to examine the effects of interfacial chemistry on the interfacial micromechanics of cellulose fiber/polymer composites. Different interfacial chemistries were created by bonding polystyrene (a common amorphous polymer) to fibers whose surfaces contained different functional groups. The chemical compatibility within the interphase was evaluated by matching the solubility parameters (δ) between the polymer and the induced functional groups. The physico-chemical interactions within the interphase were determined using the Lifshitz–van der Waals work of adhesion (W _a ^LW) and the acid–base interaction parameter (I _a?b) based on inverse gas chromatography (IGC). The micromechanical properties of the fiber/polymer interphase were evaluated using a novel micro-Raman tensile test. The results show that the maximum interfacial shear stress, a manifestation of practical adhesion, can be increased by increasing the acid–base interaction (I _a?b) or by reducing the chemical incompatibility (Δδ) between the fibers and polymer. A modified diffusion model was employed to predict, with considerable success, the contribution of interfacial chemistry to the practical adhesion of cellulose-based fibers and amorphous polymers. The increased predictability, coupled with the existing knowledge of the bulk properties of both fibers and matrix polymer, should ultimately lead to a better engineering of composite properties.     

Deformation of bowed silicon chips due to adhesion and applied pressure

One potential method to accomplish high-rate nanomanufacturing is to develop processes which allow for rapid transfer of nano-scaled devices from a template to a device wafer. In order to accomplish this transfer, the device wafer must make intimate contact with the template. A similar situation exists in wafer bonding, except that in that case the two wafers remain in bonded contact due to the work of adhesion even after the applied pressure is removed. In high-rate nanomanufacturing intimate contact must be maintained during transfer, while allowing for easy separation afterwards. Wafers typically have waviness and bow which cause a deviation of many micrometers from flatness over the 15-mm length scale of a typical chip. This non-flatness can be a serious problem in the transfer of nanometer-scale elements. In this investigation, a model is developed to examine the effects of applied pressure, bow radius and the work of adhesion on the flattening of a spherically/cylindrically bowed chip. This model uses elastic plate theory and the work of adhesion. An operating window is found which provides intimate contact while allowing for separation once the pressure is removed. It is also shown that the effect of adhesion is to produce a discontinuity in the internal bending moment, at the separation boundary, which is proportional to the square-root of the product of the work of adhesion and the flexural rigidity. This "moment-discontinuity" method can be applied to other problems involving adhesion of elastic plates.     

Evolution of the interfacial stress transfer ability between a glass fibre and a polypropylene matrix during polymer crystallization

A rigorous evaluation of the influence of a transcrystalline interphase on the adhesion between a fibre and a polymer matrix is difficult because it is generally not possible to consider this parameter without other factors. Indeed, in the case of a reinforcing material allowing spontaneous transcrystallization of a thermoplastic matrix, the inhibition of this phenomenon is, for instance, possible by modifying the surface topography or the physico-chemical nature of the fibre by appropriate coatings. In the same way, the fibres which do not intrinsically favour transcrystalline growth can behave as active substrates by applying a shear stress onto the fibre-matrix interface. In this case, however, processing of the sample for the classical micromechanical tests remains extremely difficult. In order to understand better the participation of a transcrystalline interphase in the interfacial adhesion in a polypropylene–glass fibre system, an experimental protocol has been developed which allows us to evaluate the evolution of the interfacial shear stress during the matrix crystallization, the latter being spherulitic or cylindritic. The results show that a transcrystalline interphase in a polypropylene–glass fibre composite does not significantly alter the adhesion between the two materials after total crystallization of the matrix. Nevertheless, it is shown that an important hooping of the fibre occurs during the development of a transcrystalline superstructure.     

Influence of the hard-to-soft segment ratio on the adhesion of water-borne polyurethane adhesive

Three water-borne polyurethane dispersions were synthesised by the pre-polymer mixing process. Different hard segment contents in the polyurethanes were obtained by varying the diisocyanate/macroglycol (NCO/OH) molar ratio. A decrease in the NCO/OH ratio produced an increase in the mean particle size and in the pre-polymer viscosity, as well as a decrease in the molecular weight of the polyurethane. On the other hand, the lower the NCO/OH ratio, the more crystalline the polyurethane and the lower the resistance to flow at high temperature. Lower NCO/OH ratios improved the thermal degradation stability of the polyurethane. Finally, a high initial adhesion to PVC was obtained in all joints produced with the aqueous polyurethane dispersions and the final adhesion increased as the NCO/OH ratio in the water-borne polyurethane adhesive decreased.     

Effect of the physical and mechanical properties of epoxy resins on the adhesion behavior of epoxy/copper leadframe joints

In this study, the adhesion strength of three epoxy resins, which are used as basic materials for epoxy molding compound (EMC) in microelectronics, to copper leadframe was determined using the peel test. The epoxy resins used were O-cresol Novolac (OCN), dicyclopentadiene (DCPD), and biphenyl sulfide (BIPHS) epoxy resins. It was found that DCPD showed the highest peel strength and OCN had the lowest value. The difference in the peel strength was explained by investigating the physical and mechanical properties, as well as the surface properties of the epoxy resins. These properties included the surface energy, viscosity and gelation time, fracture toughness, and the coefficient of thermal expansion. As a result of the lower viscosity of BIPHS and DCPD than OCN epoxy resin, BIPHS and DCPD have a better peel strength than OCN. The DCPD resin has a better peel strength than BIPHS because of its higher fracture toughness.     

Effect of magnetic field on deposition and adhesion of calcium carbonate particles on different substrates

Magnetic field effects on CaCO₃ adhesion to substrates were evaluated statistically as for the number of crystals, the occupied surface area per crystal and the crystal size distributions. CaCO₃ was precipitated from Na2CO₃ and CaCl₂ solutions, which prior to precipitation were or were not exposed to a magnetic field under flowing conditions. Glass, copper and stainless steel plates were used as substrates. The precipitation process occurred at 30°C, and in the case of glass plate experiments were also conducted at 60°C. The CaCO₃ deposits were photographed using an optical polarizing microscope equipped with a camera and then the images were analyzed statistically. For each system studied 14 plates were used. It was found that in all systems the magnetic field (MF) influenced the number of crystals deposited and their size distribution. The changes were statistically significant (t-Student's test). The nature of the substrate surface was found to have a significant effect on the amount of CaCO3 deposited. However, the average amount of calcium carbonate deposited on the glass surface at 30°C determined from 14 analyzed plates was only slightly less than that from MF-treated solutions (0.689 and 0.748 mg/cm², respectively), and it was practically the same at 60°C (0.539 and 0.534 mg/cm²). At this higher temperature the deposition of calcium carbonate was reduced relative to that at 30°C. From the results obtained it may be concluded that the MF affects both nucleation and crystal growth of CaCO₃.     

The effect of surface properties on the adhesion of modified polychloroprene used as adhesive

The adhesion properties of polychloroprene can be improved by addition of such materials as piperylene–styrene co-polymer (PSC), VeoVa-10 polymer, VeoVa-11/methyl methacrylate/2ethylhexyl acrylate co-polymer (VeoVa-11/MMA/2EHA) and poly(vinyl acetate) waste (wPVAc). Here, the relationship between adhesion properties and surface tension of polychloroprene was investigated. Contact angle measurements have been used to study the effects of nature and content of polymeric additives on the adhesion and surface properties of polychloroprene. Low-surface-tension VeoVa-10 polymer has the tendency to migrate to the surface of polychloroprene; thus, adhesion is determined mainly by this additive property. Enrichment of polychloroprene film bottom layer by the additive was observed using high-surface-tension PSC and wPVAc. In this case, the adhesion properties of polychloroprene depend on the interactions at the interface. Adhesion properties of polychloroprene were found to depend not only on compatibility between adhesive components, but also on compatibility between the adherend and the adhesive.     

The effects of urine and temperature on the physicochemical surface properties and adhesion behaviour of uropathogenic bacteria

Bacterial adhesion in relation to urinary-tract infections has gained importance in the last years because of the increasing catheterization in hospitals to assist post-surgery flow of urine. Since the initial adhesion of bacteria to biomaterials is governed by physicochemical forces emerging from the physicochemical properties of both interacting phases, we have investigated the physicochemical surface changes of uropathogen Enterococcus faecalis ATCC29212 bacteria due to the presence of urine in its growth medium and to the differences in the environmental temperature. Urine-grown cells were found to be less hydrophobic based on water contact angles at 22°C, while no changes were detected at 37°C. In addition, they exhibited higher acid-base surface energy component than urine-free cultured cells. These changes in surface properties were also reflected in thermodynamic predictions of the adhesion to glass and silicone, which were experimentally compared with the in vitro adhesion curves obtained in a parallel plate flow chamber. The shapes of the adhesion graphs indicated that interaction free energies should be used to describe only the initial adhesion stages. Adhesion to silicone was always enhanced by urine-grown cells, while the adhesion to glass did not seem to be affected by the urine constituents. Despite the fact that the interaction free energies were not able to explain the adhesion process in some cases, changes in the electron-donor and electron-acceptor parameters of their surface free energy due to urine addition seemed to have a relation with initial adhesion rates.     

Study of adhesion forces and mechanical properties of human skin in vivo

In this work we have studied the effect of adhesion forces on the mechanical parameters between a spherical indenter and human skin surface with an indentation test. To take into account the effect of adhesion on Hertzian contact radius, pressure and strain, a theory of adhesion contact, like that of Johnson, Kendall and Roberts (JKR) or Derjaguin, Muller and Toporov (DMT) must be used. These theories correct the errors induced by the adhesion forces on contact parameters. The change in skin surface lipid film during hydration by water is assessed by analyzing the evolution of the adhesion energy and skin stiffness.     

Temperature influence on the physicochemical surface properties and adhesion behaviour of Enterococcus faecalis to glass and silicone

The interest in studies on the physicochemical surface properties of bacteria has increased because they are related to the causes of the initial adhesion of microorganisms to biomaterials and the subsequent biofilm formation on indwelling medical devices. The determination of physicochemical parameters such as hydrophobicity or surface tension is usually done at room temperature, not taking into account the real temperature at which bacteria cause infection inside the human body. In this work, the influence of the experimental temperature on the surface physicochemical characteristics and adhesion behaviour of Enterococcus faecalis ATCC29212 to glass and silicone has been studied. Water, formamide and diiodomethane contact angles on bacterial lawns changed when the experimental temperature was increased from 22°C to 37°C. Moreover, hydrophobicity, as determined by water contact angle, increases with temperature, in agreement with the higher and lower adhesion to silicone and glass, respectively, observed at 37°C, with respect to the results at 22°C. Also, when the formamide and diiodomethane contact angles are considered, the changes in the adhesion behaviour to glass and silicone are predicted by the sum of Lifshitz-van der Waals and acid-base interaction free energies if the measurement temperature is the same as the bacterial growth temperature, i.e. 37°C.     

Influence of glass mechanical strengthening on the adhesion properties of poly(vinyl butyral) films to a float glass surface

The effects of various methods of mechanical strengthening of glass on the adhesion properties of poly(vinyl butyral) (PVB) film to a float glass surface were investigated. The mechanisms of the influence of the strengthening processes on the adhesion properties were analyzed. The influence of different types of pretreatment of the glass surface on the adhesion of the polymer films was also considered. It was shown that ion-exchange strengthening followed by treatment with an alkaline water solution provided the best combination of high mechanical strength of glass and good adhesion of the PVB films to the glass surface. Metal-oxide coatings on float glass increased the mechanical strength of glass but decreased the adhesion strength between the polymer and glass. The adhesion of PVB to the metal-oxide layers was determined not only by the chemical composition of the layers, but also by the method of layers formation, the type of glass surface pretreatment, and the nature of the intermediate layer between the metal-oxide layer and the glass surface.     

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.     

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.     

Effects of natural-resource-based scavengers on the adhesion properties and formaldehyde emission of engineered flooring

In this study we investigated the effects of using four additives, wheat flour (WF), tannin, rice husk (RH) and charcoal, to melamine-formaldehyde (MF) resin for decorative veneer and base plywood in engineered flooring in order to reduce the formaldehyde emission levels and improve the adhesion properties. We determined the effects of variations in hot-press time, temperature and pressure on the bonding strength and formaldehyde emission. Blends of various MF resin/additive compositions were prepared. To determine and compare the effects of the additives, seven MF resin blends were prepared with the four different additives: four with a wt ratio of 8:2 (MF/WF, MF/tannin, MF/RH and MF/charcoal), and three in the wt ratio of 8:1:1 (MF/WF/tannin, MF/WF/RH and MF/WF/charcoal). The desiccator and perforator methods were used to determine the level of formaldehyde emission. The formaldehyde emission level decreased with all additives, except for RH. At a charcoal addition of only 20%, the formaldehyde emission level was reduced to nearly 0.1 mg/l. Curing of the high WF and tannin content in this adhesive system was well processed, as indicated by the increased lap-shear strength. In the case of WF, the lap shear strength was much lower due to the already high temperature of 130°C. The adhesive layer was broken when exposed to high temperature for extended time. In addition, both WF and tannin showed good mechanical properties. With increasing WF or tannin content, the initial adhesion strength increased. The MF resin samples with 20% added tannin or WF showed both good lap shear and initial adhesion strengths compared to the pure MF resin.