Dimple-type failures in a polymer/roughened metal system

Roughening of metal surfaces frequently enhances the adhesion strength of polymers to metals by mechanical interlocking. When a failure occurs in a polymer/roughened metal system, the failure is prone to be cohesive within the polymer. In a previous work, an adhesion study on a polymer (epoxy molding compound, EMC)/roughened metal (brown-oxide-coated copper-based leadframe) system was carried out, and the correlation between the failure path and adhesion strength was investigated. In the present work, an attempt to explain why such failure paths occurred was made under the assumption that microvoids were formed in the EMC, as well as near the roots of the CuO needles during the compression-molding process. A simple adhesion model developed from the theory of fiber reinforcement of composite materials was introduced to explain the adhesion behavior and thereby explain the formation of failure paths. It is believed that the adhesion model developed in the present work can be used to explain the adhesion behavior of other similar polymer/roughened metal systems.     

Pull-out behavior of oxidized copper leadframes from epoxy molding compounds

Because of their high electrical and thermal conductivities, copper-based alloys have recently experienced increased demand for use in leadframes. There is, however, a concern about the adhesion of copper-based leadframes to epoxy molding compounds (EMCs), as poor adhesion has been partly responsible for delamination and cracking in plastic packages during reflow soldering. In this study, copper-based leadframe sheets were oxidized in a brown-oxide and/or a black-oxide forming solution to improve the adhesion strength between the copper and the epoxy resin. The effects of the formation of oxides on the adhesion strength of leadframe to EMC were studied using pull-out specimens. After the pull-out tests, fracture surfaces were analyzed by various techniques to find out the failure paths. Each oxidation treatment showed different adhesion behavior and failure paths according to oxidation time.     

Polymer molecular behaviors at buried polymer/metal and polymer/polymer interfaces and their relations to adhesion in packaging

Packaging materials are widely used in modern microelectronics. The interfacial structures of packaging materials determine the adhesion properties of these materials. Weak adhesion or delamination at interfaces involving packaging materials can lead to failure of microelectronic devices. Therefore, it is important to investigate the molecular structures of such interfaces. However, it is difficult to study molecular structures of buried interfaces due to the lack of appropriate analytical techniques. Sum frequency generation (SFG) vibrational spectroscopy has recently been used to probe buried solid/solid interfaces to understand molecular structures and behaviors such as the presence, coverage, ordering, orientation, and diffusion of functional groups at buried interfaces and their relations to adhesion in situ in real time. In this review, we describe our recent progress in the development of nondestructive methodology to examine buried polymer/metal interfaces and summarize how the developed methodology has been used to elucidate adhesion mechanisms at buried polymer/metal interfaces using SFG. We also elucidated the molecular interactions between polymers and various model and commercial epoxy materials, and the correlations between such interactions and the interfacial adhesion, providing in-depth understanding on the adhesion mechanisms of polymer adhesives.     

An energy-based failure criterion for delamination initiation in electronic packaging

The significance of interfacial delamination as a crucial failure mechanism in electronic packaging has been documented in many papers. A number of failure criteria have been used to solve the problems with a pre-crack at the interface. However, in real electronic packages, the size and location of the cracks or/and delamination cannot be predicted. It is not easy to use the traditional fracture criteria to deal with more complicated 3D delamination problems. The epoxy molding compound (EMC)/copper leadframe interface was selected in this study. A series of button shear tests were conducted to evaluate the interfacial adhesion between the EMC and copper. In each test, the failure load acting on the EMC of the button shear sample was measured at different shear angles and a finite element model was used to evaluate the stresses at the EMC/copper interface. In this paper, an energy-based failure criterion is proposed using both the interfacial distortional and hydrostatic strain energy densities as two failure parameters. Stresses were extracted from the numerical simulation in order to calculate the interfacial distortional strain energy density, U _d, and the interfacial hydrostatic strain energy density, U _h, related, respectively, to the shear and tensile modes. U _d and U _h were averaged within a selected region of the finite element model where it exhibits high interfacial strain energy density values.     

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.     

Study on the interfacial interactions and adhesion behaviors of various polymer-metal interfaces in nano molding

This study focuses on investigating interfacial interactions and the adhesion mechanism of polymer-metal interfaces in nano-molding. Polyphenylene sulfide (PPS), polyamide 6 (PA6), and isotactic polypropylene (iPP) were chosen as candidate polymers, and aluminum (Al), and copper (Cu) were used as metal substrates. By establishing the metal matrix composed of a rectangular pit with length, width, and depth of 4.5, 4.5, and 2.0 nm, respectively, six paired polymer-metal interfacial systems in a cuboid of 7.5 × 7.5 × 11.5 nm, consisting of metal, polymer, and vacuum layer (from bottom to top) were constructed. Molecular dynamics simulations were performed to calculate interfacial interactions and bonding processes. Results showed that wall-slip behavior was pronounced in nano-molding. Viscoelasticity and polarity of the polymers played a crucial role in interfacial interactions, which guided the wall-slip behavior and greatly affected the bolt performance. PA6 and PPS were more suitable for molding than iPP on both Al and Cu substrates. PA6 showed the best filling and bonding performances, followed by PPS, while iPP revealed the poorest performances. The Cu substrate exhibited better anchor strength and filling rate than Al substrates with the same polymer.     

Molecular dynamics simulation on the adhesion mechanism at polymer-mold interface of microinjection molding

In the demolding process of microinjection molding, the defects of microstructures are often caused by the strong adhesion between polymer and mold. In order to study the adhesion mechanism, the molecular dynamics (MD) method was proposed to simulate the adsorption of cycloolefin copolymer (COC) molecules on mold surfaces. The evolution snapshots of COC molecular chains of three interfacial models were obtained to directly demonstrate the adhesive strength of interfaces. Meanwhile, the work of adhesion, the relative concentration, the potential energy, and the radial distribution function (RDF) were calculated to explain the interaction mechanism of polymer-mold interfaces. The simulation results showed that the COC-Ni interface had the largest work of adhesion and the lowest potential energy, compared with other two interfaces. The van der Waals (VDW) energy, which mainly derived from the interaction between H atoms in COC and the mold material was the only nonbond interaction energy at the COC-Ni and COC-Si interfaces, while the electrostatic energy existed in COC-Al₂O₃ interface. In order to reduce the adhesion between polymer and mold, fluorine (F) element could be doped into the Ni mold.     

Fracture of particulate filled polymers

The addition of particulate mineral fillers to polymers confers certain mechanical property improvements automatically. Stiffness increases, creep diminishes and distortion at elevated temperatures is often reduced. However, the fracture energy of a polymer, as measured in impact, cracking or tearing tests, may vary quite unpredictably when filler is incorporated. In some special cases the fracture energy increases when small amounts of filler are added although it falls away again at higher volume loadings. This enhancement of polymer toughness by filler is an example of reinforcement. More generally the addition of filler causes a continuous and drastic reduction in fracture energy, resulting in a brittle, weak product. This paper seeks to explain the common degrading effect of filler on polymer fracture energy by considering the progress of a crack through the composite material. The crack travels through regions of polymer and also along the interfaces between polymer and filler. Experiment demonstrates that, although fracture of the polymer regions absorbs considerable energy, fracture of the interfaces usually requires very little. These weak interfaces do not resist cracking and are the cause of brittleness in particulate filled systems. This idea was quantified for thermoplastics such as low density polyethylene and poly (methylmethacrylate) filled with colloidal silica by twin-roll milling. Where the interfacial adhesive energy was much smaller than the polymer fracture energy, the composite toughness dropped as predicted when filler was added. The particle size, the nature or dispersion of the filler, and the crystallinity of the polymer used, had little influence on this phenomenon, as pointed out theoretically. The crucial parameters influencing the fracture energy of the filled polymer were found to be the volume fraction of filler and the interfacial adhesion between polymer and filler. By chemical treatments the adhesive energy between filler and polymer was raised until the interface was almost as tough as the polymer itself. In this case the filled polymer showed good fracture toughness, lending further support to the theory.     

Adhesion Strengths of Epoxy Molding Compounds to Gold-plated Copper Leadframes

In the present study, the effects of plasma cleaning on the adhesion strength of molding compounds to gold-plated copper leadframes are presented. Important process parameters such as the type of gasses used and the time exposed in air before molding are specifically evaluated. The leadframe pullout test is performed to measure interfacial bonding strengths. The liquid droplet test is used to measure contact angles and atomic force microscope (AFM) is employed to characterize quantitatively the roughness of modified surfaces to correlate with the bond strength measurements. The results indicate that plasma cleaning of leadframes has three major ameliorating effects, namely, surface cleaning due to the removal of contaminants, enhanced chemical compatibility with molding compounds and surface roughening with associated larger surface contact area for better mechanical interlocking. Exposure of plasma-cleaned leadframes in air before molding is detrimental to interface bond quality, a finding suggesting that molding operations should be carried out immediately after cleaning. The experimental results show that roughness on the nano-scale is an important surface characteristic that has a strong correlation with interface bonding strengths.     

Thermoplastic bonding to metals via injection molding for macro-composite manufacture

In this work, we explore a new method of in-situ joining of polymers to metals in injection molding to allow direct bonding between thermoplastic and metal parts. Such a method can integrate several downstream steps in product manufacture, allow optimal design of products and joints, and avoid adhesive application, assembly, and associated difficlties. A variety of process parameters and their effects upon the interface tensile strengths were examined. A full factorial experiment was conducted involving four of the critical process parameters identified. The effects upon tensile strength at break of the following process parameters were studied: (1) adherend surface temperature, (2) screw linear velocity, (3) bondline thickness, and (4) pack and hold pressure. The fracture surfaces and the thermoplastic metal interfaces were analyzed. The bonds fabricated with higher adherend surface temperatures have increased mean tensile strength and less adhesive failure. This increase in mean bond tensile strength and less adhesive failure was due to increased polymer penetration of the adherend surface roughness, at the micrometer level, as shown in the analysis of the polymer-metal interface by a scanning electron microscope (SEM).     

Improvement in the adhesion between copper metal and polyimide substrate by plasma polymer deposition of cyano compounds onto polyimide

The surface modification of Kapton film by means of plasma polymer deposition is discussed from the viewpoint of improving the adhesion between copper metal and Kapton film substrate. Plasma polymers of AN (acrylonitrile) and FN (fumaronitrile) were used for the surface modification, and the adhesion between the copper metal and the plasma polymer-coated Kapton film was evaluated by the T-peel strength measurement. The surfaces of peeled layers were analyzed by X-ray photoelectron spectroscopy (XPS) and the failure mode is discussed. The plasma polymer deposition of AN and FN shows an effective improvement in the adhesion between the copper metal and Kapton film; in particular, the AN plasma polymer deposition increased the peel strength 4.3 times. Failure occurred mainly in the Kapton film, and the adhesion between the AN plasma polymer and the Kapton film and that between the copper metal and the AN plasma polymer were found to be quite strong.     

Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy

This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liquid and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liquids. It has been found that molecular interactions at such polymer/liquid interfaces dictate interfacial polymer structures. The molecular structures of silane molecules, which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing molecular-level understanding of polymer adhesion promotion. The molecular structures of polymer/solid interfaces have been examined using SFG with several different experimental geometries. These results have provided molecular-level information about polymer friction, adhesion, interfacial chemical reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive analytical techniques.     

Polyphenyletherketone and polyphenylethersulfone adhesives for metal-to-metal joints

Two major factors play an important part in improving adhesive bonding in crystalline polyphenyletherketone ( ) and amorphous polyphenylethersulfone ( ) polymer-to-metal joint systems: (1) the mechanical strength of reaction product layers formed at polymer/metal interfaces is greater than that of the polymer itself; and (2) the extent of mechanically weak Fe₂O₃ layers on interfacial metal surfaces, which should be minimized to avoid the undesirable cohesive failure mode through these layers. As a result, the most promising failure mechanism for good bond performance was the mixed cohesive failure modes in which separation occurred in both the polymer and adhesive layers at the polymer/metal interfaces.     

Interfaces in repair,recycling, joining and manufacturing of polymers and polymer composites

A basic set of 10 thermoset polymer–polymer interfaces has been identified to play a vital role in the technical and economic aspects of composite manufacturing (RIM/RTM, compression molding, autoclave lamination), recycling, repair, welding, and joining of polymer composites. Knowledge of the chemical interactions and molecular connectivity at these interfaces and their influence on processability and mechanical properties of the polymers and polymer composite is essential, and has been the focus of this research. Presented in this report are the results of an exploratory study performed to understand the interactions at the polymer–polymer interface and their influence on the interfacial fracture toughness of a thermoset vinyl ester, which is widely used in liquid molding applications. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 775–785, 1999     

Fundamentals of Adhesion Failure for a Model Adhesive (PMMA/Glass) Joint in Humid Environments

The origins for the abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces were explored using a poly(methyl methacrylate) (PMMA) film on silicon oxide as a model system. The interfacial and bulk water concentrations within the polymer film were quantified as a function of D₂O partial pressure using neutron reflectivity. The adhesive fracture energies of these PMMA/SiO₂ interfaces at the same conditions were determined using a shaft-loaded blister test. Discontinuities in the adhesive fracture energy, bulk moisture solubility, and the width of the interfacial moisture excess near the interface were observed at the critical RH. A mechanism based on the coupling of bulk swelling-induced stresses with the decreased cohesive strength due to moisture accumulation at the interface is proposed and is consistent with all experimental observations.     

Fundamentals of Adhesion Failure for a Model Adhesive (PMMA/Glass) Joint in Humid Environments

The origins for the abrupt adhesion loss at a critical relative humidity (RH) for polymeric adhesives bonded to inorganic surfaces were explored using a poly(methyl methacrylate) (PMMA) film on silicon oxide as a model system. The interfacial and bulk water concentrations within the polymer film were quantified as a function of D₂O partial pressure using neutron reflectivity. The adhesive fracture energies of these PMMA/SiO₂ interfaces at the same conditions were determined using a shaft-loaded blister test. Discontinuities in the adhesive fracture energy, bulk moisture solubility, and the width of the interfacial moisture excess near the interface were observed at the critical RH. A mechanism based on the coupling of bulk swelling-induced stresses with the decreased cohesive strength due to moisture accumulation at the interface is proposed and is consistent with all experimental observations.     

Development of polymer‐molding‐releasing metal mold surfaces with perfluorinated‐group‐containing polymer plating

The polymer‐molding‐releasing properties of metal molds were found to be related to the following factors: (1) interfacial chemical bonding between the surfaces of polymers and metal molds and (2) a friction force or friction coefficient between polar substances and/or low‐molecular‐weight components in the polymers and physical factors on mold surfaces. We theoretically and experimentally confirmed that metal molds with good polymer‐molding‐releasing properties had very small surface free energies. We also proved that the surface free energies in the resulting polymer moldings were lower than before shaping. The molding releasing properties improved with decreasing friction force and friction coefficient between the surface of polymers and metal molds and with decreasing surface free energy. To obtain metal molds with lower surface free energies, we developed a polymer plating method with perfluorinated‐group‐containing triazine dithiol. The Metal mold treated by polymer plating had lower critical surface tension (7.5 mJ/m²) than Teflon (18 mJ/m²), indicating that the surface consisted of CF₃ ? groups. The treated mold showed excellent durability in its releasing properties, which was better than that of the untreated mold. This technique was developed for the production of molds for the Fθ lens and the naturally bright focusing screen. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2549–2556, 2003     

Adhesion and interfacial healing between supramolecular hybrid networks and glass substrates

Adhesion towards glass and interfacial healing of partially supramolecular hybrid polymer networks featuring a range of H-bonds content were investigated through two dedicated adhesion test methods. In a first series of tests, adhesion strength was measured by separating two substrates containing a cured inner resin layer, and shown to decrease with increasing H-bonds content in the polymer network (from 0 to 50%) as the mechanical strength of the polymer also decreased while the failure mechanism shifted from adhesive to cohesive due to the possibility to form hydrogen bonds with glass substrates. In a second step, the test was used to evaluate interface restoration through healing of the polymer matrices and results showed an increased from none to a tensile strength recovery up to 70% after 1 h healing time for the 50% H-bond polymer. Then, self-adhesion of freshly cut polymer surfaces to glass substrates was investigated, showing increasing tack with increasing H-bonds content. The influence of glass surface treatments on adhesion and interfacial recovery properties was also explored: while aminosilanes did not influence the interfacial behavior of partially supramolecular self-healing polymers towards glass, trimethoxy (octadecyl)silane (ODS) modification strongly hindered their adhesion abilities, further highlighting the fundamental role of hydrogen bonds interaction with the substrates.     

Enhanced adhesion of silica for epoxy molding compounds (EMCs) by plasma polymer coatings

Silica for epoxy molding compounds (EMCs) was coated via plasma polymerization using an RF plasma (13.56 MHz) as a function of the plasma power, gas pressure, and treatment time. The monomers utilized for the plasma polymer coatings were 1,3-diaminopropane, allylamine, pyrrole, 1,2-epoxy-5-hexene, allyl mercaptan, and allyl alcohol. The EMC samples were prepared from biphenyl epoxy resin, phenol novolac, triphenyl phosphine, and plasma polymer-coated silica, and the loading of silica was controlled to 60 wt%. The EMC samples were cured at 175°C for 4 h and subjected to T_g, CTE, and water absorption measurements. The adhesion of silica to epoxy resin was evaluated by measuring the flexural strength of EMC samples and the fracture surfaces were analyzed by SEM. Plasma polymer coatings were also characterized by FT-IR and coating thickness measurements. The plasma polymer coating of silica with 1,3-diaminopropane and allylamine enhanced the flexural strength of EMC samples (167 and 165 MPa), compared with the control sample (140 MPa), and exhibited a higher T_g, a lower CTE, and lower water absorption. The enhanced properties with 1,3-diaminopropane and allylamine plasma polymer coatings can be attributed to the amine functional groups in the plasma polymer coatings.