Microstructure,adhesion strength and failure path at a polymer/roughened metal interface

Metals and polymers are extensively used in microelectronics packaging where they are joined together. Since both the yield and reliability of packages are strongly affected by the interfacial adhesion between polymers and metals, extensive studies have been performed in order to improve the resistance to debonding of many resulting interfaces. In the present work, the interfacial fracture energy of representative polymer/metal interfaces commonly encountered in micoroelectronics packaging was characterized. A copper-based alloy leadframe was used as the metal and an epoxy molding compound (EMC) was used as the polymer. The leadframe surfaces were roughened by chemical oxidation in a hot alkaline solution and molded with the EMC. In general, roughening of metal surfaces enhances their adhesion to polymers by mechanical interlocking, yet often produces a cohesive failure in the polymer. Sandwiched double-cantilever beam (SDCB) specimens were employed to measure the adhesion strength in terms of interfacial fracture energy. After the adhesion test, the microstructures of metal surfaces before molding with the EMC were correlated to the adhesion strength, and the fracture surfaces were analyzed using various techniques to determine the failure path.     

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.     

Degradation and recovery of adhesion properties of deformed metal–polymer interfaces studied by laser induced delamination

Adhesion properties of polymer coatings on metals are of great interest in various industrial applications, including packaging of food and drinks. Particular interest is focused on polymer–metal interfaces that are subjected to significant deformations during manufacturing process. In this work steel samples laminated with polyethylene terephthalate (PET) were subjected to uniaxial tensile deformations followed by annealing treatments. The measurements have demonstrated degradation of adhesion of the metal–polymer interface as the strain introduced by the deformation increased. Moreover, it was observed that within the geometry used in the experiments tensile deformations of the metal substrate introduced in-plane compressive stresses in the bulk of the coating. After applying a thermal treatment restoration of the adhesion has been achieved.Laser induced delamination technique was used to monitor the adhesion properties. In this technique a coating is subjected to a series of infrared laser pulses with a stepwise increase of intensity. Upon increasing the laser pulse intensity, the pressure which is formed inside the blisters reaches a critical value, resulting in further delamination of the coating. To process the experimental data an elastic model was developed. From the analysis of the experimental data the critical stresses required for the delamination and the practical work of adhesion are derived. The model accounts for the compressive in-plane stress present in the coating of the deformed samples.     

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.     

The Effect of Immersion on the Breaking Force and Failure Locus in an Epoxy/Mild Steel System

This paper presents the effects of immersion on the adhesion behavior in a polyamide-cured epoxy system immersed in sodium chloride electrolyte adjusted to three different pH values. The strength of lap shear joints was measured before and after exposure and after redrying. The failure locus was determined on a macroscopic and microscopic level. It was found that a large adhesion loss occurred upon immersion. Most of that loss was recovered upon redrying. All of the breaking force was recovered when the immersion fluid was distilled water. The locus of failure was primarily through the bulk of the adhesive before immersion. After immersion the failure was interfacial with a thin residue of polymer remaining on the metal surface. These results are discussed with respect to earlier work on the water absorption properties of the system.     

The Effect of Immersion on the Breaking Force and Failure Locus in an Epoxy/Mild Steel System

This paper presents the effects of immersion on the adhesion behavior in a polyamide-cured epoxy system immersed in sodium chloride electrolyte adjusted to three different pH values. The strength of lap shear joints was measured before and after exposure and after redrying. The failure locus was determined on a macroscopic and microscopic level. It was found that a large adhesion loss occurred upon immersion. Most of that loss was recovered upon redrying. All of the breaking force was recovered when the immersion fluid was distilled water. The locus of failure was primarily through the bulk of the adhesive before immersion. After immersion the failure was interfacial with a thin residue of polymer remaining on the metal surface. These results are discussed with respect to earlier work on the water absorption properties of the system.     

The adhesion between electrolessly deposited copper and a photoimageable polymer

We investigated the relationship between the peel adhesion of copper, deposited by electroless plating, to a photoimageable polymer and the time of chemical etching before plating. Mechanical interlocking is generally considered as being the adhesion mechanism between deposited metals and substrates. However, we found that the peel strength decreased with an increase in the etching time though the polymer roughness did not change. The glass transition temperature of the photoimageable polymer became lower as the etching time increased. The pretreatment not only roughened the surface of the photoimageable polymer, but also affected the bulk polymer and the adhesion.     

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.     

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.     

A study of the adhesion between steel and an ethylene-acrylic acid-acrylate terpolymer

The interfacial interaction of an ethylene-acrylic acid-t-butyl acrylate (EAA) terpolymer with metals (mild steel) has been studied by spectroscopic techniques (XPS, AES, and IR), SEM, and mechanical testing. Using peel tests it was shown that the copolymerization of small quantities of acrylic acid and t-butyl acrylate with ethylene induces polymer/metal adhesion values rather higher than in plain polyethylene. The fracture surfaces were analyzed by XPS, AES, and SEM. It was shown that the fracture is cohesive and occurs within the polymer. Some specimens were aged under various conditions. In this case, the failure was located at the polymer/metal interface. In fact, SEM, XPS, and AES did not show the presence of polymer on the metal surface. Iron corrosion is then considered responsible for the failure of the joint. Since a thin layer of metal oxide covers the surface of mild steel, so to understand the polymer/ metal interfacial interaction better, IR spectroscopy was performed on polymer/metal oxide (goethite and hematite) composites. It was shown that the carboxyl groups of the polymer react with hydroxyl groups present at the metal surface. Adsorption isotherms of the polymer on iron oxides were also obtained, showing remarkable EAA terpolymer chemisorption, while there was no evidence of bonding between iron oxides and plain polyethylene.     

Relationship between interfacial phenomena and viscoelastic behavior of laminated materials

A literature review shows that the main arguments used to describe viscoelastic behavior of polymer composites are the existence of an interphase and/or physico-chemical matrix-reinforcement interactions. The purpose of this investigation was to study the influence of both of these parameters on the viscoelastic behavior of a sandwich structure. Using a theoretical approach of the mechanical coupling between phases in laminate composites, the interphase influence is shown to be negligible. In order to understand the influence of an interphase on viscoelastic features of laminates, some metal/polymer/metal laminates were processed under various conditions to obtain different degrees of metal/polymer adhesion. Dynamic mechanical spectroscopy tests reveal that both the amplitude of the main loss factor peak and the low temperature apparent modulus increase with the adhesion. Finite elements calculations show that discontinuities of displacements at the metal/polymer interface explain the loss peak changes. The continuity of displacements is ensured only from a threshold value of the peel energy.     

Metal/polymer interfaces with designed morphologies

The morphology of a metal/polymer interface is important for many properties, e.g. its adhesional strength. Starting from the basic processes occurring in the initial stages of metal/polymer interface formation, it is possible to obtain different morphologies by variation of the preparation conditions. In this report we present selected examples from our own work of how metal/polymer interfaces with different morphologies can be prepared by evaporating noble metals (Au, Ag, Cu) onto chemically different polymers, i.e. bisphenol-trimethyl cyclohexane polycarbonate (TMC-PC), pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide (PI), and on Teflon AF 1601. The interfaces were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The combination of these techniques allows one to determine morphological parameters such as the concentration and distribution of metal clusters at the surface and in the near-surface region. Using low deposition rates and elevated temperatures, spread-out metal/polymer interfaces can be formed, whereas the use of high deposition rates and moderate temperatures results in relatively sharp interfaces. Another approach to obtain a defined morphology is to form large metal clusters of 10-30 nm diameter on the polymer surface and embed them into the polymer in a controlled manner by a subsequent annealing process. First experiments on the macroscopic adhesion of Au and Cu on TMC-PC showed that the initially low peel strength could be increased substantially by subsequent annealing above the glass transition temperature.     

Surface and interface charge effects in metal-polymer adhesion

The objective of the present work was to study the electrostatic component of adhesion in metal film-polymer systems under low energy (some keV) photon irradiation. In this energy region electrostatic forces can be influenced directly by fast secondary electrons. Quantum yields of such electrons for A1, Ag, Au, polyethylene, Mylar, PVC, and Teflon were calculated with the special Monte Carlo code and used as input for an analytical model of the charge profile formation near the metal film-polymer interface. The dependence of adhesion characteristics on photon energy and atomic number of materials (metal films, polymer substrates) is discussed.     

An FT-IR Study on Interfacial Interactions in Ethylene Copolymers/Aluminium Laminates in Relation to Adhesion Properties

The effect of three different functional groups in ethylene copolymers on the adhesion with aluminium was studied. The interface in polymer/metal laminates was analyzed by FT-IR, and the adhesion mechanism for each functional group was evaluated. Laminate samples were prepared by solution casting or by hotpressing polymeric film onto the aluminium substrate. In the latter case, the interface was exposed by solvent extraction. The interfacial structures developed by the different copolymers were correlated to the mechanical strength of hotpressed laminates, which was measured by a peel test. The polymer surfaces were further characterized by contact angle measurements.

Polar functional groups, carboxylic acid and butyl ester in hotpressed laminates were found to form Lewis acid/base interactions with the aluminium oxide. The strength of the interfacial interactions was correlated to the concentration and acidity/basicity of the group, the acid group being the most efficient. A silane functional group provided strong adhesion to the laminates at a much lower concentration than the polar groups. Silanols as well as Al-O-Si linkages were detected at the polymer/aluminium interface.     

An FT-IR Study on Interfacial Interactions in Ethylene Copolymers/Aluminium Laminates in Relation to Adhesion Properties

The effect of three different functional groups in ethylene copolymers on the adhesion with aluminium was studied. The interface in polymer/metal laminates was analyzed by FT-IR, and the adhesion mechanism for each functional group was evaluated. Laminate samples were prepared by solution casting or by hotpressing polymeric film onto the aluminium substrate. In the latter case, the interface was exposed by solvent extraction. The interfacial structures developed by the different copolymers were correlated to the mechanical strength of hotpressed laminates, which was measured by a peel test. The polymer surfaces were further characterized by contact angle measurements.

Polar functional groups, carboxylic acid and butyl ester in hotpressed laminates were found to form Lewis acid/base interactions with the aluminium oxide. The strength of the interfacial interactions was correlated to the concentration and acidity/basicity of the group, the acid group being the most efficient. A silane functional group provided strong adhesion to the laminates at a much lower concentration than the polar groups. Silanols as well as Al-O-Si linkages were detected at the polymer/aluminium interface.     

Plasma polymerization coating of silica fillers for Epoxy Molding Compounds (EMCs)

Silica fillers for Epoxy Molding Compounds (EMCs) were modified via plasma polymerization coating of acrylonitile, acrylic acid and dimethyl phosphite with RF plasma (13.56 MHz). The resulting samples were characterized by DSC, FT-IR and contact angle measurements. EMC samples were prepared from silica fillers, biphenyl epoxy resin, phenol novolac and triphenyl phosphine, and cured at 175°C for 4 h. Flexural strength of the EMC samples was evaluated in a 3-point bending mode with an Instron 5567 at a crosshead speed of 1 mm/min both at RT and 250°C, and failure surfaces were analyzed by SEM. Some samples were exposed to 121°C, 2 atm pressure and 100% RH for 12, 24 and 32 h, and then to 250°C for 10 min prior to testing at RT. Plasma polymer coating of silica with acrylonitrile greatly improved the flexural strength of EMC at RT as well as at 250°C, followed by acrylic acid and dimethyl phosphite. Exposing EMC samples to 121 °C, 2 atm pressure and 100% RH for 32 h decreased the flexural strength by 13% when the silica was coated with acrylonitrile plasma polymer, compared to the 21% decrease in the control sample. Plasma polymer coating of silica also increased the Tg of the EMC, and lowered water absorption and CTE in the rubbery region. Therefore, enhanced properties by plasma polymer coating of silica with acrylonitrile or acrylic acid can be attributed to nitrile or carboxylic acid groups, as confirmed by FT-IR, which can react with epoxy groups in the base resin, as evidenced by DSC analysis.     

金属胶接接头劈裂破坏的模型分析

对金属胶接接头劈裂强度测试方法和影响劈裂强度的主要因素的作用进行了分析与讨论。本文提出了金属胶接接头劈裂破坏模型，对完善劈裂强度测试方法提出了改进意见．     

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.     

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.     

Effect of moisture‐ingress on adhesion energy in a metal oxide‐polymer system

This work quantifies the damage caused by moisture in a metal coating system under extreme weathering conditions, using Variable Radius Roll Adhesion Test (VaRRAT). Interfacial toughness (adhesion energy) between the metal oxide and the polymer in painted steel panels, studied by using VaRRAT, is observed to fall with increasing temperature and time of exposure to moisture. Possible cause for irreversible loss in adhesion energy in the paint system is attributed to the sorption of free water at the metal oxide–polymer interface. Different failure responses were observed in two different paint–metal systems. Adsorption or diffusion in the Henry's mode is rate controlling in green paints as indicated by the low activation energy of 12 kJ mol^?1. The white samples showed a high activation energy of 30 kJ mol^?1, indicating a mixed process of diffusion as well as chemical to be rate determining. Different paint/binder ratios are responsible for the different responses of these samples. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006