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.     

Oxidation of the copper alloy with pure copper plating

The oxidation failure of a copper alloy lead frame with/without a copper plating layer was investigated. The oxidation rate and adhesion strength of oxide films on copper alloy substrates were studied by measuring the thickness and by carrying out peel tests. The adhesion strength of the oxide film was mainly influenced by the composition but not the thickness of the oxide film. The highest adhesion strength was obtained when the oxide film was composed mainly of Cu₂O. When the thickness of the copper preplated layer was over 0.165?μm, the Cu atoms of the preplated copper were available for oxidation. Thus the oxidation process was within the copper preplated layer, and the main product of the oxidation was Cu₂O. It was found that the large column grain of the oxide film on the copper alloy with a copper plated layer, favored the diffusion of copper or oxygen atoms that led to the formation of Cu₂O, and lead to higher adhesion strength. This indicated that the oxidation resistance of a copper alloy lead frame can be effectively improved by electroplating copper.     

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.     

Adhesion improvement of epoxy resin/copper lead frame j oints by azole compounds

The adhesion strength of epoxy resin/copper joints is often very poor, due to the naturally formed copper oxide having a low mechanical strength. To improve the adhesion strength of epoxy resin/copper lead frame joints, copper lead frames were treated with azole compounds as adhesion promoters. The azole compounds used were benzotriazole (BTA), benzotriazole-5-carboxylic acid (CBTA), 8-azaadenine, imidazole, 2-methyl imidazole, urocanic acid, adenine, benzimidazole, and polybenzimidazole (PBI). The dependence of the adhesion strength of epoxy resin/azole-treated copper joints on the structure of the azole compound, the azole treatment time, and the azole treatment temperature was investigated. The surface coverage of azole-treated copper was examined by contact angle measurements, a surface defect test, optical microscopy, and scanning electron microscopy (SEM), and the locus of failure was studied by X-ray photoelectron spectroscopy (XPS). Triazole compounds such as CBTA and 8-azaadenine showed excellent adhesion strength; imidazole-based azole compounds did not improve the adhesion strength. However, the adhesion strength of CBTA- and 8-azaadenine-treated joints decreased with increasing treatment time, since thick porous Cu-azole complexes had a weaker mechanical strength when formed. The polymeric azole compound PBI showed the highest adhesion strength, 785 N/m, because of complete coverage of the copper surface. The thermal stability of azole compounds and epoxy resin/azole-treated copper joints was also investigated. CBTA and 8-azaadenine did not decompose up to 250°C, while PBI was stable up to 500°C in an air atmosphere.     

Surface characterization and adhesion of black-oxide-coated copper substrate: effect of surface hardening processes

The effects of surface-hardening processes on the changes in surface characteristics and adhesion of black copper oxide substrate with epoxy resins are studied. Various techniques, namely SEM, XPS, AFM, XRD, Auger electron spectroscopy, contact angle goniometry, D-SIMS and RBS, were used to identify the changes in surface characteristics. Dense, fibrillar cupric oxide crystals characterized the as-deposited oxide coating with high surface roughness. The surface-hardening process flattened and consolidated the fibrils without changing the compositional and thermodynamic characteristics of the coated surface. The surface-hardening process reduced the total thickness of copper oxide by approximately 50–150 nm. The reduction in oxide thickness was not a predominant factor for the reduced bond strength of the surface-hardened coating. The bond strengths of both the as-deposited and surface-hardened black oxide coatings increased with oxidation time, until saturation at about 120–150 s. For the as-deposited oxide coating, mechanical interlocking, high wettability and resistance to surface contamination are the three major sources for improved adhesion, amongst which the enhanced mechanical interlocking provided by the fibrillar cupric oxide is the most important. Surface hardening reduced the efficiency of mechanical interlocking mechanism. There was close functional dependence between the button-shear strength and surface characteristics, such as surface roughness, coating thickness and surface free energy.     

Surface characterization of pre-treated copper foil used for PCB lamination

Properly micro-roughened electrodeposited copper foil is used in the conventional lamination process in order to improve its bond strength. In this investigation other treatments, including pumice scrubbing, chemical etching and brush scrubbing methods, were employed in order to obtain strong bonding. The effects of these treatments are investigated in terms of copper surface morphology using optical profilometry (OP), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The microstructure of the electrodeposited copper foil surface and its bonding properties are discussed in terms of various experimental results, in order to compare it with rolled annealed copper foil. Various surface morphologies of copper foil corresponding to different treatments are observed. The pumice scrubbing showed the largest increase in copper surface roughness, which leads to the highest improvement in bonding properties. The bond strength between copper and FR-4 resin substrate was analyzed by peel strength measurements, and based on this, the optimized process to treat the copper surface is proposed.     

Surface characteristics and adhesion performance of black oxide coated copper substrates with epoxy resins

Black oxide is a conversion coating applied onto the copper substrate to improve its interfacial adhesion with polymeric adhesives. A comprehensive study is made to characterize the black oxide coating using various characterization techniques, including SEM, XPS, AFM, XRD, Auger electron spectroscopy, TEM, D-SIMS, RBS and contact angle measurements. It was found that the oxide coating consisted of cupric and cuprous oxide layers from the top surface to inside. The cuprous oxide layer was formed on the copper crystal surface, on which densely-packed fibrillar cupric oxide grew continuously until saturation. The cupric oxide had a fibrillar structure with high roughness at the nanoscopic scale, whereas the cuprous oxide was rather flat and granular. There was a continuous change in oxide composition with no distinct boundary between the two oxide layers. The bond strength between the epoxy resin and the oxide coated copper substrate increased rapidly at a low level of oxide thickness, and became saturated at thicknesses greater than about 800 nm. There were similar dependences of bond strength on surface roughness, oxide thickness especially of cupric oxide and surface energy, reflecting the importance of these surface characteristics in controlling the interfacial adhesion.     

Studies on the adhesion of polyimide coatings on copper foil

The peel strength and the color of the copper foil peeled at 90 degrees from five different polyimide films were studied. The interfacial surfaces of copper foil and polyimide were examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy dispersion analysis by X-ray (EDAX). There is a correlation between peel strength, and the color of the interfacial side copper caused by oxygen diffusion. Study of the imidization process carried out in vacuum indicates that the geometric arrangements of the atoms of polyimide also play a very important role in peel strength.     

Study on brown oxidation process with imidazole group,mercapto group and heterocyclic compounds in printed circuit board industry

The adhesion strength of the interface between copper foil and resin is an important technological parameter for applications in microelectronics. In this study, a new brown oxidation solution of copper foil, including the recipe composition and reliability tests, was fully discussed. We provided an overview of brown oxidation process used in the semi-flexible printed circuit boards production industry by investigating the brown oxide film. The morphology of the copper oxide film was changed from lamellar structure to honeycomb structure with the increasing of oxidation time. The peel adhesion strength of the Cu/polyimide laminates was increased from about 2–16 N/cm by altering the immersion time and the concentration of inhibitors in brown oxidation solution. Scanning electron microscopy, peel tests and X-ray diffraction indicated that the higher adhesion strength was resulted from the rougher surface and the proper etching depth of copper foil, which was caused by chemical reactions on the interface surface of copper foil.     

Effect of laminating parameters on the adhesion property of flexible copper clad laminate with adhesive layer

In this study, the effect of two laminating parameters, laminating pressure and holding time, on the adhesion strength of flexible copper clad laminate (FCCL) with an epoxy-type adhesive layer was evaluated. The changes in the adhesion property, fracture surface, morphology, and chemical bonding state were characterized by 90° peel test, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The results showed that the adhesion strength of the FCCL was decreased as the laminating pressure was increased beyond the suitable level. Laminating pressure exerted the greatest influence over the FCCL adhesion strength. On the other hand, the holding time did not significantly affect the peel strength of the FCCL. The fracture of FCCL occurred at the interface between the rolled Cu and the adhesive layer. In addition, the FCCL with high adhesion strength was stable with the variation of adhesion strength after temperature and humidity test.     

Surface modification of polyimide film by coupling reaction for copper metallization

In this study, Upilex-S [poly(biphenyl dianhydride-p-phenylene diamine)], one of polyimide films, was modified by coupling reactions with N,N-carbonyldiimidazole (CDI) to increase adhesion to copper for flexible copper clad laminate (FCCL). Imidazole groups show strong interaction with copper metal to make charge transfer complexes. Because polyimide film did not have active site with coupling agent, the film surfaces were modified by aqueous KOH solutions and reacted with dilute HCl solutions.Surface modified Upilex-S was analyzed by X-ray photoelectron spectroscopy (XPS) to examine the surface chemical composition and film morphology and investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Changes in the wettability were evaluated by measuring contact angle with the sessile drop method. After deposition of copper on surface modified Upilx-S, the adhesion strength of the copper/polyimide system was measured by a 90° peel test using the Instron tensile strength tester. The peel strength of the copper/polyimide system increased from 0.25 to 0.86 kg_f/cm by surface modification. This result confirmed that the CDI coupling reaction is an effective treatment method for the improvement of the adhesion property between copper metal and polyimide film.     

Temperature effect on PI/Cu interface

The interfacial reaction and peel strength of polyimide with copper foil at various cure schedules have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and peel test to determine the temperature effect on polyimide/copper interface diffusion and adhesion. SEM studies indicate that the polyimide/copper interfaces are fairly smooth for all samples studied in this experiment. The TEM microstructure reveals the existence of a copper-polyimide interaction zone at the interface when it is cured at a temperature higher than 250°C, which also results in a high peel strength. XPS spectra revealed higher copper and carbonyl carbon contents at the polyimide interface when it is cured at a high-temperature schedule (350°C). From the results of these interface studies, it is concluded that chemical bonding resulting from the interaction of copper oxide and polyimide carbonyl group provides the binding force for polyimide and copper foil. © 1993 John Wiley & Sons, Inc.     

Effect of BN or B4C content on physical and tribological properties of copper-clad laminates reinforced with carbon fiber and epoxy resin

Plasma treatment was used to improve the surface roughness of copper foil. The copper-clad laminates reinforced with carbon fiber, boron nitride (BN), or boron carbide (B₄C), and epoxy resin were prepared by hot pressing. The effect of BN or B₄C content on the physical properties and tribological properties of copper-clad laminates reinforced with carbon fiber and epoxy resin were studied. The resulting copper-clad laminate exhibited desirable properties, such as dielectric constant, peel strength, oxygen index, and arc resistance, which were influenced by the concentration of BN or B₄C particles. Additionally, the wear and friction properties of the laminate were evaluated, revealing the effects of load, sliding speed, and particle content on weight loss, specific wear rate, and coefficient of friction. SEM analysis of worn surfaces provided insight into the stages of wear, highlighting the importance of an oxide layer in reducing wear and protecting the copper surface.     

A New Approach to the Copper/Epoxy Joint Using Atmospheric Pressure Glow Discharge

A new approach for adhering copper to an epoxy resin was studied. In this new approach, the copper surface was first treated with hydrogen plasma generated by the atmospheric pressure glow (APG) discharge. Then a thin film of γ-aminopropyltriethoxysilane (γ-APS) was formed on the treated copper surface. The copper oxide formed by air on the copper surface deteriorated the adhesion by forming a weak boundary layer, part of which could separate from the surface. This oxide layer was reduced when an APG hydrogen plasma was applied for a couple of minutes at a frequency of 13.56 MHz and a power input of 200 W. The resulting peel strength at the copper/epoxy interface increased up to ca. 0.9 Kg/cm. Curing temperature of γ-APS was also an important factor in obtaining good adhesion at the copper/epoxy interface, with the highest value of peel strength occurring at a curing temperature of 120°C.     

Analysis of the Molecular Structure at the PPS/Copper Interphase and its Role in Adhesion

X-ray photoelectron spectroscopy (XPS) was used to examine the interfacial chemistry in polyphenylene sulfide (PPS)/copper bonded laminates. Several surface pretreatments were studied including a simple methanol wash, two acid etches, thermal oxidation and chemical oxidation. Peel test analysis showed poor adhesion to the methanol-washed and acid-etched foils, giving a peel strength of only 3-5 g/mm. XPS analysis of the failure surfaces revealed a large amount of inorganic sulfide at the interface with reduction of the copper oxide. Chemical oxidation using an alkaline potassium persulfate solution gave a matt-black surface consisting of primarily cupric oxide. These samples showed improved adhesion and XPS analysis of the failure surfaces revealed fracture through a mixed PPS/cuprous oxide layer. A simple thermal oxidation yielded a cuprous oxide surface layer and laminates bonded to these surfaces showed a more than ten-fold increase in peel strength. XPS analysis of the failure surfaces showed much lower amounts of interfacial copper sulfide and it was postulated that excess sulfide at the interface was responsible for the poor adhesion observed for other pretreatments.     

Improving interfacial adhesion between copper foil and resin using amino acid in printed circuit board industry

The corrosion inhibitor of nitrogen-containing amino acid histidine (His) has been applied in brown oxidation solutions and the relationships between brown oxidation solutions with the various concentrations of His and the adhesion strength of copper/resin laminates have been systematically studied by various testing techniques, including electrochemical impedance spectroscopy (EIS), peeling strength and field emission scanning electronic microscope. The result obtained from EIS suggested that His exhibited excellent anti-corrosive performance which contributed to a higher stability of the organic metallic film post brown oxidation process treatment. The roughness surface like honeycomb structure was gained and peel strength of the Cu/resin laminates was raised up to 0.71 kg/cm by altering the concentration of His in brown oxidation solutions. Moreover, theoretical calculations manifested that the studied inhibitor was almost adsorbed in parallel on the copper surface and X-ray photoelectron spectroscopy (XPS) declared that the value of adhesion strength was related to the surface chemistry of copper foil and resins after the lamination process which may attribute to chemical reactions at the copper/resin interface.     

Effect of copper oxides on the thermal oxidative degradation of the epoxy resin

The effect of copper oxides on the thermal oxidative degradation of a brominated epoxy resin–dicyandiamide system was studied using Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). The addition of small amounts of Cu₂O or CuO fillers to the epoxy resins affected the relative amounts of highly reacted cyclic species formed during thermal aging and induced catalytic degradation of the epoxy resins. The overall and initial activation energies of the degradation process were found to decrease, and the order in the degradation kinetics of the epoxy resin changed from a near zero order to negative domain (autocatalytic nature) in the presence of copper oxides. © 1994 John Wiley & Sons, Inc.     

Spray Deposition Methods for Improving Interfacial Strength of Copper–Epoxy Systems

A simple spray method using a plain orifice atomizer has been developed for depositing γ-aminopropyltriethoxysilane (APS) from solutions in water and in methanol onto copper surfaces. The evaporative patterns of the sprayed droplets were studied to determine the distribution of deposited APS and the percent coverage of the surface. The peel strengths between copper foil and epoxy resin were measured with and without APS deposition. It was shown that the application of APS resulted in a considerable increase in interfacial adhesion. APS applied from a 1 wt% solution in methanol resulted in a higher peel strength than when applied from a 1 wt% aqueous solution; the opposite was true with 0.2 wt% APS solutions, indicating a trade-off between deposited APS film thickness and surface coverage. In all cases, a higher concentration of APS gives a higher peel strength. APS was very effective when chemisorption occurred at the surface but much less effective when only physisorption took place. A study of the fracture surfaces showed cohesive failure inside the epoxy layer, and that the deposited APS on the copper surfaces had a long-range effect which was seen deep into the epoxy layer, well away from the copper surface.     

The Effect of Tungsten Sulfide Fullerene-Like Nanoparticles on the Toughness of Epoxy Adhesives

In this work the effect of inorganic fullerene-like (closed cages) nanoparticles of tungsten disulfide (IF-WS₂) on the mechanical properties and especially on the toughness of epoxy resins, was studied. The epoxy resin used was the well-known DGEBA (di-glycidyl ether of bis-phenol A) cured with polyamidoamine. The epoxy/IF-WS₂ nanocomposites were prepared by applying a high shear mixing to obtain a uniform dispersion and homogeneous distribution of the IF nanoparticles in the epoxy matrix. Two mixing procedures were used — a low shear of short duration and high shear with a long mixing time. The resulting epoxy nanocomposites were first characterized for their shear and peel strength using appropriate bonded joints. The experimental results demonstrate that enhanced shear strengths and shear moduli were achieved, together with a significant increase in the peel strengths at low concentrations of the IF-WS₂ nanoparticles (more than 100% increase at 0.5 wt% IF-WS₂). Above the threshold value of 0.5% IF-WS₂ the peel strength decreased sharply. The fractured surfaces of the bonded joints were examined by transmission and scanning electron microscopy in order to characterize the fracture mechanisms and analyze the dispersion level of the nanoparticles within the polymer. The electron micrographs indicated that the presence of the nanoparticles in the epoxy matrix induced fracture mechanisms which were different from those observed in the pristine epoxy phase. These mechanisms included: crack deflection; crack bowing; and crack pinning. Evidence for a chemical interaction between the nanoparticles and the epoxy were obtained by infrared measurements in the attenuated total transmittance mode. The data suggests the formation of new carbon–oxygen–sulfur bonds, which are most likely due to the reaction of the outermost sulfur layer of the IF nanoparticles with the reactive epoxy groups. The observed simultaneous increase in both shear and peel strengths at very low IF-WS₂ concentrations, found in this work, could lead to the development of high performance adhesives and to new types of structural and ballistic fiber nanocomposites.     

含有互穿网络结构的丁腈橡胶改性环氧/酚醛胶粘剂的合成及其应用

合成了一种含有互穿网络结构的丁腈橡胶(BN)改性环氧/酚醛胶粘剂。通过扫描电镜(SEM)分析, 证明了此胶片具有互穿网络结构。同时对胶粘剂进行了差热分析(DTA),并考察了其粘结性能以及贮存性能。最后将其成功地应用到一种厚铜箔(0.4～0.5mm)环氧玻璃布层压板上,得到的覆铜板样品具有剥离强度高、平整度好、电性能优良等特点。