Electroless Copper Films Research Articles

Enhancing Adhesion of Electroless Copper Film on Smooth Polyimide Surfaces by Photocatalytic Oxidation

The application of advanced high-frequency (5 G) requires a low-profile Cu foil to reduce the high-speed signal loss due to skin effect. However, the adhesion of electroless copper film on the smooth surface of polyimides (PI) remains a challenge. Here, a two-step composite modification method, consisting of TiO2 photocatalysis and Na2S2O8 photooxidation treatment, is successfully applied to induce a high adhesion strength of electroless copper film on the smooth PI surface. To meet the requirements of the flexible electronics, the average surface roughness of the PI film is increased from 4.2 to 11.5 nm through optimizing the modification. Whereas the surface contact angle decreases from the original 76.9° to 18.7° and the adhesion strength between the electroless copper film and PI film is increased from 0 to 0.90 KN m−1. Spectroscopic investigations reveal that the amide bonds on the surface PI get oxidized by radicals, while an abundant –COOH and –NH2 groups form. The high oxidation potential, strong stability of the sulfate anion radicals, as along with their increased concentration in the treatment solution are contributed to the extraordinary modification performance. Therefore, the TiO2 photocatalysis and Na2S2O8 photooxidation composite treatment is an effective and environmentally friendly method for surface modification of PI films.

Adhesion improvement of ABS resin by MnO2-H3PO4-H2SO4 colloid with ultrasound-assisted etching treatment

The adhesion strength between electroless copper and acrylonitrile-butadiene-styrene (ABS) resin could be improved significantly by MnO2-H3PO4-H2SO4 colloid system. In this paper, when the concentrations of H3PO4 and H2SO4 in the colloid were 4.2 and 10.4 M, respectively, the effect of the ultrasound-assisted etching (UAE) time on the etching performance of ABS resin were investigated. With the UAE for 10 min, much more uniform cavities formed on the ABS surface, the average surface roughness (Ra ) decreased from 143 to 107 nm, which was much lower than etched by CrO3-H2SO4 colloid (420 nm). Though the average surface roughness (Ra ) was decreased, the average adhesion strength between the electroless copper film and ABS substrate increased from 1.39 to 1.60 kN m−1, which was also higher than chromic acid etching treatment (1.42 kN m−1). The density of the hydrophilic groups and the uniformity of cavities on the ABS surface were increased with the UAE, which resulted in high adhesion strength and low average surface roughness.

Surface modification of polyimide by combining swelling and TiO2 photocatalytic treatments for adhesion improvement of electroless Cu

In order to enhance the adhesion strength between the PI film and the electroless copper film, a combination of swelling and TiO2 photocatalytic treatments was used to modify polyimide (PI) film. The effects of the swelling solution composition and TiO2 photocatalytic condition on the surface performance were investigated. After the optimal swelling and photocatalytic treatment, the surface contact angle of the PI film decreased from 85 to 28.7°, and the surface average roughness of the PI film only increased from 1.3 to 13.6 nm, indicating no obvious change for the surface topography of PI film after the photocatalytic treatment. However, the adhesion strength between electroless copper film and the PI film reached to 0.6 KN·m−1. The FT-IR spectra and XPS analyses indicated that –COOH group was formed on the PI surface after the treatment, and the surface hydrophilicity was improved, which improved the adhesion strength between the PI film and the electroless copper film.

Photocatalytic Surface Modification of PI Film for Electroless Copper Plating

This study investigated the surface modification of polyimide (PI) film through TiO2 photocatalytic treatment. The effects of TiO2 content, treatment duration, and UV power on the surface topography, surface contact angle, and adhesion strength of the surface-modified PI films were investigated. The results indicated that, after surface modification under the optimal photocatalytic conditions, the surface contact angle of the PI film decreased from 84.4° to 38.8°, and the adhesion strength between the PI film and the electroless copper film reached 0.78 kN/m. X-ray photoelectron spectroscopy analysis further demonstrated that carboxyl groups formed on the surface of the PI film after photocatalytic treatment. The surface hydrophilicity and adhesion strength of the surface-modified PI film were enhanced due to the numerous carboxyl groups formed on its surface. Therefore, the photocatalytic treatment is an environmentally friendly and effective method for the surface modification of PI films.

Characterization and mechanical testing of polydopamine-adhered electroless copper films

Self-assembling polydopamine has shown great versatility as a multifunctional surface coating. This study investigates the adhesive properties of polydopamine used to bind electroless copper films to several polymer substrates widely used for commercial applications, including polyethylene (PE), polycarbonate (PC), poly(methyl methacrylate) (PMMA), and nylon 6,6. Each substrate was coated in a similar manner resulting in continuous copper films of 52±12nm in thickness. Transmission electron microscopy revealed a non-preferential polycrystalline microstructure of copper grains averaging 53±17nm in diameter with a range of 21nm to 123nm, indicating there is some overlap of grains in the film. Peel testing, tensile tests, and tribological oscillating wear tests were used to characterize the mechanical performance of the adhered copper films. The nylon substrate had the highest peel strength, 49±5mN/mm, while the remaining substrates ranged from 5 to 13mN/mm on average. Coated substrates were strained up to 20% tensile strain to investigate film delamination, buckling, and rupture. Transverse buckling of the films relative to the direction of strain was observed due to Poisson's effect during tension. Films on nylon substrates resisted buckling and rupture up to 20% strain, whereas buckling, delamination, and rupture was observed in the other substrates below 20% strain. Tribological testing revealed that the coefficient of friction was governed by the polymer substrate and was not dependent upon the polydopamine or copper films. As a demonstration of the robust nature of this method, a nylon suture was coated with polydopamine-adhered copper film and tied in a knot to show a large complex deformation state without showing signs of delamination.

Environmentally Friendly Copper Metallization of ABS by Cu-Catalysed Electroless Process

Abstract An environment-friendly surface etching and activation technique for ABS surface metallization were investigated as a replacement for conventional chromic acid etching bath and palladium catalyst. After etching by H2SO4-MnO2 colloid, the ABS surfaces became rough; meanwhile the carboxyl and hydroxyl groups were formed on the surface. With absorption and reduction by a sodium borohydride solution, copper particles were deposited on the ABS surface, which serve as a catalyst replacement for SnCl2/PdCl2 colloid. The effects of CuSO4 concentration, NaBH4 concentration, reduction temperature and reduction time on the adhesion strength between the ABS surface and the electroless copper film were investigated. The average adhesion strengths reaches 0.87 kN·m−1.

The effect of TiO2 morphology on the surface modification of poly (ethylene terephthalate) for electroless plating

In this study, a surface modification of the poly (ethylene terephthalate) (PET) film using TiO2 photocatalytic treatment was investigated. In order to enhance the adhesion strength between the PET film and the electroless copper film, the effects of TiO2 crystal forms, TiO2 particle sizes, and TiO2 content, as well as treatment condition, upon the surface contact angle, surface characterization, and adhesion strength were investigated. Anatase TiO2 with a particle size of 5 nm had a high catalytic activity and dispersibility in aqueous solution. After the optimal photocatalytic treatment, the surface contact angle of the PET film decreased from 84.4° to 19.8°, and the surface roughness of the PET film increased from 36 to 117 nm. The adhesion strength between the PET film and the electroless copper film reached 0.89 KN m−1. X-ray photoelectron spectroscopy analyses indicated the carbonyl group was formed on the PET surface after photocatalytic treatment, and the surface hydrophilicity was improved. Consequently, TiO2 photocatalytic treatment is an environmentally friendly and effective method for the surface modification of the PET film.

The Impact of Hydrogen Gas Evolution on Blister Formation in Electroless Copper Films

Electroless copper plating is a key process to provide electrically conductive layers on insulating substrates for the subsequent copper electroplating of printed circuit boards (PCBs). Recently introduced substrate materials are prone to spontaneous delamination failure (blistering) of the electroless layer during deposi-tion. A higher nickel content in electroless copper baths prevents delamination failure by increasing the internal tensile stress in the copper layer. This effect is achieved by suppressing copper self-diffusion through the incorporation of small amounts of nickel in the grain boundaries and the grain boundary junc-tions. Whether the hydrogen gas that evolves inevitably during electroless copper deposition, and that is potentially trapped in the deposit may cause blistering is still under debate. Here we test this hypothesis by quantifying the generated hydrogen gas volume during copper deposition in baths with nickel concentra-tions between 0 and 1000 ppm, and observing the rate of blister formation in the deposit. We find a con-stant molar ratio of 0.65±0.05 mol H2/mol Cu independent of the nickel content in the bath, in contrast to a systematic and substantial change of the blister probability. From this lack of a correlation between the blister rate and hydrogen evolution, we rule out hydrogen evolution as a determining factor for blister for-mation.

Electroless deposition and evaluation of Cu/multiwalled carbon nanotube composite films on acrylonitrile butadiene styrene resin

Cu/multiwalled carbon nanotube (MWCNT) composite films were fabricated by electroless deposition on acrylonitrile butadiene styrene (ABS) resin substrates. Four types of MWCNTs with different dimensions were added to the electroless copper plating baths together with the dispersants sodium lauryl sulfate and hydroxypropyl cellulose. The ABS resin substrates were first subjected to a roughening treatment to improve film adhesion. The microstructure of the composite films was characterized, and it was found that the MWCNTs were uniformly dispersed. The adhesion strength for the films was as high as that for electroless copper films. The composite films had an electrical resistivity in the range of 2.39–2.72μΩcm, and their coefficient of friction was considerably lower than that for electroless copper films.

Comparative study of electroless copper film on different self-assembled monolayers modified ABS substrate.

Copper films were grown on (3-Mercaptopropyl)trimethoxysilane (MPTMS), (3-Aminopropyl)triethoxysilane (APTES) and 6-(3-(triethoxysilyl)propylamino)-1,3,5- triazine-2,4-dithiol monosodium (TES) self-assembled monolayers (SAMs) modified acrylonitrile-butadiene-styrene (ABS) substrate via electroless copper plating. The copper films were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Their individual deposition rate and contact angle were also investigated to compare the properties of SAMs and electroless copper films. The results indicated that the formation of copper nuclei on the TES-SAMs modified ABS substrate was faster than those on the MPTMS-SAMs and APTES-SAMs modified ABS substrate. SEM images revealed that the copper film on TES-SAM modified ABS substrate was smooth and uniform, and the density of copper nuclei was much higher. Compared with that of TES-SAMs modified resin, the coverage of copper nuclei on MPTMS and APTES modified ABS substrate was very limited and the copper particle size was too big. The adhesion property test demonstrated that all the SAMs enhanced the interfacial interaction between copper plating and ABS substrate. XRD analysis showed that the copper film deposited on SAM-modified ABS substrate had a structure with Cu(111) preferred orientation, and the copper film deposited on TES-SAMs modified ABS substrate is better than that deposited on MPTMS-SAMs or APTES-SAMs modified ABS resins in electromigrtion resistance.

Study of electroless copper plating on ABS resin surface modified by heterocyclic organosilane self-assembled film

6-(3-triethoxysilylpropyl)amino-1, 3, 5-triazine-2, 4-dithiol monosodium (TES) was used to fabricate self-assembled film on corona pretreated acrylonitrile–butadiene–styrene (ABS) resin surface. The self-assembled film modified ABS resin was treated by electroless copper plating. Orthogonal test was carried out to study optimal condition of the process. The surface appearance, plating rate and thickness of electroless copper films were investigated to determine the optimal time of corona-discharge, self-assembly and electroless copper plating. SEM results indicated that porous morphology appeared on ABS resin surface modified by TES self-assembled film and the surface roughness also increased. The adhesion test showed that the adhesion property between ABS resin and copper was excellent. The surface of electroless copper film had high brightness under the optimal condition of 1 min corona-discharge, 30 min self-assembly and 10 min electroless copper plating. The electroless-copper plating temperature was 55 ∼ 60°C and pH was 13 ∼ 13·5.

Surface modification of ABS by photocatalytic treatment for electroless copper plating

Surface roughness of acrylonitrile–butadiene–styrene (ABS) resin prior to metallization is treated generally with sulphuric/chromic acid system. However, the presence of chrominum (VI) ion imposes serious environmental problems. In this work, TiO2 photocatalytic treatment was used to enhance the adhesion strength between the ABS surface and the electroless copper film. Effects of the TiO2 content, irradiation time and UV power upon the surface topography, surface characterization and the adhesion strength were investigated. The results indicated that the surface hydrophilicity of ABS resin and the adhesion strength between the electroless copper film and ABS surface increased with an increase in the UV power and a prolongation in irradiation time, and did not increase linearly with an increase of TiO2 content. Though the surface topography of ABS changed little, the adhesion strength reached 1.25 kN/m, which was higher than that in the optimal H2SO4–MnO2 colloid. The surface chemistry results indicated that –COOH and –OH groups formed with the photocatalytic treatment and the absorption strengths increased with the UV power. XPS analysis results further demonstrated that the contents of C=O and –COOH reached 6.4 and 4.9% with the photocatalytic treatment, which was much higher than that of the H2SO4–MnO2 colloid (3.9 and 3.1%). The high contents of C=O and –COOH groups enhanced the surface hydrophilicity of the ABS resin and improved the adhesion strength between the electroless copper film and ABS resin. The results indicated that the photocatalytic treatment was an environment-friendly and effective method to replace the commercial wet chemical process for ABS surface modification.

A study of the environmentally friendly polycarbonate surface etching system containing H2SO4–MnO2 colloid

In this paper, an environmentally friendly etching system containing H2SO4–MnO2 colloid was used to investigate surface etching for polycarbonate (PC). The effects of swelling condition, H2SO4 concentrations and etching times on surface topography and surface roughness were studied. With the etching treatment, the surface average roughness (R a) of PC substrates increased from 3 to 76 nm and the adhesion strength between the electroless copper and PC substrate reached 1.08 KN/m. Surface chemistry of PC substrates was investigated by the contact angle measurement and X-ray photoelectron spectroscopy spectra (XPS). After the etching treatment, PC surface became hydrophilic and the contact angle decreased from 95.2 to 39.6o. XPS analyses indicate that hydroxyl and carboxyl groups are formed on the PC surface as a result of the etching treatment, which improve the adhesion strength between PC substrate and electroless copper film.

Improvement in the Etching Performance of the Acrylonitrile–Butadiene–Styrene Resin by MnO2–H3PO4–H2SO4 Colloid

The present study aimed to evaluate the surface etching of the acrylonitrile-butadiene-styrene (ABS) resin in the MnO2-H3PO4-H2SO4 colloid. To enhance the soluble Mn(IV) ion concentration and improve the etching performance of ABS resin, H3PO4 was added as a complexing agent into the MnO2-H2SO4 etching system. The effects of the H2SO4 concentration and etching time on the surface topography, surface roughness, adhesion strength, and the surface chemistry of the ABS substrates were investigated. The optimal oxidation potentials of MnO2 in the colloids decreased from 1.426 to 1.369 V with the addition of H3PO4. Though the etching conditions changed from 70 °C for 20 min to 60 °C for 10 min, the adhesion strength between the ABS substrates and electroless copper film increased from 1.19 to 1.33 KN/m after etching treatment. This could be attributed to the significant increase of the soluble Mn(IV) ion concentration in the MnO2-H3PO4-H2SO4 colloid. The surface chemistry results demonstrated that the oxidation reaction of -C═C- bonds in the polybutadiene phase was accelerated in the etching process by the addition of H3PO4, and the abundant -COOH and -OH groups were formed rapidly on the ABS surface with the etching treatment. These results were in agreement with the results of surface scanning electron microscopic observations and adhesion strength measurement. The results suggested that the MnO2-H3PO4-H2SO4 colloid was an effective surface etching system for the ABS surface roughness.

Study of an environment-friendly surface pretreatment of ABS-polycarbonate surface for adhesion improvement

In this paper, an environmentally friendly etching system containing MnO2–H3PO4–H2SO4 colloid was used to investigate surface etching for ABS- polycarbonate (PC/ABS) as a replacement for conventional chromic acid etching solutions. In order to obtain a good etching performance, a swelling system, containing tetramethylammonium hydroxide (TMAH), and 1-Methyl-2-pyrrolidinone (NMP), was used to investigate the surface swelling for PC/ABS resin. Then the effects of H2SO4 concentration, and etching time on the surface topographies and surface contact angle were investigated. After the optimal swelling and etching treatment, the surface contact angle of PC/ABS resin decreased from 95.7° to 28.3°, and the adhesion strength between electroless copper film and PC/ABS resin reached to 1.04KNm−1. The FT-IR spectra and XPS analyses indicated that hydroxyl and carboxyl groups formed on the PC/ABS surface as a result of the swelling and etching treatment, which improved the adhesion strength between PC/ABS substrate and elctroless copper film.

Aging Effect on Adhesion Strength between Electroless Copper and Polyimide Films

The effect of atmospheric aging on the adhesion strength between electroless copper and polyimide films was investigated. 100nm thick electroless copper was plated on polyimide film and 10µm of copper electroplating was performed successively. Aging was allowed to occur at room temperature in atmosphere for designated times: either between electroless deposition and electroplating or between electroplating and the peel test. The adhesion strength between copper and polyimide film increased and then saturated as aging progressed for about 1d, with rapid increase from 0.2 to 0.5d. Among the samples with equivalent total aging time, samples that were electroplated later showed higher adhesion strength. Inter-diffusion and chemical bonding between copper and polyimide were observed after aging and additionally applied electrical current increased the adhesion strength. It could be suggested that the atmospheric aging and additionally applied current enhance the interdiffusion and the chemical bonding between electroless copper and polyimide film. [doi:10.2320/matertrans.M2013044]

Strain in Electroless Copper Films: Analysis by in situ X-Ray Diffraction and Curvature Methods

Strain in chemically deposited copper films on polymer substrates was determined by means of in situ X-ray diffraction (XRD), deposit stress analyzer (DSA) and spiral contractometer (SC). The strain evolution of the films was studied as a function of copper film thickness and electroless copper bath parameters, during and after deposition. The results are not indicative of a preferred crystallite orientation or texturing in the deposit. The copper film stress is controllable over a wide range of some 100 MPa from compressive to tensile stress by appropriate variation of bath parameters (e.g. temperature, concentration of bath components such as nickel, stabilizer and formaldehyde). A higher tendency of blister generation for relaxed or compressively stressed films is apparent, which implies that a sufficient level of tensile stress throughout the deposition promotes film adhesion. An observable change from tensile to compressive film stress during the cooling of the sample from bath operation to rinse water temperature is discussed in terms of substrate-induced thermal stress to the copper film. In this context, the difference in the substrate materials required for XRD (polymer), DSA (copper) and SC (stainless steel) may be a significant factor contributing to the diverging measured stress behaviors of the methods. Moreover, it is questionable whether SC stress data can be compared with XRD and DSA stress data, due to the low resolution of the SC method (~60 MPa).

Analysis of stress/strain in Electroless Copper Films

Polymer substrates were chemically coated with copper using various electroless copper baths and the internal strain/stress, as well as the adhesion quality, in the resulting copper films were studied during and after deposition as a function of the deposit thickness and the operation parameters of the electroless bath. The appearance of internal compressive stress in the copper film correlates to the probability of buckle driven delamination failure (blistering). Based on a simple theoretical concept we derived limits of allowed compressive stress in the copper film without inducing this failure mode. Furthermore depth-resolved X-ray diffraction (XRD) measurements in up to 1 μm thick electroless films indicate an approximately linear internal stress profile from about +200 MPa tensile stress at the substrate/adsorbate interface to −100 MPa compressive stress at the surface of the deposit. This will be explained in terms of a possible composition gradient of nickel in the copper film.

Stress and Strain Evolution in Electroless Copper Films Evaluated with X-ray Diffraction and Substrate Curvature

The scope of electroless copper plating technology is evolving to applications with smaller structures and smoother substrates, where adhesion of the copper film can be hard to achieve. The extent to which different factors, such as film stress and hydrogen evolution, contribute to delamination and blister formation on smooth organic substrates is a topic of active discussion. Stress and strain evolution is reported here for a nickel- and formaldehyde-free formulation for a range of plating bath temperatures. X-ray diffraction, with the sin 2ψ method, was used to monitor the strain within the copper crystallites during and after their growth. Substrate bending measurements were used to detect the average stress of the film. This technique indicates a 60 nm wide zone of tensile stress at the interface between substrate and film. During and immediately following deposition, the average substrate stress is more compressive than implied by the strain of the Cu crystallites. After deposition, the additional compressive stress seen by substrate bending relaxes until it matches the Cu crystallite strain. Furthermore, the thermal substrate contraction effect upon termination of plating is considered, since it may influence the film adhesion.

The effect of nickel on the strain evolution in chemical copper films

Chemical (or electroless) copper films are deposited from an electrolyte on palladium-activated insulating substrates in order to construct electrical interconnects for electronic components. These films provide the electrical contacts that are required for subsequent galvanic copper plating. As smoother substrates are required for advanced applications, achieving sufficient film adhesion becomes more difficult. Nickel, initially added to the electrolytes in order to improve deposition speed, has increasingly become important to promote good film adhesion. Film stress strongly affects film adhesion. Here we studied the effect of nickel addition on the internal strain of the film during and after electroless deposition by monitoring the strain of the Cu crystals with X-ray diffraction. Films without nickel tend to have exponential relaxation of the film strain after the deposition. For a 2000wt. ppm Cu electrolyte, about 30wt. ppm Ni was required in order to prevent this from occurring. Films with higher nickel content have a columnar structure with reliable and constant tensile strain during and after the electroless deposition.