Flexible Copper Research Articles

A 32 × 32 temperature and tactile sensing array using PI-copper films

The development of a flexible 32 × 32 temperature and tactile sensing array, which will serve as the artificial skin for robot applications, is presented in this work. Pressure conductive rubber is employed as the tactile sensing material, and discrete temperature sensor chips are employed as the temperature sensing cells. Small disks of pressure conductive rubber are bonded on predefined interdigital copper electrode pairs which are patterned on a flexible copper–polyimide substrate which is fabricated by micromachining techniques. This approach can effectively reduce the crosstalk between each tactile sensing element. The mechanical and electrical properties of tactile sensing elements are measured. Also, the corresponding scanning circuits are designed and implemented. The temperature and tactile sensing elements are heterogeneously integrated on the flexible substrate. By using the integrated 32 × 32 sensing arrays, temperature and tactile images induced by the heaters/stamps of different shapes have been successfully measured. The flexible sensor arrays are bendable down to a 4-mm radius without any degradation in functionality.

Producing fine pitch substrate of COF by semiadditive process and pulse reverse plating of Cu

On the chip on film (COF) substrate, the width of the conductor of the substrate is usually smaller than 50 μm, with the smallest ones being 15 μm. It is difficult to produce it by the conventional subtractive process. In this study, the semiadditive process has been used to produce fine pitch substrates of COF. The flexible copper clad laminate (FCCL) used in the semiadditive process is a copper foil that is 2 μm thick; the FCCL with 8 μm thick copper foil is used to produce the same substrate of a COF by the conventional subtractive process. The effects of the semiadditive process and subtractive process have been studied and compared. Two sample substrates (50 μm/50 μm and 30 μm/30 μm fine pitch) have been designed. H2SO4/H2O2 etchant has been used in the etching process for the semiadditive process successfully. The test results of the metallographic slicing tester and the SEM show that the semiadditive process is more suitable to produce fine pitch pattern (below 50 μm/50 μm) than the subtractive process, especially in the application that requires an accurate impedance. In addition, the periodic pulse reverse plating can improve the thickness uniformity and physical properties of the electroplated copper layer.

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.

Improvement of Bonding Characteristics Between the Frequency Selective Surface and E-Glass/Epoxy Composites for Stealth Radomes

Radomes of weapon systems such as aircraft, warships, and missiles have been manufactured using FSS (Frequency Selective Surface) for the stealth performance and E-glass/epoxy composite to protect the radar antenna from weather and mechanical impact and to minimize the loss of EM (electromagnetic) waves transmitted through the radome. In this work, the FSS was manufactured from an FCCL (Flexible Copper Clad Laminate) composed of a copper foil of 20 μm thickness and a polyimide film of 40 μm thickness. The bonding characteristics between the FSS and E-glass/epoxy composite were determined for different surface treatments of copper such as atmospheric pressure plasma treatment and nitric acid/ferric chloride treatment. The EM transmission characteristics of the FSS were also measured by the free space measurement system before and after surface treatments. Nitric acid/ferric chloride treatment of the FSS was found to be a promising method to enhance the bonding characteristics between the FSS and the E-glass/epoxy composite without change in the EM transmission characteristics of FSS.

Adhesion characteristics of Cu/Ni–Cr/polyimide flexible copper clad laminates according to Ni:Cr ratio and Cu electroplating layer thickness

The adhesion strength of Cu/Ni–Cr/polyimide flexible copper clad laminate (FCCL) was evaluated according to the composition ratio of the Ni–Cr layer and the thickness of the Cu electroplating layer, by using a 90° peel test. The changes in the morphology, chemical bond and adhesion property were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The peel strength of the FCCL increased with increasing Cr content and increasing Cu electroplating layer thickness. This increasing FCCL peel strength was attributed to a lower C–N bond and higher C–O and carbonyl (C=O) bonds in the polyimide surface compared to the FCCL with a lower adhesion strength. The adhesion property of the FCCLs was significantly affected by the Ni:Cr ratio.

Effect of Ni-Cr Layer on Adhesion Strength of Flexible Copper Clad Laminate

In this study, the effect of a Ni-Cr layer on the adhesion strength of flexible copper clad laminate (FCCL) was evaluated after thermal treatment. The changes in the chemical composition, morphology, and adhesion properties were characterized by scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and 90 deg peel test. The results showed that both the peel strength and thermal resistance of the FCCL increased with increasing Cr ratio. The thermal treatment of the FCCL increased the proportion of C-N bonds and reduced that of the C-O and carbonyl (C=O) bonds in the polyimide. The roughness of the fracture surface decreased with increasing thermal treatment temperature and holding time. The chemical function and roughness of the fracture surface were affected by the Ni-to-Cr ratio.

Microstructural Characterization of Thick PZT films on Cu Foils Deposited by Electrophoresis

Piezoelectric and electrostrictive responses in poled and unpoled ferroelectric and relaxor ferroelectric compositions are of importance in transducers for converting electrical to mechanical impulses and vice-versa. Sensor applications make use of the very high piezoelectric constant dijk of the converse effect, which also permit efficient conversion of electrical to mechanical response. One of the most important families of materials for piezoelectric applications is Pb(Zr,Ti)O3(PZT). The most widely studied composition of PZT lies at the boundary between the tetragonal and rhombohedral phases, known as the morphotropic phase boundary (MPB) and exhibits greatly enhanced dielectric and piezoelectric properties in bulk and thin film. In modern electronic applications, pyroelectric detectors, piezoelectric microsensors, and micromechanical pumps require the integration of PZT films into a variety of device structures. To get sufficiently large piezoelectric strains for optimization of the performance and reliability of the device, thick films in the thickness range of 5–50 μm are desired. On the other hand burying the device components within the substrate is of utmost importance for miniaturization. In comparison to traditional surface mounted components embedded ones will free surface space for a higher functionality of the device, reduce solder points and increase device reliability. Additionally, to reduce the device costs the use of flexible copper foil as substrates is of particular interest. Its high conductivity and compatibility with printed circuit boards makes copper an attractive candidate substrate for embedded application. However, depositing PZT thick films on copper is not trivial, due to the conflict between the high temperature required to sinter PZT (∼1150°C) and low melting temperature of Cu (∼1050°C), in addition to the easy oxidation of Cu. As a consequence the preparation of PZT thick films on Cu involves a complex route to decrease the ceramic sintering temperature and to control the oxygen partial pressure. So far, no successful deposition of PZT thick films on copper foils was reported.

Nanocomposite stealth radomes with frequency selective surfaces

The stealth function of the radome (Radar + Dome) is to transmit or reflect the EM (electromagnetic) wave selectively through the radome. In this work, the stealth radome for aircrafts and warships was developed with the FSS (frequency selective surfaces), PVC foam, and nanoclay-dispersed E-glass fabric/epoxy composite. The water diffusivity of nanocomposites, which changes the stealth characteristics, was measured with respect to the contents of nanoclay . The EM transmission characteristics were measured by the free space measurement system in the X-band frequency range (8.2–12.4 GHz) with respect to the content of nanoclay. Also, the flexural strength of the sandwich construction composed of the nanocomposite, PVC foam, and FCCL (flexible copper clad laminate) was measured by the 3-point bending test.

Plasma surface treatment of polyimide for adhesive Cu/80Ni20Cr/PI flexible copper clad laminate

Two different methods of surface treatment, a low pressure plasma treatment and an atmospheric plasma treatment, on polyamide (PI) substrate were investigated from the point of view of adhesion of a Cu/80Ni20Cr layer on PI in flexible copper clad laminate (FCCL). The specimen treated with atmospheric plasma had high peel strength properties before and after aging. The root-mean-squared (rms) roughness with samples exposed to both kinds of plasma treatment was almost the same at 0.68 nm and 0.65 nm respectively, but the contact angle was different at 14.5° and 5.9°. We interpret this to mean that a more hydrophilic surface can be obtained using atmospheric plasma treatment. Observing the XPS spectra, more C–O binding rate was identified on the FCCL sample treated with the atmospheric plasma. From these results, it is suggested that the better adhesion achieved with atmospheric plasma treatment is not due to the mechanical roughness on the PI but due more to the activated C–O chemical bonding.

Soft lithography: masters on demand

We report an ultra-rapid prototyping technique for forming microchannel networks for lab-on-a-chip applications, called masters on-demand. Channels are produced by replica molding on masters formed by laser printing on flexible copper printed circuit board (PCB) substrates. Masters of various designs and dimensions can be individually or mass produced in less than 10 minutes. Using this technique, we have fabricated channels as narrow as 100 microm with heights ranging between 9 microm and 70 microm. Multi-depth channel fabrication is also reported, using a two-step printing process. The functionality of devices formed in this manner is verified by performing in-channel electrophoretic separations and culture and analysis of primary mammalian cells.

Fabrication of Elastomeric Wires by Selective Electroless Metallization of Poly(dimethylsiloxane)

A simple, low-cost method for fabricating flexible copper wires on elastomeric substrates is demonstrated. The use of microcontact printing and electroless metal deposition rather than conventional photolithography and physical vapor deposition of metals makes the process inexpensive and operationally simple. Copper films on elastomers may be used to fabricate flexible electronic circuits and biocompatible electronic devices.

Fabrication of 2-Layer Flexible Copper Clad Laminate by Vacuum Web Coater with a Low Energy Ion Source forSurface Modification

In order to fabricate adhesiveless 2-layer flexible copper clad laminate (FCCL) used for COF (chip on film) with high peel strength, polyimide (PI; Kapton-EN, <TEX>$38\;{\mu}m$</TEX>) surface was modified by reactive <TEX>$O_2^+$</TEX> and <TEX>$N_2O^+$</TEX> ion beam irradiation. 300 mm-long linear electron-Hall drift ion source was used for ion irradiation with ion current density (J) higher than <TEX>$0.5\;mA/cm^2$</TEX> and energy lower than 200 eV. By vacuum web coating process, PI surface was modified by linear ion source and then 10-20 nm thick Ni-Cr and 200 nm thick Cu film were in-situ sputtered as a tie layer and seed layer, respectively. Above this sputtered layer, another <TEX>$8-9{\mu}m$</TEX> thick Cu layer was grown by electroplating and subsequently acid and base resistance and thermal stability were tested for examining the change of peel strength. Peel strength for the FCCLs treated by both <TEX>$O_2^+$</TEX> and <TEX>$N_2O^+$</TEX> ion irradiation showed similar magnitudes and increased as the thickness of tie layer increased. FCCL with Cu (200 nm)/Ni-Cr (20 nm)/PI structure irradiated with <TEX>$N_2O^+$</TEX> at <TEX>$1{\times}10^{16}/cm^2$</TEX> ion fluence was proved to have a strong peel strength of 0.73 kgf/cm for as-received and 0.34 kgf/cm after thermal test.

Flexible and lithography-compatible copper foil substrates for ferroelectric thin films

A process has been developed for preparing a low surface roughness copper foil by evaporation and subsequent peel-off of copper metal layers on glass slides. These 15 micron thick substrates exhibited roughness values between 1 and 2 nm root-mean-square (RMS) and 9 nm RMS over 25 μm 2 and 100 μm 2 analysis areas, respectively. The deposition and crystallization of barium strontium titanate layers were demonstrated on these smoother variant foils. The fully processed dielectric layers exhibited field tunability greater than 5:1, and could withstand fields in excess of 750 kV/cm. High field loss tangents below 0.007 were observed, making these materials excellent candidates for microwave devices. Finally, a process of lamination and contact lithography was used to demonstrate patterning of micron-scale features suitable for microwave circuit element designs.

Assembly of Copper Column Interconnect Flip Chip

This paper addresses a new flip chip interconnection technology, flexible flip chip connection (F2C2) technology, for attaching silicon chips to a chip carrier using flexible copper wires. F2C2 is a novel approach to create area array flip chip interconnections using a matrix block of wires encapsulated with a heat-resistant, dissolvable substance. A slice from the wire matrix ingot is first attached to the chip using solder. The other end of the slice is then matched and soldered to the footprint of a substrate. Finally, the encapsulating, dissolvable substance is removed from the body of the slice, leaving the chip attached to the carrier by the inter- disposed, flexible copper wires. The compliant copper wire interconnections can accommodate the coefficient of thermal expansion (CTE) mismatch problem between die and substrate, thus eliminating the need for underfill.

Fatigue Behavior of Thin Cu Foils for Flexible Printed Circuit Board

Flexible printed circuit board (PCB), which is used for folder and slide type cellular phones, consists of flexible copper clad laminate (FCCL) and cover layer. Through it an electric current is applied to liquid crystal display (LCD) from the main board of cellular phone. In thin Cu foils of flexible PCB fatigue cracks due to repeated bending motion generate and propagate, and they cause a short circuit. Fatigue behavior of thin Cu foils being used for flexible PCB must be evaluated and confirmed to resolve this problem. It is based on findings by several researches that the mechanical properties of thin film materials differ from those of their bulk counterparts. Thin film properties have been investigated over the last years; however fatigue behavior of thin films has not yet been studied as thoroughly as monotonic behavior. In this study fatigue properties of thin Cu foils for the application in flexible PCB are obtained. Fatigue testing was performed for two kinds of Cu foils that were made by rolling and electrochemical procedures respectively. Differences of fabrications in fatigue behavior of thin foils were distinguished. Especially for rolled Cu foils, effects of rolling directions in fatigue properties were evaluated.

초고속 대면적 표면 처리 장치가 부착된 300 mm 폭 연성 동박적층 필림 제작용 진공 웹 코터

Prototype of vacuum web coater (Vic Mama) consisting of ion source with low energy less than 250 eV for high speed surface modification and 4 magnetron sputter cathodes was designed and constructed. Its performance was evaluated through fabricating the adhesiveless flexible copper clad laminate (FCCL). Pumping speed was monitored in both upper noncoating zone pumped down by 2 turbo pumps with 2000 l/sec pumping speed and lower surface modification and sputter zone vacuumed by turbo pumps with 450 1/sec and 1300 1/sec pumping speed respectively. Ion current density, plasma density, and uniformity of ion beam current were measured using Faraday cup and the distribution of magnetic field and erosion efficiency of sputter target were also investigated. With the irradiation of ion beams on polyimide (Kapton-E, ) at different fluences, the change of wetting angle of the deionized water to polyimide surface and those of surface chemical bonding were analyzed by wetting anglometer and x-ray photoelectron spectroscopy. After investigating the deposition rate of Ni-Cr tie layer and Cu layer was investigated with the variations of roll speed and input power to sputter cathode. FCCL fabricated by sputter and electrodeposition method and characterized in terms of the peel strength, thermal and chemical stability.

Interfacial Reaction of Cu-Tie Coating-Polyimide Flexible Copper Clad Laminates

Adhesion of flexible copper clad laminates (FCCL) on two different types of polyimide (PI), sputtering raw polyimide (SRPI) and casting raw polyimide (CRPI), were studied. Chromium (Cr), strongly reacts with dangling O bonds, was used as tie-coating layer in order to improve low adhesion between copper (Cu) and polyimide (PI). Polyimide surfaces were pretreated with gas plasma that was generated with a mixture of argon and oxygen before sputtering process. Adhesion measurement was performed on these samples according to IPC specification. The results show that the peel strength of the FCCL on SRPI was higher than on CRPI. This is considered to be due to the formation of Cu-Cr-O solid solution at the metal- PI interface.

High SNR flexible top hat monopole probe for 1.5 T MRI

In this paper, design principles and application of a thin and flexible intravascular top hat monopole probe with increased signal-to-noise ratio (SNR) and improved longitudinal and radial coverage are described and compared with the conventional monopole probe. The top hat probe is constructed by removing a segment of the shield of a coaxial cable. A flexible thin copper foil in contact with the inner conductor of the cable is added to the end section of the cable. The copper foil acts as an antenna top hat load increasing the uniformity of the current distribution on the probe and establishes a uniform region of SNR distribution around the probe. Improvement of the SNR distribution of the top hat probe relative to the conventional monopole probe is evaluated using quantitative indices. Experimental results show that for the top hat probe, the ratio of the maximum image SNR to its relative standard deviation (SR) is 2.73 times higher than that of a conventional probe.

Mechanical and electrical properties of electroplated copper for MR-imaging coils

Mechanical and electrical properties of electroplated copper films were investigated to be used in an active tracking system for the patient-safe localisation of a catheter in intravascular interventions using magnetic resonance imaging. For this system spiral micro coils with high aspect ratio are fabricated on a planar polyimide foil to be wrapped around a catheter tip. Due to this special assembly method the coils and the copper, respectively, have to withstand high mechanical forces. The influence of the electroplating parameters as current density and shape, temperature and sample motion on the mechanical and electrical properties is investigated. These samples were characterised using nanoindentation, tensile tests, laser profiler and four-point prober. Results show that a low constant current density at a temperature of 40°C and sample motion generate flexible and high-Q copper coils.

Carbon black pastes as coatings for improving thermal gap-filling materials

Carbon black pastes were found to be effective as coatings for improving the performance of thermal gap-filling materials, including flexible graphite, aluminum and copper. The thermal contact conductance across copper mating surfaces was increased by up to 180%. A fluidic form of carbon black paste (based on polyethylene glycol) was more effective than a thixotropic form (based on polyol esters). The carbon black pastes were much more effective as coatings than a commercial silver paste. With a carbon black paste coating, aluminum foil (7 μm thick) was a superior gap-filling material compared to similarly coated flexible graphite (130 μm thick). However, without a coating, flexible graphite was superior to aluminum. Commercial silicone-based gap-filling materials were inferior to flexible graphite or aluminum (whether coated or not).