Circuit Board Applications Research Articles

Fabrication of a 2.5 Gbps × 4 channel optical micro-module for O-PCB application

We report on the fabrication of a polymer-based 2.5 Gbps × 4 channel optical interconnecting micro-module for optical printed circuit board (O-PCB) application. An optical waveguide array is used for optical transmission from vertical surface emitting laser (VCSEL) array to photodiode (PD) array and the built-in 45° waveguide mirrors are used for vertical coupling. The optical waveguide array and the 45° mirrors are fabricated by UV imprint process in one-step. We fabricate microlensed VCSELs by micro-inkjetting method, which reduced radiation angle of VCSEL from 18° to 15° for better light coupling. We use solder ball array and pin array for alignment between O-PCB and the electrical sub-boards with alignment mismatch below 10 μm in x, y and z axis. The fabricated optical interconnection module transmits data at the rate of 2.5 Gbps per channel.

Lead zirconate titanate thin films directly on copper electrodes for ferroelectric, dielectric and piezoelectric applications

Replacement of noble metal electrodes by base metals significantly lowers the cost of ferroelectric, piezoelectric and dielectric devices. Here, we demonstrate that it is possible to process lead zirconate (Pb(Zr0.52Ti0.48)O3, or PZT) thin films directly on base metal copper foils. We explore the impact of the oxygen partial pressure during processing, and demonstrate that high-quality films and interfaces can be achieved through control of the oxygen partial pressure within a narrow window predicted by thermodynamic stability considerations. This demonstration has broad implications, opening up the possibility of the use of low-cost, high-conductivity copper electrodes for a range of Pb-based perovskite materials, including PZT films in embedded printed circuit board applications for capacitors, varactors and sensors; multilayer PZT piezoelectric stacks; and multilayer dielectric and electrostrictive devices based on lead magnesium niobate–lead titanate. We also point out that the capacitors do not fatigue on repeated switching, unlike those with Pt noble metal electrodes. Instead, they appear to be fatigue-resistant, like capacitors with oxide electrodes. This may have implications for ferroelectric non-volatile memories.

Local Time-Temperature-Dependent Deformation of a Woven Composite

Moire interferometry is utilized to investigate the time-temperature-dependent deformation of a woven composite substrate used in multilayer circuit board applications. Creep tests are performed at temperatures ranging from 27 to 70°C, and the resulting longitudinal and transverse displacement fields are measured via moire interferometry. Measured displacement fields reveal the influence of fabric architecture on woven composite response. The deformation fields in the plane of the composite for loading along both warp and fill directions consist of a periodic arrangement of high-strain and low-strain regions in accordance to the interlacing bundle architecture. The deformation fields over the cross-section of the composite indicate that neighboring unit cells are subjected to equal and opposite bending moment even when the composite is loaded in uniaxial tension.

Morgan advanced ceramics offers pressed glass spacers for circuit board applications

Morgan advanced ceramics offers pressed glass spacers for circuit board applications

Effect of temperature on elastic properties of woven-glass epoxy composites for printed circuit board applications

The effect of temperature on the elastic properties of woven-glass epoxy substrates for printed circuit board applications was investigated. Three common commercially pressed boards (1080, 2116, and 7628 woven-glass fabrics and FR4 epoxy) were used for this study. The elastic properties were identified by means of a combined theoretical and experimental vibration method. Vibration experiments were performed under reduced air pressure to reduce the influence of mass, stiffness, and damping of the surrounding air and inside an environmental chamber with varying temperature and constant humidity. Above the glass-transition temperature (Tg) of the epoxy, the in-plane shear moduli (G12) were decreased by 33–37% and the out-of-plane shear moduli (G13, G23) were decreased by 48%, whereas the Young’s moduli in the warp (E1) and fill (E2) directions were much less affected.

Towards advanced circuit board materials: adhesion of copper foil to ultra‐high molecular weight polyethylene composite

AbstractPolyethylene based composites are attractive materials for advanced circuit board applications because of their unique combination of properties: low dielectric constant and loss factor, light weight, high flexural modulus and low thermal expansion coefficient controlled in all spatial directions. This investigation describes a process to consolidate chopped fibers of ultra‐high molecular weight polyethylene concurrently with its bonding to a copper foil. Bonding is affected by a thin sheet of low‐density polyethylene, incorporating a crosslinking agent with a concentration gradient across the sheets thickness. In this single step process, the composite material is formed and bonded to the metal foil, achieving good adhesion without the use of extraneous glue. Copyright © 2002 John Wiley & Sons, Ltd.

Evaluation of adhesion between glass/epoxy composite substrate and copper foil for printed circuit board applications

Adhesion between glass/epoxy composite substrate and copper foil was evaluated by 90° peeling test. Effect of fracture behavior and the peel strength on copper foil style was experimentally investigated using copper foils with various surface roughness. As mechanical anchoring effect increases with increased surface roughness, the surface roughness increased the surface roughness of copper foil strongly affected the peeling strength and local load variation during the test. The fracture behavior was characterized by secondary electron image and reflective electron image techniques of scanning electron microscopy (SEM). All of the specimens were found to fracture at the anchor points formed in the resin layer on the surface of the substrate. The experimental results were correlated to a formulation based on the theory of beams on elastic foundation, which is presented for predicting the adhesive strength in the adherend- adhesive system. In order to apply this formulation to a no adhesive system, such as FR-4/copper foil, an analytical model was proposed. In this model, a mixture phase of the copper with the surface resin in the substrate was regarded as the adhesive. The calculated results were in relatively good agreement with the experimental results. It was confirmed that the analysis model and evaluation method is useful to predict the peel strength due to the mechanical anchoring effect.

On resonant effects in multilayer RF/microwave printed circuit board applications

Multilayer printed-circuit boards (PCB's) are nowadays widely used in high-frequency technology applications. While their small dimensions, compact design and low cost are an obvious advantage, the impact of full-wave phenomena such as electromagnetic coupling, resonance or leakage increases substantially as the used frequency spectrum shifts toward and beyond the upper UHF band and the L-band (1-2 GHz). Two types of PCB structures exhibiting resonant properties are considered in this paper: PCB shielding covers and large split-ground conducting plates. These structures can cause narrow-band (resonant) cross-talk. Their behavior is studied by both numerical and analytical approaches, and simple techniques to predict possible resonant coupling are proposed.

Thermoelastic Properties of Plain Weave Composites for Multilayer Circuit Board Applications

The thermoelastic properties of woven glass/epoxy substrates for multilayer circuit board applications were investigated and the influence of fabric geometry assessed. The woven fabric geometry of several commercially pressed boards was carefully characterized using optical microscopy. The elastic moduli and Poisson’s ratios were then measured in uniaxial tension, while the CTEs of the boards were measured using thermal mechanical analysis (TMA) at temperatures above and below the glass transition. Experimental data were compared to predictions from two new analytical models. One of the models, which does not use classical lamination theory, results in a significant improvement for the prediction of Poisson’s ratio and CTEs. Finally, a parametric study was performed to demonstrate the effect of fiber crimp on the properties of the fabric.

Creep and relaxation behavior of woven glass/epoxy substrates for multilayer circuit board applications

AbstractThe creep behavior of a common woven glass/epoxy composite substrate for multilayer circuit board applications was characterized using dynamic mechanical analysis (DMA). The creep compliance was measured in both the warp and fill directions of the composite over a temperature range of 30°C to 155°C. The creep compliance of the neat FR‐4 epoxy matrix was also characterized for comparison with the composite response. Master creep curves were obtained for the neat resin and the composite in the warp and fill directions assuming thermorheologically simple behavior and applying the time‐temperature superposition principle. The creep data was fit to a Prony series and then converted to relaxation data in the Laplace domain. Micromechanical models were developed to predict the relaxation behavior of the woven glass/epoxy composite from the elastic properties and the geometry of the glass fabric and relaxation behavior of the neat resin. Model predictions were compared with experimental data.

Effect of copper lamination on the rheological and copper adhesion properties of a thermotropic liquid crystalline polymer used in PCB applications

Attempts were made to improve the adhesion and strength properties of a copper clad thermotropic liquid crystalline polymer (LCP) for use in advanced printed circuit board (PCB) applications, through manipulation of processing/laminating conditions. Two types of copper foil were investigated. Most significantly, it was observed during these trials that the presence of the copper itself in a laminated sample dampens the modulus growth rate of the LCP during the recrystallization process, which occurs during the lamination cycle. The amount to which this dampening occurred appeared to depend both on the concentration of higher melting crystals already present in the LCP as well as the type of copper used for lamination. This observation has not been previously reported and is expected to influence modulus calculations relating to the construction of advanced PCB applications using LCPs as the substrate material.

Drum Side Treated Foil: Advanced Copper Foil Technology for Multilayer Printed Circuit Board Applications

Drum Side Treated Foil is an electrodeposited copper foil with the adhesion promoting treatment applied to the drum, or shiny side, instead of to the more typical matte side. The treated drum side is then laminated to the core prepreg, leaving the matte side exposed. Advantages of this technology, compared with standard copper foil, include: Simplified, non‐subtractive pre‐cleaning, eliminating the need for microetching, pumice cleaning or mechanical scrubbing; improved resist adhesion for higher innerlayer yield; ultra‐low profile for cleaner etching and improved line definition; higher oxide‐to‐prepreg bond strength for improved board reliability; improved controlled impedance consistency and predictability. A technical description of Drum Side Treated Foil’s inherent advantages, processing recommendations and results of several case studies comparing printed circuit board yields with Drum Side Treated Foil vs standard foil are presented in this paper.

Role of Overpotential on Texture, Morphology and Ductility of Electrodeposited Copper Foils for Printed Circuit Board Applications

A study was made to correlate different electrodeposition parameters, like, e.g., cathodic overpotential, bath composition, and bath ageing, with characteristics as crystallographic texture and roughness, and the ductility of electrolytic copper foils. Copper foils with a low (220) preferred crystallographic orientation, and a smooth surface are obtained when depositing at 87 to 113 mV cathodic overpotential from copper sulfate solutions with a low chloride content. Under these plating conditions copper foils with the highest ductility were achieved.

Instrumented puncture impact characterization of composite laminates for printed circuit board applications

One sided copper glass/epoxy laminates for printed circuit board (PCB) applications were examined using an instrumented puncture impact technique. The effect of the diameter of the support ring, the initial impact velocity, and the preence of a stress concentrator on the impact parameters were evaluated. The orientation of the copper film with respect to the impacter was also varied. The PCB laminate showed brittle failure under the different conditions examined. Results showed that a higher degree of toughness was found for the condition where the copper layer was directly impacted by the dart. The maximum force and the total impact energy were found to be independent of the strain rate. The effect of a stress concentrator on the impact parameters was also found to be minimial. Total impact energies were found to be in the range of 8.8 to 12.6 J for the different testing conditions.

Instrumented dart impact evaluation of composite laminates for printed circuit board applications

Double-sided copper film and single-sided copper film epoxy/glass fabric laminate for printed circuit board applications were examined for impact strength at different velocities of deformation and different temperatures. An instrumented dart impact tester was employed in the experiments. The single-sided copper laminates were evaluated by first placing the copper film face up facing the impacter, and second by placing the copper film face down away from the dart. It was found that the maximum force of deformation was almost independent of the initial velocity of deformation, and the total energy was found to increase with velocity. In addition, the maximum force and the total energy were found to decrease linearly with temperature for every case. A brittle feature was also prevalent in the damaged composite. A cross-type of fracture was also common where more damage was found with increasing velocity of impact. A combination of brittle failure with fibre pullout, as well as a brittle failure with debonding or matrix failure were also found by examining the fracture surfaces with a scanning electron microscope.

Stiffness and thermal expansion predictions for hybrid short fiber composites

AbstractA model is developed to predict thermal expansion coefficients and elastic moduli of multi‐component (hybrid) composites. The model includes the influences of fiber aspect ratio; isotropic and anisotropic fiber materials; planar, three‐dimensional or arbitrary fiber orientation; hollow and solid spherical reinforcements; and voids. The first step in the procedure is to predict the properties of an aligned‐fiber single‐reinforcement composite for each reinforcement type. Various micro‐mechanics approaches are used, depending on the type of reinforcement. A simplified version of Lee and Westmann's theory is found to work well for hollow spherical reinforcements. Performing an orientation average accounts for the spatial orientation of each reinforcement, then an aggregate averaging procedure combines the single‐reinforcement properties to model the hybrid. Predictions of the model compare favorably to experimental elastic and thermal properties of short fiber/hollow sphere composites designed for very high speed integrated circuit (VHSIC) board applications.

The performance of the AT&T single mode ST® connector and its latest enhanced features

Abstract The AT&T single mode ST® connector is a low-loss, low-reflection connector suitable for high-speed, wideband digital transmission. This new connector offers enhanced hardware for easier use and more stable connections. In addition, a new backplane-compatible version is introduced for circuit board applications. The insertion loss of AT&T's single mode ST connector averages 0.34 dB with a standard deviation of 0.28. The reflection of an AT&T ST connection (product against product) averages -42.8 dB with a worst case of -34.1 dB. The environmental performance of the connector is characterized by low variations in loss and refection. The AT&T single mode ST connector and its backplane version were subjected to a battery of tests whose results are discussed in this article.

Characterization of a bis-maleimide triazine resin for multilayer printed circuit boards

The thermosetting resin investigated in this study was a mixture of bis-maleimide and bis-cyanate, frequently referred to as BT (bis-maleimide triazine). For printed circuit board applications, a brominated epoxy resin was blended with BT to impart flame resistance. Resin curing was extensively investigated using a combination of thermoanalytical techniques (thermal analysis, heated-cell infrared spectroscopy, dynamic mechanical analysis, and microdielectrometry). Differential scanning calorimetry indicated a minimum of two separate reactions. Fourier-transform infrared spectroscopy provided more detailed information on the cross-linking reactions during the curing. The onset of cyclotrimerization was found to appear at 150°C, correlating with one of the peaks observed in the differential scanning calorimetry measurements. Dynamic mechanical methods were used to investigate the viscosity profile during simulated lamination temperature profiles. Microdielectrometry performed simultaneously with parallel-plate rheometry provided further insight into the physical changes that occur during lamination.

Thermal characterization of a bis‐maleimide/bis‐cyanate/epoxy thermosetting resin for composites

AbstractThe thermosetting resin investigated here was a mixture of bis‐maleimide and bis‐cyanate, frequently referred to as BT (bis‐maleimide triazine). Triazine is the reaction product of the cyclotrimerization of bis‐cyanate during curing. For circuit board applications, a brominated epoxy resin was blended with BT to impact flame resistance. Resin cure was extensively investigated using a combination of thermoanalytical techniques (thermal analysis, heated cell infrared spectroscopy, and dynamic mechanical analysis). The ultimate glass transition temperature was found to be 240°C, which could only be obtained using cure temperatures above 225°C. At lower temperatures, the reaction does not reach full conversion, since the glass transition temperature of the curing network equals or slightly exceeds the cure temperature. Differential scanning calorimetry (DSC) indicates a minimum of two separate reactions. Fourier transform infrared spectroscopy provided more detailed information on the crosslinking reactions during cure. The onset of cyclotrimerization was found to start at 150°C, correlating with one of the peaks in the DSC. At higher temperatures, the epoxide reacts with the cyanate functionality forming oxazoline ring structures. It was not possible to unambiguously assign the origins of the high temperature peaks in the DSC. These high temperature peaks may be attributed to several reactions, including epoxy homopolymerization and polymerization of bis‐maleimide. The high temperature reaction mechanisms warrant further investigation.