Himanshu Suyal

Integrated optical and electronic interconnect PCB manufacturing research

Purpose – The purpose of this paper is to provide an overview of the research in a project aimed at developing manufacturing techniques for integrated optical and electronic interconnect printed circuit boards (OPCB) including the motivation for this research, the progress, the achievements and the interactions between the partners.Design/methodology/approach – Several polymer waveguide fabrication methods were developed including direct laser write, laser ablation and inkjet printing. Polymer formulations were developed to suit the fabrication methods. Computer‐aided design (CAD) tools were developed and waveguide layout design rules were established. The CAD tools were used to lay out a complex backplane interconnect pattern to meet practical demanding specifications for use in a system demonstrator.Findings – Novel polymer formulations for polyacrylate enable faster writing times for laser direct write fabrication. Control of the fabrication parameters enables inkjet printing of polysiloxane waveguides...

A direct-writing approach to the micro-patterning of copper onto polyimide

Purpose – The purpose of this paper is to present a novel manufacturing process that aims to pattern metal tracks onto polyimide at atmospheric pressure and ambient environment. The process can be scaled up for industrial applications.Design/methodology/approach – From a thorough literature survey, different approaches were carried out for processing polyimide. Following a design of experiments for the processing and various characterisation techniques, a micro‐coil was manufactured as a test demonstrator.Findings – The characteristics of some main formaldehyde‐based electroless copper baths were compared. The quality of the sidewalls was characterised and the performance of the process was assessed.Originality/value – This paper demonstrates a high‐value manufacturing technique that is mass manufacturable, low cost and suitable for use on 3D surfaces. Criteria required for the development of a direct‐writing process have been described. The issues surrounding electroless plating on polyimide have been ex...

Integrated optical and electronic interconnect printed circuit board manufacturing

Introduction: At high bit rates copper tracks in printed circuit boards (PCBs) suffer severe loss and pulse distortion due to radiation of electromagnetic waves, dispersion and bandwidth limitations. The loss can be overcome to some extent by transmitting higher power pulses and by changing the dielectric constant and loss tangent of the PCB substrate material. However, high power pulses consume power and can cause electro-migration which reduces the board lifetime, although the copper tracks can be surrounded by another metal to prevent this at the expense of further processing steps. The use of special board materials can be costly and some materials containing high dielectric constant crystallites can cause poor adhesion. The pulse distortion, dispersion and bandwidth limitations can be overcome to some extent by the use of pulse pre-emphasis and adaptive equalisation at further cost. Electromagnetic waves are radiated efficiently at high bit rates removing power from the track so causing loss, but more importantly they are also received efficiently by other nearby and distant copper tracks on the same PCB, or on adjacent PCBs, or PCBs and other electrical conductors outside of the system enclosure. This EMI crosstalk causes increased noise and so degrades the signal to noise ratio and the bit error rate of the copper track interconnections. Therefore, the main forces driving the development of alternative interconnect technologies are the EMI crosstalk, which becomes increasingly more serious as bit rates increase for longer and denser interconnects, and secondly the cost of overcoming the other problems that occur in copper interconnects at high bit rates. Optical fibres have replaced copper cables for long distance, backbone and submarine applications where they offer wide bandwidths for low loss, produce and receive no electromagnetic interference, and are relatively low cost. Optical interconnects are beginning to penetrate the markets at shorter distances, such as in local area networks, and as their cost is reduced, will be used within the system enclosure. The use of optics is expected to occur first where the problems for copper are most significant which is for high bit rate, dense interconnections in large area backplanes within non-conducting enclosures. Optical fibres are not the most convenient for interconnections within a system as they can only bend through a large radius of about 10 cm, otherwise light escapes from the fibre core into the cladding resulting in loss and signal corruption. Fibre connectors form a major part of the cost of the optical interconnect and a system with many fibres has many costly connectors. The fibres must be individually routed and errors in routing are time consuming to debug and correct. The fibres can be laid flat on the PCB plane and even bonded together within an epoxy layer, but this is not suited to low cost mass production. An alternative technology suitable for low cost mass production is that of multimode polymer buried channel optical waveguide interconnections within layers in the multilayer PCB formed by the same, or slightly modified, processes already available within PCB manufacturing facilities. Copper tracks are still required in such substrates to transmit power through the backplane (or motherboard), Figure 1, in order to power mezzanine (or line, or drive, or daughter) boards and copper is still a practical and low cost option at low data rates. Hence, there is a need to develop a new type of multilayer hybrid PCB in which optical waveguide interconnects are used for the highest data rates, with copper tracks for lower data rates and for power lines and earth planes. These issues have been anticipated by system design companies such as Xyratex Technology, IBM Zurich and Siemens C-Labs, microprocessor designers such as Intel and materials development companies such as Dow Corning, NTT, Rohm and Haas and Exxelis, who have instituted research in their own laboratories and in associated universities into optical waveguide interconnect technology. Leading Universities and Research Institutions such as Cambridge (CAPE), University College London (UCL), Heriot Watt University, Loughborough University, National Physical Laboratory (NPL), IMEC - Ghent University, TFCG Microsystems, Belgium, Paderborn University, Germany, Helsinki University of Technology, Espoo, Finland and ETRI, South Korea are developing novel polymer materials, developing fabrication techniques, discovering design rules for waveguide layout and carrying out precision characterisation. Optical buried channel waveguides usually have a core with an approximately square or rectangular cross section made from a high refractive index (slow speed of light) material and a cladding surrounding the core of a lower refractive index (higher speed of light). They operate by total internal reflection (TIR) in a similar way to optical fibres. The cost of waveguide connectors is minimised by choosing to use multi-mode waveguides which typically have cores of 40 - 70micron width which can tolerate more misalignment than single mode waveguides. The optical buried channel waveguides are formed on a plane by a variety of fabrication techniques which can be implemented, after slight adaptation, in PCB manufacturers. Arrays of low-cost vertical cavity surface emitting lasers (VCSELs) emitting 850 nm wavelength and arrays of photodiodes operating at 1 0 Gb/s are readily available at low-cost for use in optical transmitters and receivers. At this wavelength, polymer is a convenient low-loss material for use as the core and cladding. Polymers can be chosen or designed which can be easily processed to form waveguides at low temperatures, have low cost, and can withstand subsequent high temperature reflow soldering processes. For optical printed circuit boards to be brought into widespread use, layout tools must be made readily available which design both the copper tracks and the optical waveguides [1]. In 2006 David R. Selviah of UCL, formed a large consortium of complementary universities and companies and led a successful bid to carry out a Flagship project entitled “Integrated Optical and Electronic Interconnect PCB Manufacturing (OPCB)” in the Innovative Electronics Manufacturing Research Centre (IeMRC). The consortium companies represented a complete supply and manufacturing chain and route to market for the polymer waveguide technology including companies manufacturing PCB layout tools, computer programs for modelling the behaviour of multimode waveguides, developing and supplying low loss polymer formulations, manufacturing multilayer PCBs, supplying printer fabrication equipment together with end user system companies who require optical printed circuit boards. The following sections describe the project’s objectives, the approaches being taken and some examples of what has been achieved so far in the project with an indication of future directions.

Fabrication and characterisation of direct laser-written multimode polymer waveguides with out-of-plane turning mirrors

This paper describes work on the development of a two-level lightwave circuit containing multimode polymer waveguides with 45/spl deg/ out-of-plane turning mirrors to communicate between the layers. The polymer waveguides and mirror structures are produced using a direct laser-writing system. The system is based on a He-Cd (325 nm) UV laser and a vertically mounted slotted base-plate which is used to align the polarising, routing and focussing optics that guide and focus three beams (at normal and /spl plusmn/45/spl deg/ angles) onto a computer controlled XY translation stage.

Innovative Optical and Electronic Interconnect Printed Circuit Board Manufacturing research

An overview of the pound1.3 million EPSRC and company matched funded Innovative electronics Manufacturing Research Centre (IeMRC) Flagship project between 3 UK universities and 10 companies entitled ldquointegrated optical and electronic interconnect PCB manufacturingrdquo. The project aims to develop of optical waveguide design rules, layout software, fabrication methods compatible with commercial production, characterisation techniques and optical connector design to provide a supply chain for Polymer Multimode Optical Waveguide Printed Circuit Boards (OPCB) for 10 Gb/s board-to-board interconnections.

Direct laser-writing of polymer structures for optical interconnects on backplane printed circuit boards

We have previously developed a direct UV-laser-writing technique and custom photo-polymer optimised to form multimode polymer waveguides and embedded 45deg out-of-plane mirrors. In the IeMRC funded ldquoIntegrated Optical and Electronic Interconnect PCB Manufacturingrdquo (OPCB) project the key aim is to explore how these techniques can be extended to suit real-world optical backplane applications - both in the context of scale and manufacturability. In particular, we are assessing the viability of the technique for writing polymer optical waveguides of approximately 50 times 50 mum in cross-section over large metre-scale areas and the compatibility of this optical interconnect approach with existing PCB manufacturing processes.

Direct laser written polymer structures and passive devices for photonics applications

Direct laser writing is a good alternative to photolithography for rapid prototyping as it is a maskless process. Here we describe different polymer structures and passive devices fabricated using a direct laser-writing system based on a He-Cd (325 nm) UV laser. This has been used to fabricate structures in polymer such as bumps, micro-via, mechanical guides for fibre alignment and passive devices such as beam splitters and combiners which can be used for incoherent light sources such as LEDs. A custom multifunctional acrylate photo-polymer with good thermal and mechanical properties was used to fabricate the structures.

An additive method for photopatterning of metals on flexible substrates

Here we present an additive and cost effective process for plastic electronic manufacturing. Metal tracks are fabricated on polyimide substrates via simple chemical processes combined with direct laser writing or photomask exposure. Laser write speed up to 0.5 m•s-1 and metal track linewidth as low as 5 μm were achieved. Further, this process was easily extended to 3D manufacturing; a helical silver track was written onto a cylindrical substrate. Selective electroless plating was also demonstrated on the photopatterned microstructures which showed promising conductivity close to that of bulk silver metal.

Laser ablation and laser direct writing as enabling technologies for the definition of micro-optical elements

A qualitative comparison is made between laser direct writing and laser ablation as enabling technologies for the structuring of multimode waveguides (50x50μm2) and 45° micro-mirrors into an optical layer. A small demonstrator is fabricated that allows us to couple light vertically from a transmitter into an optical layer and from the optical layer to a receiver. The optical layer, a multifunctional acrylate-based photo-polymer, is applied on an FR4-substrate. Multimode waveguides, that carry signals in the plane of the optical layer, are fabricated by means of laser direct writing, a technology that is available at HWU. The 45° micro-mirrors, that provide out-of-plane coupling, are ablated with the laser ablation set-up available at UGent. This set-up contains a KrF-excimer laser (248nm) that can be tilted, which eases the definition of angled facets. Surface roughness measurements are performed on both the optical layer and the micro-mirrors with a non-contact optical profiler. Loss measurements are performed on both the waveguides and the micro-mirrors.

Comparative performance analysis of 100% fill-factor microlens arrays fabricated by various methods

Fill-factor of microlens arrays (MLAs) is one of the most important performance criteria of microlens arrays (MLA), especially in imaging applications. Low fill-factor lenses suffer greatly from spurious light and diffraction affects and result in low contrast in a beam steering system. Contrast ratio of low fill-factor circular shaped microlens arrays is nearly one-fourth of that of the system with high fill-factor square shaped microlens arrays. In this study performance of various types of nearly 100% fill-factor spherical MLAs in beam steering applications are compared. Design and fabrication of the MLAs are studied. A new hybrid method for design and fabrication of 100% fill-factor MLAs--by combining refractive-diffractive lenses, is suggested and tested.