Selviah, D.R.

English

UCL Discovery

Integrated Optical & Electronic Interconnect PCB ManufacturingTuesday, March 16, 2010 | David R. Selviah, Dept. of Electronic and ElectricalEngineering, UCLDigital information, encoded onto light signals, is regularly sent along optical fibresover distances varying from a few metres to thousands of kilometres. Fibres havelargely replaced traditional copper cables for high performance broadbandcommunication over distances exceeding a metre, as they offer advantages such aslower cost, immunity to electrical interference and weight savings. Printed circuitboard (PCB) backplanes are widely used in the electronic cabinets, or racks, that formthe heart of a variety of IT systems and incorporate connectors to allow other PCBs tobe attached and detached at right angles (Figure 1). In the highest speed computers,for communication between the central processor arrays, hard disc storage arrays, andthrough data routing switches, there is now considerable interest in incorporating highspeed "optical wiring," by means of plastic light-guides, within large, metre-scale,electrical PCBs combining optical and electrical interconnections (OPCBs).Figure 1: Schematic view of system backplane architecture.Optical interconnections were investigated in this IeMRC "Integrated Optical andElectronic Interconnect PCB Manufacturing" flagship project for short distance, high-speed, data communication applications on PCBs to replace high data rate coppertracks which suffer severe cross-talk and increased loss and increased cost at datarates above 10 Gb/s. This three-year research project explored methods for themanufacture of optical waveguides within an optical layer laminated into the boardand investigated their compatibility with techniques already in use in commercialPCB manufacturers.Four different polymer waveguide manufacturing techniques were investigated andcompared: Photolithography, direct laser writing, laser ablation and inkjet printing.Some of these techniques were very new, while others were more established,although in all cases their extension to larger areas and adaptation for use by PCBmanufacturers remained a significant challenge. As the photolithographic fabricationtechnique was better established, so waveguides fabricated by this method at twocompanies in two polymers, were measured and used as a benchmark to compare withthose fabricated by the other methods as they were developed during this project.Another aim of the research was to take the findings and use them to adapt existingcomputer programmes, used for laying out patterns of copper tracks, so that theyincorporated new rules suitable for designing optical waveguide layouts inOPCBs. By working with materials suppliers within the consortium, new polymerformulations were developed suitable for rapid production of waveguides on largearea boards and characterised. In addition, research considered low cost methods forpolishing the waveguide end facets to increase light throughput and waveguides weretested under severe conditions of high temperature and humidity to determine theseeffects on their losses.The large team of university and industrial collaborators comprised: The Electronicand Electrical Engineering Department, University College London, UCL (Instigator,Principal Investigator and Technical Project Leader); the School of Engineering andPhysical Sciences, Heriot-Watt University, Edinburgh; and the Wolfson School ofMechanical and Manufacturing Engineering, Loughborough University--together witheight companies: Xyratex Technology (Project Manager and manufacturer of petabytedata storage systems), BAE Systems (global aerospace, security and defencecompany), Renishaw, Dow Corning USA (polymer supplier), Exxelis (polymersupplier), Stevenage Circuits Ltd. (PCB manufacturer), Cadence Design Systems(PCB layout software supplier) and National Physical Laboratory (national standardslaboratory).Figure 2: The project sponsors and the consortium.Consortium partners Xyratex, BAE Systems and Renishaw formed the end usersystem partner group. The end users provided specifications for the types of opticallyand electrically interconnected PCB that are required for practical exploitation of thisnovel technology. In addition, Xyratex was, in parallel, carrying out an internalresearch and development project called "Candeo" to develop active pluggable in-plane connectors for optical backplanes and a demonstration assembly, whichincorporated a fully operational electro-optical backplane, high speed switch testcards and the prototype connectors.Xyratex was able to provide precise specifications for prototype OPCB backplanesrequired for use in their system demonstrator. UCL, having established waveguidedesign rules from comprehensive waveguide measurements and modelling,collaborated with Cadence to use their PCB layout software to design complexwaveguide interconnection patterns and test waveguide component structures, Thewaveguide design files were supplied directly or after fabrication of photomasks to allfabrication partners for waveguide fabrication.Consortium polymer partners, Dow Corning supplied their OE4140 and OE4141photopatternable polysiloxane and Exxelis supplied their Truemode™ photosensitivepolyacrylate formulation to fabrication partners: Heriot Watt University for directlaser writing fabrication, Loughborough University in collaboration with StevenageCircuits for laser ablation, to Loughborough University for ink jet printing fabricationand to Stevenage Circuits for photolithographic, laser direct imaging and laserablation fabrication in order to develop their different manufacturing processes.In addition, Dow Corning and Exxelis themselves fabricated waveguides in their ownpolymers, in house, using photolithographic techniques. Heriot Watt Universitydeveloped their own photosensitive polyacrylate formulations to increase the speed oftheir direct laser write process. All of the fabricated waveguides were sent to UCL fordevelopment of measurement techniques and use of them to characterise and comparethe waveguide performance, as the fabrication processes were themselves developed.NPL developed an alternative measurement technique and some waveguidesmeasured at UCL were remeasured at NPL giving the same results to withinexperimental measurement accuracy so validating both techniques. UCL also sentmeasured waveguides to BAE Systems and Stevenage Circuits for accelerated agingbeing returned them to UCL at regular intervals for remeasurement to assess anydeterioration of performance.Further research interactions between the partners are also indicated below in themore detailed descriptions of the research performed.Computer Aided Design (CAD) and Test and Measurement of WaveguideStructuresUCL was technical leader of the entire project and their research included: Establishment of waveguide design rules from comprehensive measurementsand modelling of waveguide components, including bends and crossings. Modification of widely-used commercial computer aided design layout toolsoftware, Cadence Allegro/OrCAD, to enable it to design the novel opticallyand electronically interconnected PCBs (OPCBs). Measurement of waveguide insertion loss of each of the four waveguidemanufacturing techniques and comparison with National Physical Laboratory'smeasurements. Investigation and analysis of the effect of waveguide wall roughness, end facetroughness and cross sectional shape on the behaviour of light and the effect onwaveguide loss. Comparison of the benefits of different methods of milling and polishingwaveguide end facets (in collaboration with Xyratex and Stevenage CircuitsLtd.). Characterisation of the behaviour of optical waveguides in aging tests at hightemperature and humidity (in collaboration with BAE Systems and StevenageCircuits Ltd.). Development of novel connector designs suited for interfacing flip chip lasersand photodiodes to OPCBs (in collaboration with Loughborough Universityand Stevenage Circuits Ltd.). Development of low cost novel manufacturing techniques compatible withcommercial PCB manufacturing processes (in collaboration with StevenageCircuits Ltd.). Sourcing of suitable waveguide manufacturing techniques and material andtransfer of technology to Stevenage Circuits Ltd.Figure 3: 10 Gb/s Ethernet system demonstrator with an integrated optical waveguide and multilayercopper track interconnected printed circuit board backplane fully interconnecting 3 daughter (line)cards via novel 80 Gb/s aggregate self aligning optical pluggable connectors.The waveguide design rules were used to design a novel complex optical backplanelayout. The end user system industrial partners specified two main requirements fordemonstration of a practically viable system: First, the line-cards had to be directlyinterchangeable (without rotation); so all optical interfaces were designed to face inthe same direction and to be identical to each other; second, the line-cards wererequired to be closely spaced. An interconnection pattern was designed (Figure 5(c))to allow each of several daughter cards to have bi-directional low losscommunications to each of the others.Figure 4: Sample waveguide (FR4/cladding/core/cladding) manufactured using dry film: (a)photograph of a curved waveguide transmitting red laser light, sample size ca. 50 mm x 50 mm; (b)microscope image acquired in reflection mode: cross-section of a waveguide transmitting red laserlight, core size is 50 mm x 50 μm.Using an optical PCB, to this design, sourced from IBM Zürich/Varioprint, Xyratexand UCL constructed a system demonstrator (Figure 3) fully inter-linking bi-directionally three daughter cards via self-aligning pluggable optical connectors,developed by Xyratex and Samtec, containing 4 VCSELs and 4 photodiodes. Theoptical interconnections were tested by UCL and Xyratex and it was demonstratedthat 10 Gb/s Ethernet traffic could be passed error free through each of thewaveguides, obtaining open eye-diagrams. This is, to our knowledge, the world's mosthighly-integrated demonstrator built to date, incorporating connectors to allowdaughter cards to be attached and detached at right angles and to be interchanged.Later in the project, Heriot-Watt University also manufactured the UCL waveguideinterconnection design using laser direct writing (Figure 5(c)).Figure 5: The SEM image in (a) shows direct laser-written, unclad polysiloxane waveguide cores(~50 x 50 µm, on a 100 µm pitch) with lower cladding, on a ~50 µm thick polyimide sheet. Theoptical microscope image in (b) shows an end-on view of back illuminated, clad, laser-writtenmultimode polysiloxane waveguide cores on a 100 µm pitch, on an FR4 substrate. The cores shownin this figure were laser-written using a flat-top intensity profile and an optical power of 3 mW. Thephotograph in (c) is of direct laser written waveguide cores and cladding on FR4, to UCL's design,for use as an optical backplane interconnect board in the Xyratex demonstrator system. The CCDimages in (d) and (e) show an 850 nm laser coupled into the 120 mm and 170 mm long waveguideson this board.Waveguide Fabrication in a PCB Manufacturing EnvironmentStevenage Circuits Ltd., in cooperation with the universities, identified technologyprocesses required for manufacture of optical waveguides using standard PCBtechnologies. Trials involved making waveguides using: Photolithography with liquidpolymers supplied by Dow Corning and Exxelis; using dry film withphotolithography and evaluation of Nd:YAG laser ablation of the aforementionedpolymers in cooperation with Loughborough University. The dry film approach wasidentified as the best means for mass production, because the same technology is usedto manufacture standard PCBs.Using this method, Stevenage Circuits Ltd. successfully manufactured waveguides forcharacterization (Figure 4). The adhesion of the liquid polymers to the substrate FR4could be improved by engineering the polymer formulation or laminate surfaceproperties. The deposition of the liquid polymers by screen printing was consideredand it was found that there was a need to develop new polymer formulations withdifferent density and viscosity. In addition to waveguide manufacturing, StevenageCircuits Ltd. provided its facility resources for various waveguide testing purposes(polishing, inspection and environmental testing) and, in cooperation with UCL,carried out research on improvement of micromachining of the waveguide end facetsto minimise their scattering.Laser Direct Writing of WaveguidesHeriot-Watt University studied the viability of direct UV-laser writing as amanufacturing technology capable of forming multi-mode polymer waveguides overlarge metre-scale backplanes. An optimum intensity profile for the writing beam wasidentified, compatible with producing both straight and curved guides. Following upearlier work with acrylate-based polymers, excellent results have been obtained usingOE4140 and OE4141 photopatternable polysiloxane, supplied by consortium partner--Dow Corning.Studies focused on the optimisation of optical power and writing speed and led to theproduction of well-defined, low-loss waveguides written at speeds in excess of20 mm/s. The waveguide writing system permits production of guides over boards aslarge as 300 mm x 600 mm, at potential speeds of up to 2 m/s, with sub micronresolution. The waveguide writing system has been used to produce an optical-interconnect insert board, compatible with the Xyratex system demonstrator.Laser Ablation of WaveguidesLoughborough University investigated laser ablation (in cooperation with StevenageCircuits Ltd) of optical polymer materials (OE4140 and OE4141 photopatternablepolysiloxane from Dow Corning and Truemode™ from Exxelis) to form waveguides(Fig. 6a) and examined the preparation of in-plane mirrors. Excimer, UV Nd:YAGand CO2 laser systems were trialled and waveguide structures fabricated (Figure 6band c). Transmission measurements of straight waveguides prepared in this way werecarried out at UCL and showed that while signals could be transmitted, the losseswere significant, which was thought to be due to the wall roughness. Further work isneeded to fully characterise the ablation process and investigate the different processparameters, such that the transmission efficiency can be improved.Figure 6: (a) The laser ablation process to create optical waveguides; (b) a cross-section through awaveguide machined in Truemode™ using a Nd:YAG laser; (c) a cross-section through an opticalwaveguide prepared using Excimer laser ablation of Truemode™ polymer.Inkjet Printing of WaveguidesA significant part of the research considered the inkjet deposition of polymerwaveguide structures focussing principally on the structuring of the core layer on topof the lower cladding. It was found that the UV cure optical polymer materials,OE4140 and OE4141 photopatternable polysiloxane from Dow Corning andTruemode™ from Exxelis, could be inkjet printed by controlling the viscosity anddeveloping correct inkjet print-head waveforms. Using this technique, initially lines ofoptical polymer were deposited and then cured in a separate UV exposure unit.However, a key issue was preventing the spread of the material before curing, suchthat structures with a good height to width ratio (aspect ratio) were formed.An aspect of the work investigated the control of the substrate wettability (Figure 7a,b) to increase the contact angle of the printed core droplets, whilst maintaining astable parallel line structure before UV curing, but it was found that this alone did notcreate suitable structures. Better aspect ratio features, such as those shown in Figure7c, d, were obtained using different methods of UV cure that enabled multiple printand cure cycles to be carried out to build up the thickness of the deposit.Measurements of the losses of these structures were carried out at UCL and showedencouraging results, but further work is needed to reduce the width of these featuresto dimensions closer to that required for the application.Figure 7: Inkjet printed lines showing: (a) extensive wetting leading to broad features; (b) reducedspreading of an inkjet printed line of core material on a modified hydrophobic glass surface; (c)parallel lines of polysiloxane showing the effect of different cure regimes; (d) cross-section throughan inkjet printed polysiloxane waveguide, built up from multiple print and cure cycles.AcknowledgementsThe partners in the project gratefully acknowledge financial support from the UKEngineering and Physical Sciences Research Council (EPSRC) via the Innovativeelectronics Manufacturing Research Centre (IeMRC). The three universities gratefullyacknowledge the matching financial support provided by the eight companies in directand indirect contributions, which was significantly higher than originally envisaged.The project partners thank Martin Goosey and Darren Cadman, the IeMRC monitorsfor very helpful contributions.Further details of the research reported here can be found at the following links: IeMRC Flagship Project The IEMRC Opto-PCB Manufacturing Project Integrated Optical and Electronic Interconnect PCB Manufacturing(OPCB) IeMRC Conference, September 5, 2007.

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