RHM 1Aspects of Platingfor Organic SubstratesDr. Roy [email protected] Interconnect TechnologiesEndicott, NY 13760RHM 2OverviewGeneral fabrication strategiesCircuitization:Subtractive processingSemiadditive processingBuild-upAspects of selected processesPlating Fundamentals/SystemsEtchingRHM 3General ReferencesMicroelectronics Packaging HandbookTummala and Rymaszewski, Edso Electrochemical Fundamentals/Modeling Corrosion:First edition Chapter 5, Second Edition I-436ffo Additive and Subtractive Circuitization Processes:First Edition Chapter 12, Second Edition III-304ffPrinciples of Electronic PackagingSeraphim, Lasky and Li, Eds.Chapter 16 General Plating -- both electroless and electrolyticChapter 18 Metal EtchingAlso see:M. Paunovic, M. Schlesinger, Fundamentals of Electrochemical DepositionG. Mallory, J. Hajdu, Eds, Electroless PlatingRHM 4Plating used to make conducting connections x, y and z and to provide surface finishes for soldering or corrosion prevention.Circuit boards (organic substrate): Signal traces, through vias (some v. high aspect ratio), blind vias. Employs catalysts/electroless copper and lamination most often.Two sided processing.Wafers (inorganic substrate): Traces ~ trenches and blinds.Initiated with dry deposition.One sided processing.Flex (organic substrate): Signal traces, through vias, blind vias.Most often uses sputtered metals for first layer and commoning.Commonly one sided, can be two sided processing.RHM 5Circuitization Sequences:Subtractive or print and etchSemiadditiveBuild-upSubtractive Processing:Particularly useful for one sided circuitry.May be coupled with full panel plating of vias.Simplest process sequence, line shapes not as desirableCopper clad substrateDrill through viaIf 2 sided connectionDielectricRHM 6Sputter commoning layer forthrough via Full panel electrolytic copper plate. Format = mounted piece or rollApply/expose/develop photoresist—tent or plug holesEtch copper, strip resistSubtractive, continuedRHM 7Semiadditive ProcessingOften used for processing requiring electrolytic surface finishes.Fine line applications for chip carriers and flex.More complex processing than subtractive.RHM 8For plating, need thin copper as an electrical commoninglayer.For flex this is most often provided with sputtered copper over a “tie” layer of Ti, Cr, Nichrome, etc. put down for adhesion.Often the substrate dielectric is activated with a dry process to create surface active functional groups to bond to the tie layer. This can be done in a roll format. Example: Polyimide (Kapton H) untreated 20-24 g/mm for Cr tie layerO2 MW plasma (reactive neutrals) 16 g/mm for Cr O2 dc glow (neutrals+ions+photons) 50-67 g/mm for CrAr ion beam (ions only) 53-58 g/mm for Cr Concommitant water wetting angle reduction observed.(Egitto and Matienzo, IBM J Res Devt, 1994)RHM 9Drill Adhesion promoteSputter tie metal,Sputter commoning copperSemiadditiveOrganic substrateApply, expose,develop photoresistRHM 10SemiadditivePattern electroplatecopperStrip photoresistPlate protective finishNi/Au, Sn, Sn/PbFlash etch commoningand tie layersStrip photoresistEtch commoning andtie layersMinimizes attack on CuRHM 11Interconnect Schemes:1. Core and subcomposite laminationinterconnect by through hole drilling/plating.Most common for PWB.Leaves stubs at through hole interconnects2. Build-up processing.Often used for organic chip carrier applications.Exploits laser drilling or photovia formation.Sequential process requires high yield in later process steps.3. Z-interconnect.RHM 12External copperExternal copperPrepregPrepregPrepregCircuitized coreCircuitized coreLaminate, drillRHM 13DesmearPlateCircuitizeStub Blocked wiring channel Backdrillingx, y, z challengeRHM 14Build-up on core or flex, two sided shownLaminate dielectric, etch or peel carrier if needed.Laser drill, 25-100 micron viasRHM 15Sputter, photo, plate, strip, flash etchOr sputter, plate, photo, etch, strip“1 + 1”“2 + 2”RepeatWiring channel not blockedRHM 16Parallel processing allows culling of defective cores—esp. importantas the surface area of the workpiece increases.RHM 17Types of PlatingRHM 18General Principles of Plating for Electrolytic, Electrolessand Immersion Plating Main principle of plating, regardless of type:Some source supplies electrons for the reduction of metal ions from solution.Metal deposition is an electrochemical reductionMz++ Ze-----Æ M0Metal ion electrons deposited metalMetal ions in solution are surrounded by a shell of molecules or ionscalled “ligands”. During the deposition process these leavethe vicinity of the metal ion. The ligands may be water molecules:Cu(H2O)62+RHM 19Schematic Representation of Metal DepositionRHM 20Controlling FactorsThermodynamics:The change in free energy, G must be negative.The free energy change can be related to the change in the electrochemical potential for the overall reaction.G = -nfErxnChemical/Electrochemical Kinetics:The kinetics of the reactions must be fast enough to yield a substantial deposition rate.Mass transfer/Hydrodynamics:Material must reach the plating surface (or leave the surface) by migration, diffusion, or solution flowRHM 21The rate and overall performance of a plating system can be dependent on:chemical composition (ligands, pH, etc)the more strongly complexed an ion, the harder it is to reducepH can affect complexation and overall chemical and electrochemical reactionsphysical properties (temperature, agitation, or mass transporthigh temperature and agitation results in higher rates and uniformityelectrochemical properties of the metal ion the standard potential, E0the charge on the ion, Zthe rate of electron transferRHM 22Silver ion is easier to reduce than cupric ion. If a solution contains both metals, silver will be plated out first, since the std potential for silver is less negative than for cupric ion. Also, if there are no side reactions, it will take twice the number of electrons to reduce the same number of copper ions as silver ions.Cu2++ 2e Æ Cu0E0= +0.337 VAg++ e Æ Ag0E0= +0.799 VRHM 23Basic Principles of ElectroplatingFaraday’s law: The mass of the metal plated is directlyrelated to the number of coulombs passed through thesolution. Electroplating is typically done at constantcurrent, so that number of coulombs = I x t related to number of moles by Faraday constant and the number of electrons per mole of deposited metal:Moles = coulombs/(F x