Presentation on theme: "What determines impedance ?"— Presentation transcript:
1What determines impedance ? How is a PCB Made ?What determines impedance ?
2Manufacturing Processes for a Multi-layer PCB Tracks under solder maskThe following presentationcovers the main processesduring the production ofa multi-layer PCB.The diagrams which followrepresent a section througha 6 layer PCB, as indicatedin red.Via holeSMD PadSection through PCB
3Impedance Considerations Typical Layer Construction Layer PCBCopper LaminateLayer 1 (Outer)FOILPRE-PREGImpedance ConsiderationsLayer 2 (Inner)INNER LAYERLayer build / stackup is one of the most important aspects of controlled impedanceMany combinations of material thickness can be used.PCB Fabricators manufacturing techniques varyLayer 3 (Inner)PRE-PREGLayer 4 (Inner)INNER LAYERLayer 5 (Inner)PRE-PREGLayer 6 (Outer)FOIL
4Impedance Considerations Drilling of Bonded PanelCopper LaminateDrilled HoleLayer 1Impedance ConsiderationsLayer 2Press temperature and pressure have an effect on the flatness and hence impedance. This should be checked prior to drillingDrilling itself does not effect impedanceLayer 3Layer 4Layer 5Layer 6
5Impedance Considerations Electroless Copper ProcessAddition of Copper to all Exposed SurfacesCopperDrilled HoleLayer 1Impedance ConsiderationsLayer 2Electroless copper effects copper thickness on outer layers (T)Sometimes other solutions are used containing carbon etcLayer 3Layer 4Layer 5Layer 6
6Impedance Considerations Laminating and Imaging of External LayersUV sensitive film is laminated over top and bottomsurfaces of PCBIt is then exposed and developed, leaving an exposed image of the PCB patternLayer 1Impedance ConsiderationsLayer 2Does not effect impedanceLayer 3Layer 4Layer 5Layer 6Copper
7Impedance Considerations Electro-plating Process 1Additional Copper to all Exposed SurfacesLaminated FilmPlate Additional CopperLayer 1Impedance ConsiderationsLayer 2Electro-plating increases the copper thickness on outer layers (T)This will always be variations in the amount of copper added.This finished copper thickness should be used in structure calculationsLayer 3Layer 4Layer 5Layer 6
8Impedance Considerations Electro-plating Process 2Add Tin over Exposed Copper AreasTin PlatingLaminated FilmAdditional CopperLayer 1Impedance ConsiderationsLayer 2Does not effect impedanceLayer 3Layer 4Layer 5Layer 6
9Impedance Considerations Electro-plating Process 3Remove Laminated FilmLaminated Film RemovedTin PlatingLayer 1Impedance ConsiderationsLayer 2Does not effect impedanceLayer 3Layer 4Layer 5Layer 6
10Impedance Considerations Etch Process - Remove Exposed CopperCopper RemovedTin PlatingLayer 1Impedance ConsiderationsLayer 2The etch process produces an ‘etch back’ or undercut of the tracks. This can be specified by the W / W1 parametersThis means that tracks will end up approximately0,025 mm (0.001”) thinner than the original design.Layer 3Layer 4Layer 5Layer 6
11Impedance Considerations Tin Strip - Remove Tin PlatingTin Plating RemovedLayer 1Impedance ConsiderationsLayer 2The Removal of Tin will slightly reduce the copper thickness (T) on the outer layersLayer 3Layer 4Layer 5Layer 6
12PCB is now complete except for surface finishes and panel routing TracksVia HoleSMD PadLayer 1Layer 6Tracks
13Impedance Considerations Solder Mask Application- Curtain Coated MethodApply Liquid Photo-imageable Resist, then DryLayer 1Impedance ConsiderationsSome PCB Fabricators chose to check the impedance before the solder mask is addedStructures can be checked in Normal and Coated modeThickness of solder mask should be specified using H1Layer 6
14Impedance Considerations Solder Mask ApplicationImage, Develop and CureUV Image, Develop and CureLayer 1Impedance ConsiderationsDoes not effect impedanceLayer 6
15Impedance Considerations Surface Finish ProcessApply Solder to Exposed Copper AreasLayer 1Impedance ConsiderationsSurface Finish (Tin / Lead / Gold / Silver) is usually only added to padsIf board has no solder mask the thickness of finish should be added to T.Layer 6
16Impedance Considerations Component NotationSCLImpedance ConsiderationsDoes not effect impedance
17Impedance Considerations Routing (includes second stage drilling)Impedance ConsiderationsControlled Impedance coupons are routed from the panelGood controls are necessary to ensure that coupons can be matched to manufacturing panels
18Impedance Considerations Process finished PCB and coupon for testingImpedance ConsiderationsControlled Impedance coupons are routed from the panelControls are necessary to ensure that coupons can be matched to manufacturing panels this should be performed on trial panels prior to production ramp up.
19Impedance Considerations Why as a designer do you need to discuss your design with your PCB fabricator?PCB manufacture is a process, it uses materials which are not “Ideal”FR4 for example is a glass resin mix made of two substances with differing electrical properties.PCB Manufacturers need to make small adjustments to designs to maximise yieldsImpedance ConsiderationsGlass Er 6Resin Er 3 (FR4)Resin Er < 3 (High performance laminates)
20Impedance Considerations Why as a designer do you need to discuss your design with your PCB fabricator?Process varies from one fabricator to another.Press pressures temperatures may varyPre preg and Core may vary from oneSupplier to another.Impedance ConsiderationsSupplier variations
21Impedance Considerations Process finished PCB and coupon for testingImpedance ConsiderationsControlled Impedance coupons are routed from the panelControls are necessary to ensure that coupons can be matched to manufacturing panels this should be performed on trial panels prior to production ramp up.