Presentation on theme: "PRINTED CIRCUIT BOARDS"— Presentation transcript:
1 PRINTED CIRCUIT BOARDS The PCBBasically consists of a planar (flat) substrate which has electronic components mounted on it that are interconnected by conductive tracks.
2 PRINTED CIRCUIT BOARDS The PCBAdvantagesLarge number of components can be fitted and connected together on a small, flat substrate – aided by advancements in component reduction, thin line widths of interconnects.Multi layer possibilities allows more complex circuitry without taking up more room.Mass production – high volume throughput, reduces cost tocustomer.
4 The PCBPopulated PCB showing conductive traces, through-hole paths into the opposite side and element montageOne side of a motherboard PCB, showing conductive traces, vias and solder points for through-hole components on the opposite side
5 PCB applications for high frequency telecom and sensor modules
6 The rise of PCB in the 1950’sPCB became commonplace in consumer products aided strongly by US releasing invention for common use in 1948In the 1950’s every electronic component generally had wire leads and PCBs had holes drilled for each wire of each componentThe components were then soldered to the PCB, this is through-hole construction. This was done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machineThrough-hole mounting is still useful in attaching physically-large and heavy components to the board, but wires and holes are wasteful, it costs money to drill holes, and wires are merely cut off
7 SMT In the 1960’s Surface Mount Technology SMT developed: components made with small contact pads are physically held by solder to the conductorsSolder paste is generally applied by screen printing process and components mounted on, solder paste also acts as temporary adhesivePCB are passed through an IR oven to cure the solder
8 Advantages of Surface Mount smaller componentsno need to drill holes through abrasive boardssimpler automated assemblysmall errors in component placement are corrected automatically, as the molten solder pulls the component into place by surface tensioncomponents can be fitted to both sides of the circuit board
9 Copper Foil Bonding Patterned Conductive Interconnects Subtractive technique of copper foil bonding to whole substrate and removing unwanted metal became generally adopted. Copper foil is typically tenths of microns in thickness.Substrates developed are compatible with this technology and these are generally matted glass fibres with epoxy based – e.g. FR4, FR5. Copper foil is generally pressed while heated onto the substrate with ‘adhesive’.Substrates developed which are made of epoxy and glass fibres to which this process can be done – Bonding aided by Tg of the epoxy. Substrates such as FR4 became prominent.
10 Copper PatterningUnwanted copper is etched away with ferric chloride, and photolithography exploited and developed to produce patterns.Light sensitive photoresists are used as patternable barriers for etch and design technology developed to produce circuitry as exposable medium for transfer into photoresist.
11 Photolithography with a Photomask Copper clad substrate is covered with photoresist and a photomask, clear acetate with dark emulsion as opaque regions, is placed over photoresist during exposure.After exposure, photoresist is developed, copper is left exposed in regions to be removed.Exposed copper is etched away with ferric chloride andphotoresist is then removed.
12 Photolithography with a Photomask A basic photomask for a simple PCB can be made today with an inkjet transparency and a printer. Circuit design is made on pc.Photoresist covered, copper clad FR4 can be bought from RS.A light box can be easily made for exposing, but in industry much more sophisticated equipment is used, with much better resolution and quality.Photomask acetates are designed on CAD packages and printed.
13 Alternatives for Patterned Resist Techniques also developed since the 1960’s to screen print resist onto copper layer – no exposure undertaken.Resolution not as good as photolithography in terms of resolvedfeature sizes but a good method for mass production of low cost boards.A milling tool can be used to ‘cut’ away the unwanted copper to leave a desired pattern.This requires sophisticated plotting equipment in either X,Y or X, Y, Z axis control.This is another Subtractive Method
19 PCB Substrates Fabrication of the Laminate After cooling the sheets are trimmed, inspected for adequate quality, cut into smaller sheets about 1 meter square and vacuum sandwiched between clear plastic sheeting.This provides protection for the copper surface, particularly from oxidation by airBoards are rejected at inspection stage if they have warp and twist, imperfection in the copper surface, or poor bonding between the copper and the laminate.
20 PCB Substrates Fabrication of the Laminate The final properties of the laminate depend upon the materials used and process control during manufacture. In addition to electrical properties such as dielectric strength and constant, dissipation factor, insulation resistance, resistivity (both surface and volume), there are physical characteristics. These include flexural strength, punching and drilling qualities, flame resistance, and water absorption
21 PCB SubstratesOne important non-electrical characteristic is the maximum temperature at which boards can be operated. This is also relevant for the curing of printed epoxies and thick film pastes on the boards.ResinReinforcementMax T 'CPhenolicPaper100Glass250EpoxyGlass120PolyesterGlass120SiliconeGlass250PTFEGlass200
37 Copper Foil Copper is the dominant metal for interconnection use. The copper foil is normally specified by weight i.e. 1/2 oz/ft2 (152.5 g/m2), 1 oz/ft2 (305 g/m2); they correspond to foil thicknesses of 17.5 and 35 microns respectively.It is usually produced by electrolytic deposition on a flat mandril, it is about 99.8% pure and has a tolerance on thickness of about ± 10%.
38 Why Copper? Copper is an excellent electrical conductor Electrical resistivity is approx 1.6 ohm cmA not so expensive metalSoft and easily workableEasily processed and patternable by photolithography, using relatively benign chemicals
39 Copper FoilDesired thickness of copper foil will be dependant on required circuitry, i.e. resistance of tracks etc.Problems exist on high frequency communication boards with respect to the skin effect in a ‘thick’ conductor.Growing interest in an ability to deposit thinner copper and other metals onto PCB substrates to try and alleviate this problem.
40 Copper FoilCopper can be easily electrodeposited onto a PCB Laminate – problems however!ADHESION!The adhesion is far superior from bonding copper foil sheets by the combination of heat, pressure and adhesive.Common adhesion tests include measurement of Peel Strength – ‘how easy is it to peel the copper foil off’ and Pull off Strength – how easy is it to pull up the copper foil from the substrate
41 Copper Foil Some Typical Adhesion Values for Bonded Foil EP–GC–Cu, 152g/m2Pull off Strength = 60NPeel Strength = 1.1kN/mPF-CP-Cu, 152g/m2Pull off Strength = 45NPeel Strength = 1.05kN/m
42 Copper Foil Typical Resistance of 305gm-2 foil on laminate 3.5 m Often tend to speak about a sheet resistance measured in ohms per squareR = l / ARs = / tSEE 4 POINT PROBE ANALYSIS FOR FURTHER DETAILS ON MEASUREMENT
43 Reproducibility and robustness of Boards Reproducibility is extremely important in terms of producing ‘identical’ laminates, i.e. adhesion parameters must be consistent, along with resistivity of copper, change in resistivity of copper as board is bowed or flexed, thickness of copper, dielectric constant of laminate. Boards are therefore produced within specific tolerances.The produced laminate boards must be robust enough to withstand abuse during processing stages and in later life on the finished product. Abuse can include:Changes in temperature – sometimes rapid and hot and coldFlexing – the boards may have to be flexed, particularly during processing stages.Resistance to chemicals, e.g. solvents, fluxes, acids etc.Physical abuse – to a certain extent being dropped!
44 Copper Foil Copper layer can be patterned into circuitry Generally done by photolithography as discussed previouslyOn a double sided laminate, both layers are patterned and aligned on top of each other for subsequent interconnection.Alignment or Fiducial Marks are used for this
46 Double Sided PCB BoardTop layer is patterned in alignment with bottom layer.Holes are drilled through the board and then copper iselectrodeposited to interconnect top and bottom layers.
47 Multilayer BoardsOn multilayer boards, the individual, copper patterned laminates are stacked on top and bottom of each other. Insulating layers between the conductors can also be added.Often begin with double sided board in middle and work outwards.Again alignment marks are used to position each layer on top of the next.Laminates then pressed, heated and bonded in a similar process to before.Holes then drilled in appropriate places for subsequent electrodeposition of copper to interconnect layers.
49 Importance of Thermal Expansion Because of T used in processing the fabrication of multilayer boards it should be obvious now that the thermal expansion of each of the laminates and copper must be closely matched.Any mismatch in thermal expansion will cause a mismatch in alignment of each layer.Laminate materials have therefore been designed with this factor in mind.
50 FLEXIBLE PCB More and more interest being shown to flexible PCB’s Applications include flexible interconnections. Advanced applications include small screens, e.g. mobile phones – and large ‘roll up’ screensCommon substrate choices include polyester, polyimide etc.Typical problems – flexing substrate can cause metal tracks and components etc. to ‘peel’ offResistance of metal conductors can also be different when substrate is flexed
51 FLEXIBLE PCB Flexible circuits can reduce weight of PCB Flexing can allow easier installation and servicingProduction can be inexpensive
52 FLEXIBLE DISPLAYSFlexible Displays can take up less space (rolled up), they are more portableActive elements can be deposited by vapour deposition (amorphous silicon) – also printed organic materials
53 FLEXIBLE PCBSubstrates such as polyester, polyimide etc. tend to have smooth surfaces.Adhesion of metal for conductive tracks, solder etc. is generally very poor on a smooth substrate.Substrate is roughened typically by chemical means – e.g. immersion in hot permanganate.Metal can be deposited by evaporation (for thin layers) or foil can be bonded to substrate or deposited by ACD electroless and ECD electrodeposition.ACD electroless metal deposition on plastics developed since 1960s