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					                                                       DO   -   IT   -YOURSELF




              HOW TO MAKE GOOD
               HOMEMADE PCBs?                            INDRANI BOSE

 Do not use sodium hydroxide for developing photoresist laminates. It is completely a dreadful stuff
    for developing PCBs. Apart from its causticity, it is very sensitive to both temperature and
                      concentration, and made-up solution doesn’t last long


H
          ere is a guide to produce high-      image with a very solid opaque black.         but through-linking will be harder.
           quality PCBs quickly and effi-           Nowadays, artwork is drawn using         65mil round or square pads for normal
           ciently, particularly for profes-   either a dedicated PCB CAD program            components and DIL ICs, with 0.8mm
sional prototyping of production boards.       or a suitable drawing/graphics package.       hole, will allow a 12.5 mil, down to 10
Unlike most other PCB homebrew guides,         It is absolutely essential that your PCB      mil if you really need to. Centre-to-cen-
emphasis is placed on quality, speed, and      software prints holes in the middle of        tre spacing of 12.5mil tracks should be
repeatability rather than minimum ma-          pads, which will act as centre marks          25 mil—slightly less may be possible if
terials cost, although the time saved by       when drilling. It is virtually impossible     your printer can manage it. Take care
getting good PCBs every time usually           to accurately hand-drill boards without       to preserve the correct diagonal track-
saves money in the long run.                   these holes.                                  track spacing on mitered corners; grid
    With the method described here, you             If you’re looking to buy PCB soft-       is 25 mil and track width 12.5 mil.
can produce repeatedly good single- and        ware at any cost level and want to do             The artwork must be printed such
double-sided PCBs for through-hole and         hand-prototyping of boards before pro-        that the printed side is in contact with
surface-mount designs with track densi-        duction, check that this facility is avail-   the PCB surface when exposing, to avoid
ties of 40 to 50 tracks per inch and 0.5mm     able. If you’re using a general-purpose       blurred edges. In practice, this means that
SM pitches. Only photographic methods          CAD or graphics package, define pads          if you design the board as seen from the
have been dwelt in depth because other         as either a grouped object containing a       component side, the bottom (solder side)
methods such as transfers, plotting on         black-filled circle with a smaller con-       layer should be printed the ‘correct’ way
copper, and ‘iron-on’ toner transfer are       centric white-filled circle on top of it,     round, and the top side of a double-sided
not really suited for fast, repeatable use.    or as an unfilled circle with a thick black   board must be printed mirrored.
    The problem with toner transfer sys-       line (i.e. a black ring).
tems is that the ‘expensive part’ is the            When defining pad and line shapes,
film, and you can’t really feed much less      the minimum size recommended for vias                          Media
than an A5 sheet through a laser printer,      (through-linking holes) for reliable re-      Artwork quality is very dependent on
so you waste a lot on small PCBs. With         sults is 50 mil, assuming 0.8mm drill         both the output device and the media
photoresist laminate and cheap trans-          size; 1 mil = (1/1000)th of an inch. You      used. It is not necessary to use a trans-
parency media, you only use as much of         can go smaller with smaller drill sizes,      parent artwork medium—as long as it
the expensive part (the board) as you                                                        is reasonably translucent to UV, its fine-
need, and offcuts can usually be used                                                        less translucent materials may need a
later for small boards.                                                                      slightly longer exposure time. Line defi-
                                                                                             nition, black opaqueness, and toner/ink
                                                                                             retention are much more important.
          Artwork generation                                                                      Tracing paper has good enough UV
You need to generate a positive (copper                                                      translucency and is nearly as good as
black) UV translucent artwork film. You                                                      drafting film for toner retention. It stays
will never get a good board without good                                                     flatter under laser-printer heat than poly-
artwork, so it is important to get the best                                                  ester or acetate film. Get the thickest
possible quality at this stage. The most                                                     you can find as thinner stuff can crickle.
important thing is to get a clear sharp                                                      It should be rated at least 90 gsm; 120

                                                     ELECTRONICS FOR YOU „ NOVEMBER 2000                                            37
                                                              DO   -   IT   -YOURSELF

       gsm is even better but harder to find. It          Instagraphic Microtrak board devel-        sided PCBs, it may be worth making a
       is cheap and easily available from office      ops really quickly, gives excellent resolu-    double-sided exposure unit, where the
       or art suppliers.                              tion, and is available in thin (0.8mm) and     PCB can be sandwitched between two
                                                      heavy copper flavours. On using spray-on       light sources to expose both sides si-
                                                      photoresist, you will always get dust set-     multaneously.
                   Output devices                     tling on the wet resist. So it is not recom-       To find the required exposure time
       Laser printers offer the best all-round        mended unless you have access to a very        for a particular UV unit and laminate
       solution. These are affordable, fast, and      clean area or drying oven, or you only         type, expose a test piece in 30-second
       good-quality. The printer used must have       want to make low-resolution PCBs.              increments from 2 to 8 minutes, develop,
       at least 600dpi resolution for all but the                                                    and use the time which gave the best
       simplest PCBs, as you will usually be                                                         image. Generally speaking, overexpo-
       working in multiples of 0.06cm (40 tracks
                                                                        Exposure                     sure is better than underexposure.
       per inch). 600 dpi divides into 40, so you     The photoresist board needs to be ex-              For a single-sided PCB, place the
       get consistent spacing and line width.         posed to UV light through the artwork,         artwork’s toner side up on the UV box
           It is very important that the printer      using a UV exposure box. UV exposure           glass, peel off the protective film from the
       produces a good solid black with no toner      units can easily be made using stan-           laminate, and place its sensitive side down
       pinholes. If you’re planning to buy a          dard fluorescent lamp ballasts and UV          on top of the artwork. The laminate must
       printer for PCB use, do some test prints       tubes. For small PCBs, two or four 8-          be pressed firmly down to ensure good
       on tracing paper to check the quality first.   watt, 30.5cm tubes will be adequate. For       contact all over the artwork.
       If the printer has a density control, set it   larger (A3) units, four 38cm tubes are             To expose double-sided PCBs, print
       to the blackest. Even the best laser print-    ideal. To determine the tube-to-glass          the solder-side artwork as normal and the
       ers don’t generally cover large areas well,    spacing, place a sheet of tracing paper        component side mirrored. Place the two
       but usually this isn’t a problem as long       on the glass and adjust the distance to        sheets together with the toner sides fac-
       as fine tracks are solid.                      get the most even light level over the         ing, and carefully line them up, checking
           When using tracing paper or drafting       surface of the paper.                          all over the board area for correct align-
       film, always use manual paper feed and             Even illumination is a lot easier to       ment, using the holes in the pads as a
       set the straightest possible paper output      obtain with 4-tube units. The UV tubes         guide. A light box is very handy here,
       path to keep the artwork as flat as pos-       you need are sold as replacements for          but exposure can also be done with day-
       sible and minimise jamming. For small          UV exposure units, ‘black light’ tubes         light by holding the sheets on the sur-
       PCBs, you can usually save paper by cut-       for disco lighting, etc. These look white,     face of a window.
       ting the sheet in half. You may need to        occasionally black/blue when off, and              If printing errors have caused slight
       specify a vertical offset in your PCB soft-    light up with a light purple. Do not use       mis-registration, align the sheets to av-
       ware to make it print on the right part of     shortwave UV lamps like EPROM                  erage the errors across the whole PCB,
       the page. Some laser printers have poor        eraser tubes and germicidal lamps that         to avoid breaking pad edges or tracks
       dimensional accuracy, which can cause          have clear glass, because these emit           when drilling. When these are correctly
       problems for large PCBs. But as long as        shortwave UV which can cause eye and           aligned, staple the sheets together on
       any error is linear, it can be compensated     skin damage.                                   two opposite sides, about 10 mm from
       by scaling the printout in software.               A timer that switches off the UV           the edge of the board, forming a sleeve
           Print accuracy is likely to be a no-       lamps automatically is essential, and          or envelope. The gap between the board
       ticeable problem when it causes mis-           should allow exposure times from 2 to 10       edge and staples is important to stop
       alignment of the sides on double-sided         minutes in 15- to 30-second increments.        the paper distorting at the edge. Use
       PCBs—this can usually be avoided by            It is useful if the timer has an audible       the smallest stapler you can find, so that
       careful arrangement of the plots on the        indication when the timing period has          the thickness of the staple is not much
       page to ensure the error is the same on        completed. A timer from a scrap micro-         more than that of the PCB.
       both layers; for example, choosing             wave oven would be ideal.                          Expose each side, covering up the
       whether to mirror horizontally or verti-           Use glass sheet rather than plastic        top side with a reasonably light-proof
       cally when reversing the top-side art-         for the top of the UV unit, as it will flex    soft cover when exposing the underside.
       work.                                          less and be less prone to scratches. A         Be very careful when turning the board
                                                      combined unit, with switchable UV and          over, to avoid the laminate slipping in-
                                                      white tubes, doubles as an exposure unit       side the artwork and ruining the align-
             Photoresist PCB laminates                and a light-box for lining up double-          ment. After exposure, you can usually
       Always use good-quality, pre-coated pho-       sided artworks. If you do a lot of double-     see a faint image of the pattern in the
       toresist fibreglass (FR4) board. Check                                                        photosensitive layer.
       carefully for scratches in the protective
       covering and on the surface after peeling
       off the covering. You don’t need darkroom                                                                    Developing
       or subdued lighting when handling boards,                                                     Do not use sodium hydroxide for devel-
       as long as you avoid direct sunlight,                                                         oping photoresist laminates. It is a com-
       minimise unnecessary exposure, and de-                                                        pletely and utterly dreadful stuff for de-
       velop immediately after UV exposure.                                                          veloping PCBs. Apart from its caustic-

        38                                                  ELECTRONICS FOR YOU „ NOVEMBER 2000



CMYK
                                                        DO   -   IT   -YOURSELF

                    ity, it is very sensitive   drate type of ferric chloride, which            with hot water to warm it up. Putting a
                    to both temperature         should be dissolved in warm water un-           PCB in cold tinning solution will usu-
                    and concentration, and      til saturation. Adding a teaspoon of table      ally prevent tinning, even if the tem-
                    made-up solution            salt helps to make the etchant clearer          perature is subsequently raised.
                    doesn’t last long. When     for easier inspection.                              For a good tinned finish, strip the
                    it’s too weak it doesn’t         Avoid anhydrous ferric chloride. It cre-   photoresist thoroughly. Although you
                    develop at all, and when    ates a lot of heat when dissolved. So al-       can get special stripping solutions and
                    too strong it strips all    ways add the powder very slowly to wa-          hand applicators, most resists can be
                    the resist off. It is al-   ter; do not add water to the powder,            dissolved off more easily and cleanly us-
                    most impossible to get      and use gloves and safety glasses. The          ing methanol (methylated spirit). Hold
                    reliable and consistent     solution made from anhydrous ferric             the rinsed and dried PCB horizontal,
results, especially when making PCBs in         chloride doesn’t etch at all, so you need       and dribble few drops of methanol on
an environment with large temperature           to add a small amount of hydrochloric           the surface, tilting the PCB to allow it
variations.                                     acid and leave it for a day or two.             to run over the whole surface. Wait for
    A much better developer is a sili-               Always take extreme care to avoid          about ten seconds and wipe off with a
cate-based product that comes as a liq-         splashing when dissolving either type           paper towel dipped in methanol.
uid concentrate. You can leave the board        of ferric chloride, as it tends to clump            Rub the copper surface all over with
in it for several times the normal devel-       together and you often get big chunks           wire wool until it is bright and shiny.
oping time without noticeable degrada-          coming out of the container and splash-         Wipe with a paper towel to remove the
tion. This also means that it is not tem-       ing into the solution. It can damage eyes       wire wool fragments and immediately
perature critical—no risk of stripping at       and permanently stain clothing.                 immerse the board in the tinning solu-
warmer temperatures. Made-up solution                If you’re making PCBs in a profes-         tion. Don’t touch the copper surface af-
also has a very long shelf-life and lasts       sional environment, where time is               ter cleaning, as finger marks will im-
until it’s used up. You can make the            money, you should get a heated bubble-          pair plating. The copper should turn sil-
solution up really strong for very fast         etch tank. With fresh hot ferric chlo-          ver in colour within about 30 seconds.
developing. The recommended mix is 1            ride, a PCB will etch in well under five        Leave the board for about five minutes,
part developer to 9 parts water.                minutes. Fast etching produces better           agitating occasionally; do not use bubble
    You can check for correct develop-          edge-quality and consistent line widths.        agitation. For double-sided PCBs, prop
ment by dipping the board in the ferric         If you aren’t using a bubble tank, you          the PCB at an angle to ensure the solu-
chloride very briefly—the exposed cop-          need to agitate frequently to ensure            tion gets to both sides.
per should turn dull pink almost in-            even etching. Warm the etchant by put-              Rinse the board thoroughly and rub
stantly. If any shiny copper-coloured ar-       ting the etching tray inside a larger tray      dry with paper towel to remove any tin-
eas remain, rinse and develop for a few         filled with boiling water.                      ning crystal deposits. If the board isn’t
more seconds. If the board is under-ex-                                                         going to be soldered for a day or two,
posed, you will get a thin layer of resist                                                      coat it with either a rework flux spray
which isn’t removed by the developer.
                                                                 Tin plating                    or a flux pen.
You can remove this by gently wiping            Tin-plating a PCB makes it a lot easier
with dry paper towel, without damag-            to solder, and is pretty much essential
ing the pattern. You can either use a           for surface mount boards. Unless you                            Drilling
photographic developing tray or a ver-          have access to a roller tinning machine,        If you have fibreglass (FR4) board, you
tical tank for developing.                      chemical tinning is the only option. Un-        must use tungsten carbide drill bits.
                                                fortunately, tin-plating chemicals are ex-      Fibreglass eats normal high-speed steel
                                                pensive but the results are usually             (HSS) bits very rapidly, although HSS
                 Etching                        worth it.                                       drills are alright for odd larger sizes
Ferric chloride etchant is a messy stuff,           If you don’t tin-plate the board, ei-       (>2 mm). Carbide drill bits are expen-
but easily available and cheaper than           ther leave the photoresist coating on           sive and the thin ones snap very easily.
most alternatives. It attacks any metal         (most resists are intended to act as sol-       When using carbide drill bits below 1
including stainless steel. So when set-         dering fluxes) or spray the board with          mm, you must use a good vertical drill
ting up a PCB etching area, use a plas-         rework flux to prevent the copper from          stand—you will break drill very quickly
tic or ceramic sink, with plastic fittings      oxidising.                                      without one.
and screws wherever possible, and seal              Room-temperature tin-plating crystals           Carbide drill bits are available as
any metal screws with silicone. Copper          produce a good finish in a few minutes.         straight-shank or thick (sometimes called
water pipes may get splashed or                 There are other tinning chemicals avail-        ‘turbo’) shank. In straight shank, the
dripped-on, so sleeve or cover them in          able, some of which require mixing with         whole bit is the diameter of the hole, and
plastic; heat-shrink sleeving is great if       acid or high-temperature use.                   in thick shank, a standard-size (typi-
you’re installing new pipes. Fume ex-               Ensure that the temperature of the          cally about 3.5 mm) shank tapers down
traction is not normally required, al-          tinning solution is at least 25oC, but not      to the hole size. The straight-shank
though a cover over the tank or tray            more than 40oC. If required, either put         drills are usually preferred because they
when not in use is a good idea.                 the bottle in a hot water bath or put           break less easily and are usually
    You should always use the hexahy-           the tinning tray in a bigger tray filled        cheaper. The longer thin section pro-

                                                      ELECTRONICS FOR YOU „ NOVEMBER 2000                                             39
                                                              DO   -   IT   -YOURSELF

       vides more flexibility.                        other and insert a 0.8mm track pin in
           Small drills for PCB use usually           two opposite corners, using the pins as
       come with either a set of collets of vari-     pegs to line the PCBs up. Squeeze or
       ous sizes or a 3-jaw chuck. Sometimes          hammer the pins into the boards, and
       the 3-jaw chuck is an optional extra and       then into the remaining holes. The two          reheating the joint on the solder side.
       is worth getting for the time it saves on      PCBs are now ‘nailed’ together accu-                 For vias, holes which link sides with-
       changing collets. For accuracy, however,       rately and can be drilled together.             out components, use 0.8mm snap-off
       3-jaw chucks aren’t brilliant, and small                                                       linking pins. These are much quicker
       drill sizes below 1 mm quickly form                                                            than using pieces of wire. Just insert
       grooves in the jaws, preventing good
                                                                            Cutting                   the bottom of the stick into the hole
       grip. Below 1 mm, you should use               A small guillotine is the easiest way to        and bend over to snap off the bottom
       collets, and buy a few extra of the small-     cut fibreglass laminate. Ordinary saws          pin. Repeat the process for other holes
       est ones, keeping one collet per drill         (bandsaws, jigsaws, and hacksaws) will          and then solder both sides.
       size, as using a larger drill in a collet      be blunted quickly unless these are car-             If you need ‘proper’ through-plated
       will open it out and it no longer grips        bide-tipped, and the dust can cause sink        holes—for example, to connect to inac-
       smaller drills well.                           irritation. A carbide tile-saw blade in a       cessible top-side pins, or for underneath
           You need a good strong light on the        jigsaw might be worth a try. It’s also          surface-mount devices—Multicore’s
       board when drilling, to ensure accuracy.       easy to accidentally scratch through the        Copperset system works well, but the
       A dichroic halogen lamp, under-run at          protective film when sawing, causing            kit is very expensive. It uses bail bars
       9V to reduce brightness, can be mounted        photoresist scratches and broken tracks         consisting of a rod of solder, with a cop-
       on a microphone gooseneck for easy po-         on the finished board. A sheet-metal guil-      per sleeve plated on the outside. The
       sitioning. It can be useful to raise the       lotine is also excellent for cutting boards,    sleeve is scored at 1.6mm intervals, cor-
       working surface about 15 cm above the                                                          responding to the PCB thickness. The
       normal desk height for more comfort-                                                           bar is inserted into the hole using a
       able viewing. Dust extraction is nice,                                                         special applicator and bent over to snap
       but not essential—an occasional blow                                                           off the single bail in the hole. It is then
       does the trick! A foot-pedal control to                                                        punched with a modified automatic cen-
       switch the drill ‘off’ and ‘on’ is very con-                                                   tre-punch, which causes the solder to
       venient, especially when frequently                                                            spray over the ends of the plated sleeve
       changing bits. Avoid hole sizes less than                                                      and also pushes the sleeve against the
       0.8 mm unless you really need them.                                                            side of the hole. The pads are soldered
           When making two identical boards,                                                          on side to join the sleeve to the pads,
       drill them both together to save time.                                                         and then the solder is removed with
       To do this, carefully drill a 0.8mm hole       provided the blade is fairly sharp.             braid or a solder sucker to leave a clear
       in the pad near each corner of each of              To make cut-outs, drill a series of        plated hole.
       the two boards, getting the centre as          small holes, punch out the blank, and                Fortunately, it is possible to use this
       accurate as possible. For larger boards,       file to size. Alternatively, use a fretsaw      system for plating standard 0.8mm holes
       drill a hole near the centre of each side      or small hacksaw, but be prepared to            without buying the full kit. You can buy
       as well. Lay the boards on top of each         replace blades often. With practice it’s        the bail bars separately as refills. For
                                                      possible to do corner cutouts with a guil-      the applicator, use a 0.9mm automatic
                                                      lotine but you have to be very careful          pencil that works much better than the
                                                      that you don’t over-cut!                        original applicator, as you get one bail
                                                                                                      for every press of the button. Get a small
                                                                                                      automatic centre-punch and grind the
                                                                  Through-plating                     tip off to make it completely flat—this
                                                      When laying out double-sided boards,            works fine for punching the bails. For an
                                                      give some thought to how top connec-            anvil, use a thick flat piece of metal. Plate
                                                      tions will be made. Some components,            all the holes before fitting any compo-
                                                      such as resistors and unsocketed ICs,           nents so that the bottom surface is com-
                                                      are much easier to top-solder than oth-         pletely flat. Holes must be drilled with a
                                                      ers (radial capacitors). So, try to make        sharp 0.85mm carbide drill to get the
                                                      the top connection to the easier compo-         hole size right for the plating process.
                                                      nent. For socketed ICs, use turned-pin               Note that if your PCB package
                                                      sockets, preferably the ones with thick         draws pad holes of the same size as that
                                                      pin section under the socket body. Lift         of drill, the pad hole can come out
                                                      the socket slightly off the board, solder a     slightly larger than the drilled hole,
                                                      couple of pins on the solder side to tack       causing connection problems with the
                                                      it in place, and adjust so that the socket      plating. Ideally, the pad holes should
                                                      is straight. Solder all the solder-side pins,   be about 0.5 mm, regardless of hole size,
                                                      and then the required top-side pins by          to make an accurate centre mark.           t

        40                                                  ELECTRONICS FOR YOU „ NOVEMBER 2000



CMYK

				
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