Daqarta for DOS Contents
PRINTED CIRCUIT CONSTRUCTION TECHNIQUES:
INTRODUCTION:This section provides instructions for direct-drawn circuit boards, where a felt-tip pen provides a "resist" that keeps etchant from removing the copper beneath the lines. This is usually much simpler than the commercial photo-resist method when you are making only a few small boards.
You don't need a darkroom, UV lamp, special pre-sensitized boards, carcinogenic or caustic developer / stripper solutions, or multiple attempts to get the correct exposure either, so it's also cheaper to get started. But if you later go into volume production, many of the following instructions will apply as well.
There are also some laser printer methods available, where instead of felt-tip resist you print the desired circuit on special resist film that can then be laminated directly to the copper-clad circuit board before etching. These systems can be quite temperamental to get the lamination working properly, especially with a clothes-iron as the heat source. If you want to experiment with this method, the ready-made MUX demo board layout files contain suitable artwork that you can print out directly.
CIRCUIT LAYOUT:Lay out the design on 0.10" grid tracing paper (frosted mylar is better) as though you were viewing the circuit from the top (component side) of the board, looking through it with your X-ray vision to see the copper conductors or "traces" on the bottom side. (What, you don't have X-ray vision?) Use a normal black pencil for the bottom conductors and red for the top-mounted components. Mark a black dot where each hole will be.
Don't agonize over the getting "perfect" layout: You may need to use one or more plain wire jumpers where you can't easily avoid the need for two conductor traces to cross.
Be sure to consider how the finished board will be mounted, and mark mounting holes as well... typically one at each corner.
Standard Dual Inline Package (DIP) integrated circuit (IC) chips have 0.10" spacing between pins and 0.30" between rows. The parts are shipped with a slightly greater row spacing and must be bent to fit the 0.30" spacing when installed.
Don't try to run traces closer than 0.10" center-to-center. That will allow the lines to be drawn later with a standard felt-tip marker and still leave reasonable spacing between them. Commercial boards can use narrower spacing, and even run "sneak-through" traces between the pins of a chip, but that would be very difficult with the hand-drawn method.
Allow at least 0.40" between the holes for each standard 1/4 watt resistor if you want it to lay flat in the normal manner. Alternately, though not recommended, you can choose to stand these on end for much closer spacing, at the cost of greater height of the finished circuit.
Frosted mylar has a big advantage over tracing paper for the layout, since you can erase and redraw repeatedly as you experiment with different arrangements. Look for this at a major art supply store, especially one that caters to a university clientele. If you can't find it with a 0.10" grid, you can always tape plain frosted mylar over graph paper which will show through while you draw.
If you are only going to make one small circuit, it's probably not worth a big quest to get this stuff... thin graph paper will work just fine, or even thick graph paper if you have (or can rig up) a simple light box to view the final result from the back.
The very best bet is to use a Computer Aided Drafting (CAD) program, which can keep track of conductors and components on separate "planes" of the drawing, and can print out normal or reversed views on standard paper. CAD makes it MUCH simpler to rearrange things as you work out your design. Just make sure that you can get the 0.10" spacing correct on the final printer output, or the pins of the IC chips won't fit!
A CAD program is worth learning for future projects, but if you don't already have one and know how to use it, it's probably overkill just for a small project.
Complete CAD layouts for the MUX board are provided for standard printers. These can serve not only as examples of a typical board layout, but also can be used directly to allow you to jump into board construction.
BOARD SELECTION:Cut a sheet of single-sided copper-clad circuit board to the proper dimensions, as needed. You can cut larger sheets with a hacksaw, or by scribing heavily on both sides and snapping, or with a heavy-duty industrial sheet-metal shear if you have access to one. Sheets in standard sizes like 4" x 6" are available from Digi-Key, Jameco, or Radio Shack. (See Resources section at end.)
The industry-standard board material is "1 oz. FR-4 Epoxy" or words to that effect. The "1 oz." refers to the weight of copper per square inch of board surface. Don't get 2 oz, since it takes just about forever to etch and is hard to solder because it is such a good heat sink.
"FR-4" is the designation for the Fire Retardant standard, and "Epoxy" or "Epoxy-Glass", etc, indicates that the board itself is made from fiberglass mat laminated with epoxy resin. You might also find "Phenolic" instead of epoxy. This is OK for small low-voltage circuits, and is gentler on drill bits, but it is prone to warping.
The board thickness will typically be 1/16" (0.062), but this is rarely critical for small projects.
MARKING DRILL HOLES:Tape the finished layout right-side up to the top (unclad) side of the board. Actually, since the layout will typically be on a sheet that is much bigger than the board itself, you will be taping the board to the back of the sheet. Alignment is much easier if you have included the board outline on the layout, since from the back you can't see the circuit portion to center it... it will be covered by the opaque board.
Place a sharp point (centerpunch, scriber, or even a sharpened nail) over each hole location and gently tap through the pattern to mark the spot. You can use a small scrap of wood for a "hammer"... a real hammer will probably be too heavy.
DRILLING:Next, you must drill each hole you have marked. If you are using standard fiberglass epoxy board, it will simply eat up any normal drill bit, assuming you even have one small enough. You might get away with this if you have phenolic board, or for a few (larger) mounting holes in epoxy, but in general you need CARBIDE bits for the circuit holes.
And there is a MUCH better way than conventional drilling: Use a small high-speed hand tool like a Dremel, and use a carbide ball-tip dental bur instead of a drill bit. You can get these from dental supply houses, or maybe you can work out a deal with your dentist for the small quantity you will need.
The style of dental bur you should get has a standard 1/16" cylindrical shaft to fit in the corresponding collet of your Dremel tool. The shaft tapers down to where the actual ball is attached. The ball has 6 tiny curved blades or flutes.
These typically come in standard diameters that are numbered in tenths of a millimeter, so that number 010 = 1.0 mm. That is a good general-purpose size that will handle the thick leads of most large components, including TO220 power transistors and voltage regulators. But for most holes, including integrated circuit pins and standard component leads, you should use an 007 or 008 bur.
In general, you should use a hole size that just fits the leads or pins. If it's much larger the holes will take up so much space that you won't be able to fit much copper pad around them without running into adjacent traces. That would make it doubly hard to solder, since the solder would need to bridge a larger gap from the wire lead to the edge of the hole, plus there wouldn't be a good anchor for it there.
You may, however, want to get a larger bur like 012 or 013 for carving slots for larger tab-mount components like certain trimmer potentiometers.
If you have seen commercial carbide printed circuit drill bits for sale... AVOID THEM LIKE THE PLAGUE! If you try to use these in a manual setup like a Dremel tool, or even in a small Dremel drill press, they will break before you get to the third hole. The tips are too long and fragile, and any sideways force will snap them instantly... carbide is super-hard, but it's also super-brittle.
Dental burs, on the other hand, are totally forgiving: You can use them hand-held with impunity, even if you don't start the hole out straight. You can even "carve" with them to enlarge or elongate holes... no problem. (Think about what your dentist does to teeth!) They last almost forever.
Place your circuit board over a piece of scrap wood while drilling, and allow the bur to go right through into the wood a little. (Save the scrap wood for future drilling... it may end up looking like Swiss cheese, but it will still work fine for this purpose.)
Since you will be drilling tiny holes that are closely spaced, you will need good lighting. Your face will be very close to the work, so be sure to wear safey glasses or goggles. Pay attention to the air inlet and exhaust on your handpiece, to avoid blowing dust into your face. You may need to pause frequently to blow the dust from the circuit board so you can see the marks for the remaining holes.
When you are done drilling, take a moment to protect your dental bur. Although the ball tip is very hard, the shank is mild steel and will bend easily if you drop the handpiece while the bur is attached. This is about the only reason you will ever need to discard a bur... it won't "wear out".
If you can find a piece of metal tubing that is just the right diameter to fit over the end of the handpiece, you can make a slip-on cover that will shield the bur. (You can hacksaw a slit into one end of the tubing if it's too tight to fit.) Or you could make a custom wooden tool-holder for a regular workstation. But even if you have to dismount the bur from the tool, you should get in the habit of protecting it.
After all the component holes are drilled, you can use a conventional low-speed drill for the few mounting holes.
BOARD PREPARATION:The holes on the metal side of the board will now have rough edges from the bur piercing through. You must sand these smooth to the touch, without scratching up the rest of the copper surface. The best way to do this is by wet-sanding with a small piece of "Wet-or-Dry" sandpaper, 400 grit or so. (This is the black or dark gray paper.) Rinse frequently while you are sanding. The same scrap of paper can be used repeatedly for many boards.
Next, the copper surface must be scoured clean to remove any oils or oxidation that didn't come off in the sanding. Use a regular kitchen-type steel wool / soap pad like "Brillo" or "SOS" for this, then wash the board with a drop of dish detergent and rinse thoroughly to remove traces of soap. You may want to try other scouring pads like "Scotch-Brite", etc, which can be used directly with dish detergent. Dry with a clean towel... the copper should have a soft shine. Avoid touching the surface.
DRAWING THE RESIST LINES:The actual circuit lines can now be drawn directly on the copper surface with a "permanent" felt-tip marker. The marker acts as a "resist" to prevent the etchant from affecting the copper beneath the lines. Make SURE the marker is a permanent type, since it must withstand immersion in etchant. "Sharpie" fine point permanent markers work well. Use only BLACK... other colors, even in the same marker family, are typically not as resistant to etchant. Avoid the "extra fine tip" versions... they dry out too fast during use. You may also want a large flat-tipped marker for ground, power, mounting, or unused areas that surround the circuit.
Of course, since you will be drawing on the copper side of the board, you must flip the layout over to use the reversed image as a guide... that's why you used tracing paper or mylar. If you used a CAD system, you can just print out a reversed version for this purpose. If you used heavier graph paper, you will need to place it over a light box to see the reversed pattern.
Since you have already drilled the holes, this is now simply a case of "connect the dots" with your marker. The reason you had to sand the edges of the holes is to prevent snagging the fiber tip of the marker and pulling out wisps of felt. Also, the little "mounds" around each hole would have prevented the marker from contacting the surrounding surface. You need a small area of copper "pad" that extends all the way around each hole to aid soldering.
As you work, try to avoid contacting the board with your bare hands. You don't want to smudge the wet lines you've just drawn, of course, but you also don't want to leave hand oils in undrawn areas that will cause the marker to skip. Try working toward yourself from the furthest edge of the board, with a sheet of blank paper setting on top the undrawn portion to shield it where you must rest your hands.
While you are drawing the lines, you will undoubtedly discover a hole that you missed marking or drilling. Relax. Just draw a little pad in the proper spot, and you can drill it later, after the board is etched. Since you will usually have plenty of other points around the location, it's easy to "eyeball" where it should be drawn relative to the rest.
If you draw a line in the wrong place, let it dry for a minute and then use an ordinary pencil eraser to remove it. Make sure the eraser is clean by first rubbing it on a piece if plain paper.
If the error is a small one in a dense area of the board, you may instead opt to scrape it off gently with a sharp blade, such as an eXacto knife. This is useful, for instance, if you accidentally let two adjacent pads touch... you can just scrape a clean path between them.
After you have drawn all the traces, go back and widen the power and ground areas, possibly with a separate marker for "fill". Typically you will have a large ground area that extends from the circuit region out to the edges of the board. You can fill this in to provide a "ground plane" for better conduction and shielding, and also to reduce etchant waste.
Consider how this ground area will interact with your mounting holes. If you will be attaching the circuit to a metal case, do you want the circuit to be grounded to that case? This is typically desired for shielding purposes... but if you don't want that here, be sure to isolate pads around the mounting holes that are large enough to clear screw heads, etc.
ETCHING:When you are all done and the ink is dry (a few minutes) it's time to etch. The proper etchant is ferric chloride, which is available from your board supplier. Use a glass or plastic tray for etching (NOT metal!), and dedicate it for this purpose alone... ferric chloride is not only TOXIC, it also stains many things permanently. Old darkroom trays work well. You will never get your tray really clean afterward, so don't even think about borrowing kitchen utensils here!
Ferric chloride works best if it is warmed to about 100 degrees F or so, but this is not critical. You can set the tray on an old heating pad if you are working in a cool area. Be sure to remove any fabric cover from the heating pad first, and since you will certainly slop some etchant onto it you will probably want to dedicate the pad for etching only, just like the tray.
Store your etchant in the bottle it comes in, and fill the tray about a half inch deep or so for each etching session. The circuit board will float on the surface if you set it down carefully... copper side toward the etchant, of course! To avoid having to touch the etchant with your hands, use a strip of masking tape wider than the tray, and affix it to the board. Then just hold the ends of the tape and lower the board into the etchant. Immerse one end first and tilt to immerse the other, squeezing out any trapped air bubbles.
Slosh the board back and forth gently to dislodge any residual bubbles, then stick the ends of the tape to the outside edges of the tray. Check the board every 10 or 15 minutes to monitor the etching process: Simply peel back the tape ends and tilt the board enough to see if the unmarked copper is gone yet. This typically takes 20 to 40 minutes. It's OK if you leave it in a little longer, but since eventually the marker lines will dissolve, you don't want to just go away and forget it.
Here's an important tip about an amazing etchant phenomenon: New etchant is typically packaged at a higher concentration than ideal. While this does mean you are getting a "good deal" for your money, it will actually etch SLOWER than the proper dilution... sometimes ridiculously slower! So you should be prepared to adjust the concentration as needed. Also, as the etchant ages and some evaporation takes place, you may need to adjust it again. (That's why you should store it in it's airtight bottle instead of just putting a lid over the tray.)
The proper concentration is a specific gravity of 1.30, or 30% more dense than water. You can measure this with a device called a hydrometer, but this is not something most people have lying around the house. If you have access to a good balance, you can determine the weight of a known volume of etchant and add water accordingly. Water has a specific gravity of 1.00 by definition, which means 100 ml of water should weigh 100 grams, hence 100 ml of etchant should weigh 130 grams.
But if you want to avoid having etchant in contact with your measuring vessels, you can use the trial-and-error method with fair results: If the etch time exceeds 40 minutes, add about 5% water before you use it again.
When the board is done etching, rinse thoroughly in tap water. Remove the marker ink with the same scouring pad used to clean the surface earlier, following this as before with a drop of detergent and a water rinse before drying.
TINNING THE BOARD (OPTIONAL):To give your board a professional look, prevent future oxidation of the copper, and make soldering easier, you can "tin" your board all over with solder before you install any components. To do this you will first need to coat the raw copper with a thin film of rosin-based liquid soldering flux, which you can apply with a disposable tissue. (NEVER use acid-based plumbing-type flux.) Rosin flux is very sticky stuff, so use an old magazine or newspaper to protect your work area.
Then hold your soldering iron tip flat against the board and apply normal rosin-core electronic solder. It will spread as it melts, and you can "paint" it back and forth across the board quite easily to plate all the copper.
Hold the board up to the light and view from the top side to locate any holes you have plugged with solder... they can be easily unplugged by passing the iron over them again.
You may sometimes get an imperfect etch, where there are small "freckles" of copper left between the traces. Now is the time to remove these: They are so small that the soldering iron tip can easily overheat them to the point where they unbond from the board, since there is no surrounding copper to act as a heat sink. Just "scrub" them gently with the iron until they lift, and sweep them away.
Since solder smoke may contain trace metals and other things that it would be better not to breathe, you should not only work in the proverbial "well-ventilated area" but also use a small table fan to pull smoke away from the work area. Don't blow the fan onto the board: It will cool down and make soldering very difficult. Ideally, you should have a carbon filter to pull the smoke into before running it up a vent hood, if you are going to do go into circuit production.
When the board is all tinned, there will be patches of dark hardened rosin on the solder, and sticky rosin on the bare board. Ordinary rubbing alcohol will remove these. One trick you may want to try is to keep a whole bottle just for this purpose. Put the board in a tray and pour the alcohol over it, then scrub with an old toothbrush. Return the alcohol to the bottle afterward and you can reuse it many times.
Rub the board dry, and it will have a dazzling chromed appearance. Even if tinning didn't have any other advantages, it might be worth it just for aesthetics alone!
COMPONENT INSTALLATION:Insert all the IC chips first, without soldering. They will almost always have their two rows of pins spread more than the proper 0.30" when you get them. One simple way to install them is to insert one row, and then push the whole chip gently against that side as you slip the pins of the other side into their holes one by one. Or you can try bending all the pins on a side by pushing the chip against the top of the work surface.
To avoid overheating the ICs, solder only one or two pins at a time on each chip, then move to the next chip in rotation until all pins are soldered on all chips. Transistors, diodes, and LEDs can be handled the same way.
Insert several resistors, and splay the leads slightly to hold them in place while you solder. Solder one lead on each before returning to do the remaining leads. This is not just to keep them from overheating, but to prevent bad solder joints: If you try to solder the second lead right after the first, you may end up putting a load on the first joint before it is properly solidified. Capacitors are handled just like resistors.
After soldering each little group of components, snip off the excess leads with diagonal cutters as close to the board as possible. Now it will be easier to solder the next group. (This does not apply to integrated circuit chips... they don't stick through the board enough to matter anyway.) Save the wire clippings in case you need little jumpers elsewhere on your board.
If you discover when you are all done that you have made an error in the circuit layout, you can often repair it just by carefully cutting unwanted traces with your Dremel tool, and soldering small pieces of insulated wire as jumpers to the correct locations. If the board is very complex, this will be the method of choice to add design modifications as well.
Another repair / modification method is to build a "piggback" circuit on a small scrap of board. This can be fabricated via the same printed circuit methods, or directly wired on a little piece of perfboard. Attach it to the "mother" board by stubs of wire directly soldered to each. The wire clippings from resistors and capacitors that you have already installed are ideal for this.
Since the "daughter" board will typically need power, ground, and an input and output, there will be at least four points of support. Keep the wire stubs short by planning the daughter board placement over the proper spot on the mother board. You want room to get the soldering iron in to attach these stubs, but not much more than a half inch for maximum stiffness.
ABOUT SOCKETS:Avoid these except for expensive parts. Not only do they add to the expense, but they actually reduce reliability by providing many little connections that can oxidize. Also, they won't work at all for double-sided boards using these simple construction methods without plated-through holes.
DOUBLE-SIDED BOARDS (ADVANCED):For more complex projects, you may find that your layout has more jumpers than you can tolerate. This is particularly likely with digital circuits, since there are no resistors that act as natural jumpers for other traces to pass beneath.
When you reach your personal jumper limit, the next step is to use a double-sided circuit board. Layout is similar to a single-sided board, except you should use a different color (like blue) to distinguish top traces from bottom traces. For CAD designs, the top traces will be on a different plane of the drawing.
You should still try to keep the top (component side) traces to a minimum, simply to aid in soldering later. The top and bottom traces will be connected where component leads pass through. For places where top and bottom traces must connect but no component is present, just put a hole. This will become a pass-through or "via" by soldering a stub of wire there later.
Mark the holes and drill and sand as before. You will find that you need to sand BOTH sides now, since the copper will acquire a little rounded "volcano" around each punch-mark. Then scour and clean both sides of the board.
The top side must be protected while you work on the bottom side. Although you can use a broad-tip marker to cover the whole surface, this coating is quite fragile and will suffer little scratches and pinholes. Apply a better protective layer over it in the form of vinyl contact paper, wide masking tape, or wide cellophane tape. Whatever you use, be sure to burnish it down well along any seams and in areas where there are drill holes. This will keep etchant from leaking in while the bottom is being etched. Since capillary action will try to wick the etchant under the tape around the holes and along the edges, it is a good idea to use both marker AND tape to be safe.
Now draw the bottom side and etch as usual. Rinse, remove the tape, and rinse again to get rid of etchant that seeped under.
Now scour the marker from the top side. Leave the marker on the finished bottom side for now, and touch up any areas that have been damaged during etching, before applying protective tape as you did earlier for the top. Don't do any tinning yet... wait until both sides are done.
Now draw the top side and etch and rinse as before. Scour the marker from both sides and you are ready for tinning or final assembly.
Note that unlike commercial double-sided boards this method does not have "plated-through holes", so you will need to solder BOTH sides of any component that connects to top and bottom traces. This can be particularly tricky around IC chips, since you don't have access under the package itself.
If the trace connects to a pin on the top side and continues under the chip while remaining on the top side, you need to insure that there is a good connection to the trace covered by the chip. The drill hole must be smaller than the trace, so that an adequate skirt of copper surrounds the hole.
Note that you can't use ordinary sockets with this method, since they don't leave any top surface of the pins exposed for soldering. If you are installing an expensive part and feel you really must have a socket, you will need one of the more exotic socketing methods like separate pin sockets or all-metal breakaway socket strips.
Digi-Key Corporation (Components, incl 1% metal 701 Brooks Ave, South film resistors, boards, Thief River Falls, MN 56701 supplies) Order: 1-800-344-4539 FAX: 218-681-3380 Web: http://www.digikey.com Dremel (High-Speed Hand Tools) Web: http://www.dremel.com (See local hobby shops and craft stores.) Jameco Electronics (Components, 1355 Shoreway Road boards, Belmont, CA 94002 USA supplies) Order: 1-800-831-4242 FAX: 1-800-237-6948 Web: http://www.jameco.com Radio Shack (Components) Web: http://www.radioshack.com (Find your local store via locator on Web.)
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