PCB Processes

8/14/2019 PCB Processes

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PCB - Copper Printed Circuit Boards

Solderless breadboards are the fastest and most flexible way to design andexperiment. When the circuit is ready for more permanent use, there are severalmethods of connecting components together that are appropriate for one-timedesigns in robots. Point-to-point soldering can result in a rat's nest problem.Homemade printed circuit boards are a good solution.

I purchased and read the wonderful book Making Printed CircuitBoards. The book covers much more than is described on this page. I highlyrecommend you read the book before attempting to make your own circuitboards.

Choosing a Sample Circuit

Although I eventually want to use surface-mounted components and double-

sided circuit boards, I decided to eliminate additional new variables for my firstattempt. I limited the first project to through-hole components (such as standardDIPs) on a single-sided circuit board.

I'm experienced with through-hole components and I've soldered 0.1-inch point-to-point perf boards. I saved myself a lot of trouble and frustration in learning anew technology by building on top of what I already know well.
http://www.robotroom.com/BugdozerBrains.html#RATSNESThttp://www.amazon.com/exec/obidos/ASIN/0070027994/robotroom-20http://www.amazon.com/exec/obidos/ASIN/0070027994/robotroom-20http://www.robotroom.com/BugdozerBrains.html#RATSNESThttp://www.amazon.com/exec/obidos/ASIN/0070027994/robotroom-20http://www.amazon.com/exec/obidos/ASIN/0070027994/robotroom-20


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The chosen circuit isn't important, although it should be fairly simple for your firstattempt. I decided to create a daughterboard that contains complete circuits for avoltage regulator and two DC motor drivers (H-bridge motor driver) forSweet, aline-following robot. This board includes resistors, small DIP sockets, capacitors,a switch, an LED, Molex connectors, diodes, a jumper, and a few crossover

wires. An excellent variety of common components with which to experiment!

Free CAD Software

Before writing this web page, I'd never used a PCB CAD layout program before. I

don't own any PCB CAD software and I didn't want to pay hundreds of dollars tobuy a package. One popular freeware choice is CadSoft Eagle.

ExpressPCB also has free Windows OS software that is quite good and easy toget started with. The advantage to using their software and mini-board size isthat, if desired, you can order professionally made boards, 3 for $59 (the pricehas varied slightly over the years). This is a nice deal for robot hobbyists thatdon't need production quantity runs. Their boards are of higher quality (greateraccuracy, plated through holes, and tin coating) than is usually possible to makeat home. Over the years, I've ordered over a thousand boards from ExpressPCBand have been very pleased with their on-time service, reasonable prices, andexcellent product. (No, I don't get any special deals or discounts.)

The ExpressPCB online help is written with beginners in mind, the internetordering option is convenient, the software has never crashed, and they updatethe software regularly with new features.

The disadvantage to using their software is that its breadth of capabilities andcomponents library are somewhat limited, although you can create customcomponents. Also, you're stuck with their file format, which is a problem if youwant to order production boards from someone else. One day I decided todecode their file format and was bothered to find that it was encrypted. No bigdeal, I managed to trace through their code and extract their decryption routine.It's a shame though, because I wanted to create and post some utilities foradding artwork, spilling ground planes, or printing cleanly at home. Now I don'tfeel comfortable about doing so since they apparently felt they had a reason forprotecting their file formats.

For all the positive reasons, I chose to use the free ExpressPCB software, and Ihappily continue to do so to this day.
http://www.robotroom.com/HBridge.htmlhttp://www.robotroom.com/Sweet.htmlhttp://www.cadsoft.de/http://www.expresspcb.com/http://www.robotroom.com/HBridge.htmlhttp://www.robotroom.com/Sweet.htmlhttp://www.cadsoft.de/http://www.expresspcb.com/


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Avoiding False Starts

When trying to design a circuit, I got overwhelmed on the first couple of attempts.I discarded several windows and files. I was trying to make too large of a leap bydesigning with surface-mounted components, two-sided boards, components onboth sides, and very thin traces.

To get beyond this mental obstacle, I designed the circuit by physically laying outcomponents on an ordinary perf board with pre-drilled holes every tenth of aninch. Then I copied the patterns into the ExpressPCB software and switchedback-and-forth as the circuits progressed. On the screen I could see efficientchanges, which I then tried on the physical layout. Sometimes I would notice aproblem on the physical layout that I needed to correct on the screen.

I worked on a single functional unit at a time (like the voltage regulator) and onlyproceeded after I had perfected that section. This allowed me to experiment,apply, and learn layout skills without having to go back and throw out a lot of prior

work.


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Final Circuit

The picture isn't to scale. The final board is 3.8 inches by 2.5 inches to comply

with the size of ExpressPCB's mini-board service. Of course the software cancreate other board sizes.

The green lines are copper connections on the bottom (lower layer) of the board.Notice the green text appears flipped since it is being seen through the invisibleboard.

The red lines and circles (through-hole vias) are on the top layer of the board.Because through-hole components are being used instead of surface-mountedcomponents, each red circle represents a hole that needs to be drilled (ick!).

The yellow lines and text are component outlines that won't actually appear onthe copper of the board. Instead, more expensive boards can be ordered with theyellow portion printed in ink. For my purposes, the yellow is ignored.

The white dots won't appear on the copper either. They're used as on-screenguides for positioning components at tenths of an inch.

Red and green circles appear at all four corners for drilling holes for mounting


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screws.

My finished layout reflects simple, beginner choices:

.040" line (trace) widths are used instead of smaller .012". The thickerwidths aren't electrically necessary, but they provide a margin of error if Imistakenly etch the board for too long. Thinner traces might disconnect(open).

Each .056" round via (.035" hole) is supplemented with a larger 0.030"

radius circle of 0.020" line thickness . This enlarges the pad tocompensate for manual drilling errors and improve solder contact area(more on this later).

All holes and component connections are centered on the 0.10" grid. This

allows the layout to exactly mimic a prototype breadboard. The corners of traces (lines) aren't squared. That is, I tried to avoid turn

angles of 90 degrees or less . Although not technicallynecessary with such thick traces, I wanted to practice rounding corners

to reduce noise that will occur on smaller, thinner lines.

Only four connections are made on the top layer (red). Since a single-

sided board is being created, these top connections are made with point-to-point hand wiring.

Plenty of space is reserved between components. Plenty of space isreserved along the board edges. This board is going to be hand populatedand soldered, so I'm saving myself frustration by spacing generously.

Almost the entire circuit exists on the lower layer (green). Don't design onthe top layer (red) and then mirror the image using a paint program tomove it to the bottom layer. I did that and it ruins the left-to-right order onthe chip connections. Put another way, if the circuit is intended for the

bottom of the board, design it on the lower layer (green) to begin with.

Using available space, label major connections and components with texton the top (red) or lower (green) layer. Consider adding other information(copyright, date, board function, or revision number). Don't use thesilkscreen (yellow) layer for text unless you don't want it to appear on yourprintouts.


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Align all components vertically, with polarized (+/-) components in thesame orientation. Consistent direction makes it easy to see on the finishedboard if any component is placed incorrectly (horizontal or diagonal orinverted polarity).


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Printing Around ExpressPCB

Although it would be nice to print from within ExpressPCB, printing from their Filemenu includes a grid and grayscale background that messes up the copperlayer. (Version 2.1.0 through 4.1.2 tested.)

I wonder if this was on purpose to prevent hobbyists from creating their ownhomemade printed circuit boards using ExpressPCB software? I don't see how ithurts them for newbies to learn PCB layout on their software. To the contrary,this prevents potential customers from testing their circuit before ordering. (DearExpressPCB, If you're reading this, please clean up the printing! Be confidentthat the quality of your product and service is enough to attract regular businessaway from home-etched boards as people get tired of etching their own or createa successful design that they want multiple copies of.)

No problem, you just need to take a screen shot and print from a graphicsprogram! Here's how:

1. Choose Options in the View menu.2. Turn off "Show Grid".3. Turn off "View top copper layer" or "View bottom copper layer" depending

on which you don't want to print.4. Turn off "View silkscreen layer".5. Turn off "View corners".6. Switch to the Colors tab.


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7. Change Background to white.8. Change Top layer to black.9. Change Bottom layer to black.10.Change Board edges to black (if you want a border to help align or cut

on).

11.Click the OK button.

Now the window appears exactly as it needs to be printed. All extraneous partsare hidden and all copper parts are in black.

12.Zoom (enlarge) until the board image is as large as can possibly fitcompletely within the window.

13.Take a screen shot (press both the Alt and Print Screen keyssimultaneously).

14.Run a paint program. For example, Microsoft Photo Editor or Paint ShopPro.

15.Choose Paste (or Paste As New Image) from the Edit menu. The screenshot should now appear in the paint window.16.Crop as desired.17.Print from the File menu. Microsoft Photo Editor's print dialog allows a final

size to be specified in inches, which is handy. Otherwise use the percent% scaling option until the printed image matches the desired size.

What a pain! Okay, that's what I get for using free software.

Creating Transfer Film

I purchased a five-sheet package of Techniks Press-n-Peel PnP-BLUE PrintedCircuit Board Transfer Film for Laser Printers and Photocopiers. At ElectronixExpress it costs $9.90 (#N03PNPB5). The cost decreases and approaches adollar a sheet in larger quantities.

After testing the printout on inexpensive plain paper, the circuit should be printedto the dull side of the PnP-BLUE transfer film.
http://www.elexp.com/http://www.elexp.com/http://www.elexp.com/http://www.elexp.com/


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I made a couple of mistakes with mirror images and I made some changes to thecircuit. As a test, I cut out the used portions of the blue paper and then re-fed theremainder back into the printer. Although I can't guarantee this because it mightharm your printer, the final circuit transferred just fine even though the same

sheet of blue paper (becoming ever smaller as I cut off the printed portions) hadmade three passes through the printer. I've repeated this technique over theyears, and it continues to allow me to reuse the unprinted portions of the sheet,saving money.

As an alternate to Press-n-Peel, many people have had good luck with cleartransparency sheets or glossy paper.

Transferring to the Copper Board

Plain, copper clad PC board can be purchased from many different retailers.Expect to pay $2 for a 4-inch by 4-inch board.

If you're using transfer film or press-on labels, then don't buy pre-sensitizedboards. They're for transferring the image photographically.

Try not to use double-sided boards if only one side is going to have a circuit. Theunused copper must be removed by the etching solution. It takes longer to etchand uses up the etching solution sooner if large, unused areas of copper areexposed.

If necessary, cut down the board to the approximate size. A table saw with adiamond blade or a shear are the best choices (Micro-Marksells both). However.a Dremel with cut-off blade or a hacksaw works okay. In any case, be sure towear goggles and a breathing mask.

Always clean the board with water (perhaps some cleaning solution) and ascouring pad. The scrubbing removes grime and oxidation, thus preparing thesurface for a crisp transfer and ultimately firm soldering.
http://www.micromark.com/http://www.micromark.com/http://www.micromark.com/


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After drying the copper board, place the blue transfer paper with the image(toner) side against the copper. Carefully position the image -- the clinging of theplastic to the clean copper helps. Then place a piece of ordinary white paper overthe blue paper. (The white paper helps the iron glide without catching or

smearing the blue paper.)

Follow the transfer paper's directions for ironing. Usually a low synthetic to woolsetting for a couple of minutes (it usually takes me up to five minutes); no steam.

Pressure

Although you want to cover the entire board with enough heat and pressure, theboard edges just seem to be the most difficult to transfer correctly. For bestresults, concentrate the tip of the iron on the outer edges while the rest of theboard receives the less intense (but more constant) heat and pressure of themiddle portion of the iron. With the iron tip, carefully rub each and every detail

that appears near and edge or corner.

In my first attempt, I failed to iron the very edge of one corner of the board. Eventhe thinnest lines of the middle-located text transferred well, but the missededges failed to adhere to the board.

Either let the board and blue paper cool off or rinse them in cool water. I triedboth methods and they worked equally well (I tend to run the board under waterso that I can see the results right away). When the board is cool, slowly peel offthe blue paper to reveal the transferred image.

Double-check the inverted image on the blue paper for failed toner transfers. Anyfailed transfer will appear as black toner remaining on the blue paper. This will bethe most common problem encountered.


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Filling In Gaps

Use a permanent marker or rub-on patterns to replace missing toner. Thepermanent marker portions must dry for a few hours and usually works betterwith a second coat. My initial attempt failed as the "wet" marker portions etchedaway almost as much as the completely exposed areas.

Etchant

The etchant is a chemical that reacts with exposed copper and removes it fromthe PC board. Copper covered with resist (toner, rub-on patterns, tape?, andpermanent marker) won't be removed, although it will begin undercutting if leftsoaking for too long.

Ferric chloride (iron chloride hexahydrate; ferric trichloride hexahydrate; FeCl36H2O) is a popular chemical for etching. So, too, is ammonium persulfate. Assoon as you begin handling chemicals, be sure to wear protective rubber gloves,old clothing, and even safety goggles. Make sure you're in a well-ventilated areawith a source running of water.

I use ferric-chloride crystals. When I added them to the water, they reactedviolently. The reaction released vapor, noise, and heat. Add the crystals slowlyand carefully!


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Store the etchant in a non-metallic jar with a non-metallic lid. Label the etchantand store it away from children.

This is corrosive and poisonous!

Only properly equipped, educated adults should perform the etching portion of

circuit board creation.

Search the web for product data safety sheets. Print the safety sheets, readthem, and then store them with the chemicals. Discuss the contents of the safetysheets with your family and those that may run across the chemical.

Etching

Warm the jar of etchant in a bucket of warm water (or use an aquarium heater).

Don't heat the etchant above 115 degrees Fahrenheit or vapors may bereleased.

If desired, a very small aquarium pump can be used to provide aeration duringetching. Aeration greatly improves etching speed. Pumps and plastic aeratingblocks can be purchased from retailers that sell the etchant chemicals andcopper boards.

Pour just enough etchant into a plastic tray that contains small plastic risers toprevent the circuit board from sitting directly against the bottom of the tray. Thecircuit board should face down, so that the copper drops off as it is etched. Thetray may already contain a plastic block with holes and a tube to an aquariumpump.

Using plastic tongs, place the circuit board into the tray. Turn on the aerationsystem (if desired). Use the tongs to inspect the progress of the board and toagitate the board and tray if necessary.

With bubbling etchant and close board inspection, it should be clear why rubber


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gloves, goggles, fresh air, and old clothes are necessary. Using all plastic partswhen handling a metal-corroding chemical should now seem obvious. Be sure tosegregate all the parts that have been exposed to etchant away from other uses.In other words, don't stick that etchant tray back into the kitchen cupboard.

The etching should take 5 to 20 minutes depending on board size. If it takesmuch longer, the etchant may have dissolved its fill of copper, and should bedisposed of properly.

All exposed copper should now be etched away on the board, with only resist(blue film or toner) remaining.

Clean Up

I'm not sure of the legal and proper method of disposing of used-up etchant. Iwould be happy to link to an EPA page or other official document if you runacross one. Obviously, proper disposal is going to depend on the type of etchantchemical. Whatever you do, don't pour it down into anything with copper pipes!

For ferric chloride, I've heard that you should neutralize the acid by adding abase (sodium carbonate, sodium hydroxide, or calcium carbonate). This alsosolidifies the liquid, and after drying, it can be disposed of as solid waste.

Store any partially-used (but still good) etchant in a recycled sealed plasticcontainer separate from the completely-unused fresh etchant (if any remains).Although you can always "top-off" a partially-used batch of etchant with freshetchant, you don't want to put partially-used etchant back into the remaining freshetchant. It just seems like etchant "spoils" (becomes an ineffective sludge) a fewmonths after being exposed to copper.

Wash all the parts, trays, and circuit boards with plenty of fresh water.


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Drilling Holes

Some people like to drill the holes before etching. I prefer to etch first to allow theetchant to carve starting center holes. The holes tend to guide the drill bit to thecenter of the via.

A drill press and a 1/32 bit works okay for most resistor and DIP holes. A #66 bit iseven better. Micro-Mark sells #66 bits for $6.95 a half-dozen.

I used my Dremel drill press at low speed. I broke three bits. I guess I wassupposed to use it at high speed and also prevent the board from shifting. Anexpensive and inconvenient lesson.

I wore goggles to protect against flying debris and a filter mask to protect mylungs from board dust.

Use Scotch-Bright (green color) pads or steel wool to remove the blue resist anddrill dust, exposing the clean, copper circuit.
http://www.micromark.com/http://www.micromark.com/


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Oops!

Okay, you've used up transfer film, etchant, and expensive tiny drill bits. You'vealso cut boards and spent all sorts of time cleaning, ironing, etching, cleaningagain, drilling, and cleaning again.

Miss anything?

Perhaps you didn't double-check your test plain-paper printed pattern on somechips to make sure you didn't have the entire thing mirror-image flipped?

That's right. The first time I did this I designed the whole thing on the top layer(red) even though I planned to have the circuit on the backside of the board(green). Thinking I was smart, I simply flipped the design in the paint programbefore printing. Although the lettering transferred in the correct orientation, thecircuit no longer matches the left-to-right pin order of the chips. If I had correctlyused the CAD software, the reversed lettering on the screen would have

transferred and etched in proper orientation, as well as the rest of the circuit.

(for comparison, the GND hole isn't drilled)


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Vias Too Small

Because I went ahead and finished the board with the mirror-image mistake in it,I learned some additional lessons before starting a fresh board.

It's tough to line up the drill with the via holes for the through-hole components.Being slightly off can pull-up or drill-out a lot of the copper pad. This reduces thecontact the solder and through-hole wire can make with the circuit. With this inmind, extend the diameter of via pads for hand drilling.

Even though I could use thinner traces (lines) for low current digital signals, Iswitched the entire board over to thicker traces. Since the board had the room,why not? (As long as adequate spacing exists between traces.) It saves onetchant, prevents breakage, and may improve noise tolerance.

However, don't make the trace lines as thick as the pads or via through holes.The circles should be a little bigger than the lines they connect to, so that thesolder wicks around evenly rather than heading down a line.

After mounting and soldering components, I have the first circuit board I've evermade!


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Now I've got the materials (except drill bits) and I've learned the process. Makingreliable, repeatable printed circuit boards is much faster than point-to-pointsoldering. My choice of components has expanded to include surface-mounteddevices with tiny pins. I can make lighter, stronger Sumo bots. I can make a line-following-robot circuit board, test it, order high-quality boards from ExpressPCB,

a maybe make a kit for my local robot club.

My technology has made it into the 1970s. Now on to double-sided circuit boards
http://www.robotroom.com/PCB2.htmlhttp://www.robotroom.com/PCB2.html

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