A few years ago, I started experimenting with homemade prototype PCBs as an alternative to professionally fabricated PCBs from board manufacturing company. My company was flexible enough to give me some resources and time to explore the subject matter. What I discovered was that with a small initial investment, you can make reasonably high quality two sided boards. In addition, all the equipment needed was easily accessible. I’ve decided to put my findings into this guide. Hopefully some of my fellow hobbyists will find the information useful.
Introduction
Ever since I started working, I have always been amazed at how fast technology moves. Being in the technology business certainly exposes you to vulnerabilities of this trend. A few of the projects of the past have been victims of this “greatest and latest” arms race. As a direct result, these projects have failed miserably.
There are other problems with technology, not the least of which is that the state of the art is always very expensive. I cannot speak for other fields and disciplines, but this is almost always true of consumer products. The driving factor behind these products is silicon, because without it, there would be no electronic revolution.
Within the discipline of electronics engineering, itself a sub-discipline of electrical engineering, one of the major tasks is to connect the silicon. Typically, all electronic products of today have one or more PCB (printed circuit board) connecting various ICs (integrated circuits, or chips), resistors, capacitors and other passive components to make a circuit that performs amazing feats. However, just as with any other field, the state of the art of PCB can be quite expensive. NASA’s 10+ layered PCBs used in satellites usually costs tens of thousands of dollars for the board alone, and that’s NOT including the parts! Well… we call them NASA boards in the industry, because only NASA can afford them; I don’t actually know what goes into a NASA PCB. All joking aside however, PCB’s is a major cost factor during manufacturing. These factors must be considered during the design phase. For a small company or a hobbyist, the prototyping cost of a PCB is an even greater financial detriment. The board below is a video processing board that I designed. This was a 4 layer board with full PCB specs, and was about 300 dollars each.
A few years ago, I started experimenting with homemade prototype PCBs as an alternative to professionally fabricated PCBs from board manufacturing. My company was flexible enough to give me some resources and time to explore the subject matter. What I discovered was that with a small initial investment, you can make reasonably high quality two sided boards. In addition, all the equipment needed was easily accessible. I’ve decided to put my findings into this guide. Hopefully some of my fellow hobbyists will find the information useful. The circuit below is a VGA to DVI converter that I designed, and fabricated in the kitchen sink. Took a whole afternoon to drill all those holes.
A word of advice, if you’d rather just shell out a bit of money for your boards, it’s really not that expensive for the lower end of the PCB spectrum. Advanced, for example, sells full featured 2 layer boards for only $33 each. For barebone boards, I’ve purchased them for as little as $11 dollars, with minimum quantities of 5 or more.
On the higher end of the spectrum, San Francisco Circuits provides advanced technology circuits such as fine line/HDI, PCB assembly and flex boards. You can also get consulting services from the higher end board houses. I’ll explain what the terms “layers”, “full featured” and “barebone” means later in the guide if you are unfamiliar with PCBs, but just keep in mind that if you are having trouble making the boards yourself, you can always buy them pretty cheap.
However, the biggest motivation to make the boards at home is the turnaround time. Usually the cheap economy services at the board houses are 5+ business days, plus shipping. This means that from the time you submit your design to the time that you get your board in the nice Fedex package, two weeks could have gone by. I can usually make a two layered board in 2 hours or less. In the world rapid prototype iterations, it doesn’t get any better than that. This means that if I screwed up the design in the morning, I can cover my ass by the afternoon. What more reason do you need?
So, without further ado, let’s get on with the business of making PCBs. Before you can make them however, you got to know what printed circuit boards are.
PCB Basics
Before we can make a PCB, let’s take a look at its composition. If you deal with PCBs all the time (as in, you do PCB layout all day long), you can probably skip this section. However, if you’re a bit shaky on PCB concepts, it’s best to at least skim through this section.
PCBs stands for printed circuit boards. They are called “printed” because you print your circuits out onto the copper. With the design printed, you then either mill or etch your prints into the copper. The general process is actually quite complex, especially with quality control considerations and efficiency measures implemented in large fabrication houses. However, the process can be simplified into manageable steps such that home fabrication is possible. We will also be skipping a ton of steps, just because some of the more complicated features such as silkscreen and multilayer (more than two layers) is impossible at home. But I digress, here’s what you need to know about the PCB itself.
The surface of the PCB has several features. You will probably notice right away, when picking up any typical PCB, that the majority of the surface is covered with green stuff. This is called the soldermask, and it is a dielectric (insulator). It actually has several specific tasks. First, it is there to prevent corrosion, as the oxygen in our atmosphere is quite toxic to the copper on the top and bottom layers. Next, it has the job of preventing accidental shorts from occurring. The exposed copper is very vulnerable to paper clip drops and loose screws. Best to cover it up with green stuff that won’t conduct.
The next thing you will notice are the tiny lines that run across the surface of the board (albeit covered in green). They are the copper that reside on top of the PCB. This is how electrical connections are created from one electronic element to another. The term used to describe these lines is “signal trace” or just “trace”; they describe the trace that a copper takes from one point to another. Next, there are the pads. These are exposed bits of metal covered in tin (through electroplating). They are exposed so that the pins on your ICs and your resistors can be soldered onto the board. The tin does not oxidize, but is still conductive. This property protects the underlying copper, while still allowing an electrical connection to occur to the component being soldered. A plus side is that the metal tin is a major component in modern solder, such that the flow of the solder is facilitated by the tinned pads. Lastly, the colored letters and markings seen on top of the soldermask are called the silkscreens. They are aptly named since the markings are applied to the soldermask through a silkscreen process. It is essentially a stencil made with a thin membrane, onto which colored ink is applied. This layer allows the PCB designer to label the components, and indicate switches and functionality.
However, there are things underneath the surface that cannot be seen with the naked eye, but play a key role in the functionality of the board. Below is an example of a 4 layer PCB, typically very cheap to manufacture.
On any given PCB, you can only see the top and bottom copper traces. Underneath however, there may be many layers of copper creating connections between components. The cost of a PCB is generally dictated by the number of layer. These layers increase the number of possible connection options between components by allowing traces to intersect one another without shorting out. For very dense circuits such as mobile devices, more layers are need since the number of connection per area is high. On circuits with lower densities, a lower number of layers is preferred since it reduces manufacturing costs.
The round circle like things that can be seen on the surface of PCBs are called vias.
These are drilled holes that create the connections between the layers. The holes are actually drilled after the copper traces are created, and synthesized through copper electroplating. With a combination of traces and vias, the PCB designer is allowed to create circuits in three dimensions.
Finally, the layers between the copper (labeled “core” and “prepreg” in the above picture) are FR4 (most of the time). The abbreviation stands for Flame Retardant 4, created out of fiberglass and resin. These insulators create the structure of the board, and gives it rigidity. The copper on each of the conductive layers are grown onto the FR4, then etched off in acid to create the traces. Each stack, consisting of one layer of copper and one layer of FR4, are then put together on a heated vacuum press, and allowed to meld together into a single board structure.
The goal of our manufacturing process is much less ambitious. We will be constructing a two layer board, with no soldermask, no pad tinning and no silkscreens. The process is usually called “barebone” since it only contains the bare essentials of a functioning PCB. As long as the signals pass through, it can be technically called a circuit board.
I am always amazed at these marvels of manufacturing technology. I think a lot of people take for granted the plethora of electronic devices that are used in our daily lives, simply because there are so many of them. However, the production of a PCB is by no means an easy task. Next time you type on your keyboard or power up your favorite MP3 player, please refrain from being impressed by the functionality of the gear. Take a moment and marvel at the humble PCB that is surely lodged inside their plastic shells.
In the next section we will make preparations for the manufacturing process.
Preparations
There are a few basic precautions when working with wet chemistry. If you think back really hard, long ago, in a galaxy far away… …chemistry class in high school, I’m sure you are reminded of the basics of lab safety. Always wear safety goggles, and wear protective gloves. Also don’t smell stuff from beakers whose contents you do not know. In fact, just don’t smell beakers, they usually don’t smell very good.
Basic Process
The basic process goes something like this. We will be buying the boards with resist already coated. For the technically uninitiated, a resist is a thin layer of chemical that is deposited on a substrate (in this case copper) which will mask it from the subsequent steps. We will then expose the copper using printed transparencies, and develop the exposure using a commercially available developer solution. Then we mix our own solution and etch. The result is a kitchen made PCB that should be good enough for most small to medium sized hobby level projects. It doesn’t cost too much either.
Materials
Anyways, you will need some basic equipment to make the PCBs. Here’s a shopping list.
Hydrochloric acid 30% – available at home improvements stores, sold as masonry cleaners, and usually labled as “muriatic acid”. This is basically hydrochloric acid (HCl) at around 31%. If you have access to HCl, you can mix it yourself. Remember if you are mixing the HCl yourself, ALWAYS add the acid to the water, not the other way around. The acid is required to lower the Ph so that the copper will oxidize.
Hydrogen Peroxide 3% – we need an oxidizer that will provide the oxygen. H2O2 can be found at the pharmacy, and is sold as ointment to clean newly opened wounds. You can also buy it off of McMaster in crates.
Sodium bicarbonate – baking soda is available at any old grocery store. Good for putting out fires and neutralizing acids (I’ll let you guess which function we’ll be using it for).
MG Chemicals 600 series – this is a commercially available copper board with the resist already coated, sold by MG chemicals, 600 series, available at Digikey. You can pick and choose which size fits the prototype best. I usually get a few single and double sided boards if I don’t know the complexity of the project beforehand. 1.6 mm thickness is the standard size for most enclosures.
MG Chemicals positive developer 418 – this is the developer that goes along with the 600 series boards. As you will see later, the resist that is coated on the board needs to be developed (like a photo), and this solution will do just that. Available at Mouser.
UV lamp – this is usually not that hard to find. You will need to expose the resist somehow. Any source of stable and reliable UV is fine. The sun is NOT a stable and reliable of UV (ever tried getting a tan in the winter?). Try to have a fixture for the lamp so that it is raised about 5 – 8 inches from the table. MG Chemicals sells a convenient little kit if you are short on time/creativity.
Thick piece of glass – I cannot stress how important this material is. When we are going to do the exposure, the transparency will be taped to the PCB. However, we need to make sure that the transparency is as close to the PCB as possible so that the shadows are crisp. Get a piece of glass and lay it on the mask, this will sharpen your shadows and make the etching much easier. Available at McMaster, or just break a window and grab a piece, preferably not your own (but don’t blame me when the police comes).
Two trays – You will need a container to do the developing and etching. Glass do not react to too many chemicals so they are a good candidate.
Chemical flask or graduated cylinder – needed for measuring the solutions and chemicals, available at McMaster.
Chemical squeeze bottles – after mixing the solutions, a good way to dispense them is through a squeeze bottle, for easy clean up and convenient use, available at McMaster.
A good printer – since PCBs stand for printed circuit boards, you need a printer. This is where it gets tricky. I started using laser printers, but I discovered that when using transparencies, the heat tends to shrink the transparencies just slightly. The result was unreliable footprints and scaling. For this reason, I strongly recommend inkjet printers. I use a Canon PIXMA 4500 series.
Transparencies – inkjet transparencies are different from laser transparencies. Make sure you get the right one for your printer.
Isopropyl alcohol – or any kind of cleaning solution. Acetone would work too, so would nail polish remover (which is diluted acetone).
Magic marker – for correcting errors. I use black.
Preparing Solutions
Two of the three solutions used can be prepared ahead of the time. Unfortunately, the most critical solution, the acid-oxide, cannot be prepared ahead of time because the oxygen tends to escape and does not provide enough oxides to scrub off the copper. However the developer and neutralizer can be prepared and stored for months. Follow the directions provided by the manufacturer and load the solution into a squeeze bottle (usually something like add 10 part water to 1 part developer). Next drop two table spoons of baking soda into hot water. Mix until no visible baking soda is present and load into another squeeze bottle. This is your neutralizer. Should you spill the acid on the table, in the sink or on your skin, squeeze the acid with this solution as soon as possible. Don’t worry, it’s just baking soda (unless you are allergic to baking soda, then you’re on your own).
Another thing, make sure you label your squeeze bottles. You don’t want to be spraying the acid when you should be neutralizing (something this author knows a thing or two about).
In the next section we’ll take a look at ECAD considerations.
ECAD for Home Fab
Before making the mask, we need to prepare our artwork that will be transferred to the copper. Because we are making circuits at home without the help of precise machinery, we need everything to have loose tolerances. Loose tolerances help to increase yield (percentage of successful circuits).
Standardizing Measurements
For the sake of being on the same page, the circuit industry has some measurement conventions. 1 mil is 0.001 inches. This is the de-facto standard for all measurements on a circuit board. Metric values are starting to become popular, but as of right now, many fabrication houses still uses mils. I prefer to use millimeters, but it still good to be able to convert back and forth.
The thickness of copper placed on top of the FR4 material is measured in Ounces/(square ft), or just oz. A piece of circuit board with 1 oz copper means that 1 oz of copper was rolled out on 1 square foot of circuit board, which is roughly 1.4 mil thick.
Circuit Board Specifications
A PCB fabrication specification of 6/6 mil/1 oz means that the smallest possible feature on a circuit board is 6 mil of copper and 6 mil of spacing between coppers, on a 1 oz thick copper board. Below is a diagram of the type of circuits you can build using the above specs on a 2 layer board.
As an example, here’s a screenshot of the 33 each promotion by Advanced Circuits. It specifies 6 mil trace/6 mil space/1 oz copper. This is a typical spec for low-end circuit board services (as for October 2012).
If we were to layout our circuits using 6/6 mil then etch the circuits using 1 oz copper, I would estimate that my home made circuit yield is about 50%. Etching 0.006 inch traces of copper is rather difficult. 50% yield is not a great number, since it means that I have to make two circuits every time I want one single functional circuit. If I increase the spec to 8/8 mil, then etch using 1 oz copper, the yield goes up dramatically, to maybe about 80-90%. This is a pretty good number, and is the specs that I usually use for home made circuits. If the circuit uses large components and I’m not too worried about space, I can increase the yield even more by using 10/10 mil on 1 oz copper. This almost guarantees that I get a fully functional circuit, but the larger dimensions make circuit layout a bit more difficult.
Transferring Specs to ECAD
Modern electronic computer aided design (ECAD) software has the ability to store “rules”. These are checks that the software applies to your artwork to make sure that you meet the specifications of the board house. Since our board house is the sink in our kitchen, we have a set of specs that we’d like to meet. In my ECAD software suite, I have rules for several board houses, as well as a special set of rules for home made circuits. Make sure you apply these rules before printing your artwork mask. ECAD software suites usually have a design rule check (DRC) step which checks your design for problems with respect to the specs designated in the rules list.
I want to make the point that streamlining the circuit fabrication process is very important. Because we are experimenting with tolerances, we need to document the relative quality of the circuits for each rule set. We will then know which set of rules will give relatively good yield, and which rules will produce poor yield. For my equipment, the specs of 8/8 mil with 1 oz gives a relatively good yield. I have discovered this after many cycles of experimentation. However for others trying to replicate the results, with different printers, different exposure equipment, tighter or looser tolerances may be applied. Your initial circuits will NOT look good, but do not get discouraged. Keep changing your process or the rules and think of ways to improve the quality of your circuit build process. After a few iterations, the circuits will improve dramatically.
Ground Fills
What are ground fills? Well, imagine a circuit board with just traces. There is usually a lot of wasted space. If we were trying to etch the circuit, we need a lot of etching solution to erode all that copper. However, we can minimize the amount of copper that needs to be etched by filling the GND node of the circuit for all unused space. Below is a proximity sensor circuit that I designed for home fabrication. It is shown without ground fills, with the GND node highlighted.
You can see that the most of the space is wasted and requires removal. We can add a step in our ECAD process to add ground fills for the GND node to minimize the amount of copper that needs to be removed.
After the ground fill, the circuit looks like the above pictures. For home made circuits, ground fills are good. We will use a lot less etching solution to create the circuit, and we’ll have a smaller mess to clean up.
Panelizing
The last point I’d like to touch on is a process called panelization. Once the tolerances are well understood and a prototype circuit is debugged and confirmed to be working, we might be tempted to make several circuits in one go. Many ECAD suites has the panelize function which allows you to create copies of your circuit multiple times. An example of a panelized accelerometer sensor circuit is shown below.
Once the ECAD process is finished, it’s time to print a mask. We’ll take a look mask preparation in the next section.
Mask Fabrication
Photo-lithography has got to be one of the coolest inventions ever, and mask making is one of the most important steps in the process. Photo-lithography is the reason why computer chips are so cheap, and why circuit boards are a dime a dozen. In this section we will look take a look at how to make an impeccable mask, and be on our way to making our own printed circuit board.
The Process
In case you are new to the idea of making PCBs, the process is rather simple. Below is a simplified version of the steps that we will be taking to get a finished PCB.
Since the traces on the copper is of a scale that is printable from an ink-jet printer, we will print a pattern onto a transparency. Next, we transfer the pattern to a pretreated piece of copper covered in photo reactive acid resistive chemical. This chemical is commonly called “resist”. The mask is placed on top of the PCB and exposed to UV rays. Once the resist is exposed to UV rays, it changes properties. The areas that are exposed to the UV rays can easily be washed off with a special solvent. The portions that is not exposed to UV rays will not wash off. This step is call “strip”. Once the stripping is completed, the circuit is ready for etching. It is dipped in acid until the copper is etched off. Remember that some of the resist is still applied onto the copper. The acid will only attack the areas where the copper is exposed and unprotected by the resist. After about 10 minutes, all the copper will be eaten away and you are left with a circuit. The remaining resist is then removed and the circuit is ready for assembly.
Quality Print
Because the quality of the pattern transfer directly affect the etching of the board, the making of the mask and the exposure onto the pretreated PCB is probably the trickiest step in the whole process. A screw-up during this process will result in a broken trace or unwanted shorts in the finished product, so pay attention! Every little detail counts.
First, wear some gloves when touching any of the materials. I prefer textured nitrile because they give the fingers a bit of grip, and they don’t stretch that much. Any oil that is transferred to the mask and the copper will show up as unwanted artifacts on the final circuit. You know those Intel commercials where the technicians are wearing bunny suits? It’s exactly like that. A clean work environment will give you better looking circuits and less screw-ups. It’ll also protect you from the various chemicals used during the fabrication process.
Whatever ECAD program you use, all of them will have the option to create a Gerber file. You can use a free Gerber viewer and then print your pattern, or these days, a lot of ECAD programs will have the option of being able to conduct scaled prints. Use the HIGHEST quality setting on your ink-jet at 1:1 scale and make sure the ink-jet transparency is on the correct side! Print your mask in black.
In case you are wondering, you really do need an ink-jet. I’ve experimented with laser printers in the past, and the problem is that when the ink gets heated up, the pattern shrinks by a bit, and unevenly in different direction. This causes the pattern to be transferred wrong, and your fine pitch IC patterns will not match the copper. Since ink-jets do not heat the ink, the pattern remains true. I use a Cannon Pixma 4500 series and it is one of the best cheap printers for this kind of jobs. Also a word on transparencies. You need to get ink-jet transparencies. Both laser and felt-tip pen transparencies will cause the ink to run. Ink-jet transparencies have tiny little holes that will trap the ink, resulting in a dry print (rather than black ink all over the place). These transparencies will have a printing side and a non-printing side. Make sure you use the correct side. If you are careful with all of these details, you should end up with a mask for your circuit that looks like this:
As you can see, I’ve taken the liberty of panelizing my circuit such that I get 16 circuits in one go. This is common practice, and definitely worth the time if you are making more than one circuit. Also note that on my artwork, I have flooded the entire circuit with ground fill (copper tied to ground). Since the area covered by the mask will be protected from the etching process, the ground fill reduces the amount of etching solution that we need to mix. This is a good idea if you want to reduce the amount of “wet work” that needs to be done for each PCB fabrication cycle. It only takes 2 minutes on the ECAD program.
That’s it for now, next we expose the mask.
Pattern Transfer
We need to transfer the artwork on the mask to our copper. I’ve included photos in this part of the tutorial for easy reference. Note that the photos are from two batches of circuits to show various aspects of the exposure and development process.
Cutting the PCB and Taping the Mask
Now that artwork has been printed, we need to prepare our PCB. First, we want to reduce the amount of copper that is exposed to the etching solution. This will help us conserve the etching solution and reduce clean up. We first cut the PCB to the shape our mask.
We will be using the MC Chemicals 600 series PCB material, single sided, 1 oz copper, available on Digikey. The boards come with a sticker on the copper side to prevent any light from deteriorating the sensitized resist applied on top of the copper.
Measure and mark out the shape of the circuit. A felt-tip pen works best because it will not damage the sensitized resist.
Next, cut the PCB into the shape of the mask. I use a hand saw handle that can take hacksaw blades.
Once the PCB is cut, the copper will have a very rough edge. Take a file and de-burr the edge so that there are no sharp edges left on the cut material.
The little bumps on the edges of the copper will prevent the mask from sticking on the copper directly, and will cause diffraction patterns on the copper during exposure. If we skip this step, the edges of transferred pattern will not be sharp, and as a result, the copper etch will not yield high quality circuits.
We will then peel the protective layer on the PCB material then align the mask onto the PCB.
You must do this step quickly, and away from any source of UV rays, such as sunlight. Take some masking tape and affix the mask to the PCB.
Exposure
Now we are ready to expose the pattern to the chemical. Before exposure however, make sure you have mixed your developer solution and have the developing tray ready. We need to develop the pattern right after exposure.
If you are using a UV lamp, you’ll need to expose the circuit for about 8 minutes. My lab has a UV exposure oven used to trigger UV glue, but I found that it is great for making circuits as well. This is probably my favorite machine in the lab, right up there with the fancy oscilloscope. The UV oven allows me to expose the circuit for only 2 seconds, which means the lamp in the oven is about 240 times more powerful than a typical consumer level UV lamp. Before the lab got the powerful UV oven, I just used a consumer level black light. The results are about the same, but the oven allows me to create circuits much faster.
Place the PCB and mask directly under the UV lamp, then place your heavy glass over the mask. Make sure there is no debris between the mask and glass. The heavy glass will press the mask as close as possible against the copper.
Because the pattern is pressed against the copper, the artwork will transfer flawlessly to the copper. If there is debris, or a gap between the mask and the glass, artifacts and blurry areas will be transferred to the copper.
Development
As soon as the exposure is complete, remove the mask from the PCB. The masks can be reused over and over again, until the pattern wears out from use. I have my collection in a filing cabinet and find myself reusing some of the masks maybe a handful of times a year.
Place the PCB in the developer tray and spray it with the developer solution.
This will wash away the areas that was exposed to the UV rays and leave the rest of the areas intact. You will see the pattern develop on the PCB after 10 to 20 seconds of contact with the developer solution.
As soon as you see the pattern emerge, you need to remove the solution from the surface of the copper by running the circuit in water. If the copper is left in the developer for longer, all patterns might be washed away, including the masked areas.
Let the developed PCB air dry and keep all objects and hands away from the delicate surface. Once all water has been removed, we can inspect the pattern.
Touch Ups
As careful as you might have been during this process, there might be small artifacts and imperfections in your PCB pattern. A fine tipped marker will act as a decent resist and will protect the copper from being etched. If you spot a broken trace for example, you can bridge the broken trace with a small dot on the offending area. This can potentially save you from doing another exposure if your transferred pattern is of inadequate quality.
Now that the pattern is transferred to the copper, we can proceed to the etching portion of the fabrication process.
Etching
The removal of the copper is accomplished through a step called etching. We will mix a solution of hydrochloric acid (HCl) and hydrogen peroxide (H2O2), which, when exposed to copper, will oxidize the copper and strip it from the FR4 material. With the artwork pattern transferred to the copper, the PCB is now ready to be etched. The remaining resist left on the copper will protect the pattern from being etched, while the exposed copper will be stripped off.
Safety
ALWAYS wear goggles when playing with HCl. Goggles are a requirement for this step. Acid is no joke and can be very corrosive, so please be careful. I usually wear gloves, goggles and long sleeves for the etching step. Always have a squirt bottle of baking soda and water ready. If you do get some acid on your skin, use the baking soda/water solution on the exposed skin as soon as possible. This will neutralize the acid and save you from a nasty irritation and/or minor scarring. Since the HCl is only 30%, it is more or less diluted, but care should still be taken.
Etching Solution
The mixture should be 2 parts H2O2 to 1 part HCl. Use a graduated cylinder for this step. For small circuits up to 2 square inches, 10 ml should be enough solution for the etch.
For larger circuits up to 10 square inches, you might need to mix up to 50 ml of solution. Scale up as required by the size of the circuit. If ground fills are used, this should scale up pretty nicely. However, if there are large areas that requires complete copper removal (for example in RF circuits), you’ll need a bit of extra solution to remove the extra copper.
Etching
Once the mixture measured, pour the solution over the PCB in the etching tray.
The copper will turn pink immediately. This is the oxidation process in progress.
Gently sway the tray in a circular motion. The key word here is “gently”, you don’t want to get the etching solution everywhere. The slight movement will allow the etching solution to circulate and speed up the oxidation. The etching solution will start turning green.
After about 5 to 8 minutes (for 1 oz copper), parts of the circuit will have etched through. The etched through areas will dark at first, then clear up. Wait until all exposed copper is etched, then wait 1 more minute to make sure that all unwanted copper is etched off. The exposed areas will be translucent as the FR4 material is exposed.
Drain the etching solution in a glass jar. Do NOT pour the solution down your sink as you might etch some pipes down the line. I keep the solution in a large jar and dispose of it once a year at a facility that takes chemical waste. There are ways to keep this solution and reuse it over and over. I’ve experimented with these techniques and I have to say that they are rather cumbersome to exploit. The oxygen in the solution gradually escapes, which makes the solution less effective in scrubbing the copper off. The basic idea is to blow air into the solution either with a straw or a aquarium pump to regenerate the oxygen in the solution. I would recommend experimenting with the technique if you are producing large quantities of PCBs. For my personal use, I find that quickly mixing a new batch has more benefits.. There is also a lot less maintenance of the chemicals by mixing a new batch (ie. testing for temperature, acidity, specific gravity then adjusting for variations from the ideal conditions). Overall, for small batches it is cheaper, faster and less of a headache to mix the solution anew. The trade-off of course is that you need to dispose of the solution every once in a while.
Use the baking soda and neutralize the PCB. Then washing the PCB thoroughly under running water until all traces of acid has been disposed.
Stripping
Now that the copper has been etched, we need to remove the remaining resist. I usually use a foam brush, acetone and some paper towels. Alcohol and nail polish remover might also work, but might be a bit slower to remove than acetone.
Once the resist is stripped, the circuit is complete.
The whole process usually takes 30 to 45 minutes. It sure beats sending a design out and waiting 2 weeks for the boards to be delivered. This particular circuit was a USB joystick controller. I was able to get 15 working circuits out of the 16. This is a yield of 94%! The non functioning circuit was fixed with a small wire jumper, so I still ended up with 16 working circuits. Not bad for the kitchen sink.
You can use the handsaw to de-panel the circuit. I find that a hand saw works better than mechanized saws. Something about the FR4 tends to heat up the blade on mechanized saws, which leads to premature warping and dulling.
Multi-Layer PCBs, Tips & Tricks
So you want to make more complex circuits and a single layer is just not enough. What can you do? Well, Digikey has a selection of double sided pre-sensitized boards for etching. We’ll just need to create two masks and repeat the process on both sides. Then we need to connect the two layers.
Mask Preparations
A major difference between a single layered board and two layered board is that we need two masks that are aligned together. Since vias are used in multi-layered boards, we can usually line them up using the via patterns. Print the top and bottom masks as you would a single layered board, then tape them together, using the vias as alignment cues. I usually use Kapton Tape for this step. You’ll end up with a mask like this:
Next, I either oversize the PCB by a little bit, or cut a small corner off of the mask on one size. We need to make sure that the PCB does not move when we flip the mask and PCB over during the exposure step. You’ll notice on the mask photo, I’ve cleared two of the corners. Once the PCB is placed under the corner, tape it to the mask using these exposed corners.
Next, expose as you would a single layer board. Flip the mask and PCB sandwich and do the same on the other side. Etch as you normally would and you are well on your way to a two sided board.
Drilling
My drilling set up is a Dremel with a mini drill stand.
The drill bits are available from Think and Tinker. You’ll need to get the drills sizes corresponding to the vias that you will be making. I generally find that 0.65mm, 0.80mm, 0.85mm and 1 mm are the most useful.
I get the drill bits with the stop rings attached, which makes changing the drill bits a lot faster. Manual drilling, to say the least, is a very slow and painful process. To make things easier, if your ECAD tool can put a small void in the middle of your vias and through-hole pads, the etched away copper will actually help guide the drill bit to the center of the drill location. This will make the drilling a lot easier and faster, and basically will “center punch” your drill locations during the etching process.
Making Via Connections
There are several options for how to connect these two layers. The simplest is to drill a hole, solder a wire solder on one end, clip, then solder the other end. This is great if you only have 10 or less holes, but if you have 200 holes, this is not very practical (my neck gets cramped pretty quickly when doing this).
The fastest option I’ve found is to use a product called “Track Pins” made by a company called Harwin, available from Newark.
These are tiny pins that are tapered on one end and has a “head” on the other. They come in a long strand so you can just poke and break them off. Make your vias 1.65 mm (65 mils) wide and use a 0.8 mm (31 mil) drill bit. Solder both ends and you have homemade vias.
However the Harwin track pins are rather large and cumbersome. They are extremely difficult to layout because they require so much space. They also protrude high above the surface of the PCB so you cannot put components over vias. An alternative is the PCB Rivets by Favorit, available through Mega Electronics.
The website says they require a tool to close the rivets, but I just hammer them down manually. Solder both ends for a snug fit. There is still a bit of a bump with these rivets so avoid using them under components. Since they need to be placed individually into the drilled holes, they are a lot more cumbersome to use. Make your vias 1.1 mm (45 mil) wide and drill using a 0.65 mm (26 mil) drill bit. They are much smaller than the track pins, which makes the layout a lot easier. The tradeoff is how difficult they are to drop into the drilled via holes.
Tips and Tricks
Personally I like to avoid making two layer boards as much as possible. When it is that much trouble to make a board, I’ll just shell out the money and wait two weeks. The time saved on layout and fabrication can be put to other engineering efforts. However, there are some tricks you can use to make complicated circuits even on a single layered PCB.
Mixing through-hole with surface mounts – If you are using both through-hole and surface mount components on your board, mount them on opposite sides. This way the copper connecting the components can all reside on the surface mount side. It’s near impossible to solder through-hole components on the component side anyways.
Using jumpers for a few connections – Sometimes you just need one or two connections on the second layer. Instead of making a complex two layer board, just use a few jumpers. The effort used to solder a few extra passives is greatly less than the fab work for making a two layered PCB.
Unused pins on microcontrollers (or other programmable devices) – If you are having a hard time making connections to a dense pitch IC, remember the unused pins on the microcontroller can be set to “input” mode and accept voltages up to the device’s maximum operating voltage. This is a neat trick if you need to run a track “through” an IC, or connect two ground fills together “through” a microcontroller. Just make sure that the pin is set to input so that you don’t burn out any output drivers on the microcontroller.
That’s all for now. I’ll post more if think of any.
