Most hobbyists say that it is easier to build a functional prototype of an electronic device, than to make the enclosure for it. You could say that there are a lot of ready-made enclosures on the market, but they are never exactly what you need. You could also use a 3D printer to build a custom enclosure, but high-end 3D printers are too expensive, and the cheaper ones produce housings which are often not robust enough, and also require a lot of additional treatment.
Another way is to build the enclosure out of FR4, a material which is commonly used in PCB production. Such enclosures are low-cost, with thin walls but yet very strong, nice looking, pleasant to the touch and have excellent thermal and moisture stability. FR4 offers some more possibilities – efficient wiring with no wires inside the housing, integrated UHF or SHF antennas or RFID coils, capacitive switches, electrical shielding, selective semi-transparency, water or air tightness, and even integration of complex mechanical assemblies.
Here I shall explain the process of building those “magic” enclosures. It is based on nearly fifty years of personal experience and more than a hundred enclosures, built for most of my projects. Here are two examples – this case for a hardware password manager is just a few centimeters long, while the other one (protective transportation cover for my son’s synthesizer) measures 125cm (about 49 inches), and yet both of them are strong enough to withstand a grown man standing on top of them.
The global approach is simple – you take the sheet of single-sided copper clad FR4, cut it and solder the parts together. That sounds simple, but there are a lot of details which should be met if you want to get top results. Please read about them carefully. You might be tempted to skip some of the steps described here, but if you do so, you will most likely end up being disappointed with the results.
Tools and Materials
- A lot of single-sided FR4, preferably 1.5 mm thick (it is classified as 1.6 mm, but when you measure it, it always turns out to be 1.5). In some cases, you will need some other thickness.
- Medium power soldering iron (about 40 W, or 60W if it is temperature controlled).
- Soldering wire with rosin core, about 1 mm diameter
- Soldering flux
- Transparent self-adhesive laminate
- Safety blade cutter with a lot of spare blades
- Fret saw frame
- Fine rasp
- Fine sandpaper
- Ferric chloride solution
- Undiluted alcohol
There are several general design concepts for enclosures made of FR4. The first uses the main PCB, with all electronic components, as a mechanical base on which four walls will be soldered. Typically all electronic components should be soldered first, then PCB is tested and finally the walls soldered together.
Main PCB must have a copper area around 4 mm wide along all of it’s edges and large pads for spacers. Top and bottom covers are very simple and they need no copper areas, so you can also use acrylic, aluminum, plastic, etc. This concept is suitable for simple projects, which contain only one PCB. It is not advisable for thick enclosures, as it would be inconvenient for maintenance.
There is another approach which is suitable for bigger and more complex devices, as there is a lot of free space which can be organized according to the requirements of the project.
You should leave copper areas for screw nut soldering, but do not solder them before the whole enclosure is finished. After the job is done, you can close it and bore the holes at the inside walls, so they will fit perfectly with the outer holes. Then you can solder screw nuts, using a lot of flux.
Note that not all screw nuts are easy for soldering, so you will need some experimenting. In my experience, nickel plated brass screw nuts are the best in this regard, but sometimes they can be hard to get. So I use M3x5 mm spacers (but not those made of aluminum), which are very easy to solder.
If you want to make an enclosure without screws, you can use friction to hold two subassemblies firmly together. Finish the upper (outer) part of the enclosure first, and then adjust positions of inside walls on the lower cover. Outer walls will always be slightly inclined to the center (the reason for this will be explained later) and it will create some friction when you close the box.
To adjust the tension, solder the inner walls to the lower plate, but only at the central point, trying to maintain the right angle between the walls and the bottom plate. Then try to put on the top cover. It might be too loose or too tight, so you have to tune up the right position for a tight fit. Repeat this for shorter walls and then solder the length of the walls to the lower plate, and to each other. You should also rasp the connecting edges of the inner walls, to make space for solder which keeps the outer walls together.
The tightest joint between inner and outer walls should be at the center of the walls, with the possibility of a gap close to the edges, so the inner walls need to be slightly curved, like show in the center image above.
Generally, all plates should be made of single sided FR4, but in some special cases you’ll need double sided. Try to find the double sided wall on the drawing show to the right in the gallery above.
Those were general concepts, but all combinations are also possible. Soon we shall see that some applications are so complex that it is hard to classify them.
To etch or not to etch
Although it is possible to build the usable housing without copper etching, it is highly recommended to etch all plates, and to leave only areas (about 4 mm wide) which will be soldered. This will bring a lot of advantages. First, it is much easier to solder the narrow copper areas, when the surrounding copper does not consume a lot of heat, so the risk of overheating and damaging FR4 base is much lower. Second, you will get a neater interior. Also, if there is no bare copper, no corrosion will take place. The danger of short circuiting is lower, and so on.
If there is a demand for a shielded case, for HF applications or a light shielded area, this pattern is recommended. The only disadvantage of etching is that you must plan all details while designing the enclosure, as there is no undo after etching. If you omit some area that should be used for soldering, you can not add the copper layer later.
Take some time to design the enclosure carefully, as every miscalculation in this step may ruin your effort. Take good care of which piece will overlap with another one, as it will affect the position of copper traces. The thickness of FR4 used must be taken into calculation for the dimensions of overlapping plates.
Use some CAD program, or draw the whole enclosure with colored pens. It will serve as a useful reference while cutting and assembling the parts. It is good practice to build at least a few millimeters larger enclosure than you planned at first. In most cases, it will pay off later – no matter how carefully you measure and plan, sooner or later you will need some extra space.
There are a total of 10 sheets here. Draw each of them in detail, including etched copper positions and all dimensions, or at least those which are not considered.
If you are fortunate enough to have a CNC cutter, then use it to cut FR4 parts. Any mechanical or water jet cutter will be fine, but do not use a laser cutter, as the reflection from the copper surface can damage it. Otherwise, you can use some hand tools, like a fret saw frame. A safety blade cutter can also be used for cutting straight lines, but the demand for significant force and accuracy can be frustrating if you are not experienced enough.
Try it, just press a metal ruler firmly with one hand, take the safety blade cutter in the other and make a V-cut. You should make about 5-10 blade passes on each side of the FR4, starting lightly until you make the good scratch, and then increase the pressure until you use as much strength as you can. Make the V-cut on the copper side first, as FR4 is transparent enough to let you see the line on the other side, so you can align both V-cuts precisely.
If the blade is sharp enough and you use significant force, grooves on both sides will be deep enough and if they are well aligned, it will be easy to break the sheet by hand. The cut line will be rough, so you should treat it with the fine rasp.
Of course, you can’t cut curved and inner lines with a safety blade cutter, so in that case you have to use the fret saw frame.
Check the dimensions carefully after each step, and do your best to get as accurate dimensions as possible (try to limit the error to less than half a millimeter). After cutting and treating, wash the copper side with some powder detergent, abrasive enough to clean the surface but not to damage it.
Note: FR4 is made of fiberglass matting which has been impregnated with epoxy resin. Small particles of glass, which are released during treatment, may cause temporary skin irritation to those who have sensitive skin. In that case, use protective gloves and wash your hands up to the elbows after work.
Laminating and copper etching
The goal of this step is to create the mask for copper etching. You can also apply the photoresist process, but it is much cheaper and easier to use any kind of self-adhesive laminate, preferably transparent.
When FR4 pieces are dry and clean, stick the laminate sheets on the copper side, avoiding bubbles and dirt. Use a super-thin permanent marker to draw all lines precisely on the laminate, according to the drawing mentioned above in Design Concepts and use the safety blade cutter or art knife to cut the laminate. Peel off all areas which should be unprotected during etching.
Use Ferric Chloride solution to etch the copper layer. This process is well known to all hobbyists who make their own printed circuit boards. After etching, remove the laminate and wash all pieces with powder detergent.
Hold your breath, this is a crucial step. All copper surfaces should be tinned before final soldering; use a lot of flux, as a lot of flux means a good joint, and the good joint is what we need here. Don’t attempt final soldering without tinning, as it could result in a poor joint, damage the FR4 base, and reduce your ability to maintain the exact positions in assembly process.
It is hard to ensure alignment on all axes (X, Y and two angles) while soldering FR4 plates together but, if you follow the procedure, you can tune one parameter at a time.
First, take care of X position (black arrow) only and ignore the accuracy for all others. Solder two plates at one point, close to one of the edges.
Next, you can tune the YB position at the other end of the soldering line. FR4 is elastic enough to compensate for the initial misalignment, but X position, which was already adjusted, will remain intact.
Now it’s time to readjust the YA position at the first point, without worrying about first two tuned positions (X and YB), as there is no way to spoil them any more. This should lead to the perfect alignment of the whole soldering line.
The only thing left is the angle between two plates. Use a square or triangular ruler for reference, but do not adjust the exact right angle between plates, but rather make it slightly obtuse. You can fine-tune the angles at each edge separately, especially if the plate is long enough to enable bending between them.
When all positions are adjusted and checked, you can put more soldered points between plates, checking the angle for each of them. For shorter plates it should be somewhere in the middle, or at every 4-5 cm (1.5-2 inches) if the soldering line is longer. Then you solder the whole line, but try not to melt the already soldered points, as you could spoil the plate angle and position. When you have finished, you can return and solder the missed points.
While the solder is cooling, it shrinks and pulls the upper plate for about two degrees, so the final angle will be close to 90 degrees. If it is obtuse still, make one more pass with the soldering iron (with no force applied), and look how it shrinks. If it is acute, then make one more pass, applying some force with your hand.
Apply the same process for all walls. After that, you can solder the walls mutually. At this step, you can’t start from an obtuse angle, so you will get slightly acute angles at the end. You can ignore this, or even use it to obtain a tight joint, mentioned above.
Take care not to be too slow while soldering, as you could overheat the FR4, and especially not to press the soldering iron too hard. This will deform the FR4 base, or even unstick the copper. FR4 is transparent, so if you see the white areas from the other side instead of dark brownish copper, you can be sure that the copper is detached from the base. It may be useful to do some experimenting with FR4 leftover parts. If you want to see for yourself how much mistreatment the material can endure, you can try to break the bonds with your hands after proper soldering, or even test the bonds where you used too little or too much solder, or where you held the soldering iron too long or pressed it too hard.
It might also be helpful to make a special wooden tool for assembling. You probably get the idea from this drawing. Soldered joints will be even stronger if plates are held about 45° in relation to the horizontal plane, so that the molten tin connects to both panels equally.
When the assembly is complete, use the rasp to process all edges, then polish them with fine sandpaper. Wash the inner side with alcohol, to remove all remaining rosin core and flux, if you used it. At the last step, wash the whole enclosure with detergent and a lot of water, dry it and enjoy the fruits of your labor. If you followed all steps carefully, you will have a good reason to feel proud.
Cutting, laminating and etching are the hardest parts of the process, so why not just order all the sheets from your PCB supplier? Of course you can do it, if you don’t mind the price. In some cases (for prototyping and very small scale production), the final price of the unit could justify the cost of PCBs. It sounds like a dream come true – avoid the worst part of the job, and get all the parts with perfect accuracy, heavy duty paint and overlay letters and signs. This is one of such project.
You can also make the printed conductors inside the enclosure walls, so you will have even complex wiring with no wires. This, of course, requires careful planning during the design process. Here is part of an enclosure in which copper layers are used both for mechanical subassembly and electrical interconnection.
It is also easy to implement capacitive switches, inductive couplers, reed switches, Hall Effect sensors and RFID, UHF or SHF antennas in enclosure walls. Printed wiring for LED indicators can contain SMD current limiting resistors and soldering pads which are close to LEDs.
Copper is not transparent, but FR4 has diffuse transparency at the etched areas. Some thin (0.8 or 1 mm) FR4 boards add a nice milky or yellowish tint which can look good when used for indicator LEDs, or even night lamps or some LED Art gadgets. The following examples combine transparent front boards with conductors which are implemented not only in FR4 copper traces, but also in the kite’s tail (which is AC cord) and black sticks (M3 spacers dressed in heat shrink tubes). Black borders are copper traces left for soldering.
Unfortunately, most manufacturers put the coloured logo marks on FR4 epoxy base. There is no way to erase the ink, so this material is hardly usable for such applications. To make things worse, there is no way to see those marks before you etch the board. Yet some FR4 boards do not have those marks and, with a certain amount of luck, you can find and purchase them.
FR4 boards can be bent to a certain extent, so it can be used for softly curved walls. Here is a digital clock bent like a pancake, an homage to Salvador Dali’s famous picture The Persistence Of Memory.
The front board is made of .8 mm FR4, which is quite easy to bend. The 7-segment areas were designed in a CAD program, used to cut self-adhesive laminate for etching process. All other boards are 1.5 mm thick.
When you solder such curved walls together, be very careful not to overheat the boards, especially if they are 1 mm thick or less. Mechanical tension of the bent FR4 plate makes it extremely vulnerable to overheating, and even the slightest deformation is impossible to repair. You need to fix the plates in their final position and shape, and briskly solder them together.
It is even possible to build the mechanical units, possibly combined with some prefabricated metal or plastic parts. This is particularly valid for projects which contain both electronic and mechanical components, as it enables efficient placing of drivers and connectors close to motors and solenoids, sensors for home detection of moving parts, wiring using printed conductors, easy placing of indicator LEDs for debugging, integration of electronic circuits on the enclosure walls and so on.
Here are some examples of such assemblies, which explore many possibilities of FR4 enclosures.
Proper use of FR4 can greatly improve projects and make them look like high end products, even if they are built in amateur conditions. It should not be neglected as a building material for hobby projects or for small scale production.
Of course, craftmanship is of vital importance, so you will need some experience under your belt to get top results. Don’t give up, and soon you’ll be proud of the appearance of your projects!
Voja Antonic works as a freelance microcontroller engineer in Belgrade. His first microprocessor projects, based on Z80, date back to 1977, just a few years after the appearance of the first Intel’s 4004. He assembled the firmware manually, by pen and paper. In 1983, he published his original DIY microcomputer project called Galaksija, which was built by around 8000 enthusiasts in the former Yugoslavia. To date he has published more than 50 projects, mostly based on microcontrollers, and released all of them in the public domain.