So you need a chemical tank but don’t want to spend many hundreds of dollars on the pre-made PCB tanks commonly found on electronic supplier websites. I used to (notice I say “used to”) have two professional tanks but now use custom build tanks because, when it comes to special requirements and performance, a custom tank will always win, hands down.
These tanks are ideal for PCB production and below is a list of the possible tanks you can incorporate into your own workshop:
- Development tank — For developing photo-resists
- Etching tank — For etching off the copper
- Stripping tank — For stripping the resist
- Tin tank — For tinning the copper once cleaned
Firstly, you can decide what your tank needs so features such as heating, draining and air agitation can be added at almost no extra cost.
Secondly, a custom tank is a fraction of the cost of a professional tank so you can get more tanks for the same price of one tank (in my case I need at least four tanks and the total material cost has been around $120 which includes heaters, piping, plastic and much more).
Thirdly, and this is the most important point, why pay money for something that you can easily build yourself!
For those who want to use a plastic box (like Tupperware), you should exercise caution. The reason has nothing to do with quality or ability— it's mainly because they are made of non-stick plastics and therefore, if you want to add a tap or pipe, you will struggle to find a glue and sealant that will provide a water tight bond (hot glue peels off Tupperware and sealants actually fall off).
If you are worried about the cost, don’t be! You can easily build a minimal tank for less than $30 (if you already have tools and equipment). The three pieces of A4 Plexiglas per tank only cost around $11, which means your wallet can stay full for a little longer!
The design for a typical chemical tank is shown in the diagrams below.
- The tank width (50mm) is approximate
- ALL circular dimensions (holes) are measured as a diameter!
Overall layout for chemical processing tank
Layout for lid and bubbler in the chemical tank base
Chemical tank top view with emphasis on the silicone sealant applied to the external surface of the four corner joints
The spray tank is almost identical—the difference is that the only addition is a spray pipe. Note that if you do not have access to running water and sewer drainage, you will need two additional plastic containers where one holds clean water while the other holds waste water. Then, when convenient, these can be filled and emptied.
Overall layout for spray cleaning tank
Layout for spray pipe
The dimensions given here can be taken with a pinch of salt as the only pieces being cut are the bottom, front, and back pieces. When the pieces are glued together, only the flat edges of the Plexiglas will be used and not the cut edges because cut edges are never straight and thus create large gaps. Remember, surface tension plays a big role in gaps smaller than 1mm so the smaller your gap between the pieces is, the less likely that fluid will get through.
The tap needs to be as low down in the tank as you can get it, but don’t allow it to hang lower than the tank unless you plan to have the tank on the edge of a table. But if the tap is too high then the tank won’t fully drain and you will need to tip the tank to empty the remainder. It’s a difficult choice to make but, personally, I prefer to have the tank on a table away from the edge and thus have the lowest part of the tap slightly higher than the bottom of the tank.
Depending on the tank's function, certain features can be left out. For example, an etching tank will need the drainage, air agitation, and heater whereas a tinning tank would only need drainage. The cleaning tank is designed to spray water onto PCBs to clean and so only a drainage and spray pipe are needed.
Bill Of Materials (BOM)
Depending on the size of tank that you require, the pieces of Plexiglas (or Perspex) should be the size of the largest face. In the case of this article and tank requirements, the standard A4 size has been chosen which has dimensions of 297mm x 210mm x 3mm. The reason for this is as follows:
- It reduces the need for cutting (which is difficult, even with a band saw)
- Cheaply available in this size (~$4 per A4 piece at 3mm)
- When gluing, the flat uncut edges will prevent most leakage
For a tank container:
- 3 x A4 size Plexiglas (3mm thick)
- Water butt tap
Spray cleaner tank extras:
- 12V DC mini brushless pump (240L / Hr)
- Plastic PVC pipe (internal diameter 8mm)
- Two plastic containers (storage of clean water and storage of dirty water)
Chemical tank extras:
- Aquarium air pump (6mm output)
- Aquarium air pump pipe (6 mm)
- Aquarium heater (preferably 100W)
Tools and items needed for any construction:
- Hot glue gun
- Cutting tool (hacksaw, band saw, or Dremel)
- Sandpaper or sanding tools
- Drill with appropriate bits
- Aquarium silicone sealant
- Super glue (pound shop glues work fine here)
- Masking tape (for protection of Plexiglas)
- PPE (Personal Protective Equipment: gloves, eye protectors, etc.)
- Ruler, pencil, right angle square, permanent ink
The tank lid is up to you, but two possibilities are either a wood lid with a plastic underpart or a Plexiglas lid. Personally, I prefer wood because it looks nicer, is cheaper, and I have loads lying around. There is no need to spend more than what I am already spending!
Superglue notice: If you spill glue on your fingers, use acetone.
Cutting the Plexiglas
Getting the pieces of Plexiglas cut to size is very easy because two pieces are already at the correct size (the two sides) and one piece is cut into three rectangles of equal width. When cutting the Plexiglas, make sure to wear the appropriate PPE as it produces sharp pieces of material that also smell quite strong (if using a powered tool such as a band saw).
So to start, cut one sheet of A4 Plexiglas into three pieces of approximately 7cm each. The reason why these pieces are bigger than the needed tank width of 5cm is so that there is excess material between the two sides and the front / back pieces. This excess will provide 90 degree corners that will accept the aquarium sealant perfectly. The reason why the sealant is not inside the tank is because 5cm is too small to get to all the internal corners.
Once cut, you should carefully sand down all the edges so that sharp points are removed. The sanding also improves visual performance because a jagged tank can be unsightly.
Drill the Pieces
With the pieces cut, it is time to drill the necessary holes into the front and back pieces. The holes that are needed depend on your tank requirements but typically you'll find that specific tanks require the following holes:
- Chemical Tank
- Two holes in the back (air and heater wire)
- One hole in the front (tap)
- Spray Tank
- One hole in the front (water spray pipe)
Plexiglas cutouts and markings
Base showing where sides will be glued
Adding the Extras!
Air agitator pipe
This is done by applying superglue to the Plexiglas, breathing on the pipe, and then securing it into place (masking tape helps here). Why breath on the pipe? Superglue requires moisture to activate and so breathing on the pipe adds a layer of moisture for the superglue to dry both quicker and stronger!
Make sure that the pipe is secure and DOES NOT MOVE during the drying. Once dry, carefully make holes with either a 1mm bit or a pin and then a small screwdriver to help widen the hole slightly. I personally use a sharp pointed tool for the widening.
Clamps used to keep pipe secure
This is also easy to do: Just unscrew the plug so that the wire end is just wires and glue the heater (not the heating element) to the Plexiglas with superglue. When the glue is dry, apply a generous amount of hot glue (just to give it a bit more mechanical stability) and pull the wire (carefully) through the hole. At this point, fill the hole with sealant and reattach the plug. If the air agitator pipe is dry (and is needed) pull the pipe through the bottom hole, then apply sealant and allow to dry.
The tap should have a rubber O-ring and a nut. Place the O-ring on the threaded end, apply some superglue, and push it through the hole. Then, as tightly as you can, screw the nut so that the tap is correctly positioned and tightly fitted. When the glue between the O-ring and the Plexiglas is dry (around 10 minutes), apply sealant both inside and outside the tank.
This does not get added yet because this is glued in when the tank is completed.
Putting the Tank Together
With your five sides ready, it’s time to stick them together. Before you jump straight in, please see the warnings and reminders below!
- Use super glue on all joints and ensure that you superglue the ENTIRE length of the material!
- Use masking tape to prevent the superglue from going onto areas which should not have glue on them!
- Make sure that the pre-cut edges are the only glue points! You must not use a custom cut edge for gluing because there will be plenty of gaps (unless you can cut THAT straight).
- The air agitation pipe MUST go vertically upward and be glued to the back piece so that the pipe goes above the fluid line in the tank. This is because a syphon effect will drain the tank when the pump is turned off. Also, connect a valve to the pipe just after the air pump in case of backflow.
So here is the general procedure!
- Stick the two sides onto the base piece, making sure the two sides are perfectly parallel. The easiest way to do this is to cut a piece of Plexiglas or wood 5cm wide and making sure that it just touches both sides everywhere inside the tank
- Stick the front and back onto the sides and base piece, ensuring there is at least a 0.5cm lip between the front / back pieces and the sides. Remember that this lip is needed for the aquarium sealant.
- With all the pieces dry and set you can now use the aquarium sealant on all joints. Apply in the same way that you would apply bathroom silicone sealant and make sure to keep the draw as consistent as possible. If you have a silicone sealant tool, you can use it here to get a nice finish!
- If you want a second layer of protection, you can also apply hot glue over the silicone sealant; but ensure that the sealant has had the full required curing time (typically 12-24 hours).
For neatness, you can apply a layer of black electrical tape to hide the sealant. If you feel really brave, you can cut off the excess material (be careful to leave the material with sealant alone).
The cleaning tank has its spray pipe added after the sides and front / back faces are glued because the spray pipe needs to be fitted at the top. Before the pipe is inserted and glued, it needs to have the holes added first. To do this, drill holes in the pipe as shown in the previous design plans. Remember, the holes need to be in-line and on the same bend so that when you place the pipe, the holes are pointing at the same angle! This can be tricky, so take time when choosing the hole locations.
Once you have the holes sorted, use the hot glue gun to pump glue into the end of the pipe that sits inside the tank. This is so that water does not gush out the end and defeat the whole point of having spray holes.
Once the pipe is drilled, you can glue it in place with superglue and use clamps to keep the pipe from moving. Once this is done, all you need to do is connect the 12V pump to both a power supply and spray pipe, then submerge the pump in the water feed tank. The tap on the cleaning tank then needs to be connected via a pipe (e.g., a garden hose pipe) to a second container (preferably the tanks with screw caps) for waste storage that can be emptied appropriately when full.
When the power supply is turned on, you will watch water be pumped from the feed tank into the spray pipe. This will create (if done properly) jets of water that can be used to clean a PCB very quickly. The waste water will trickle down to the bottom of the tank, collect, and then eventually go down the tap into the waste container.
It's coming together! The sides are glued first
The front and back are then glued into place
While we wait, a simple lid made out of wood and scrap PCB material
The nearly completed tank
It is also a good idea to have the tanks on joists instead of flat on a table. I'd also recommend putting some type of removable kitchen towel underneath. This helps because if a leak should occur, you can spot it on the towel very quickly in the form of a daily inspection. This also prevents your work surface from being ruined by the tanks (I have wood surfaces that go black when in contact with ferric chloride).
Method to enable inspection of a single tank
Same method applied to many tanks
10 Minute Test
The first test you will need to do is fill the tank with water and either place in a container or place on highly absorbent kitchen towel. Leave for 5 to 10 minutes and check to see if there is any visible water on any of the joints. If you spot water then you will need to either dry and apply more sealant or carefully remove the old sealant, add new sealant and allow to dry for the required time.
1 Hour Test
Once your tank passes the 10-minute test, you can then move on to the 1-hour test. Perform the same procedure as before and check for leaks.
The 24 Hour Test
Once done, you can perform the 24-hour test, but this time make sure the tank is sitting on a fresh kitchen towel and this time add food colouring to the water. In my first prototype test, I actually used ferric chloride instead which is NOT advised because if it does spill or leak the result could be catastrophic.
Final Chemical Test
Once 24-hours are done, you can lift the tank and observe the kitchen towel to see if you can see any colour in the material. If you do, then there was a “micro leak” and this can be easily found because A) the leak will be right where the colour spot was found and B) the tank will (hopefully) have food colouring on the outside where there was a leak.
With the 24-hour test completed, it's time to do a second test but this time with the chemical you intend to put in the tank. If your tank has a heater, make sure this is OFF first! Then when the tank with the chemical passes, do a third test with the heater on. This is because when heat is added, plastics and glues are more flexible and leaks may form.
So now that you have your chemical / spray tank working, it is time to put it to good use. But remember that the tank is tall and narrow, so adding feet would be a good idea or using fixtures to prevent it falling. If you have multiple tanks, you can stick them together so that they prevent each other from toppling over.