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DIY PCB Etch Tank - Construction

This article provides the details of the materials and the build method used in the construction of the PCB Etch Tank introduced previously. This article could be seen as one part of the trilogy of articles that could aid the hobbyist design engineer in producing PCB's at home. The articles in the trilogy are:

  1. Homemade PCB Etch Tank
  2. Direct Inkjet to PCB Printing.
  3. Toaster SMT Reflow Oven.

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Figure 1 : PCB homemade design equipment trilogy.

 

 Introduction

As intimated previously the homemade PCB Etch Tank could be considered as the 1st of three stages in producing, quite quickly, a hobbyist project's prototype PCB. As most hobbyist design engineers have discovered the most difficult entry point into testing or prototyping ideas is the relatively high, and sometimes repetitive cost, of producing small one-off PCBs. 

A search on the Internet for homemade PCB Etch Tank construction techniques should lead to a wonderful variation in materials used and construction methods. To contain the etchant I decided to build a tank from acrylic sheet. I decided to do this firstly because it is a readily available material and secondly because it is an important material to use in a variety of hobbyists projects and the sooner one gets to grips with cutting and working with acrylic in general the better.

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Figure 2 : Ammonium Persulphate (1) is a PCB etchant that could be used. A two-litre bottle, (2) is also useful in storing the etchant after use rather that leaving it in the etch tank.

 

I should say that the PCB Etch Tank build described here  has been designed to contain a radical initiator etchant like Ammonium Persulphate, (NH4)2S2O8,  as opposed to ferric chloride, FeCl3,  which is generally considered to be quite messy in use.  Also, Ammonium Persulphate has the advantage of being transparent which helps in gauging how far the PCB etch process has gone. Being transparent also helps in producing great PCB etch process photographs for a website site, like this one!

The Design Goals

Given all of the information above we could describe the design goals, of the project, to be the following:

The aim of this project is to create a PCB Etch Tank, for an etchant like Ammonium Persulphate, to etch copper clad PCBs.  The preferred temperature to etch copper clad boards, when using Ammounium Persulphase, is between 40o and 50o  Celsius. Hence, a method of heating an etchant solution to its recommended operating temperature is required [DG01]. 

As a consequence of the first design goal a thermometer could be required to monitor the temperature of the etchant solution. In other words, a thermometer that is immersible, in a PCB etchant solution, should be used to monitor and possibly regulate the temperature of the etching solution [DG02].

Again, as mentioned previously, to contain the etchant solution at its operating temperature a material that can resist etching should be used [DG03]. Research, i.e "googlling" would suggest that plastic or acrylic should be suitable to perform the task of containing the etchant solution, Ammonium Persulphate.

Generally, agitating the etchant solution is considered to quicken the PCB etching process as well as helping in maintaining an even distribution of temperature within the etch tank. Hence, an agitator should be considered to enhance the etching process [DG04].

Since it has been decided to store the PCB etchant solution, when it is not in use, in a 2-litre soft-drinks bottle, it is not unreasonable to suggest that the PCB etch tank should be capable of containing at least 2 litres of etchant [DG05].

Bearing all of these design goals in mind the materials that I used to construct the etch tank can be seen in Figure 3.

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Figure 3 : (1) Aquarium Heater: A 6in, 50W heater that is regulated to provide heat to a maximum temperature of 37o Celsius. Some blog sites suggest altering the heating element's temperature regulator to provide a maximum temperature of between 40o and  50o Celsius. I have been able to attain a maximum temperature of approximately 38o Celsius without altering the temperature regulator. Since I do not know the heating tolerances of the heating elements, I have not attempted to adjust the temperature regulator, yet.! After some trial PCB etching tests, at a temperature of approx. 35o - 37o Celsius, I will decide whether altering the regulator is necessary or not.

(2) Aquarium Thermometer : "Analog" aquarium thermometers tend to be made of glass and generally have a scale to measure temperatures up to about  40o Celsius, which is quite reasonable since higher temperatures could be inappropriate for their originally intended use. Digitally thermometers that are mounted externally, like the one in the picture, can be used to measure operating temperatures up to 50o. Its use however depends on the etchant not ruining the thermometer's probe when it is immersed in the etchant. After visual inspection of the probe this does not seem likely as the water-proofed probe appears to be housed in plastic.  Again tests will decide whether to use a glass thermometer or stick to this one.

(3), (4) The aquarium air pump and airline could be bought as a unit or separately. I bought whatever I could find that was reasonably priced from that online megastore and I am reasonable impressed with the results. A point to note is that airlines appeared to be classified in 4mm or 6mm diameter types although it appears that both types would fit the airline pump that I purchased.

(5) I used two-part epoxy resin to adhere the acrylic parts together, although forums and other knowledgeable websites would suggest that using a plastic cement\glue would have achieved better results. The reason for this is that a plastic cement\glue actually fuses the two acrylic halves creating a "welded" joint" that ultimately produces a better result. The etched tank when resin is used to glue the acrylic pieces together tends to adhere less well once the solution contained within it has been heated. Once this lesson had been learnt it required a lot of sealant and patience to repair all of the resultant leaks. 

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Figure 4: Mechanical drawing of the PCB Etch Tank.


Acrylic sheets, (6), used to construct the etching tank. The acrylic material that I bought were A4 sized (297mm x 210mm) sheets that were 12mm thick. This meant that to contain the 2 litres or 2 dm3 of fluid an inner volume that measured 297 mm x 210 mm x 32mm, at least, would be required. After considering the diameter of the heater I decided to construct an etch tank that is 297mm x 210mm x 50mm (not including the acrylic thickness).  Using the full area dimension of the acrylic sheets to construct the etch tank helps to minimize the amount of cutting. Constructing an etch tank with thicker than average acrylic sheets seemed to be a good idea from a health and safety perspective. However, cutting sheets of this thickness was no trivial task especially with a junior hacksaw, although doing so in the cold weather was motivational!  The hacksaw provided a smoother edge compared to using a jigsaw which tended to fuse the edges making them unsightly.

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Figure 5: Junior Hacksaw (a) and Jigsaw (b).

 

(7)  Aquarium sealant: This product literally saved the construction as it was used to seal leak after leak as the solution in the etch tank kept on re-emerging at weak points  in the  resin-sealed acrylic joints. This occurred each time a leak had been sealed and the solution had been heated. Finally, after a long time, considering that ideally the sealant should be left for 24-hours before being used, I managed to seal all of the leaks. How long exactly did it take? Well, let’s just say that given the number of man-hours spent on this part of the project it could have been cheaper to get boards manufactured professionally! 

Conclusion 

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Figure 6: "Now that's a nice one" [ Quote from Ratchet and Clank, Playstation 3, Sony Corp].

This project  has been realisable due to the ready availability of aquarium parts probably not manufactured in the U.K! It is relatively cheap to construct the PCB etch tank, although experimentation with heater and thermometer types could be required, which adds to the cost. One part of the project that I would like to revisit, if I make another one, is the inner width of the tank which at 50mm made it difficult to effectively apply the sealant. This has been a great project and I can now consider prototyping small boards with quick turn around times. The next article in the series should provide the test results.

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