PCBEasy

How To Etch A PCB: Part 2

Etch a PCB using Sodium Persulphate

Johann Wyss

Issue 37, August 2020

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In this issue, we show you how to etch a PCB at home using photoresist and sodium persulphate etchant.

In the previous issue, we discussed the toner transfer method to etch a PCB at home. This required you to use a laser printer and glossy paper to mark the tracks on a standard copper-clad circuit board, and then use ferric chloride to etch the exposed copper away.

In this issue, we are going to demonstrate another type of etching at home process called the photoresist method. This method consists of using a mask and a photosensitive layer on top of a copper-clad PCB substrate. Ultraviolet light shines through the exposed areas of the mask and contacts a photosensitive film on the surface of a copper-clad PCB. You then use a developer solution which causes the areas of the photosensitive layer that were exposed to UV light to solidify to the copper on the PCB. You can then etch your PCB using your chosen etchant. For this example, we are going to use sodium persulfate, but you could also use the ferric chloride we discussed in the last issue.

For the mask, we will use an overhead projector (OHP) transparency. It’s likely that people under the age of 20 have never heard the term OHP but these were a staple in most classrooms up until the early 2000’ when they were replaced with smaller and lighter digital projectors that could display anything from a computer, including video.

IMAGE CREDIT: cavalierart.com.au

These OHP machines used a transparent film that you could place into your laser or inkjet printer and print text and images in black or colour. You would then place the film on a projector platform. The platform was essentially a Fresnel lens which allowed the light from a very bright lamp to be evenly distributed across the surface of the OHP film. This light then entered a reflector that projected the image onto a wall or screen. This would allow the teacher to fill a projector screen with information and images without manually writing it on the chalkboard / whiteboard or defending their artistic talents.

Whilst the technology is now fairly obsolete, the OHP sheets can, surprisingly, still be purchased from stores like Officeworks. We can print our circuit board design onto these sheets, and they will work as the mask. The toner printed onto the transparent OHP sheet will block light from travelling through that section, and thus, will prevent the UV light from curing the photosensitive layer.

For the photosensitive layer, we will use a pre-sensitised positive photosensitive PCB. These PCBs come with a photosensitive layer already attached and are specifically designed for this process. If you don’t have access to such a PCB, you can also purchase a positive resist photosensitive film that can be applied to a usual copper-clad PCB.

For the developer, we will use caustic soda / sodium hydroxide. This, along with the etchant sodium persulfate and the pre-sensitised positive photosensitive PCB came in a handy kit, which you can purchase from Jaycar (Part no. HG9990).

The kit comes with nearly everything you need for the process, but we strongly recommend you get yourself a funnel and a couple of 1-litre glass bottles that you can use to store the liquid chemicals in after the process.

These bottles should be very clearly marked and stored as per the recommendations in the Material Safety Datasheets (MSDS) for the respective chemical. We recommend non-food containers, such as Turpentine/metho bottles, which already look different. Glass bottles are good if stored where they won't get knocked over. We will also be experimenting with the Kinsten pre-sensitided PCBs, the Kinsten DP-50 developer and Ammonium Persulfate etchant form Altronics.

Safety

Before beginning, we should read and understand the MSDS for the sodium persulfate or ammonium persulfate etchant you are using. You should not proceed without first understanding the risks associated with the chemicals and implementing control measures to mitigate said risks. This also applies to Acetone and other chemicals you may be using for cleaning.

We recommend you follow these general chemical handling safety tips:

  • All work must be done in a in a well ventilated clean and tidy environment, free from any trip hazards including other people and pets.
  • You must not mix incompatible chemicals or use the same containers without cleaning first.
  • Read the material safety datasheet (MSDS) for the chemical you are using.
  • You MUST wear basic PPE including long sleeves, nitrile gloves, and eye protection.
  • Cover all work surfaces in a disposable drop sheet, newspaper at a minimum.
  • Do not dispose of this material into the garbage or drains. It MUST be stored in a clearly marked bottle and disposed of at a chemical disposal / recycling facility as per your local council regulations.

What you will need

For this tutorial, we used the PCB etch kit from Jaycar which contains the pre-sensitised PCB, etchant, developer tray, and tweezers that we will need.

We also used pre-sensitised Kinsten PCBs along with the Kinsten DP-50 developer and ammonium persulfate etchant, all from Altronics. We found that both developers and etchants had their strengths and weaknesses and we will discuss them as we go.

Whilst the kit comes with much of what you will need, there are a few things missing which you will need:

  • Bottles to store the chemical solutions in. The kit comes with sachets of sodium persulfate and sodium hydroxide. Both of these chemicals will need to be mixed with water, and thus, you will need two containers to hold the solutions when done.
  • A funnel to aid in filling the bottles.
  • Kettle to heat water. The etchant works best with elevated temperatures. Thus, we recommend using a kettle to heat the water prior to mixing.
  • Photo frame with glass window. You position the mask onto the pre-sensitised board in a dark room and then move it into the sunlight or UV light. To ensure the mask does not move, we used a photo frame to hold the two layers firmly together.
  • OHP transparency film suitable for your printer i.e. laser or inkjet.
  • Laser or inkjet printer.
  • Nitrile gloves.
  • Eye protection.
  • Paper towel / Newspaper, and a suitable drop sheet.

If you didn’t get the etching kit, you will need to get a couple of trays to work with and tweezers.

Printing onto the transparency film

For this tutorial, we will once again use the squarewave generator PCB design that we created in Issue 35’s ‘How to design a PCB in EAGLE’ tutorial. We need to print this onto the OHP transparency. To make it easier to print multiple copies onto a single transparency, we used a program called InDesign to duplicate the design and then verified that the dimensions of the PCB matched the design in EAGLE CAD. We will include this file on the website to allow you to download the PDF and print it at home, saving you the need to use InDesign.

Once we had confirmed that each PCB maintained the correct dimensions of 51.6mm x 80.8mm, we cut out a single PCB print. Sadly, our laser printer defaulted to a toner saver mode, and thus, the print was a little blotchy and quite a bit transparent as you can see here.

This blotchiness will allow light to travel through the mask and cure the photoresist in areas we don’t want. To rectify this, we stacked two copies of the PCB design on top of each other and aligned them so they matched. We then secured the two layers using tape. This helped to reduce the blotchiness significantly and would likely be sufficient as shown here.

Aligning the two layers to be perfectly aligned was no easy task. You may notice on the name, for example, that we did not get it perfect. This imperfect alignment could result in undesired short circuits. As a precaution, we printed the transparency once more on a different printer which produced slightly better results, albeit still not perfect as you can see here.

Better results were achieved when using a different laser printer.

This was a significantly better print and we likely could have used this directly or used a felt tipped pen to mark over the lighter areas to make it perfect but we decided to once more attempt to align two copies of the print which much greater success as shown here.

Note: For this process, we were using a laser printer and laser OHP sheets. This is not mandatory as you can also use an inkjet printer, but you will need to use OHP sheets designed specifically for inkjet printers. This is actually an advantage as inkjet printers are arguably easier to come across as a hobbyist than a laser printer. They also produce a darker mask, making it less likely to need to align two layers.

With the mask side completed, it was time to turn our attention to the pre-sensitised PCB. This is the blue PCB in the kit. The blue cover is a protective film over the PCB which you MUST NOT remove in a brightly lit room as doing so will cause the photosensitive layer to react prematurely, possibly destroying the PCB. Your first task is to cut the PCB down to suit the size of the circuit board pattern size. Due to time constraints, we had to skip this step.

Note: Since the process is time-sensitive and needs to be done in a dark room, we will be taking demonstrative photos with the protective film still on. This will allow us to get good photos without damaging the film. However, this film must be removed when actually etching the PCB.

Remember you need to peel the protective sheet off before the following process and this process must be done in a darkened room. We decided to use a photo frame to assist with aligning the mask on the PCB and holding it in place during the UV curing process. We simply put the mask inside of a photo frame so that the non-printed side was facing the glass.

IMPORTANT NOTE: Some photo frames use an acrylic sheet in place of glass. It’s possible that this acrylic sheet can have a film added which reduces the amount of UV light that can pass through it, presumably to protect the document or photo inside. Naturally, this would significantly reduce the effectiveness of the UV transfer. Our first few attempts failed completely when using an A4 document frame with such an acrylic sheet. Replacing it with a photo frame containing a glass front created much better results. Whilst we have no way to know for sure, we suspect that this was the cause, and thus, recommend you avoid photo frames with an acrylic sheet for your first attempt.

We then positioned the PCB on top of this so that the printed side of the OHP sheet was contacting the photosensitive layer of the PCB. We then closed the photo frame. This keeps the mask firmly pressed against the PCB and ensures that shadowing can’t happen and also ensures that the mask can’t move on the PCB.

You then expose the PCB to a strong UV light source. The lighting we had at our premises were all LED types (We are committed to reducing our energy consumption) and thus, not suitable for the curing process. This left us no choice but to resort to using sunlight, and whilst this will work, the timing is heavily dependent on so many factors. This makes it impossible for us to make a prediction on how long is optimal for the process.

At the time of writing this tutorial, we had a clear 23°C day and a moderate UV index of 4.4. We achieved satisfactory results leaving the Kinsten PCB in the frame placed in direct sunlight for 30 mins, before using the developer. This will vary depending on what PCB you use, temperature, the number of clouds, and the UV index in your area.

Using sunlight will obviously provide inconsistent results, so if possible, use a halogen or compact fluorescent lightbulb (CFL). This will produce significantly more consistent results and be something you can repeat with consistent results.

Developing the PCB

After exposing the PCB to a strong UV light source for sufficient time you will need to develop the PCB. We experimented with two different chemicals with two different PCB types. If you’re using the Jaycar etching kit, you will need to mix the sachet of sodium hydroxide developer with 1-litre of room temperature water.

Note: You need to add the satchel to the water, as combining water and sodium hydroxide has a strong exothermic reaction. We made the mistake of adding water to the crystals and it caused the crystals to stick to the bottom of the glass bottle and heat up to over 50°C.

Similarly, if you’re using the Kinsten DP-50 developer, mix the entire satchel with 1-litre of water. This chemical did not seem to have a strong exothermic reaction but it’s not a bad idea to add the crystals to the water and not visa versa.

We found when using the sodium hydroxide crystals from the Jaycar etching kit with the Kinsten photosensitive PCB from Altronics that the developing time was incredibly quick, taking only about 30 to 60 seconds. However, there was also the risk of over-developing and removing the mask when using the sodium hydroxide developer.

Using the Kinsten DP-50 developer from Altronics with the Kinsten photosensitive PCB, we found that whilst developing took significantly longer, there was much less risk of over-developing and wasting the PCB. Thus, we recommend using the Kinsten DP-50 developer when using the Kinsten pre-sensitised PCB.

We did not have any luck working with the two PCBs that came in the Jaycar etching kit, and were unable to get the boards to develop properly using either the DP-50 Kinsten or sodium hydroxide developer.

In either case, to develop the exposed PCB, immerse it into the developing solution. While gently rocking the solution back and forth you should see the image darken on the PCB surface. Using the Kinsten PCB and developer took between 15 and 30 minutes but time varies based on several factors. You can tell the board has finished developing when only the areas covered by the mask remain visible.

Etching Process

Once the PCB has been fully developed it’s time to etch the exposed copper from the PCB. Now, this can be done using the ferric chloride we used in the last issue but this time around, we are going to try a couple of different chemicals.

The Jaycar etching kit comes with sodium persulfate. This is actually quite a nice etchant to work with compared to the ferric chloride we usually use. It does not stain surfaces and does not have any noticeable unpleasant odors. It’s a relatively fast etchant, and like ferric chloride and ammonium persulfate, works better when heated. The instructions recommend mixing the entire 100g packet to 500mL of 50°C water. We don’t recommend you do this unless you’re likely to make a lot of PCBs. The persulfate etchants, unlike ferric chloride, have a short shelf life of about 30 days when dissolved in water. Whereas, in an airtight container, the crystals will last a number of years. Thus, it’s a better idea to mix just enough etchant for your needs. You want to mix at a rate of 10g of sodium persulfate to 100mL of water. This will extend the life of your etchant as much as practical.

If you’re using ammonium persulfate, mix it at the same ratio i.e. 1-part ammonium persulfate to 5-parts water, and only mix enough for immediate use.

This ammonium persulfate etchant must be kept at an elevated temperature to work effectively. If you can’t easily maintain a temperate of 70°C, you should avoid using this etchant.

This etchant quickly became our least favorite etching solution. It puts off a very pungent urine / urea smell that rapidly filled the work area. It’s also the least powerful, taking much longer than both ferric chloride and sodium persulfate.

IMPORTANT NOTE: In no way should you allow the etching chemicals to come into contact with the developing agents. Mixing sodium hydron unpredictable chemical reaction, whilst mixing sodium hydroxide with ammonium persulfate will produce sodium sulfate and ammonia.

In either case, you want to mix just enough of the etchant for immediate use and to keep the unused crystals in an airtight container.

Keep the etchant at the desired temperature (50°C for sodium persulfate and 70°C for ammonium persulfate).

You should also continuously agitate the solution to ensure the constant motion of the solution is flowing over the surface of the PCB.

The board will begin to etch from the outside of the PCB inward and around the edges of the mask out. Maintain a close eye on the process as you don’t want the etchant to seep under the copper and begin corroding the traces from beneath. This is called under etching, and if left unchecked, can cause pads and traces to lift from the PCB substrate.

We found this to be a particular issue with the ammonium persulfate as we had to leave the PCB in the solution for well over an hour due to an inability to keep the solution at the desired 70°C.

Once the PCB is etched, you can simply remove the mask using acetone / nail polish remover, and your PCB will be ready for drilling and populating.

We had quite a few challenges creating a PCB using this technique and the results, while completely useable, are not ideal. As you can see in the image, the far right and the bottom of the PCB have a mottled appearance. This seems to be a result of overexposure to UV light or light passing through the mask in those sections. Despite this, the results are very encouraging when using the Kinsten PCB with sodium hydroxide as the developer and sodium persulfate as the etchant.

If you plan to use this method regularly in the future, you could consider making a UV lightbox to expose the PCB, and make a device that keeps the etchant at the desired temperature and gently agitates the solution for best results.

If you can master the UV exposure timing and a way to keep the etching solution at the right temperature, then it would make for a better and cleaner method compared to using ferric chloride.

Part 1

Johann Wyss

Johann Wyss

Staff Technical Writer.