We had a full saturday of tests and one of the test is etching process to make the prototype of our PCB for BadPrinter2 and for future project. We already had experience on etching our own PCBs, but this time we change a lot of material for the process, so we want to take a scientific approach and verify the correct exposure times and acid concentration in order to obtain the best results. This is also a driving criteria for designing the PCBs in DipTrace and Eagle.
First of all the materials: we change from single layer light pre-sensitive PCBs to Bungard double layer UV pre-sensitive PCBs. This mean that change completely the way in which the photo impression has to be made on the PCB. So we have to change the light source from visible light to UV; we found that common 36W UV lamps for nail reconstruction works very good and they are easy to find and cheap on ebay. We bought two of them and we intend to modify them to impress contemporary the two layers of the PCB.
And for everyone was asking… we deliberately buy them Pink 🙂
Anyway, for the tests we use only one of this; it came also with a useful timer of 120 seconds. We prepare some test transparencies with lines from thicker to thin and with space between lines again from wider to very thin (from 100 mil to 10 mil size), then a small example circuit that use one SMD component footprint and some very thin traces.
The “Lines” circuit was exposed at intervals of 10 seconds covering some part of the circuit with a thick piece of paper in order to have a sort of graduated scale of exposure times. We made one sample starting from 10 seconds to 120 seconds and another from 120 seconds to 240 seconds. Then we expose the “SMD” sample in three different time exposure 120, 240 and 360 seconds.
The first chemical operation that we have to do is to remove the impressed photo-resist with sodium hydroxide and we are lucky to find a good concentration of 5 grams in 0,5 liter of water (we have granular sodium hydroxide). Always pay attention with this chemicals because too much concentration means a lot of heat produced when mixing up the grains with the water, and also it became a lot corrosive and also remove all the photo-resist on the PCB (exposed and not) so be careful starting from a weak concentration and make some tries. Obviously it is mandatory to use protective gloves and wear glasses for eye protection (try this at home only if you know what you are doing).
It can be seen that the right part of the lower copper strip is very blurry; this represent the starting time (every vertical score is 10 seconds), so the minimum time that can be considered good is about 60 seconds.
The results are very promising and we then go to the next phase of etching: we switch from ferric chloride to the sodium persulfate that is colourless and does not stain clothes, moreover became blue when is exhausted. The suggested concentration is 100 grams per 0,5 liters and we discover that is correct; it takes about one hour and half for complete etching, but this done at ambient temperature and without air circulation (normally the etching acid have to be warmed at about 40 °C to speed up the reaction). Again, when use chemical acids it is mandatory to use protective gloves and wear glasses for eye protection and pay attention.
The etching result are very good and we can find out that the an exposure time of about 200 – 240 seconds is fine (less means to much photo-resist, while more time means that the thinner lines disappear during etching)