WEEK 6 - UV BOARD

WEEK 6

PCB Preparation

The PCB layout paper is drenched with sunflower-seed oil. Sunflower-seed oil is common available from your local grocery or wall market. Superfluous oil should be removed carefully with tissue paper. The sunflower-seed oil is used to make the white part of the layout paper transparent for light.

If you prefer to use the PCB layout more than once let the drenched PCB layout paper dry at least 48 hours. The layout paper should be carefully dried on forehand as much as possible with tissue paper. Sunflower-seed oil is a `drying' oil. Exposed to the air over a number of hours, the layout paper becomes rigid again. A kind of polymerization takes place. You will get a lot less or no greasy fingers anymore afterwards.

Other mineral or vegetable oils might work as well to obtain light transparency. However, they might not be `drying' oils. When we started experimenting, sunflower-seed oil was the first oil we used and it worked fine. So we didn't try any other oils. Using water does not work. The layout paper crumples up a bit.

PCB UV Exposure 

 

The protective plastic layer is removed - peeled back - from the photosensitive PCB. The toner side of the greased layout is placed on the copper of the PCB. Captured air-bubbles are gently pressed away from underneath the layout. The PCB with the layout is now covered with an appropriate sized windowpane and placed on a piece of plain polished tile or marble. The tile or marble absorbs the heat coming from the UV bulb, which is significant. Three to four minutes 300W bulb UV exposure from a distance of 30-40 cm will do the photo process. Take care when finished and removing the PCB, it gets hot!

PCB Etching

The developed PCB is etched with a 220 g/l solution of ammonium peroxydisulfate (NH₄)₂S₂O₈ a.k.a. ammonium persulfate, 220 gram added to 1 liter of water and mix it until everything is dissolved. Theoretically it should be possible to etch slightly more than 60 grams of copper with 1 liter etching solution. Assume an 50% efficiency, about 30 grams of copper. With a thickness of 35 µm copper on your PCB this covers a copper area of about 1000 cm². Unfortunately the efficiency of the etching solution degrades, dissolved ammonium peroxydisulfate decomposes slowly. You better make just enough etching solution you need to etch. For an etching tray of about 20 x 25 cm a minimum practical amount is 200-250 ml solution. So you dissolve about 44 grams ammonium peroxydisulfate into 200 ml or 55 grams into 250 ml water.

Etching at ambient temperature might take over an hour, it is better to heat up the etching solvent to about 35-45 degrees Celcius. The etching solution heating up could be done in a magnetron, this takes about 40 to 60 seconds in a 850W magnetron depending on the initial temperature of the etching solution (hint: first try this with just water to determine the timer setting of the magnetron). The etching - rocking the etching tray - takes about 15-30 minutes at this temperature. If you have a heated, air-bubble circulated etching fluid tank available, this is probably the fastest way to etch. At higher temperatures the etching performance decreases. The etching process is an exothermic reaction, it generates heat. Take care, cool your etching tray when necessary! You should minimize the amount of copper to etch by creating copper area in your PCB layout as much as possible. When starting the etching process and little to etch it is difficult to keep the etching solution at 35-45 degrees Celcius. It helps to fill for example the kitchen sink with warm water and rock the etching tray in the filled kitchen sink.

When the ammonium peroxydisulfate is dissolved it is a clear liquid. After an etching procedure it gradually becomes blue and more deeper blue - the chemical reaction creates dissolved copper sulfate CuSO₄. Compared to other etching chemicals like hydrated iron (III) chloride FeCl₃.6H₂O a.k.a. ferric chloride or the combination of hydrochloric acidHCL and hydrogen peroxide H₂O₂, using ammonium peroxydisulfate is a clean and safe method. Did you ever spilled dissolved iron chloride on your clothes or your assumed stainless steel kitchen sink? Do you really want to keep concentrated hydrochloric acid and hydrogen peroxide at home? So, without doubt ammonium peroxydisulfate is the best choice for etching at home. However, copper sulfate is a poisonous substance and should be treated as chemical waste.

This is the step of etching,we must do it carefully to make it successfully function and not broken.All the step of how to do this is state above but you all must do it carefull.