Now you are ready to expose the PCB. The apparatus I used was an 11-watt CFL bulb fitted in an ordinary reading lamp (see Fig. 23). Natural light can expose (and harm) the pre-sensitised board, so perform this step either at night when there is no natural light or in a dark room setting during the day. In fact, just as a sidebar, even sunlight can be used to expose the PCB, but this method is contingent on getting the settings right for the amount of sunlight and the duration of exposure. You can use a small red-coloured bulb in the working area without harming the PCB. Now arrange the apparatus as shown in Fig. 23, ensuring that the CFL is around 5cm above the glass.
It will be nice if there is someone to help you hereafter. Keep a timer next to you; I used the timer utility on my smartphone. Before exposing the PCB, switch on the CFL bulb for around 30 minutes while keeping your sandwich away from it in a dark area. Next, in a dark room setting, slowly remove the protective opaque sticker, ensuring that the PCB does not move from its place.
Ensure that the artwork is aligned with the PCB. Place the glass piece on top of the artwork. Then either use rubber bands or the commonly available paper binder clip to hold the sandwich together. This will ensure that the glass presses the artwork flush against the PCB which has been fastened to the Perspex sheet. Just to reiterate, the final sandwich will have the glass on the top, the artwork transparency below it touching the PCB’s photo-sensitised side with the laminate’s back against the Perspex sheet.
Next, switch off the CFL bulb, arrange the sandwich and the table lamp. Switch on the CFL bulb while setting the timer for 10 minutes. After the time elapses, turn off the light and prepare for the next step.
You must be wondering why to use a CFL bulb instead of a UV light source. There are two reasons: One, CFL bulbs also emit some UV light along with the visible light. Two, these are readily available. The text on the protective foil of these PCBs states that you can expose them either using UV light for a duration of 60 to 90 seconds or use a CFL bulb for six to ten minutes. That suggests that the quantity of UV light emitted by a CFL bulb in a time window of around 10 minutes is equal to that from a UV bulb in around 90 seconds.
Step 5: Developing
During the ten minutes you turn on the CFL bulb, prepare fresh ‘developer’ solution. To do this, take a photographic developing tray. If that’s not available, use a plastic or glass container with a capacity of about 750ml and dimensions of around 15x20cm2 (6×8 inches). Pour 200ml of water in the container. Add half a teaspoon of caustic soda (NaOH) flakes to the water and stir the solution until the flakes fully dissolve in the solution. Ensure that you do not exceed the caustic soda quantity. As caustic soda is a hazardous chemical, use protective gloves and spectacles. You may be able to purchase caustic soda online if you look for flakes of drain cleaner for making soap. Alternatively, try at the local departmental store.
Note that the sequence is critical. Never put caustic soda first and then add water, as it may cause spurting and even violent eruption. If you use caustic soda in powder form, the solution will be too weak to develop the PCB.
After ten minutes of exposing the PCB laminate, shut off the CFL bulb and, in a dark-room setting, carefully remove the glass and the transparency to remove the ‘developed’ PCB. Immerse the PCB in the developer solution in the container with the pre-sensitised side of the PCB facing upwards. Keep stirring the solution; preferably with a 1cm broad hobby paint brush. PCB tracks will soon start becoming visible as green lines.
It may take as much as ten minutes to develop the PCB completely; the time may vary depending on the strength or potency of the flakes and the resulting developer solution. Patience is the key here; do not be tempted to add more flakes to accelerate the development as it may ruin the PCB. Continue stirring the developer solution until the PCB looks like the one in Fig. 26. It is fully developed now.
Remove the PCB and rinse it under running water. Wipe the PCB with a tissue paper and let it dry. Carefully inspect the PCB with a magnifying lens. If there is an undesirable bridge formed by the hardened green developer, remove it carefully using a sharp object like a pin. If copper is exposed in an area where it should not be, touch it up using a permanent marker pen with a narrow nib. You are all set for the next step now.