Presensitized PCB : Artwork transfer technique for a consistent and good results

Fabricating your own Printed Circuit Board PCB is almost an inescapable task all electronic constructors must somehow go through. The PCB is both important and essential part of the electronic circuit assembly. The final performance and reliability outcome of the circuit being built is dependent to a large extent how well the PCB was built. That being said, DIY PCB proficiency should rank among the most important skill every electronic circuit constructor must hone, perhaps equally important as with the soldering skill.

If you have to DIY a PCB layout, the most practical and easiest way to do it is with the use of a presensitized PCB. A presensitized PCB is a factory prepared PCB surface coated with a photo sensitive emulsion. Kinsten PCB will allow you to hand made PCB with dense patterns and spacing in the order of 0.2mm, and it is easy! Few actually believe a circuit this dense can be handmade using presensitized PCB. But if you follow attentively the procedure presented in this blog, you will be pleasantly surprise to find just how easy it is.

This blog describes the image transfer procedure we use in our lab that always brings us good results. The presensitized PCB used in the exercise is a Kinsten Brand positive acting PCB sold at e-Gizmo shop.

Photo 2. The double side PCB artwork we will work on for our discussion. This circuit uses dense package QFN48 and SSOP28 ICs, with 0.2mm trace width and spacings. A standard 8 pin DIP IC is put along side the artwork for a sense of scale.

Artwork

In computer lingo, there is very popular phrase that goes as “Garbage In, Garbage Out”. This rule applies very well with PCB making. If you start your work with a bad artwork, it could only go worse.

Selection of suitable printing media is the first important step. Although there are several available media suitable for printing PCB positive artwork, we consistently get good results with artworks printed on a good quality tracing paper.

Examine carefully the tracing paper before using it. Illuminate one side with a sufficiently bright light source, and inspect the paper by looking through the opposite side. You want one free of dark spots and blemishes. Dark spots will mask some areas of the artwork that could create short circuiting path in crowded parts of the PCB artwork.  

Photo 3. When working with two layer PCB, the top and bottom artwork should be aligned carefully and then taped together, allowing for an opening on a side to give path for the PCB to slide through.

Printing

Laser generated positive artwork produces fine, crisps, and solid black prints, making them our top choice for generating PCB artworks.  Inkjet inks tend to blot with tracing paper media; that will result in prints with bleeding and blurred edge. This in turn will put a limit to the workable spacing between PCB pads and traces. In some occasions, bleeding could short circuit closely spaced patterns. If you have to use an Inkjet, print with pigment type of ink.  

Photo 4. Cut a PCB slightly larger than the artwork PCB size. Do not remove the PCB protective film until right the moment the PCB is processed for exposure.

PCB Preparation

The presensitized PCB surface is protected by a thick gray film both from unwanted exposure and contamination. As long as the film is not removed, the PCB can take rough handling during cutting with minimal risk of damage. You should not remove the film until it is set for exposure.

Cut a PCB with two opposite sides extending at least 5mm more. The extended portion will be used later to mount the clip that will serve as the PCB stand-off during developing and etching process (This extension is not necessary when processing single side PCB). Burrs and rough edges may prevent the artwork from lying perfectly flat; remove them by grinding or with the use of a hand file.

Photo 5. A home brew exposure box will expose the top and bottom side of the PCB at the same time with its six 10-watt fluorescent bulb. The glass top is clean thoroughly each time in preparation for use.

Exposure

Clean the exposure box glass surface with a cloth damped with mild detergent. Wipe it dry with clean dry cloth. Like what is mentioned for the media, it is equally important to have the glass surface free of smudges and particles to obtain the best image transfer.We are now ready to set the PCB for exposure. From this point onward, everything must be done under subdued light conditions. Bright light may unintentionally expose the PCB, essentially ruining your work.

Photo 6. Once the protective film is removed, the PCB is vulnerable to just about anything. Work in subdued light. Do not touch its surface and keep it safe from contaminants.

Working with the PCB artwork ready and within reach,  remove the protective film of the PCB. Hold the PCB along its edges, do not touch the any part of its now exposed surface. Avoid putting down the PCB if at all possible, not until the next step is completed.

Photo 7

Gently slide the PCB in between the assembled PCB top and bottom  artwork.  Position the PCB so that the artwork is centered and aligned with equal distances from the opposite PCB edges.

Photo 8. The top glass is mounted to keep the artwork pressed against the PCB surface during exposure.

Set the PCB inside the exposure box. To obtain a sharp image transfer, the artwork must lie flat and in contact with the PCB surface. A clear glass is put on top of the artwork/PCB for this purpose (Photo 8). Lightly press the glass over the artwork/PCB area. If you can feel loose movements with the application of pressure, the glass does not rest flat over the artwork/PCB. Use just enough weights to correct this.  Weights too heavy can flex the glass, aggravating the problem instead of solving it.

Photo 10. Use a clean plastic container for the developer bath. Metal containers may react and ruin the solution.

Switch ON the exposure box for an 8 minutes exposure. Optimum exposure time may vary, depending on how the exposure table is built. The construction detail of the exposure box used in this discussion will appear in a separate blog shortly.



Prepare the developer bath solution by dissolving three teaspoonful of developer powder in a clean plastic container filled with a cupful of water. Agitate the solution by gently rocking the container until the developer powder is fully dissolved.

Photo 11. PCB standoffs that protects the PCB surface from scratches during the developing process can be easily made out of ordinary PVC folder clips.

After the 8 minute exposure, retrieve the PCB from the exposure box and separate it from the artwork. As before, grab the PCB only by the edges; avoid touching any part of its surface.


Attach the standoff as shown in photo 11. The standoffs not only ensures the flow of developer solution on both side of the PCB during bathing, rather more importantly, it will prevent the surface of the PCB from rubbing with the surface of the container. Without the standoff, you will for sure, produce a PCB full of scratched patterns – that could only end up with broken patterns once the PCB is etched.


A standoff is not necessary when working with a single side PCB.

Photo 12. Agitation of developing bath through gentle rocking ensures uniform and fast developing.

Immerse the PCB in the developer bath you previously prepared. If you are working a single side PCB without using a standoff, you should immerse the PCB with green side up. The PCB should be fully submerged in the bath. Add water if necessary, just enough to submerge the PCB.

Gently rock the developer container to agitate the bath. You should notice the PCB green photo resists slowly washed away. Thirty seconds into the solution, the transferred artwork should gradually appear on the PCB surface. If you can’t see any pattern at all, remove the PCB from the bath and wash it with clean running water. Add more developer powder to the bath and agitate until the developer powder is fully dissolved. Put the PCB back in the bath and see if the pattern appears within 30 seconds. Repeat the steps to add more developer powder if necessary.

Continue agitating the bath. Stop every fifteen seconds or so to examine the exposed copper areas of the PCB.

If you find any traces of green photo resists over the exposed area, even what may look like faint green stain, you are not done yet. Bathe the PCB some more until only the PCB pattern itself remains. Once the satisfied, it is a good idea to continue the bathing the PCB for another for 30 seconds to 60 seconds. This extra time will dissolve unwanted photo resists that you might have missed during the inspection. Over development seldom causes any problem, while under development always do.

Wash the PCB thoroughly with running water. Your PCB is now ready for the etching process.