The Guide to Presensitized Printed Circuit Board(PCB)

What is Presensitized PCB?

Presensitized PCB come in all kinds of electronic devices, such as smartphones and tablets, and supports their functions in the same way as capacitors and resistors. Here, we will explain the characteristics, types, and manufacturing processes of this type of printed circuit boards.

Photoresist, unlike LUT, is a more reliable method with which you can make printed circuit boards of almost factory quality. At first glance, for a photoresist, you need to prepare a bunch of tools and do a lot of actions, but believe me, it's worth it. Only with a photoresist can you make boards with the thinnest traces.

Characteristics and Types of Printed Circuit Boards

Presensitized PCBs are generally plate-shaped, and circuit patterns are formed with copper foil on the surface and inside of the board, which is an insulator. Soldering electronic parts functions as electronic circuits and incorporates into various devices.

Printed circuit boards can be classified into single-sided boards. It has electronic components mounted on only one side; double-sided boards. Moreover, they have electronic components mounted on both sides; and multi-layered panels, which have multiple board layers.

According to subtractive technology, the pattern of printed circuit boards comes by etching copper foil. It comes over a protective image in a photoresist or over a metal resist deposited on the surface of galvanically formed conductors in a photoresist relief on foil dielectrics.

The Method of Tenting in Presensitized PCB

This is so-called process of "tenting," or the method of forming curtains over the holes. Moreover, holes drill in the blanks of the foil dielectric. Similarly, after chemical metallization of the walls of the holes, electrolytic growth of copper carries out to the required thickness (35-40 microns) in the holes and on the surface of the foil on the entire blank of the foil dielectric.

After that, photoresist layers obtain a protective image of the circuit and protective curtains over the plated holes. According to the obtained protective image in the film photoresist, copper is etched from the gaps in the circuit.

Analysis of Linewidth Measurement in Presensitized PCB

An analysis of linewidth measurements after copper foil etching based on the protective image in the film photoresist shows. The scatter interval of measurement values ​​increases with increasing foil thickness.

For example, when etching a 5 µm thick foil, the spread of the width is about 7 µm. When etching a 20 µm thick foil, the spread is 30 µm. Moreover, when etching a 35 µm thick foil, the spread is about 50 µm. Distortions in the width of copper conductors about the dimensions of the width of the images of the latter in the photoresist and on the photomask - a negative shift towards narrowing.

How to Prepare the Surface of Presensitized PCB?

Preparing the surfaces of workpieces for layering a film photoresist is important. It is to remove burrs of drilled holes and build-ups of galvanic copper carries out by mechanical cleaning with abrasive wheels. Furthermore, it follows by chemical treatment in an ammonium persulfate solution or mechanical cleaning with aqueous pumice slurry.

Such preparation options provide the necessary adhesion of the film photoresist to the copper pcb board surface of the substrate. The chemical resistance of protective images appears during development and etching operations. In addition, mechanical cleaning with pumice gives a matte, uniform surface with low light reflection. Moreover, it allows more uniform exposure of the photoresist.

A film photoresist with a thickness of 15-50 microns is good to use to obtain images. The thickness of the photoresist in the case of the "tenting" method dedicates to the requirements for the integrity of the protective curtains over the holes in the development and etching operations. Moreover, this carries out by spraying solutions under a pressure of 1.6-2 atm or more.

Photoresists with a thickness of less than 45–50 µm are destroyed during these operations over the holes. To ensure reliable "tenting", the diameter of the pad should be 1.4 times the diameter of the hole.

General Printed Circuit Board Manufacturing Process

In the manufacture of printed circuit boards, a method called "etching" is mainly good to use.  Here, unnecessary parts of the presensitized PCB are dissolved by chemicals.

Here, we will explain the general printed circuit board manufacturing process using etching.

As an example, the manufacturing flow of a multilayer board is below.

 Inner layer board pattern formation (etching process)

• A photosensitive film resist attaches to the substrate.
• A mask (mold) on which a circuit pattern is printed and exposed to ultraviolet light for a printing process called "exposure."
• After baking, it places in a special solvent to remove the unnecessary dry film. This is called "development ".
• Dissolve the unnecessary copper foil part with the etching liquid. After that, the dry film that has been baked onto the circuit pattern is good to remove.

How to Make Presensitized PCB?

When it comes to electronic work, you may have a strong image of using a lug board or a universal board. You can make even more amazing things by making presensitized PCB.

Nevertheless, I feel that there are few craftsmen who make their own printed circuit boards.

Regardless of a simple circuit, when I see an example of a complicated circuit assembled with a universal board, I think, "That must be hard work."

Of course, we sometimes use universal boards, but as someone who is good to use to making printed circuit boards, I often find the soldering work to be troublesome.

On the Internet, there seems to be a tendency to write that it is difficult to make a printed circuit board yourself. Various tools are important to create a fine pattern, but that is not the case. Why do you want to raise the threshold so much?

How to Mask Presensitized Printed Circuit Board?

It is a classic method that has been in good use for more than 40 years. It comes by applying a masking mask to a solid copper foil substrate and writing a pattern with a permanent marker.

Although this method is easy, if you make a mistake, it is difficult to modify it, and if the circuit becomes complicated, it will be difficult.

Also, the edges of the pattern are not smooth, so the finish is not beautiful, and it is almost impossible to draw surface mount patterns.

It may be good if it is a solid pattern such as a high-frequency circuit, but I do not recommend it.

How to Use a Photosensitive Substrate?

It is a method of baking a pattern using a photosensitive substrate. In fact, this type of photosensitive substrate already existed about 40 years ago, but patterns draw with lettering and tracing paper at that time, which often led to failures.

Today, patterns can be created by editing CAD on a computer and printing them on a special film using an inkjet printer. Even complex circuits can be created cleanly and easily.

KiCad screenshotKiCad

A famous software that has become popular in recent years. It is an integrated environment that includes not only a circuit board editor but also an electronic circuit editor, etc., and although it is free, there are no functional restrictions and it is highly functional and convenient. Japanese documents are also substantial.

Until a long time ago, it was often good to use among electronic craftsmen. It is good to use this but stopped using it because we couldn't undo it. However, it seems that a major redesign carried out some years ago and an Undo function was added.

Presensitized PCB Exposure Time

It irradiates ultraviolet rays stronger than fluorescent lamps, so it takes less time.

However, even if the exposure time is as per the profile, there will be occasional underexposed areas and it will fail, so it is not perfect. It seems that a little adjustment will affect the UV rays. Exposure comes using ordinary fluorescent lamps or a dedicated exposure machine.

Exposure work using fluorescent lamps normally, I would use a fluorescent light stand, but I didn't have one at home, so it is good to use to do it like this.

The exposure time is 8 minutes and 30 seconds for a board that has just been manufactured, and 9 minutes and 30 seconds for a board that has been manufactured for nearly a year. The success rate is over 95%. By the way, recently, it seems that the method using a fluorescent lamp remove from the method of making photosensitive substrates.

It's probably because the exposure time is not clear and the failure rate is high when it's the first time. It may also be that LED lighting is becoming more popular.

In addition, a photosensitive substrate that has been manufactured for one year needs to expose for about one minute longer (in the case of our fluorescent lamp) then a new one. So adjust it according to the age.

Also, exposure to sunlight has a high failure rate, so it is not important.

Positive Acting Presensitized PCB

• positive photoresist

A photosensitive resist that dissolves when exposed to ultraviolet light.

A photoresist that becomes more soluble in a developer when exposed to light and the exposed areas remove.

• positive data

It is the data that entered the data necessary for the board in the artwork design.

The wiring layer with silk, signal lines, component pads, etc. is designed and output in the positive.

The use of multilayer elements makes it possible to reduce the cycles of mounting the printed circuit board - dismantling it. These shortcomings are less significant than those of their predecessors, which makes multilayer boards more reliable.

Photo Positive Presensitized PCB

A photoresist is a light-sensitive substance that polymerizes when exposed to light. Recently, several types of photoresists in aerosol packaging have appeared on the global market, for example, Positiv 20 and Positiv Resist (Cramolin®).

These photoresists are liquid, therefore they good to apply to the board by spraying near the surface of the foil textolite. Film photoresists, which come in the form of a transparent flexible sheet on both sides with a protected film, which must remove when pasted onto a textolite, are widely good to use, especially for the manufacture of presensitized PCB.

Depending on the properties of the photoresist, there are two categories: positive and negative. Positive corresponds to an opaque area on the conductive track, negative - transparent. For our fee, we will choose the negative one.

Fabrication of a Board Using a Film Photoresist

The basis of the technology for manufacturing boards with a photoresist is the formation of a pattern on the surface of a foil textolite by applying a layer of photoresist, followed by exposure through a photomask. Illuminated (or unilluminated) areas remove with a solution of soda ash.

We start the board manufacturing process with a photomask, through which we will expose the photoresist with a pattern of conductive tracks. From any available presensitized PCB design program, we print on transparent film.

It is better to print on an inkjet printer, since the pattern is more contrasting, since the ink, unlike a laser printer, is good to apply in a denser layer. In extreme cases, you can use a laser. When printing, it is important to set the correct print settings.

Print options vary depending on the selected program. But it is enough to know a few simple rules for everything to work out as intended. First, it is necessary to consider the type of photoresist - positive or negative.

Secondly, if necessary, turn on image mirroring (in the case of making a two-layer board or depending on which side you will apply the patterned film to the photoresist surface). Thirdly, it is necessary to set the maximum value of the intensity (contrast) of the image. After printing, do not forget to dry the ink so as not to smudge.

Then we prepare textolite, the surface of which must be cleaned of oxides. You can use fine sandpaper or even an eraser. We achieve a shiny clean surface. We glue the photoresist onto the board, freeing its sticky surface from the protective film.

Film Photoresist: Making Presensitized PCB

Ironing laser technology will not be good due to the complexity of the required quality achievement. I have nothing against it, but LUT no longer suits me in terms of quality and repeatability.

Board with a Photoresistive Layer

We cut out the photomask along the contour of our drawing. We apply it to the photo resistive layer and press it with glass for better contact with the surfaces. Exposure carries out with an ultraviolet lamp, it is possible with a lamp for drying varnish. There is also ultraviolet in the spectrum or

We select the exposure time experimentally. It comes on a separate plate of a photoresistive film. I comes in sequentially opening sections of the photosensitive layer and turning on the lamp for the same time intervals. After exposure, we develop the photoresist in a solution of soda ash (for 0.5 l of water - a teaspoon of soda).

As a result, the photoresist will dissolve in places where we should not have tracks. Next, we wash the presensitized PCB under running water and place it in another solution - ferric chloride for etching copper.

Advantages of Presensitized PCBs

1. Increased density of conductors and components compared to its predecessors, which makes it possible, which makes it possible to significantly reduce the dimensions of the board, and, as a result, the finished equipment;
2. Shortened conductor length for more efficient operation of the equipment;
3. A complex of heterogeneous components allows you to enhance the beneficial properties inherent in each of them;
4. Effective AC circuit shielding;
5. Optimal heat dissipation from the board;
6. Increased stability of elements when subjected to various influences;
7. Increased reliability and durability due to the applied assembly methods;
8. Making improvements to its composition, if necessary.

Most people who make PCBs themselves, or at least have an idea of ​​how to make PCBs, will be familiar with some form of presensitized PCB manufacturing process. This process consists in exposing a special photosensitive coating to ultraviolet radiation through a transparent mask before proceeding to etch.

Older hams are likely to make their masks by hand with acetate paper tape. Younger ones may be laser-printed with drawings from their CAD programs. But this photoresistive PCB fabrication method is easy to improve. How about a process improvement that eliminates the acetate mask?

Conclusion

With the spread of modern technologies, the production of presensitized PCBs using embedded components is gaining more and more popularity. These types of systems are easy to find, among others, in mobile products, such as smartphones and tablets, where device size is one of the most important parameters.

This technology, which has been good to use by large manufacturers for some time, is now becoming widely available also for smaller projects. More and more printed circuit board manufacturers include this type of service in their offers. Design tools have also been widely available to support the use of embedded components for some time.

It is worth remembering that the incorporation of some elements into the structure of the printed circuit board. It allows for reducing the size of the designed device and improving its electrical parameters.