Importance of Photoresist in a Printed Circuit Board

Photolithography is not only widely known, but the only technique through which semiconductors are manufactured. This is the method used to manufacture PCBs, ICs, and other electronics components. The most important thing to see, however, is how the lithographic process functions and where the photoresist fits in. A mask layer is used when making a trace through the etch process. This photoresist is then applied and allowed to cure, etching the uncovered parts and creating traces. We also carry out the same action throughout the PCB copper etching procedure. Here are a few typical uses:

• Pattern Transfer for Copper Etching
• Via and Pad Formation
• Electroplating Masking

The significance of photoresist in PCB manufacture is examined in this article, along with its types and effects on board performance and quality.

What is Photoresist?

A light-sensitive polymer substance called photoresist is employed in the photolithography step of PCB production. Photoresist undergoes chemical changes that enable selective etching or metal plating when exposed to particular light wavelengths, most commonly ultraviolet. In essence, it defines circuit patterns on a substrate by acting as a stencil or mask. To enhance its qualities, the solution is combined with a number of other substances. The following are its primary contents:

• Resin: The main component that gives stickiness and other qualities is resin.
• Solvent: The solvent serves as the resin's carrier and dissolves it.
• Sensitizer: The photoactive substance that reacts to light is called a sensitizer.
• Additives: They are used to enhance resistance to the surface.

Negative and Positive UV Exposure:

Negative UV Exposure:
It is the type of photoresist that has been exposed to UV light solidifies. These hardened areas stay on the PCB after development. The unexposed areas are removed throughout the developing process. Negative type solder resist is the most often used solder resist in PCB fabrication because it performs well on small traces and sensitive features.

Positive UV Exposure:
The exposed sections expand and dissolve when exposed to positive UV radiation. Exposed regions don't get hardened so they can be removed easily by the developer. Positive resists are frequently used for high-precision photolithography, such as circuit patterning, rather than solder masks.

Photolithography Fabrication Process:

Etching a PCB board with a photosensitive film or ink involves a number of processes in the photolithography fabrication process. When traced back to board cleaning to thwart application and development, it entails the following.

1) PCB Preparation: The copper-clad board must first be cleaned and examined for impurities. Before moving on, we must clean and remove any dirt or oxidation.

2) Photoresist Application: As previously mentioned, this type of polymer is often cured by light. However, we must first apply this to copper-clad boards before proceeding. There are various ways to apply a wet photoresist material, such as squishing or spin coating. On the other hand, the thin film covered with photoresist needs to be laminated to the copper surface before the lamination process is carried out when dry film photoresist is used.

3) Exposure: Get a mask ready. This kind of mask layer is designed to keep light from getting to the photoresist. The pattern of traces you wish to place on the PCB will be contained in the mask.

4) Development Solution: To remove the uncured photoresist from the PCB upon exposure, a dissolver is required. In the negative one the exposed region gets hardened so don’t remove it. On the other hand unexposed ones are removed. And opposite in case of positive resist.

5) Etching Process: In photolithography, this is essentially the procedure used to remove metal, particularly when discussing PCBs and copper. When the photoresist hardens, it will hold the copper underneath it securely in place while removing any excess copper.

6) Stripping: A photoresist is applied in order to etch the PCB; after etching, the photoresist must be removed in order to recover the copper layers for additional processing. The next sections go over the various techniques for removing cured photoresist from a PCB.

The Chemical Structure of Photoresist

Based on their chemical makeup and light-reaction characteristics, PCB photoresist application techniques can be divided into three primary categories. These consist of photo-decomposing, photo-polymeric, and photo-crosslinking kinds.

1. Photo-Crosslinking Resist: As the name suggests, this chemical activates crosslinking links when light strikes it. It hardens the particular area and creates a chain that keeps aching when exposed to light. Thus, it is classified as negative photoresist.

2. Photo-Polymeric Resist: When exposed to light, the same negative kind of photoresist solidifies. The substance becomes less soluble in the developer solution. However, this time it starts a polymerization of monomers instead of cross link connections.

3. Photo-Decomposing Resist: This kind of positive photoresist hardens as the light-exposed portion dissolves in the developer solution. Azide quinine is one type of this photoresist. It is not a type that is frequently used.

Methods to Apply Photoresist on A PCB?

Achieving precision and circuit accuracy requires the proper application of resist. Every technique has advantages and disadvantages. One of the following techniques is used in the process:

Wet type photoresists: It uses spray coating, which allows us to apply a standard photoresist on a PCB using a spray. Spraying is less expensive and faster. Additionally, it creates consistent coatings and enables accurate thickness control.

Electro-Deposition: This technique coats the board by use of an electrochemical reaction. It creates a high-resolution, thin layer. With multilayer PCBs, it is frequently used.

Hot Roller Film: Dry film photoresist is applied using this technique. The film must be heated by a hot roller in order for it to adhere to the PCB.

Methods to Remove Photoresist From PCB?

The PCB photoresist needs to be completely removed following the etching procedure. Chemicals or mechanical techniques can be used to accomplish this. Thermal stripping and the application of plasma energy are further techniques. Developer solutions are typically needed in order to remove uncured photoresist. And in order to remove a cured photoresist, we require:

Chemical Stripping: The most popular technique, which is comparable to the developer solution, is chemical stripping. The boards are submerged in a chemical solution that contains methylene chloride, acetone, and other substances.

Mechanical Stripping: High-pressure water jets must be used to scrape the resist. Heat the board to high temperatures in order to thermally remove the resist. The resist is vaporised by the heat that results. However, this is typically not recommended as it could harm the material.

Plasma Stripping: This technique functions similarly to the thermal approach. It breaks down the resist chemical structure and turns it into a gas using low-pressure plasma. It's the most efficient approach.

Conclusion

Circuit board traces are printed using PCB photoresist, a light-sensitive substance. On copper-clad substrates, it functions by obscuring the trace pattern prior to etching. The proper thickness and resolution levels are ensured by choosing the appropriate kind. Reactive substances and organic solvents are found in photoresists. Environmental harm or exposure concerns could result from improper treatment. Manufacturers adhere to strict regulations for:

• Safe storage and disposal
• Controlled cleanroom environments during application
• Proper developer and stripper chemical handling

Photoresist plays an increasingly important role in attaining manufacturing excellence as PCB designs grow more compact.