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What is PCB Photoresist And Why is It Important?

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PCB photoresist is invaluable when using photolithography to print circuit boards. It reacts to light and helps mask desired areas, preparing the board for the etching process, but what is this material and what is it made of? Here is more, including how to use it when making printed circuit boards.

What is PCB Photoresist?

PCB photoresist is a photoactive material used to mask sections of the copper layer when printing circuit board trace patterns. Its solubility changes when radiated UV light or other EM wave sources like X-rays or an electron beam.

Using this property, manufacturers apply the material to cover some parts of copper-clad boards. The hidden areas remain after etching, creating the desired circuit traces. The resist is available in dry or wet form.

  • Dry Photoresist – dry resist is typically a light-sensitive compound sandwiched between a PE film and a protective polyester or Mylar layer on the top.
  • Wet Photoresist – the wet version is a viscous or lacquer-like fluid. It’s also called liquid photoresist or circuit board ink and is applied by spraying or electrical deposition.

What is PCB Photoresist Made Of?

The photoresist for PCB photolithography is a polymer in a solvent solution. The solution is mixed with various other compounds to improve its properties. It mainly contains the following:

  • Resin – the primary ingredient that provides adhesion and other properties.
  • Solvent – the solvent dissolves the resin, acting as its carrier.
  • Sensitizer – the photoactive material that responds to light.
  • Additives – various ingredients added to improve the resist properties. They increase its flexibility, improve its adhesion capabilities, make it easy to strip, etc.
PCB Photoresist
An example of a photo-decomposing resist type in dry film form
Resource: https://www.youtube.com/watch?bAH5A_sarBg

Types of PCB Photoresist

There are three main types of PCB photoresist application methods, with their classification guided by their chemical composition and reaction to light. They include photo-crosslinking, photo-polymeric, and photo-decomposing types.

Photo-Crosslinking Resist

As its name implies, exposing this material to light activates a cross-linker. The linker joins individual resin chains to produce longer chains. This process creates material that the developer cannot dissolve.

Photo-Polymeric Resist

The photo-polymeric resist is a negative type that hardens in light. It works by starting a polymerization of monomers, causing the material to become less soluble and resistant to the developing chemical.

Photo-Decomposing Resist

The decomposing type is a positive photoresist that undergoes a reduction process when radiated with light, producing a structure that dissolves in the developing chemical. It’s not a commonly used type.

Positive vs. negative resist diagram
Positive vs. negative resist diagram
Resource: https://www.researchgate.net

Positive vs. Negative Photoresist for PCB

PCB photoresist compounds are either positive or negative depending on whether they become more or less soluble when exposed to light. Here is what you need to know about the two.

Negative Photoresist

The negative resist becomes tougher when exposed to light, either through a crosslinking or polymerization process. The unexposed parts dissolve when you develop the board, leaving the masked regions intact.

Negative resist offers excellent adhesion and produces thinner films or higher resolution masking needs. It also costs less, making it a more preferred choice.

Positive Photoresist

Positive means increased solubility. The material weakens on exposure to light. The exposed parts dissolve in the developer solution, leaving the unexposed sections.

The PCB positive photoresist is a more expensive option. It also offers thicker films and reduced adhesion, making it a less popular photolithography material.

Using PCB dry film photoresist to print circuits
Using PCB dry film photoresist to print circuits
Resource: https://www.youtube.com/watch?2hQfGtSFe_0

How is PCB Photoresist Applied?

Proper resist application is crucial to achieving precision and circuit accuracy. The process involves one of the following methods: spraying, electrically, or with a hot roller. Each method has its good and bad sides.

Spray Coating

A spray nozzle atomizes the liquid and applies it as a thin film on the PCB. Spraying is quicker and less costly. It also produces uniform coatings and allows the precise control of thickness.

Electro-Deposition

This method uses an electrochemical process to coat the board, producing a thin, high-resolution coating. It works great when coating multilayered circuit boards.

Hot Roller Film

This method applies dry film photoresist. A hot roller is required to heat the film and cause it to stick to the PCB, or you can use a hot shoe laminator.

PCB before and after resist application
PCB before and after resist application
Resource: https://youtu.be/sPnnOcxqUhc?si=9FN_WcLdt9ocrcZu

Using Photoresist in the PCB Manufacturing Process

Several steps define etching a PCB board using a photosensitive film or photosensitive ink. It involves the following when traced from board cleaning to resist application and development.

Step 1: Board Preparation

  • The PCB panels, which are copper-clad substrates, are cleaned in alkaline solutions
  • This is done to rid them of organic materials that may hinder the application process
  • They’re also micro-etched to make their surfaces rough and improve resist adhesion

Step 2: Applying the Photoresist

  • The panels are heated in an oven to drive off moisture
  • They are then coated with photoresist film or ink, ensuring correct and uniform thickness
  • In large manufacturing plants, the boards are taken through automated spray tunnels or spun to improve uniformity

Step 3: Drying and Baking

  • The boards are taken into convection ovens and dried
  • They are then cooled to solidify the coating fully
  • After the film has solidified, the panels are baked to remove excess solvent

Step 4: Visual Examination and Testing

  • Workers examine the coated board to ensure an even coat
  • Tests are carried out to determine coating thickness and other requirements

Step 5: Photoresist Exposure

  • The panels are covered with a photo mask and exposed to light or another radiation source
  • When using direct laser imaging, the photo tool is not required. The laser directly prints the circuit on the resist
  • Depending on the resist type, the exposed parts may harden or become soluble

Step 6: Develop the Photoresist

  • To develop the resist and reveal the circuitry, the boards are processed in a chemical solution
  • The solution dissolves the soluble parts, leaving the hidden part covered by the insoluble resist

Later, the boards are etched using chemicals, removing the exposed copper parts. The remaining pattern contains the conductive traces to connect components on the assembled board.

Thin PCB photoresist film
Thin PCB photoresist film
Resource: https://www.youtube.com/watch?2hQfGtSFe_0

How Do You Determine the Thickness of PCB Resist?

Resist thickness is a crucial factor when producing circuit boards. It helps determine the amount of radiation energy required to activate the resist and helps avoid exposure problems like failed traces.

The dry film type comes with a predetermined thickness ranging between 1 and 2 mil — or slightly thicker if meant for hole-tenting. Wet resist does not; its thickness depends on the application method.

While the amount of resin could help gauge the thickness, errors may arise due to uneven board surfaces or solvent evaporation.

Various measuring techniques, such as White Light Interferometry (WLI) and Atomic Force Microscopy (AFM), can verify the thickness. They offer greater accuracy and do not interfere with the resist’s structure.

Photoresist removal using sodium hydroxide
Photoresist removal using sodium hydroxide
Resource: https://www.youtube.com/watch?IPsE64PMzMg

How Do You Remove Photoresist From PCB?

After etching, the PCB photoresist requires complete removal. You can do so using chemicals or physically using mechanical methods. Other methods include thermal stripping and the use of plasma energy.

Chemical stripping is the most used method. It involves immersing the boards in a chemical solution containing acetone or methylene chloride and other compounds.

Mechanical stripping requires scrubbing the resist or blasting it off with abrasive particles or high-pressure water jets. To remove the resist thermally, heat the board to high temperatures. The resulting heat vaporizes the resist.

Plasma stripping works like the thermal method. It employs low-pressure plasma to break down the resist chemical structure, converting it into a gas. It’s the most effective method.

Conclusion

PCB photoresist is a light-sensitive material used to print circuit board traces. It works by masking the trace pattern on copper-clad substrates before etching. Different resists are available, each offering various benefits. Selecting the right type ensures the correct thickness and resolution levels.

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