In the complex process of printed circuit board (PCB) manufacturing, photomasks are divided into two types—positive and negative—each serving a specific purpose at different stages of production. In this article, we will take an in-depth look at the differences between positive and negative photomasks and their respective applications.
An Introduction to Positive and Negative Film
Photomasks are one of the basic tools used in printed circuit board (PCB) manufacturing; they are used to define the circuit patterns and component placements on the board. Positive and negative photomasks are two different types of photomasks that serve distinct purposes in the manufacturing process.
Production and Applications of Positive Film
Positive film is typically used in the following situations:
- Manufacturing of Outer-Layer Printed Circuit Boards: For outer-layer circuit boards, positive film is typically used. A key characteristic of positive film is that during the exposure and development process, the transparent areas of the dry film undergo a chemical reaction and harden, while the circuit areas to be retained—which are masked by the black shadow—do not harden. During development, the uncured dry film is washed away, and the board is then placed in an electroplating tank, where the areas not covered by the dry film are plated with tin to protect the circuit patterns. This is because alkaline etching solutions are typically used in the fabrication of outer-layer PCBs, and these solutions can corrode the uncured dry film.
- Solder Mask Production: Solder mask is a surface coating used to protect printed circuit boards, typically applied using a positive film. The black shaded areas usually cover the pads on the circuit or the pads at BGA locations. After exposure and development, the uncured dry film is rinsed away, leaving only the circuit areas that need protection. These areas are not covered with solder mask ink so that components can be soldered in place.
The Production and Applications of Negative Film
Film negatives play an important role in the production of inner-layer printed circuit boards. Their specific applications are as follows:
- Manufacturing of Multilayer Printed Circuit Boards: Negative film is typically used for inner-layer circuit boards. Compared to positive film, the process for manufacturing inner-layer circuit boards using negative film is simpler. After exposure and development, the uncured dry film is rinsed away, and the board is then placed in an acidic etching bath to etch away the copper layer not covered by the dry film, leaving the desired circuit traces.
- Acid etching solutions cannot dissolve dry film, so they are suitable for inner-layer printed circuit boards.
Frequently Asked Questions
Q: What is the main difference between positive film and negative film?
Answer: Positive films and negative films differ significantly in their manufacturing processes and applications. Positive films are typically used for outer-layer circuit boards and solder mask production, while negative films are typically used for inner-layer circuit boards.
The key difference lies in how they are processed after development and the types of etching solutions used.
Q: Why are positive films used in solder mask application?
Answer: The reason positive film is used in solder mask production is that the solder mask ink needs to cover specific areas of the printed circuit board, such as the pads on the circuit or the pads at the BGA locations.
The black shaded areas on the film cover the regions that do not require solder mask, ensuring that they are not affected during the soldering of components.
