Introduction

In the previous articles, we discussed the fundamental concepts of multi-layer PCBs and the initial stages of their manufacturing process, including panel preparation and inner layer printing. Now, we will delve into the heart of the multi-layer PCB manufacturing process – the inner layer fabrication.

The inner layer fabrication is a crucial phase that involves several intricate steps, each of which plays a vital role in ensuring the quality and reliability of the final PCB product. This article will provide a comprehensive overview of the inner layer fabrication process, covering topics such as oxide treatment, pattern plating, resist stripping, and etching.

Oxide Treatment

The first step in the inner layer fabrication process is oxide treatment. This step is essential for preparing the copper surface of the printed circuit board for subsequent processes, such as pattern plating and resist stripping.

Purpose of Oxide Treatment

The primary purpose of oxide treatment is to remove any contaminants or impurities from the copper surface and create a uniform, slightly roughened surface. This roughened surface improves the adhesion of the plated copper during the pattern plating process, ensuring a strong bond between the inner layer copper and the plated copper.

Types of Oxide Treatment

There are two commonly used methods for oxide treatment:

1. Oxide Alternative Process (OAP): In this process, the copper surface is treated with a specific chemical solution that creates a thin, uniform oxide layer. The OAP solution typically contains sodium chlorite, sodium hydroxide, and other proprietary components.
2. Conventional Oxide Process: This process involves treating the copper surface with an oxidizing solution, such as alkaline sodium chlorite or acidic hydrogen peroxide solutions. The oxidizing agents create a thin layer of copper oxide on the surface.

Both processes aim to achieve a consistent, slightly roughened copper surface that is ready for subsequent processes.

Pattern Plating

After the oxide treatment, the next step is pattern plating. This process involves electroplating additional copper onto the printed circuit pattern, effectively building up the copper thickness on the inner layer.

Importance of Pattern Plating

Pattern plating serves several important purposes:

1. Increased Copper Thickness: The plated copper increases the overall thickness of the copper traces, improving their current-carrying capacity and mechanical strength.
2. Improved Etch Factor: A thicker copper layer provides a better etch factor, which is the ratio of the etchant’s lateral etch rate to its vertical etch rate. A higher etch factor leads to better pattern resolution and reduced undercutting during the etching process.
3. Enhanced Reliability: Thicker copper traces are less susceptible to mechanical stress, thermal cycling, and other environmental factors, improving the overall reliability of the PCB.

Pattern Plating Process

The pattern plating process typically involves the following steps:

1. Electrolytic Copper Plating: The printed circuit panel is immersed in an electrolytic copper plating bath, where copper ions are deposited onto the exposed copper surfaces through an electrochemical process.
2. Current Density Control: The current density during the plating process is carefully controlled to ensure uniform copper deposition across the panel.
3. Plating Thickness Monitoring: The plating thickness is continuously monitored to achieve the desired copper thickness, typically ranging from 10 to 35 micrometers (0.4 to 1.4 mils).
4. Rinsing and Drying: After the desired plating thickness is achieved, the panel is rinsed and dried to remove any residual plating solution.

The pattern plating process is crucial for ensuring the electrical and mechanical integrity of the inner layer copper traces, ultimately contributing to the overall reliability and performance of the multi-layer PCB.

Resist Stripping

Once the pattern plating is complete, the next step is resist stripping. This process involves removing the remaining photoresist from the copper surface, exposing the underlying copper pattern.

Importance of Resist Stripping

Resist stripping is a critical step in the inner layer fabrication process because any residual photoresist left on the copper surface can interfere with subsequent processes, such as etching and lamination. A thorough resist stripping process ensures a clean copper surface, allowing for proper adhesion and preventing potential defects or issues in later stages.

Resist Stripping Methods

There are several methods used for resist stripping, including:

1. Chemical Stripping: This method involves immersing the printed circuit panel in a chemical solution that dissolves or breaks down the photoresist material. Common chemical strippers include alkaline solutions, organic solvents, or a combination of both.
2. Plasma Stripping: In this method, the panel is exposed to a reactive plasma environment, typically consisting of oxygen or a combination of gases. The plasma breaks down the photoresist through a chemical reaction, effectively removing it from the copper surface.
3. Dry Film Stripping: For dry film resist systems, a specialized stripping process is used. This typically involves applying a specific chemical solution or a combination of chemical and mechanical stripping techniques.

Regardless of the method used, the resist stripping process must be carefully controlled to ensure complete removal of the photoresist without damaging the underlying copper pattern.

Etching

After the resist stripping process, the next step is etching. Etching is a crucial step in the inner layer fabrication process, as it defines the final copper pattern on the inner layer.

Purpose of Etching

The primary purpose of etching is to remove the unwanted copper from the printed circuit panel, leaving behind the desired copper pattern. This copper pattern will form the conductive traces, pads, and other features on the inner layer of the multi-layer PCB.

Types of Etching

There are two main types of etching processes used in the inner layer fabrication:

1. Subtractive Etching: In this process, the exposed copper (i.e., the areas not protected by the plated resist) is removed using a chemical etchant. Common etchants used in subtractive etching include ammoniacal etchants, cupric chloride etchants, and alkaline etchants.
2. Pattern-Plated Etching: In this process, the plated copper is first removed using a chemical etchant, followed by the removal of the underlying copper layer using a different etchant or a combination of chemical and plasma etching processes.

The choice of etching process depends on various factors, such as the desired feature resolution, the complexity of the copper pattern, and the specific requirements of the PCB design.

Etching Process Steps

Regardless of the etching method used, the etching process typically involves the following steps:

1. Etchant Preparation: The appropriate etchant solution is prepared according to the manufacturer’s specifications and the desired etch rate.
2. Immersion Etching: The printed circuit panel is immersed in the etchant solution, where the unwanted copper is slowly dissolved or removed.
3. Etch Rate Monitoring: The etch rate is carefully monitored to ensure consistent and uniform etching across the panel.
4. Rinsing and Drying: After the desired etching is achieved, the panel is rinsed and dried to remove any residual etchant and prepare for subsequent processes.

Etching is a critical step in the inner layer fabrication process, as it directly determines the accuracy and quality of the copper pattern on the inner layer. Proper etching techniques and controls are essential for ensuring the electrical and mechanical integrity of the final PCB product.

Frequently Asked Questions (FAQs)

1. What is the purpose of the oxide treatment step in the inner layer fabrication process? The oxide treatment step serves to remove contaminants and create a slightly roughened copper surface, improving the adhesion of the plated copper during the pattern plating process.
2. Why is pattern plating important in the inner layer fabrication process? Pattern plating increases the copper thickness on the inner layer, improving the current-carrying capacity, mechanical strength, and etch factor of the copper traces. It also enhances the overall reliability of the PCB.
3. What are the common methods used for resist stripping? The common methods for resist stripping include chemical stripping (using alkaline solutions or organic solvents), plasma stripping, and dry film stripping.
4. What is the difference between subtractive etching and pattern-plated etching? In subtractive etching, the exposed copper (not protected by the plated resist) is removed using a chemical etchant. In pattern-plated etching, the plated copper is first removed, followed by the removal of the underlying copper layer using a different etchant or a combination of chemical and plasma etching processes.
5. Why is etching an essential step in the inner layer fabrication process? Etching is a crucial step because it defines the final copper pattern on the inner layer, forming the conductive traces, pads, and other features. Proper etching techniques and controls are essential for ensuring the electrical and mechanical integrity of the final PCB product.

By following the inner layer fabrication process outlined in this article, manufacturers can produce high-quality, reliable multi-layer PCBs that meet the demanding requirements of various industries and applications.