Introduction:

Rigid-flex PCBs combine the durability of rigid PCBs with the flexibility of flexible PCBs. Among all types of printed circuit boards, rigid-flex PCBs are the most resistant to challenging environmental conditions, making them a preferred choice for manufacturers of medical and military equipment.

Compared to standard FR-4 rigid-flex substrates, aluminum rigid-flex substrates offer superior heat dissipation and a longer lifespan. The bonding strength of the PCB plays a critical role in the product’s reliability. Therefore, it is essential to study surface treatment methods for aluminum substrates, selecting the most effective bonding strength treatment to enhance PCB reliability and meet the high standards of electronic product durability.

Common Surface Treatment Methods for Aluminum Substrates:

Surface treatment methods for aluminum substrates can be divided into physical grinding & brushing methods and chemical etching methods. Both techniques aim to create a roughened surface to improve adhesion between the aluminum substrate and other materials.

Physical Grinding & Brushing Methods:

Physical grinding and brushing methods involve applying pressure with grinding brushes, sanding belts, or abrasive powders to the surface of the aluminum, creating a rough texture.

Common physical grinding and brushing methods include:

• Nonwoven fabric grinding brushes
• Volcanic ash grinding brushes
• Sanding belt grinding

Chemical Etching Methods:

Chemical etching involves a reaction between a solution and the aluminum substrate, which creates a roughened surface. This process typically uses an alkaline solution. However, since aluminum is not resistant to strong alkalis, operators must carefully control solution concentration and reaction time to achieve the desired results.

Measuring Surface Roughness of Aluminum Substrates:

Surface roughness measurement methods are classified into contact and non-contact techniques.

Contact Measurement: This method typically uses a probe with a small diameter that slides across the surface of the test material, detecting micro-movements. A magnetic sensor then converts these movements into roughness data.

Non-Contact Measurement: Non-contact measurement methods indirectly gather information about the surface’s topography without physically touching the surface, preventing damage to the surface and minimizing potential measurement errors.

Conclusion:

• The roughness of the aluminum substrate’s surface treatment should not be excessively high. Factors like surface morphology and uniformity are essential. Alkaline etching provides the most uniform micro-rough surface.
• Based on tests, aluminum substrates treated with alkaline etching before compression exhibit high anti-aging properties and the best adhesion strength, ensuring high reliability for end products.
• There are several surface treatment options available. For mass production, high-yield, high-efficiency, and cost-effective methods are preferred. When necessary, composite treatments can be used to meet the reliability requirements of the product.

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