1. PI substrate: For FPC flexible circuit board substrates, there are two types: adhesive substrate and non adhesive substrate. PI polyimide is essential for both adhesive and non adhesive substrates.

2. PI covering film: Its main function is for circuit insulation.

3. PI reinforcement: often used in the area on the back of FPC gold fingers. PI reinforcing ribs are used to increase the thickness and hardness of fingers, making it easy to insert and remove.

Differences in thickness between the substrate and covering film of FPC flexible circuit boards:

1. PI substrate: For PI in the substrate, 1/2mil and 1mil are common, and their thickness can even reach 4mil and 5mil (DuPont adhesive free PI material).

2. PI coating film: There are two options for the PI thickness on the coating film, 1/2mil or 1mil.

3. PI reinforcement: There are multiple options for the thickness of PI reinforcement according to customer needs, such as 0.075mm, 0.1mm, 0.125mm, 0.175mm, 0.2mm, 0.225mm, 0.25mm, etc.

Differences in color selection between the substrate and covering film of FPC flexible circuit boards:

1. PI substrate: No color selection.

2. PI covering film: PI covering film has three colors to choose from: yellow, white, and black.

3. PI reinforcement: Changes with thickness, with thicker colors becoming darker. For example, a 0.075mm PI reinforcement looks brown, while a 0.25mm PI reinforcement is closer to black.