What are the advantages of bidirectional polyimide film compared to unidirectional stretching PI fil
1. Mechanical properties are balanced in both directions, with no obvious shortcomings
Unidirectional tensile PI only has high strength in the tensile direction (MD longitudinal), but poor transverse TD toughness:
Bidirectional membrane: The longitudinal and transverse tensile strength, modulus, and tear strength are almost the same, with minimal anisotropy;
1. Mechanical properties are balanced in both directions, with no obvious shortcomings
Unidirectional tensile PI only has high strength in the tensile direction (MD longitudinal), but poor transverse TD toughness:
Bidirectional membrane: The longitudinal and transverse tensile strength, modulus, and tear strength are almost the same, with minimal anisotropy;
Unidirectional film: easy to tear horizontally, poor tensile strength, and prone to cracking when bent horizontally.
Practical use: BO-PI is less prone to cracking and burrs during multi-directional bending, cutting, and die-cutting processes.
2. Significant improvement in high-temperature dimensional stability and lower thermal shrinkage
Unidirectional film has not undergone transverse stretching, resulting in significant transverse shrinkage at high temperatures, and is prone to warping, wrinkling, and copper stripping during processing
BO-PI undergoes vertical and horizontal bidirectional high-temperature shaping, and the bidirectional thermal shrinkage rate can be controlled within ≤ 0.8%;
The transverse thermal shrinkage of MDO unidirectional film can reach 3%~6%, and severe deformation occurs after reflow soldering, pressing, and baking.
Compatible with high-precision thermal processes such as FPC, foldable screens, and chip packaging.
3. Multi directional repeated bending has a longer lifespan
Unidirectional films can only withstand bending along the stretching direction, and will break after folding several times perpendicular to the stretching direction;
The bidirectional PI molecular chain has a two-dimensional uniform orientation and can be repeatedly bent tens of thousands of times in any direction without cracking. It is a necessary material for folding screens, rotating axis cables, and flexible circuits.
4. Better thickness uniformity, ultra-thin and high-precision process adaptation
The bi-directional synchronous stretching process can correct thickness deviations, resulting in smaller thickness tolerances for ultra-thin films ranging from 5 to 25 μ m;
Unidirectional stretching can only be controlled by longitudinal stretching, with large fluctuations in thickness in the transverse direction. After photolithography and copper-clad coating, the accuracy of the circuit is poor, and line width deviation is prone to occur.
5. High flatness, more sufficient release of internal stress
There is a huge lateral residual stress inside the unidirectional membrane, which will naturally curl after cutting and copper coating;
BO-PI bi-directional stretching+high-temperature setting, bi-directional internal stress cancellation, smooth film formation without curling, high flatness of copper-clad FCCL substrate, and no bubbles during bonding and compression.
6. Unify the coefficient of thermal expansion (CTE) in both directions and match the copper foil substrate
FPC copper foil CTE and PI film need to be matched, otherwise high and low temperature cycling will cause delamination and wire breakage:
The values of bidirectional PI longitudinal and transverse CTE are close, and the deformation of the entire substrate is synchronized;
The unidirectional film has a low longitudinal CTE and a high transverse CTE, and the cold and hot cycles are not synchronized, making the circuit prone to breakage.
7. Wide processing adaptability, high yield of die-cutting and slitting
Die cutting punching type: unidirectional horizontal prone to edge breakage and film tearing; Both horizontal and vertical cutting are neat;
Drilling and etching: The film is subjected to uniform force and will not stretch or deform, causing misalignment of the circuit;
Copper lamination: No substrate tensile displacement caused by unidirectional stress.
8. Electrical performance is stable in all directions
The directional arrangement of unidirectional membrane molecules can cause differences in dielectric and insulation properties in both horizontal and vertical directions;
The bidirectional PI two-dimensional structure is uniform, with almost no difference in dielectric constant, breakdown voltage, and loss factor horizontally and vertically. The performance consistency of 5G high-frequency lines and insulation components is better.
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