What should be noted when using unidirectional polyimide film
Full precautions for using unidirectional polyimide (single pull PI) film
The characteristic of unidirectional PI is anisotropy: the longitudinal MD (tensile orientation direction) has much better strength, modulus, flexural resistance, and thermal dimensional stability than the transverse TD; the transverse MD is prone to tearing, has higher elongation, and thermal expansion. The first pri
Full precautions for using unidirectional polyimide (single pull PI) film
The characteristic of unidirectional PI is anisotropy: the longitudinal MD (tensile orientation direction) has much better strength, modulus, flexural resistance, and thermal dimensional stability than the transverse TD; the transverse MD is prone to tearing, has higher elongation, and thermal expansion. The first priority of all operations is to distinguish the tensile direction.
1、 Direction determination and force application (core)
1. Distinguish between MD vertical/TD horizontal
MD (machine stretching direction): high tensile strength, not easy to break, resistant to repeated bending, heat shrinkage reduction;
TD (Horizontal): Low strength, high brittleness, easy to crack with slight pulling, high elongation, and obvious high-temperature shrinkage.
The application of force, wrapping, bending, and tension must be arranged along the MD; It is strictly prohibited to place the main force and repeatedly bend it horizontally.
2. Tension control
Low and uniform tension throughout the entire process of slitting, laminating, winding, and die-cutting; Horizontal tension cannot be applied, thin (≤ 25 μ m) can be torn directly by slight pulling in the horizontal direction; MD can withstand standard process tension, but it is prohibited to pull and impact instantly.
3. Cut and open holes
The horizontal edge is a weak point, and the cutting and die-cutting blade cannot vertically span a large area of horizontal stress zone; The length of the long strip material must be arranged in the MD direction to reduce the lateral short edge stress.
2、 Key points for processing die-cutting, cutting, and drilling
1. Mechanical die-cutting
Thin sections<25 μ m are prone to wrinkling and edge breakage; The cutting edge of the mold is sharp, the gap is small, and the downward pressure is moderate. Under heavy pressure, it is prone to thin deformation and delamination; Slowly discharge waste after die-cutting, and the waste edges are prone to tearing horizontally.
2. Laser cutting
Priority UV picosecond/cold laser; CO ₂ thermal laser can easily cause edge carbonization and microcracks, and transverse cracks will rapidly propagate and expand along the transverse direction; Cut the path with as few horizontal long cutting lines as possible.
3. Drilling/slotting
The edge of the hole is prone to stress cracking horizontally, and it is recommended to make small rounded corners on the hole edge of high-precision parts; The high-speed drilling speed is moderate, and the feed rate should not be too fast.
3、 High temperature, hot pressing, and curing conditions
1. Temperature resistance benchmark
Long term use -200-260 ℃, short-term peak ≤ 400 ℃; The thermal expansion coefficient of unidirectional membrane MD is much lower than TD, and there is a large difference in longitudinal and transverse dimensions under high and low temperature cycling. Composite substrates (copper foil, adhesive, glass fiber) need to match the thermal expansion difference, otherwise warping, delamination, and internal stress cracking will occur.
2. Hot pressing, pressing, baking
Slow heating/cooling rate (5-10 ℃/min), rapid cooling and heating amplify the longitudinal and transverse shrinkage differences; The compression pressure is uniform, and local high pressure is prone to transverse wrinkling and thinning; Long term baking above 200 ℃ will cause slight thermal aging and a faster decrease in lateral strength.
4、 Adhesive bonding, overlay, and coating
1. Inert bonding itself
The surface energy of PI is low, and the adhesion of direct adhesive is poor; Before compounding and pasting, it is recommended to use plasma corona discharge and polish the primer to improve adhesion.
2. Control humidity with adhesive PI film
Storage humidity: 40% -60% RH; high humidity>70% RH. The adhesive layer absorbs water and is prone to bubbles and peeling during bonding; Low humidity<30% RH leads to severe static electricity, causing dust adsorption and insulation breakdown.
3. Matching of overlay tension
Copper foil FEP、 When composite with fiberglass, MD tension matching is consistent; Inconsistent lateral tension can cause the overall product to bow and curl.
5、 Attention to Electrical Insulation Scenarios
1. Surface cleanliness
Oil stains, fingerprints, and dust will significantly reduce the breakdown voltage; Clean the insulation working surface before assembly, and do not touch it directly with bare hands.
2. Thickness selection
The higher the voltage, the thicker the specifications should be selected; The insulation stability of unidirectional film MD with the same thickness is better than TD, and MD winding is necessary for high-voltage winding.
3. Electrostatic protection
Dry environments are prone to static electricity accumulation, and electronic FPC and chip insulation processes require ion air and anti-static countertops to prevent electrostatic breakdown of precision components.
6、 Storage and environmental storage
1. Temperature and humidity
Storage at 15-30 ℃, 40% -60% RH; long-term storage above 40 ℃ accelerates molecular degradation, with a significant decrease in lateral mechanics; Below 0 ℃, it becomes brittle and the film is prone to micro cracking during die-cutting.
2. Avoid light and seal
Direct exposure to ultraviolet radiation can cause yellowing, embrittlement, and insulation degradation; Whole roll aluminum foil vacuum moisture-proof packaging, use up as soon as possible after opening; Long term moisture absorption will slightly affect interface adhesion.
3. Stacking
Place the roll vertically to avoid horizontal compression and wrinkling of the inner film; Thin rolls are prohibited from stacking heavy objects.
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