Polyimide tape for flexible circuits
Amber polyimide tape is a familiar consumable on flexible printed circuit (FPC) lines. Polyimide film withstands reflow temperatures of 260°C, provides good electrical insulation and is used to mask non-soldered areas such as gold fingers, connectors and tooling holes.
Simple as it is, choosing the wrong thickness or adhesive system can leave adhesive residue, damage gold-plated surfaces or cause the tape to lift in the oven. This article summarizes practical experience with polyimide tape for FPCs.
Properties of polyimide film
Polyimide maintains dimensional stability and mechanical properties over a wide temperature range, from cryogenic temperatures up to about 260°C for short periods. The film has high breakdown voltage, does not propagate flame and resists most solvents. It is also the base material of flexible circuits, so the two are well matched in thermal expansion.
Applications on FPC lines
- Masking gold fingers and connectors during plating or protective coating.
- Securing FPCs to carrier trays during solder paste printing and reflow.
- Insulating between the FPC and metal parts in a module.
- Reinforcing flex areas and connector attachment areas.
- Masking during conformal coating spraying or selective wave soldering.
Choosing film thickness and adhesive system
Film thickness
25 µm film with 35–40 µm adhesive is the most common choice for masking. The 12.5 µm grade suits applications that need thin, flexible tape, such as inside wearable devices. Grades of 50 µm and above are used for insulation or mechanical reinforcement.
Silicone or acrylic adhesive
Silicone adhesive withstands higher temperatures and peels cleanly after reflow with little residue, making it suitable for temporary masking. Acrylic adhesive bonds strongly and is stable over the long term, suitable for permanent bonding but with lower heat resistance. Note that silicone adhesive can affect the adhesion of coatings or adhesives in later processes if it leaves traces.
Tips for residue-free use
- Apply with even pressure and avoid air bubbles that cause localized heating of the adhesive.
- Peel the tape while the board is still warm, at around 40–60°C, so the adhesive releases cleanly.
- Peel at a low angle and a steady speed.
- Store tape at room temperature, away from direct sunlight.
Die-cutting to shape
For complex masking areas, polyimide tape is die-cut into parts on a liner, allowing workers to apply it quickly and accurately, or for use with automatic applicators. This reduces handling time and avoids cutting with knives directly on the board, which can easily scratch coatings and sever traces.
AMS supplies polyimide tape from 3M, DuPont Kapton and Asian manufacturers, available slit into narrow rolls or die-cut to shape. Contact AMS Vietnam to receive samples for your FPC line.
