Chip-on-Flex

What Is Chip-on-Flex?

Chip-on-FlexChip-on-FlexHardwareDie-on-flex packaging for ultra-thin smart card modules.Click to view → (COF) is a semiconductor packaging technique that bonds a bare smart card die directly onto a flexible printed circuit (FPC) substrate, eliminating the rigid lead frame used in conventional smart card modules. This produces an ultra-thin package (as low as 0.3 mm total thickness) suitable for form factors where the standard 0.76 mm module height is prohibitive -- including biometric payment cards, wearable devices, and thin-film smart labels.

How COF Differs from Standard Modules

In a conventional smart card module, the die is mounted on a rigid copper lead frame with wire bonds connecting pads to the contact pad surface. The entire assembly is encapsulated in epoxy, resulting in a module thickness of approximately 0.4-0.5 mm.

In COF packaging, the die is flip-chipped or wire-bonded directly onto a flexible polyimide tape:

Applications

COF technology enables several emerging smart card form factors:

• Biometric payment cards -- the fingerprint sensor module and secure element must fit within the 0.76 mm card thickness alongside a battery, leaving minimal space for the chip module. COF packaging is essential for biometric payment cards.
• Wearable devices -- smart rings, wristbands, and watch modules that embed payment or access control chips in curved, flexible housings.
• Smart labels and tags -- thin-film packages bonded to adhesive labels for supply chain authentication.
• Metal-core cards -- premium payment cards with metal substrates require thinner chip assemblies for proper lamination.

Manufacturing Process

COF production requires specialized equipment:

1. Die preparation -- the silicon die is thinned to 50-100 um (compared to 150-200 um for standard modules).
2. Flex tape preparation -- copper traces are etched on polyimide film matching the die pad layout.
3. Die attachment -- the thinned die is placed on the flex tape using thermocompression bonding or conductive adhesive.
4. Wire bonding or flip-chip -- electrical connections between die pads and flex traces.
5. Encapsulation -- a thin protective coating is applied over the die.

The resulting COF module is then embedded into the card body during the standard chip embedding process, though the cavity depth is shallower due to the reduced module height.