The substrate holds all other tag components together. The tag antenna is deposited or printed on the substrate, and the IC is then attached to this antenna. A substrate is usually made from flexible material such as thin plastic, but it may also be made from rigid material. Most passive tags use substrates made from flexible material with a thickness of 100 to 200 mm. The substrate material must be able to withstand various environmental conditions through which the tag may pass during its lifecycle. Some of the materials used for the substrate are polymer, PVC, Polyethylenetherephtalate (PET), phenolics, polyesters, styrene, and even paper. The substrate material must provide dissipation of static buildup, a smooth printing surface for antenna layout, durability and stability under various operating conditions, and mechanical protection for the antenna, chip, and their interconnections. Some of the environmental conditions that can affect the substrate are heat, moisture, vibration, chemicals, sunlight, abrasion, impact, and corrosion. The substrate material may affect the design frequency of the antenna; therefore, the effect of substrate material must be considered during proper tuning of the antenna.
One side of the substrate is usually coated with an adhesive material to attach the tag to an object. The adhesive material must be able to withstand appropriate environmental conditions. Sometimes, a protective overlay made from materials such as PVC lamination, epoxy resin, or adhesive paper is added to protect the tag from environmental effects.