PET vs. Polyimide Flexible PCB

Polyimide film tolerates short-term temperatures well above 250°C, which is why it survives lead-free SMT reflow profiles (which peak in the 240–260°C range) without distortion. PET, by contrast, has a much lower continuous-use temperature—commonly cited in the 60–105°C range depending on the specific grade—and generally cannot be put through a standard reflow oven without softening or distorting.

This single property explains most of the application split between the two materials: anything that needs standard SMT reflow assembly needs polyimide. PET is reserved for constructions that are either not reflow-assembled at all, or assembled with lower-temperature joining methods such as conductive adhesives or low-temperature solder pastes designed specifically for heat-sensitive substrates.

PET shows up in membrane switch circuits, flexible printed heaters, some capacitive touch and antenna circuits, and other constructions where components are attached with conductive adhesive, crimped, or not attached at all (the circuit itself is the functional element, as in a heater trace). These applications value PET’s cost, clarity, and dimensional stability at room temperature without needing to survive a reflow oven.

It is not a substrate most fabricators treat as a default flexible-PCB material, because the overwhelming majority of flex PCB demand is for reflow-assembled circuits where polyimide is the only practical choice. If a project genuinely calls for a PET-based circuit—because it is not reflow-assembled and the cost or optical properties matter—confirm with your fabricator whether that falls within their process capability rather than assuming it is interchangeable with a standard polyimide flex quote.

It can be tempting to look at PET’s lower material cost and ask whether it could replace polyimide on a standard flex PCB to save money. For any board that goes through SMT reflow assembly, the answer is no: the substrate would not survive the process, independent of cost. The cost comparison only makes sense for the specific application categories—heaters, membrane switches, adhesive-assembled circuits—where PET’s thermal limits are not a constraint in the first place.