PET vs Polyimide Insulation for Compact Robot Controllers

Start with the real operating temperature, not the datasheet headline

The first decision boundary is continuous operating temperature at the insulation location. Selected PET dielectric films are published to about 125°C, while polyimide may support higher-temperature designs, but the adhesive and complete assembly set the final limit.^[1]

A robot controller mounted near motor drivers or power stages can see local hot spots well above the enclosure ambient. If the insulation sits against a heatsink or a switching node, measure the actual surface temperature during worst-case load. PET is often adequate for logic boards and low-power sections, but polyimide becomes the safer default when the film must survive reflow, solder rework, or continuous exposure above PET's practical ceiling.

• Measure the insulation surface temperature, not just the enclosure air temperature.
• Check whether the film must survive solder reflow or rework cycles.
• Confirm the adhesive's continuous-use rating, not only the film's rating.

Dielectric strength is not a single number

Published dielectric strength values are always tied to a specific thickness, test method, and temperature. Selected PI grades show about 154–315 kV/mm, while selected PET dielectric grades can reach about 290–350 kV/mm under their stated tests.^[1]

A thinner film does not automatically give a proportionally lower breakdown voltage because breakdown strength often increases as thickness decreases. However, the actual withstand voltage of a die-cut part depends on edge quality, burrs, adhesive squeeze-out, and any pinholes or scratches introduced during converting. Always specify the test method and accept that material-level data is a screening input, not a system rating.

• Request dielectric strength with the exact film thickness and test standard.
• Account for edge damage and adhesive flow in the final part.
• Treat published kV/mm values as screening data, not a guaranteed part rating.

Creepage, clearance, and edge coverage decide more than the film choice

Creepage is the distance along the surface of the insulation between conductors, while clearance is the shortest distance through air. Both are hard pass/fail criteria in safety testing and depend on working voltage, pollution degree, and isolation class.^[2]

A die-cut insulation part can increase creepage by wrapping over a board edge or covering a connector pin, but only if the cut geometry is accurate and the adhesive holds the film in place. PET and polyimide can both be die-cut with tight tolerances, but polyimide's higher dimensional stability at temperature makes it more predictable when the part must maintain a precise edge position near a high-voltage node.

• Define creepage and clearance requirements before selecting film type.
• Use edge wrapping or folded tabs to extend creepage without adding board area.
• Verify the die-cut part's edge position after thermal cycling.

Thickness, space, and bend radius drive the practical choice

Representative PET electronics films range from 0.012 mm to 0.500 mm, while representative high-performance PI films range from 0.0127 mm to 0.125 mm.^[1]

In a compact robot controller, available height is often the binding constraint. If the design can accept a 0.05 mm or 0.075 mm PET layer, the cost advantage is significant. If the insulation must fold around a tight corner or fit under a connector with almost no clearance, polyimide's thinner available gauges and better flex endurance become decisive. Always check the minimum bend radius for the chosen grade and adhesive stack.

• Compare total stack height including adhesive and liner.
• Check the minimum bend radius for folded or wrapped parts.
• Use polyimide when the available thickness is below about 0.025 mm and temperature is elevated.

Cost comparison: PET wins on price, polyimide wins on performance density

PET remains common because it offers low cost, dimensional stability, and useful dielectric strength. Polyimide film serves higher-temperature and thinner-gauge applications but typically costs several times more per square meter.^[3]

The total cost difference is not only the film price. Polyimide often requires more careful handling, slower cutting, and tighter process controls, which can increase conversion cost. However, if polyimide allows a thinner part, a smaller board, or elimination of a secondary insulator, the system-level cost may favor it. Evaluate both material and conversion cost against the full assembly.

• Request quotes for both film types at the same thickness and part geometry.
• Include scrap, handling, and inspection costs in the comparison.
• Consider whether a thinner polyimide part can reduce overall enclosure size.

Decision matrix for PET vs polyimide in robot controllers

Use this matrix to shortlist the film family before requesting detailed data. The final selection still depends on the exact grade, adhesive, and validated assembly.

• Continuous temperature below 125°C and cost-sensitive: PET
• Continuous temperature above 125°C or solder reflow exposure: Polyimide
• Available thickness 0.05 mm or greater and no tight folds: PET
• Available thickness below 0.025 mm or tight bend radius: Polyimide
• High-volume, low-complexity flat parts: PET
• High-voltage nodes with tight edge position tolerance: Polyimide

Validation protocol before releasing the drawing

A practical validation sequence for a die-cut insulation part in a robot controller should include electrical, mechanical, and environmental checks. Start with a withstand voltage test on the actual part at the maximum operating temperature, not just at room temperature.

Then perform thermal cycling from the minimum storage temperature to the maximum operating temperature for at least 10 cycles, checking for delamination, edge lifting, or adhesive squeeze-out. Finally, verify creepage and clearance on the assembled board with the insulation in place, because the part's final position after assembly is what matters for safety.

• Withstand voltage test at maximum operating temperature
• Thermal cycling with visual and adhesion checks
• Creepage and clearance measurement on the assembled board
• Adhesive peel test on the actual substrate after aging