flexpcb.orgPolyimide Flex PCB Datasheets: Read the Test Conditions
UPDATED October 21, 2026
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A polyimide datasheet can contain accurate numbers that answer the wrong question for a particular flex PCB. A film tested by itself is not the same specimen as a completed copper circuit. A dielectric value measured at one frequency is not automatically an input for a different frequency. A typical value is not automatically a purchasing limit.
The practical task is to preserve the conditions attached to each number before using it to compare materials or support a design decision. This article provides a comparison worksheet and two hypothetical calculations for that task. Use the flex PCB materials guide for the broader construction choices; use the workflow here when reviewing the evidence behind a specific property.
Identify exactly what was tested
Record the manufacturer, product grade, document revision and specimen description before copying a property into a spreadsheet. Distinguish uncoated film, adhesive-coated film, copper-clad laminate and completed circuit. Two documents can both use the word polyimide while describing different articles with different interfaces and processing histories.
Then identify the property being reported. Tensile modulus, tensile strength and elongation at break answer different mechanical questions. A high strength value does not itself specify stiffness, and an elongation measured at failure is not an allowable repeated strain. Write the intended use of the input beside the property so that the distinction remains visible during review.
The current Qnity Kapton HN document identifies film gauges, temperatures, test methods and specimen conditions in its property tables. Its electrical table specifies temperature and relative humidity, and gives a frequency for dielectric constant and dissipation factor. Its closing notes characterize the data as typical and caution against using it alone to establish design or specification limits. See Kapton HN data sheet, QE-10206, March 2026.
That structure suggests a useful reading order: specimen first, property second, conditions third, value last. It prevents a familiar material name from becoming a shortcut around the information that determines whether the value is relevant.
Build a comparison row that can survive review
The following original worksheet is deliberately more detailed than a material ranking chart. Fill one row for each property and candidate. If a condition is missing, enter unknown and create a question for the supplier. An empty cell should never quietly inherit the conditions of the neighboring candidate.
| Field | What to retain | Decision it protects |
|---|---|---|
| Material identity | Grade, form, thickness and document revision | Whether both values describe the proposed material |
| Measurement definition | Property, units, method and relevant specimen geometry | Whether the reported quantities are comparable |
| Environment and history | Test temperature, humidity, conditioning and prior exposure where given | Whether the result represents the intended state |
| Direction and frequency | Material axis and electrical frequency where applicable | Whether the input belongs in the intended model |
| Statistical meaning | Typical value, guaranteed bound or reported range | Whether it can support screening or acceptance |
| Application gap | Conditions that differ from the finished circuit | What requires clarification or representative verification |
Add the page and table reference to the saved record. A reviewer should be able to reconstruct the comparison without searching the entire supplier website or guessing which edition was downloaded. Keep the original units alongside converted units when conversions are necessary.
Keep thickness attached to the property
A thickness column is part of the specimen identity, even when the property is already expressed in normalized units. Do not assume that normalization removes every dependence on specimen geometry or test setup. Select the applicable gauge in the published table and document any gap between that gauge and the proposed construction.
Consider a hypothetical breakdown test reported as an electric field of 200 kilovolts per millimetre for a specimen 0.025 millimetres thick. Multiplication gives 5 kilovolts for that test specimen under the assumed uniform-field interpretation. This arithmetic checks units; it does not authorize a five-kilovolt operating rating for a flex PCB. The example values are invented and are not taken from either linked material sheet.
A circuit-level insulation decision would still need the actual construction, conductor geometry, interfaces, environment and applicable product requirements. Do not treat a film test as a completed insulation assessment. Similarly, multiplying a published field value by an arbitrary thicker film does not establish what a different specimen would achieve in a real test.
For a thickness-sensitive decision, put the proposed finished section beside the property row. The stackup guide helps identify which dielectric and interface the model actually contains. A value for one film should not be assigned to the whole stack merely because that film is its most recognizable constituent.
Track direction through the drawing and model
The Qnity Kapton EN data sheet separates machine-direction and transverse-direction entries for several mechanical and thermal properties, and distinguishes product gauges. It also identifies the methods associated with those rows. These distinctions provide a concrete reason to retain direction and grade instead of storing one generic polyimide value. See Kapton EN data sheet, QE-10173, April 2026.
When a model needs an axial property, ask how its axis maps to the film orientation in the manufactured part. Record that mapping if it is controlled and relevant. If the manufacturing orientation is unknown, mark it as an unresolved model input. Averaging two directional values does not establish that the average represents the loading direction.
Keep this question separate from a general material ranking. One candidate may have a favorable value along a reported axis while the product geometry loads a different axis. The useful comparison concerns the applicable direction, not the largest number in each document. Where orientation does not affect the decision materially, document that conclusion and its basis rather than silently discarding the field.
Match electrical frequency before ranking materials
For electrical screening, record the frequency and method with each dielectric value. A spreadsheet that places two values in the same column does not make their measurement conditions equivalent. If the required operating range is absent from a document, request appropriate data or retain the candidate as unqualified for that particular comparison.
Use an original idealized capacitor example to see why both material and geometry matter. Assume a uniform dielectric between perfectly parallel plates, neglect fringing and all other materials, and hold plate area constant. The model gives capacitance proportional to relative permittivity divided by separation. Assume candidate A has relative permittivity 3.2 and thickness 25 micrometres, while candidate B has 3.5 and thickness 30 micrometres, all at identical hypothetical test conditions.
The ratio of B capacitance to A capacitance is (3.5 / 30) divided by (3.2 / 25), approximately 0.911. Under these assumptions B has the higher relative permittivity but the lower modeled capacitance because the separation also changed. This is a geometry lesson using invented inputs, not a comparison of commercial films and not a transmission-line simulation.
If the two assumed permittivity inputs instead came from incompatible frequencies or methods, even that narrow comparison would need qualification. Do not add decimal places to conceal the mismatch. Separate uncertainty in the input from precision in the arithmetic, and avoid using this simple model to claim insertion loss or impedance for a patterned flex route.
Separate exposure history from test temperature
A specimen can be conditioned at one temperature and measured at another. It can also be tested before or after a thermal exposure. Keep these events in sequence. A property measured while hot answers a different question from a room-temperature property measured after heating and cooling.
For a proposed comparison, write a short timeline: initial specimen state, conditioning environment and duration if specified, processing or exposure, waiting period, then measurement environment. Only fill in information actually provided or confirmed. If the data sheet omits a duration or transfer condition needed for the decision, ask for it rather than assuming a common laboratory practice.
This timeline is especially useful when someone proposes using an as-received film value to explain a finished part after several manufacturing steps. List the differences explicitly. The result may remain useful for early screening, but its role should be labeled accordingly. Additional testing should address the important differences rather than repeat a convenient measurement that does not represent them.
Distinguish a typical value from an acceptance criterion
A typical value can help establish the scale of a property and identify candidates for investigation. It does not, by itself, tell the buyer the guaranteed minimum, maximum or distribution for every delivered lot. Do not turn a typical number into a purchase limit simply by copying it into a drawing.
Suppose a design requires a lower bound on a property, while a candidate document provides only a typical result above that bound. The honest comparison outcome is that the guarantee remains unresolved. It is not a pass based on the typical value, and it is not automatically a rejection of the material. Ask which specification controls the supplied product and what evidence supports the required bound.
Preserve this distinction in a material scorecard. Use separate fields for published screening data, contractual requirement and verification evidence. A single green cell can hide that only the first field has been filled. The material selection tool can help organize candidates, but final acceptance still requires evidence applicable to the actual requirement.
Close the comparison with an engineering decision
Classify each property comparison as comparable for the stated purpose, useful only for screening, or unresolved. Explain the reason in one sentence. This is more actionable than forcing every candidate into a numerical ranking when the underlying conditions differ.
Retain the original documents, completed worksheet and the questions that changed the selection. If a supplier later changes the proposed grade, review the affected rows rather than assuming that the common polymer family preserves every property. Revisit the model inputs and any qualification evidence tied to the previous construction.
For a project review, provide the intended function, proposed section, operating conditions and the specific datasheet rows that need clarification. Include the original test conditions with the numbers in your flex PCB material review request. That makes it possible to discuss the remaining evidence instead of debating an isolated property value stripped of its meaning.