What material is a flex PCB made of?

The base film is usually polyimide (PI), sometimes polyester (PET) for low-temperature static applications. Copper foil carries the circuit, a coverlay protects the traces, and PI, FR4 or stainless-steel stiffeners add local rigidity at connectors and component areas.

PI or PET - which base film should I use?

PI is the default for anything that will be soldered, run hot, or bend repeatedly. PET is cheaper and fine for simple, static, low-temperature connections, but it cannot take soldering temperatures the way PI can.

How is flex PCB thickness calculated?

Total thickness is the sum of every layer in the stack-up: base film, adhesive, copper, coverlay adhesive and coverlay film, plus any stiffener or shielding. It is not a single material property, which is why two '0.1 mm' flex PCBs can behave differently if the layer breakdown differs.

What copper weight should I specify?

Common choices are 1/3 oz (about 12 um), 1/2 oz (about 18 um) and 1 oz (about 35 um). Heavier copper carries more current but is harder to bend and limits achievable trace and space, so the right value depends on current and bend requirements together.

Do I need to send a stackup drawing?

Yes, for anything beyond a simple single-sided board. A stackup drawing tells the fabricator the layer order, material and thickness of each layer, which is what makes the quote and the build comparable with your other suppliers.

Home/Flex PCB Material & Stackup

Flex PCB Material and Stackup: How to Choose

A flexible circuit is not defined by a single material - it is defined by how five elements are stacked: base film, adhesive, copper, coverlay and stiffener. Change one and the bend performance, solderability and thickness all change with it. This guide explains the choices and how to write them down so the part you receive matches the part you designed.

The five elements of a flex stackup

Every flexible circuit is built from the same set of decisions. The order and thickness of these layers is the stackup:

Base film - the insulating carrier, usually polyimide
Adhesive - bonds copper to film, and coverlay to copper
Copper - the conductive layers, in a specified weight
Coverlay - protects the traces and defines the pad openings
Stiffener - local reinforcement at connectors and component areas

Base film: polyimide vs polyester

Polyimide (PI) is the default for most flexible circuits. It withstands soldering temperatures, has good dimensional stability and holds up under repeated bending. Common PI film thicknesses are 1/2 mil (about 12.5 um), 1 mil (25 um) and 2 mil (50 um).

Polyester (PET) is cheaper and adequate for simple, static, low-temperature connections where nothing is soldered directly to the flex. It should not be chosen for reflow assembly or for parts that bend repeatedly.

Adhesive: bonded vs adhesiveless

Adhesive-based laminate - the traditional construction: copper bonded to PI with an acrylic or epoxy adhesive layer. Lower cost, and the adhesive adds thickness.
Adhesiveless (2-layer) laminate - copper bonded directly to PI without a separate adhesive layer. Thinner, better for fine lines and repeated bending, and generally more expensive.

For a dynamic bend or a very thin stack-up, the adhesiveless construction is usually worth discussing. For a simple static part, adhesive-based may be sufficient.

Copper: rolled annealed vs electrodeposited

Electrodeposited (ED) copper - the standard choice. Good for static applications and fine lines, lower cost.
Rolled annealed (RA) copper - grain structure is more ductile, so it tolerates repeated bending better. Common in dynamic flex designs.

Copper weight is specified separately: 1/3 oz (about 12 um), 1/2 oz (about 18 um) and 1 oz (about 35 um) are the usual choices. Heavier copper carries more current but reduces bend performance and limits trace and space.

Coverlay: the layer people forget

The coverlay is a PI film with adhesive that is laminated over the finished traces, with windows opened over pads and fingers. It protects the copper from oxidation, handling damage and moisture, and it is a structural part of the bend area.

Opening size matters - too small and the pad is partly covered; too large and the coverlay edge sits close to a solder joint.
Keep it continuous through bends - a coverlay edge inside the bend zone is a stress riser.
Photoimageable coverlay is an alternative for fine openings, but it is a different material system and should be specified deliberately.

Stiffeners: where and what

PI stiffener - thin, flexible-ish reinforcement; good where a small increase in stiffness is needed.
FR4 stiffener - rigid and dimensionally stable; common behind connectors and SMT areas.
Stainless steel - the stiffest option; used where the part needs real mechanical support.

A stiffener should stop before the bend zone and transition gradually. Putting a stiffener directly in a bend prevents the circuit from flexing where it was designed to.

How thickness is actually calculated

Total thickness is the sum of every layer - it is not one material spec:

Base film (for example 25 um PI)
Adhesive layers (top and bottom, if adhesive-based)
Copper (12 / 18 / 35 um, per layer)
Coverlay adhesive and coverlay film
Any stiffener, shielding or additional laminate

This is why a target such as “0.1 mm total thickness” must be accompanied by a layer breakdown. Two suppliers can both hit 0.1 mm with different copper and adhesive combinations, and the parts will not behave the same in a bend.

Specifying the stackup on your drawing

A layer table: layer name, material, thickness and copper weight, from top to bottom
Total thickness with tolerance
Coverlay type and opening requirements
Stiffener material, thickness and position
Surface finish
Bend area, if the part bends, including radius and static or dynamic
Any impedance target and reference layer

A stackup drawing is what makes quotes comparable. Without it, each supplier quotes a different construction and the prices are not measuring the same product.

Common material and stackup mistakes

Specifying only total thickness and leaving the layer breakdown to the supplier
Choosing ED copper for a dynamic bend because it is the default
Putting a stiffener or coverlay opening in the bend zone
Forgetting adhesive thickness when targeting a thin stack-up
Mixing material systems between prototype and volume without updating the drawing
Leaving the surface finish unspecified, then comparing quotes that assume different finishes

Where this fits in the project

For the product range the stackup applies to, see our flexible PCB (FPC) page. If the part bends, read flex PCB bend radius before finalising the layer table. For the data package a quotation needs, see how to prepare Gerber and stack-up. For customs documentation on a finished part, see flexible PCB HS code. Send drawings through the contact page.

Frequently Asked Questions

What material is a flex PCB made of?: The base film is usually polyimide (PI), sometimes polyester (PET) for low-temperature static applications. Copper foil carries the circuit, a coverlay protects the traces, and PI, FR4 or stainless-steel stiffeners add local rigidity at connectors and component areas.
PI or PET - which base film should I use?: PI is the default for anything that will be soldered, run hot, or bend repeatedly. PET is cheaper and fine for simple, static, low-temperature connections, but it cannot take soldering temperatures the way PI can.
How is flex PCB thickness calculated?: Total thickness is the sum of every layer in the stack-up: base film, adhesive, copper, coverlay adhesive and coverlay film, plus any stiffener or shielding. It is not a single material property, which is why two '0.1 mm' flex PCBs can behave differently if the layer breakdown differs.
What copper weight should I specify?: Common choices are 1/3 oz (about 12 um), 1/2 oz (about 18 um) and 1 oz (about 35 um). Heavier copper carries more current but is harder to bend and limits achievable trace and space, so the right value depends on current and bend requirements together.
Do I need to send a stackup drawing?: Yes, for anything beyond a simple single-sided board. A stackup drawing tells the fabricator the layer order, material and thickness of each layer, which is what makes the quote and the build comparable with your other suppliers.

Send your stackup for review

Contact us with your project requirements through the contact page. Our team will explain how to submit Gerber files securely for review, then confirm production feasibility, lead time and quotation.