How to Easily Distinguish Between Adhesive-Based and Adhesive-Free FPC Substrates

Published Feb 27, 2025, updated Jul 31, 2025

3 min

1. Structure of Adhesive-Based and Adhesive-Free Substrates  
2. Differences Between Adhesive-Based and Adhesive-Free Substrates  
3. How to Identify Adhesive-Based and Adhesive-Free Substrates

FPC substrate is crucial to the performance of flexible circuit boards! It directly affects the reliability and lifespan of your product.

Currently, the most commonly used FPC substrate (also known as FCCL) on the market is Polyimide (PI), which has excellent heat resistance, chemical stability, and mechanical properties, allowing it to maintain stable insulation performance in high-temperature environments.

However, please note that PI substrates are categorized into adhesive-based and adhesive-free types, each with significant differences in material composition, performance, and application.

Currently, all JLCPCB FPCs use adhesive-free substrates, except for transparent PET materials. Below, we explain the differences and how to identify FPC substrates.

1. Structure of Adhesive-Based and Adhesive-Free Substrates

Adhesive-Based Substrate: Mainly consists of a three-layer structure: PI film/adhesive (AD)/copper foil. It is made by coating, pressing, and baking. The adhesive is typically epoxy resin or acrylic adhesive.

Adhesive-Free Substrate: Composed of PI film and copper foil. There are two production methods:

One method is laminating PI film with copper foil through hot pressing. This type of PI requires special treatment and is also called TPI, which is considered a trade secret. JLCPCB FPC substrates use this technology.

The other method is applying liquid PI onto the copper foil.

2. Differences Between Adhesive-Based and Adhesive-Free Substrates

Since adhesive-free substrates do not require adhesive as a bonding agent, their performance is superior to adhesive-based substrates. Currently, 90% of products on the market require adhesive-free substrates, with the specific differences listed below:

3. How to Identify Adhesive-Based and Adhesive-Free Substrates

Although most products in the market require adhesive-free substrates, many manufacturers use inferior materials to cut costs, passing off adhesive-based substrates as adhesive-free. Below are two quick identification techniques:

1. Check the Thickness

Standard FPC substrate PI thickness is 25um, while extra-thick FPC PI thickness is 50um. Copper thickness options: 12um, 18um, 35um. Coverlay thickness options: 27.5um and 50um (white coverlay requires an additional 10um).

Formula for finished board thickness: Copper thickness + Substrate + Coverlay = Finished Board Thickness

If the actual measured FPC thickness deviates significantly from the calculated thickness, it is likely using an adhesive-based substrate. Example: For a standard FPC double-sided board with 12um copper, the structure is as follows:

From above picture ,total calculated thickness = 0.104mm . If the actual measured board thickness is 0.13mm, then it is likely an adhesive-based substrate.

2. Cross-Section Analysis

Cut a cross-section of the FPC and observe it under a microscope. Check whether there is AD adhesive between the PI and copper layers. This method provides a more accurate judgment of whether the board uses an adhesive-based substrate.

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