Investigating the impact of irreversible heat shrinkage of PET on surface metallization (SMT)

In the world of flexible printed circuit boards (FPCs), material selection and structural configuration are not only cost issues, but also fundamental to product lifespan. The price difference between these two often reflects a vast difference in physical characteristics: one is designed for ultimate performance, while the other exists for extreme cost control. From a process perspective, many projects seem to go smoothly in the prototype design stage (EVT), but fail when entering the SMT (surface mount technology) high-temperature reflow soldering stage. The losses caused by the thermal failure of the substrate are often several times greater than the initial material price difference.

1. Thermal stability: A severe test of instantaneous high temperatures

The choice of FPC substrate directly defines the product's process tolerance limit. Under the industry standard of lead-free, the instantaneous high temperature of the soldering process is a severe test for polymer materials.

PI (Polyimide): An irreplaceable process benchmark

Polyimide (PI) has an extremely stable molecular chain structure, with a glass transition temperature (Tg) typically above 280°C. When subjected to the high temperatures of 240°C to 260°C during mainstream reflow soldering, PI substrates maintain excellent modulus stability. This means that the circuitry and solder pads do not shift due to thermal softening and deformation of the substrate, ensuring high yield rates for precision component mounting. Given the premise of stable operation, PI is the ideal choice for automotive, medical, and high-end communication products.

PET (Polyester): A compromise solution for specific scenarios

The physical properties of PET limit its "thermal boundary". Its long-term continuous operating temperature is usually difficult to exceed 105°C. Once it enters a high-temperature welding furnace, the molecular structure of PET will undergo violent rearrangement, causing the substrate to shrink rapidly, curl, or even melt and deform. Therefore, PET is commonly used in low-temperature processes, such as membrane switches or simple connectors using cold pressing technology. Its main advantage lies in cost, rather than environmental resistance.

2. Dimensional stability: Excluding material's response to temperature and hygroscopic properties

The precise alignment of precision circuits is a core challenge in FPC production. The material's response to environmental humidity and temperature directly determines the final yield.

PI Moisture Management: Although PI has excellent heat resistance, its polymer properties result in a relatively high moisture absorption rate. When moisture enters high temperatures, it rapidly vaporizes and expands, potentially leading to board explosion if not properly managed. This is why standardized "pre-baking" before SMT is emphasized to precisely control dimensional expansion and contraction.

PET Irreversible Heat Shrinkage: While PET has a low moisture absorption rate, its heat shrinkage is large and irreversible, making it unsuitable for SMT processes.

3. Structure and Function

Even the same material plays drastically different functions at different layers of an FPC structure:

• Substrate (FCCL): As a carrier, it bears the transmission of circuit signals. Structurally, double-sided boards typically combine copper and the substrate.
• Coverlay: Provides chemical and electrical insulation and reduces the risk of stress concentration and breakage of copper conductors during flexible board bending.
• PI Stiffener: Selectively adhered to the back of the FPC, it acts like a "skeleton" within the soft tissue, providing necessary mechanical strength for the gold finger insertion/removal areas or component soldering areas. With advancements, PI stiffeners can also be made of stainless steel.

4. Thickness and Space

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Pursuing consistent quality requires attention to "structural symmetry." When the thickness of the cover film on both sides of the substrate is inconsistent, the mismatch in the coefficient of thermal expansion (CTE) can lead to severe warpage, a fatal flaw in automated assembly. Selecting suppliers with end-to-end process control capabilities is the only way to ensure long-term, stable FPC quality.