Breaking the "Thermal vs. Yield" Paradox: How Micro-Level PI Thermosetting Films Enable Next-Gen Flexible Connections in Foldable Screens and Automotive Electronics

With the rapid evolution of smartphones toward foldable screens, automotive electronics entering the multi-screen connected era, and wearable tech striving for extreme thinness, Flexible Printed Circuits (FPCs) are undergoing an unprecedented technological revolution. Miniaturization, multi-layer integration, and high-frequency, high-speed transmission have become mandatory industry benchmarks.

However, during the precision lamination process of FPCs, material engineers have long faced a difficult physical paradox: to achieve exceptional bonding strength, the adhesive must flow sufficiently under high temperatures and pressure; yet, excessive flow (squeeze-out) leads to the contamination of micro-pitch circuits, directly plummeting production yields.

To break this industry bottleneck, Shenzhen Tunsing Plastic Products Co., Ltd. (Tunsing) has redesigned the molecular flow mechanics of bonding agents to launch a new generation of PI Hot Melt Adhesive Films (Polyimide Thermosetting Sheets). This product represents not just a material upgrade, but a complete processing breakthrough for modern precision flexible connection technology.

I. Overcoming the "Glue Overflow vs. Registration" Dilemma in the Micro-Trace Era

In traditional multi-layer FPC lamination or stiffener bonding, even a micrometer-level deviation or a minor overflow of adhesive can contaminate adjacent solder pads, leading to costly defects.

Tunsing's PI Hot Melt Adhesive Film addresses this via precise rheological tuning, achieving a unique characteristic of "micro-fluidity under heat, immediate cross-linking and lock-in under pressure":

• Micrometer-Level Overflow Control (Trading Space for Precision): Taking the flagship DSP12525 as an example, under standard lamination at $180^\circ\text{C}$, the edge squeeze-out is strictly confined between $0.10 \text{–} 0.15\text{ mm}$ (and $0.13 \text{–} 0.18\text{ mm}$ for the DSPI5030 model). This allows circuit designers to confidently place delicate components or pads closer to the board boundaries, maximizing space utilization inside highly integrated consumer electronics.

• Superior Dimensional Stability: In high-density, multi-layer registration, microscopic displacement caused by uneven thermal expansion is a primary cause of misalignment. Tunsing’s thermosetting sheets match the thermal expansion coefficients of the PI base and adhesive layer, ensuring high geometric stability even under intensive molding pressures to guarantee flawless mechanical alignment.

II. Extreme Heat Resistance: Unfazed by the Intense Thermal Stress of Lead-Free Reflow

In modern SMT (Surface Mount Technology) processes, the peak temperature of lead-free reflow soldering routinely exceeds $260^\circ\text{C}$. This stage is the ultimate test for adhesive films, where lower-grade materials often fail due to bubbling, delamination, or peeling.

Tunsing utilizes a specialized thermosetting modified adhesive. Once the post-curing stage is complete, the molecular chains of the adhesive cross-link to form a dense, three-dimensional Interpenetrating Polymer Network (IPN):

• Absolute Protection at $290^\circ\text{C}$: Tested under the rigorous IPC-TM-650 2.4.13 standard, Tunsing's PI film withstands extreme thermal shock at $290^\circ\text{C}$ for up to 60 seconds with zero bubbling or delamination, keeping the physical structure entirely intact.

• Uncompromising Peel Strength: Post-curing peel strength remains highly stable at $1.0 \text{–} 1.3\text{ N/mm}$ (exceeding the IPC-TM-650 2.4.9 requirement of $\ge 1.0\text{ N/mm}$). This ensures connections stay absolutely secure even when subjected to continuous thermal vibrations in demanding environments like automotive engine bays.

III. Standard Operating Procedure (SOP) for Maximizing Yields

To help engineers unlock the full physical potential of these advanced materials, Tunsing’s technical team recommends a standardized "Chemically Latent Lock-in" lamination workflow:

Storage (Below 10°C) ──> 1. Two-Stage Quick-Press (Air Evacuation ──> Molding Bond) ──> 2. Post-Baking Curing (160°C/60min)

1. Locking in Chemical Activity

Because the thermosetting adhesive contains active cross-linking agents, the rolls must be stored in a cold, dry environment below $10^\circ\text{C}$ and under $70\%\text{ RH}$. This effectively "freezes" premature chemical reactions, ensuring zero performance degradation for up to 3 months.

2. Two-Stage Quick-Press Process

• Stage 1: Pre-Pressing (Air Evacuation): Apply a temperature of $180^\circ\text{C}$ at $0\text{ Kgf/cm}^2$ for 10 seconds. This brief step allows the polymer chains to soften slightly, smoothly evacuating tiny air pockets trapped between layers and preventing inner micro-voids in the final product.

• Stage 2: Molding Press (High-Pressure Bonding): Immediately ramp up pressure to $35\text{ Kgf/cm}^2$ (corresponding to a machine gauge pressure of approximately $100\text{ Kgf/cm}^2$) and hold for 100 seconds. This drives the adhesive to completely wet and fill the microscopic surface roughness of the FPC.

3. Complete Polymer Curing (Baking)

Upon exiting the quick-press machine, transfer the laminated parts to a curing oven and bake at $160^\circ\text{C}$ for 60 minutes. This crucial post-bake triggers the ultimate cross-linking reaction, optimizing the material's chemical resistance, electrical insulation, and peel strength.

IV. Comprehensive Specifications & Global Customization

To meet diverse design requirements—from ultra-thin wearable sensors to heavy-duty power electronics—Tunsing offers a versatile, structured product lineup:

• Ultra-Thin & Lightweight: DSP11315 ($13\mu\text{m}$ PI base + $15\mu\text{m}$ adhesive, total thickness of only $28\mu\text{m}$).

• Balanced & Versatile: DSP12525 ($25\mu\text{m}$ PI base + $25\mu\text{m}$ adhesive, total thickness of $50\mu\text{m}$).

• Heavy-Duty & High-Voltage: DSP15020, DSP15025, and DSPI5030 series (featuring robust $50\mu\text{m}$ PI bases with adhesive thicknesses up to $30\mu\text{m}$).

Additionally, to integrate seamlessly into proprietary automated machinery globally, Tunsing offers full customization of widths (e.g., $250\text{ mm}$ or $500\text{ mm}$), adhesive thicknesses, and custom roll lengths.