Coverlay lamination: How to precisely control the double-edged sword of "adhesive overflow"?

In the FPC manufacturing process, if etching is for defining the circuit, then coverlay lamination is for putting a protective coat on the circuit. However, the key to whether this protective coat is applied well does not lie in the PI film itself, but often in the control of "gluten overflow" under the PI.

1.  What is glue overflow? Why is it a quality killer?

Coverlay (CVL) consists of a PI film and a layer of semi-cured adhesive (usually epoxy or acrylic). During the lamination process, high temperature and high pressure are used to make the adhesive layer flow and fill the gaps between the circuits, which also allows air to be expelled.

Negative impact of excess adhesive on subsequent assembly

If the adhesive layer is subjected to excessive pressure, it will overflow from the opening edge of the CVL, covering the solder pads or gold fingers that should be exposed. For engineers, excess adhesive can obstruct soldering area or cause component misalignment after SMT; for procurement, this means that the defect rate in later SMT stages may increase.

2. Three major variables affecting glue overflow

To accurately control glue overflow, collaborative optimization is necessary from three aspects: production parameters, material selection, and front-end design.

The balance between process and design

• Pressing parameters: Temperature settings and pressure distribution of the high-pressure press. Excessive pressure will cause glue to spray out, while insufficient pressure will produce bubbles.
• Selection of auxiliary materials: "Release paper" and "buffer pads" are used during lamination. The buffer pads can evenly distribute pressure and effectively suppress local glue overflow.
• Design with reserved space: When designing CVL window openings, a "glue overflow compensation" design must be reserved to provide reasonable error tolerance for the manufacturing end.

3. Value equilibrium point

The stability of the material and the precision of the design determine the upper limit of adhesive overflow control. When facing high-density requirements, it is recommended to refer to the following dialogue logic:

Decision-making considerations from a business and technology perspective

• Procurement Perspective: Second-tier adhesives with poor stability exhibit unstable viscosity under high-temperature lamination, easily leading to uncontrolled adhesive overflow (spraying). While this may superficially save on initial procurement costs, it actually introduces a significant risk of high-value scrap during later SMT processes.
• Process perspective: In high-density areas, if adhesive overflow is unavoidable, consider using a "photosensitive cover film (PIC)". It produces images through exposure and development, offering extremely high resolution and eliminating adhesive overflow issues. Although more expensive, it is the preferred choice for areas with high-density precision circuitry and components.

4. Key Control Points: Comparison of the Impact of Glue Overflow Control on Yield

In practical manufacturing processes, the amount of excess adhesive directly determines the effective soldering area of ​​the solder pads and the electrical reliability of the product.

Conclusion: Precisely define the upper limit of yield.

In FPC manufacturing, cover film lamination is not just about "wrapping" the conductors and preventing oxidation; it's a precise control process involving temperature, pressure, and adhesive overflow, directly reflecting the manufacturer's attention to detail. For customers, partnering with companies that allow for compensation and possess stable lamination parameter control at the manufacturing stage is the best way to ensure zero defects in subsequent SMT processes and reduce overall anomaly costs.