Coverlay Adhesive squeezeout on flexible circuits

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What is Coverlay Adhesive Squeezeout?

Coverlay adhesive squeezeout is a phenomenon that occurs in the manufacturing of flexible printed circuits (FPCs). It happens when excess adhesive used to bond the coverlay insulating layer to the circuit seeps out from under the coverlay film. This adhesive squeezeout can cause issues with the functionality and reliability of the flexible circuit if not properly controlled.

Key Aspects of Adhesive Squeezeout

  • Occurs when bonding coverlay film to flex circuit
  • Excess adhesive seeps out from under coverlay
  • Can lead to issues with circuit performance and reliability
  • Needs to be minimized and controlled in manufacturing process
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Flexible Circuit Construction and Coverlay Application

To understand coverlay adhesive squeezeout, it’s important to first look at how flexible circuits are constructed and how the coverlay layer is applied.

Typical Flexible Circuit Construction

A basic flexible circuit is made up of several key layers:
| Layer | Description |
| Base film | The substrate, typically polyimide, that provides structure |
| Copper traces | Thin copper conductors bonded to the base film to form the circuit |
| Insulating layers | Coverlay or Solder Mask to insulate and protect copper |
| Adhesive | Used to bond insulating layers to base film and copper |

The copper layer is etched to form the circuit traces. Then an insulating layer, either a coverlay film or solder mask, is applied on top to protect the copper. Coverlay is more common for dynamic flex applications.

Coverlay Film Application Process

Coverlay is a polyimide film with a b-staged (partially cured) adhesive on one side. It is applied to the flex circuit in a heat and pressure lamination process:

  1. Coverlay is positioned over the etched copper traces
  2. The assembly is placed in a heated press
  3. Heat and pressure are applied to bond the coverlay
  4. Adhesive flows and creates bond between coverlay and circuit
  5. Excess adhesive squeezes out from under the coverlay edges

The lamination process needs to be tightly controlled to achieve a strong bond while minimizing adhesive squeezeout. Excessive squeezeout can occur if parameters like temperature, pressure, and time are not optimized.

Causes of Excessive Adhesive Squeezeout

Several factors in the coverlay lamination process can lead to excessive adhesive squeezeout:

Lamination Press Settings

Parameter Effect on Squeezeout
Temperature Higher temperatures reduce adhesive viscosity and increase flow
Pressure Higher pressure forces more adhesive out from under coverlay
Time Longer lamination times allow more time for adhesive to flow

The press settings need to be dialed in to sufficiently bond the coverlay while minimizing squeezeout. The optimal settings depend on the specific coverlay material, adhesive, and circuit design being used.

Coverlay and Adhesive Material Properties

The rheology (flow characteristics) of the b-staged coverlay adhesive has a significant impact on its tendency to squeeze out. More viscous adhesives are less prone to excessive flow and squeezeout.

Additionally, the thickness and conformity of the coverlay film itself affects how well it contains the adhesive during lamination. Thicker, more rigid coverlays tend to allow less squeezeout than thinner, more flexible ones.

Circuit Design and Feature Sizes

The design of the flexible circuit also plays a role in adhesive squeezeout. Finer pitched traces and smaller features leave less space for adhesive to flow. This can actually make squeezeout more likely as the adhesive is forced out to the sides.

On the other hand, wider coverlays that extend well past the traces provide more room for adhesive to spread before reaching the edges and squeezing out.

Effects of Coverlay Adhesive Squeezeout

Excessive adhesive squeezeout can negatively impact the performance and reliability of flexible circuits in several ways:

Electrical Issues

Adhesive is typically non-conductive, so squeezeout doesn’t directly cause short circuits. However, it can still lead to electrical issues:

  • Alters impedance if adhesive contacts traces
  • Outgassing during Reflow Soldering can cause opens
  • Absorption of moisture can degrade high-frequency performance

Mechanical Problems

The squeezed out adhesive is not as strong or tough as the coverlay film itself. This can cause mechanical reliability issues:

  • Reduced peel strength at coverlay edges
  • Cohesive failure of squeezed out adhesive
  • Debris generation from cracking and flaking of exposed adhesive

Processing and Assembly Difficulties

Finally, adhesive squeezeout can make later processing and assembly of the flexible circuits more difficult:

  • Impedes solder mask adhesion over squeezed out adhesive
  • Interferes with solder pad access and SMT component assembly
  • Blocks access holes and other machined features
  • Makes circuits harder to clean

Because of all these potential issues, it is critical for flexible circuit manufacturers to take steps to prevent excessive adhesive squeezeout.

Preventing Excessive Adhesive Squeezeout

There are several best practices that can be employed in the coverlay lamination process to minimize adhesive squeezeout:

Optimize Lamination Press Parameters

Dialing in the correct temperature, pressure, and time settings is critical. The goal is to use the minimum amount of heat, pressure, and time that will still achieve a reliable coverlay bond.

Typical starting parameters for coverlay lamination are:
| Parameter | Value Range |
| Temperature | 300-400°F (150-205°C) |
| Pressure | 300-500 psi (21-35 kg/cm²) |
| Time | 1-2 hours |

However, the exact optimal settings will depend on the specific materials and press being used. Lamination trials should be run to dial in the best parameters for each particular scenario.

Select Appropriate Coverlay and Adhesive

Using a coverlay film and adhesive combination that is less prone to squeezeout can make the process more robust. Thicker polyimide films and more viscous adhesive formulations tend to have less problem with excess squeezeout.

The coverlay and adhesive suppliers can provide recommendations on which of their products are best suited for a given application. They may also have tips on processing parameters.

Design Circuits to Minimize Squeezeout

Certain flexible circuit design choices can help reduce the risk of adhesive squeezeout:

  • Avoid overly tight trace and space widths to provide room for adhesive flow
  • Extend coverlay edges well past traces where possible
  • Provide non-functional border areas specifically to absorb excess adhesive
  • Place coverlay seams away from critical circuit features

While the electrical functionality has to be the top priority, smart circuit design practices can help make the coverlay lamination process go smoother.

Implement Robust Process Controls

Finally, having a well controlled and monitored coverlay lamination process is essential for minimizing adhesive squeezeout. Some key aspects include:

  • Develop detailed lamination process specs and work instructions
  • Train operators on correct lamination parameters and techniques
  • Perform regular preventative maintenance and calibration on presses
  • Visually inspect circuits after lamination for squeezeout
  • Track and review process data to spot trends and out-of-spec conditions

By tightly controlling the lamination process, flexible circuit manufacturers can produce high quality and reliable products with minimal adhesive squeezeout issues.

Frequently Asked Questions

What is a coverlay in a flexible circuit?

A coverlay is an insulating layer bonded on top of the etched copper traces in a flexible circuit. It is typically a polyimide film with a b-staged adhesive on one side. The coverlay protects the copper conductors and provides electrical insulation.

Why is adhesive used in the coverlay lamination process?

Adhesive is needed to bond the coverlay film to the rest of the flex circuit. The adhesive is typically a thermosetting type that is b-staged (partially cured) onto one side of the coverlay film. When heat and pressure are applied, the adhesive flows and creates a permanent bond between the coverlay and circuit.

What causes coverlay adhesive to squeeze out?

Adhesive squeezeout occurs when too much heat, pressure, or time is used in the coverlay lamination process. This causes the adhesive to flow excessively and seep out from under the edges of the coverlay film. Overly thin or flexible coverlay materials and tight circuit designs can also make squeezeout more likely.

How does adhesive squeezeout affect flexible circuit performance?

Excessive adhesive squeezeout can negatively affect the electrical, mechanical, and processing characteristics of a flexible circuit. The squeezed out adhesive can absorb moisture, outgas during soldering, reduce high frequency performance, weaken coverlay peel strength, and interfere with solder mask adhesion and component assembly.

What are some ways to prevent excessive adhesive squeezeout?

Adhesive squeezeout can be minimized by optimizing the coverlay lamination process parameters (temperature, pressure, and time), selecting appropriate coverlay and adhesive materials, designing circuits with wider spaces and coverlay edges where possible, and implementing robust process controls and monitoring.