What is coverlay adhesive squeezeout?
Coverlay adhesive squeezeout refers to the phenomenon where excess adhesive seeps out from under the edges of the coverlay during the lamination process. When the coverlay is pressed onto the flex circuit under heat and pressure, the adhesive can be forced out, forming an uneven, messy edge around the perimeter of the coverlay.
This squeezeout not only affects the aesthetic appearance of the flex circuit but can also cause functional problems. Excess adhesive can interfere with the operation of components placed near the edge of the coverlay or cause short circuits if it bridges adjacent traces. In severe cases, the squeezed-out adhesive may even delaminate over time, compromising the integrity of the coverlay.
Factors contributing to coverlay adhesive squeezeout
Several factors can contribute to the occurrence and severity of coverlay adhesive squeezeout:
Adhesive type and quantity
The type of adhesive used and the amount applied play a significant role in squeezeout. Some adhesives are more prone to flowing and squeezing out under pressure than others. Applying too much adhesive increases the likelihood and extent of squeezeout.
Common types of coverlay adhesives include:
| Adhesive Type | Characteristics | Squeezeout Tendency |
|---|---|---|
| Acrylic | High strength, flexibility | Moderate |
| Epoxy | High temperature resistance, durability | Low to Moderate |
| Pressure-sensitive | Easy application, lower bonding strength | High |
| Thermoplastic | Reflow capability, ease of repair | Moderate to High |
Choosing an adhesive with the right viscosity and flow properties for the specific application and carefully controlling the amount used can help minimize squeezeout.
Lamination process parameters
The lamination process, where the coverlay is bonded to the flex circuit under heat and pressure, directly affects adhesive squeezeout. Key parameters include:
- Temperature: Higher temperatures lower the viscosity of the adhesive, making it more likely to flow and squeeze out.
- Pressure: Excessive pressure can force more adhesive out from under the coverlay edges.
- Dwell time: Longer dwell times allow the adhesive more time to flow and squeeze out.
Optimizing these parameters for the specific adhesive and coverlay material can help reduce squeezeout. Conducting trial runs and making incremental adjustments is often necessary to find the best settings.
Coverlay and circuit design
The design of both the coverlay and the underlying flex circuit can influence the occurrence of adhesive squeezeout. Factors to consider include:
- Coverlay edge design: The shape and profile of the coverlay edges can affect how the adhesive flows. Beveled or chamfered edges can provide a smoother transition and reduce abrupt squeezeout.
- Clearance between coverlay and components: Adequate spacing between the coverlay edges and nearby components can accommodate some squeezeout without causing interference.
- Trace routing: Routing traces a sufficient distance away from the coverlay edges reduces the risk of short circuits caused by adhesive bridging.
Close collaboration between the circuit designer and the Flex Circuit Manufacturer is essential to optimize the design for minimal squeezeout.
Techniques for mitigating coverlay adhesive squeezeout
Several techniques can be employed to mitigate coverlay adhesive squeezeout and its negative effects:
Adhesive application methods
The method used to apply the adhesive to the flex circuit or coverlay can significantly influence the amount and uniformity of the adhesive, and consequently, the extent of squeezeout. Common methods include:
| Method | Description | Advantages | Disadvantages |
|---|---|---|---|
| Screen printing | Adhesive is forced through a mesh screen onto the substrate | Precise control over adhesive placement and thickness | Limited to simple geometries, requires stencil creation |
| Roll coating | Adhesive is applied using a roller | Fast, suitable for large areas | Less precise control over thickness and edge definition |
| Spray coating | Adhesive is sprayed onto the substrate using a nozzle | Suitable for complex geometries, thin and even coatings | Overspray can cause contamination, requires masking |
Choosing the most suitable application method and optimizing the process parameters can help achieve a more controlled and uniform adhesive layer, reducing the likelihood of squeezeout.
Edge sealing and containment
Techniques that focus on sealing or containing the adhesive at the coverlay edges can effectively reduce squeezeout. Some approaches include:
- Edge sealing tape: Applying a thin, non-conductive tape along the perimeter of the coverlay before lamination can create a barrier that prevents adhesive from squeezing out.
- Containment dams: Creating raised features or walls around the coverlay edges, either on the coverlay itself or on the flex circuit, can help contain the adhesive within the desired area.
- Selective adhesive application: Applying the adhesive only to the central portion of the coverlay, leaving a small adhesive-free margin around the edges, can minimize the amount of adhesive available to squeeze out.
These techniques require careful design and process control to ensure proper alignment and effectiveness.
Post-lamination cleanup
Despite best efforts to prevent coverlay adhesive squeezeout, some amount of excess adhesive may still occur. In these cases, post-lamination cleanup techniques can be used to remove the squeezeout and improve the appearance and functionality of the flex circuit. Options include:
- Manual removal: Careful scraping or trimming of the excess adhesive using a sharp blade or specialized tool.
- Laser ablation: Using a focused laser beam to vaporize the excess adhesive without damaging the underlying circuit.
- Chemical dissolution: Applying a solvent that can dissolve the adhesive without affecting the coverlay or circuit materials.
The choice of cleanup method depends on the specific adhesive and coverlay materials, as well as the extent and location of the squeezeout. Proper safety precautions and process controls are essential to avoid damaging the flex circuit during cleanup.
Impact of coverlay adhesive squeezeout on flex circuit reliability
Coverlay adhesive squeezeout can have several negative impacts on the reliability and performance of flexible circuits:
- Electrical shorts: Excess adhesive bridging adjacent traces can cause short circuits, leading to device malfunction.
- Component interference: Squeezeout around component pads can prevent proper component placement and soldering, affecting circuit functionality.
- Delamination: Poor adhesion or uneven distribution of the adhesive due to squeezeout can lead to delamination of the coverlay over time, exposing the underlying circuitry to damage.
- Environmental contamination: Outgassed volatile compounds from the excess adhesive can contaminate sensitive components or cause corrosion.
- Mechanical stress: Thick or uneven adhesive layers resulting from squeezeout can create stress concentrations that lead to cracking or fatigue failure of the flex circuit during repeated flexing.
To ensure the long-term reliability of flexible circuits, it is crucial to minimize coverlay adhesive squeezeout and address any instances that do occur through proper design, process control, and post-lamination cleanup.
FAQ
1. What is the main cause of coverlay adhesive squeezeout?
The main causes of coverlay adhesive squeezeout are excessive adhesive application, non-optimal lamination process parameters (temperature, pressure, dwell time), and inadequate design considerations for the coverlay and circuit layout.
2. Can coverlay adhesive squeezeout cause functional problems in a flex circuit?
Yes, coverlay adhesive squeezeout can cause several functional problems, including short circuits (due to adhesive bridging adjacent traces), component interference (preventing proper placement or soldering), and delamination (exposing the underlying circuitry to damage).
3. What adhesive application methods can help reduce squeezeout?
Screen printing, roll coating, and spray coating are common adhesive application methods that can help reduce squeezeout by providing more precise control over the adhesive placement and thickness. The choice of method depends on the specific requirements of the application, such as substrate geometry and desired adhesive layer properties.
4. How can the lamination process be optimized to minimize adhesive squeezeout?
The lamination process can be optimized to minimize adhesive squeezeout by carefully controlling the temperature, pressure, and dwell time. Lower temperatures, reduced pressure, and shorter dwell times can help reduce the flow and squeeze-out of the adhesive. However, the optimal settings depend on the specific adhesive and coverlay materials used and often require trial runs and incremental adjustments.
5. What post-lamination cleanup methods are available for removing coverlay adhesive squeezeout?
Post-lamination cleanup methods for removing coverlay adhesive squeezeout include manual removal (scraping or trimming with a sharp tool), laser ablation (vaporizing the excess adhesive with a focused laser beam), and chemical dissolution (applying a solvent to dissolve the adhesive). The choice of method depends on the adhesive and coverlay materials, the extent and location of the squeezeout, and the potential risks of damaging the flex circuit during cleanup.
