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Fixing Bubbling and Delamination in Multilayer Flexible Circuits

FPC Delamination Bubbling Questions And Solutions

Introduction

In the manufacturing of FPC, the most dreaded issues are bubbling and delamination. Why does this happen? Because FPC boards are soft and are not vacuum-sealed when they leave the factory. They easily absorb moisture from the air before entering the SMT assembly line. pre-baking treatment is necessary to slowly force out the moisture. Otherwise, during the high-temperature shock of reflow soldering. the absorbed moisture rapidly vaporizes into steam. causing bubbles and delamination in the FPC. Now, let’s find the reasons and solutions for bubbling and delamination in flexible circuits

Bubbling and Delamination in Multilayer Flexible Circuits
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What Causes Bubbling and Delamination of Flexible Circuits?

In the manufacturing process of Flexible Printed Circuits. bubbling and delamination are common quality issues caused by a series of complex factors. that interact with each other, affecting both the product’s quality and reliability. Here’s an explanation of these factors and how they contribute to these issues:

Water absorption:

FPC boards, due to their soft material properties. tend to absorb moisture from the environment more easily. Typically, FPC boards do not use vacuum packaging during production and shipping. which allows them to absorb humidity during transportation and storage. This moisture absorption sets the stage for potential bubbling and delamination issues.

Improper pre-baking treatment:

If the FPC boards do not undergo proper pre-baking treatment before entering the SMT line. or if the baking conditions are inadequate (such as temperature and time). the moisture trapped inside the boards cannot be fully removed. This residual moisture rapidly turns into steam. under the high temperatures of the subsequent reflow soldering process. increasing internal pressure and forcing the material structure to separate. causing delamination and bubbling.

baking for multilayer flexible circuit

Thermal shock during reflow soldering:

The high temperatures FPC boards face during reflow soldering cause any absorbed moisture to  vaporize. This sudden increase in internal pressure can lead to separation. between material layers and also push up the material surface to form bubbles.

Improper lamination:

If the lamination process fails to adequately remove air, moisture, and contaminants. these unwanted substances get trapped inside the board. These trapped gases form bubbles during subsequent heating and pressure applications.

Insufficient heat and time during lamination:

Insufficient heat or inadequate lamination time can result in curing of the pre-preg material. leading to insufficient bond strength between the layers and increasing the likelihood of delamination.

Poor black oxide treatment on internal layers:

Incomplete or contaminated black oxide treatment of the internal circuitry. That can reduce the bonding effectiveness between the layers, raising the risk of bubbling and delamination.

Contamination of internal layers or pre-preg material:

Contamination of the board material or pre-preg sheets during fabrication. or handling severely affects their bonding quality, making the layers prone to delaminate.

Insufficient resin flow:

If the resin flow during lamination is inadequate, it fails to fill the gaps between layers sufficiently. leading to poor bonding and making the layers prone to delaminate under stress or heat.

Excessive resin flow:

Conversely, if too much resin from the pre-preg sheets gets squeezed out during lamination. the remaining amount between the layers may not be enough to maintain adequate bonding strength. affecting layer-to-layer adhesion.

Reducing large copper areas:

Since large copper areas bond less effectively with resin compared to resin-resin bonding. it’s advisable to minimize the use of large copper areas to reduce the risk of delamination.

Insufficient vacuum pressure during lamination:

If the pressure used during vacuum lamination is too low. it affects the resin flow and bonding strength. Moreover, boards laminated under low pressure have less residual stress. which might affect their structural stability and induce delamination.

These detailed analyses help us understand more deeply the causes of bubbling. and delamination phenomena in FPC manufacturing. providing a basis for improving manufacturing processes and enhancing product quality.

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How to Fix Bubbling and Delamination in Multilayer Flexible Circuits

Here are several solutions for bubbling & delamination issues during the production of FPC. aiming to ensure the quality and reliability of the circuit boards:

Improving Moisture Handling:

Use vacuum or moisture-proof packaging during the storage. and transportation of FPC materials to limit moisture absorption.

Implement standardized pre-baking processes to ensure all materials undergo sufficient baking. to remove moisture before entering the production line. Optimize baking conditions (temperature and time) based on material characteristics and environmental conditions.

multilayer flexible circuit lamination

Suggested baking methods :

 

  • Single and double-sided circuits: Bake at 105 to 115°C for 2 to 3 hours.
  • Three-layer polyimide circuits: Bake at 105 to 115°C for 3 to 4 hours.
  • Four layers and above: Bake at 105 to 115°C for at least 4 hours.

After baking, avoid storing FPCs in environments with humidity over 50% over 8 hours. Consider vacuum storage or keep the FPC board in an oven at 35°C.

Optimizing Reflow Soldering Processes:

Design a reasonable temperature curve to reduce thermal shock and expel moisture from the material. preventing sudden vaporization that causes internal pressure. Use low-temperature soldering techniques to lower peak temperatures during reflow soldering.

Improving Lamination Techniques:

Ensure adequate pressure and heat applied during the lamination process. and for an appropriate duration to cure the pre-preg and enhance interlayer bonding strength.

After lamination, check the Tg of multilayer boards. or review temperature records of the lamination process. post-cure the semi-finished products at 140°C for 2-6 hours to continue the curing process.

Enhancing the Quality of Inner Layer Processing:

Perform thorough and uniform black oxide treatment on inner layers. strictly control the process parameters of the oxidation and cleaning tanks. and enhance the inspection of the board surface quality. try using double-sided treated copper foil (DTFoil). Ensure the consistency and cleanliness of the black oxide layer. to improve its bonding strength with the resin. Strengthen the cleaning procedures for inner layers and pre-pregs. using suitable detergents and methods to thoroughly remove contaminants.

Controlling Resin Flow and Pressure:

To ensure precise control of resin flow in the pre-preg. consider the adjustment of pressure and the choice of pre-preg. Properly adjust the pressure to suit different types of pre-preg. to optimize resin flow and improve bonding:

  1. For scenarios requiring reduced resin flow or accelerated curing. choose pre-pregs with lower resin flow or shorter gel times. Adjust the pressure to ensure it’s not too high, reducing residual stress.
  2. For scenarios where enhanced resin flow and interlayer bonding strength are needed. choose pre-pregs with higher resin flow or longer gel times. Increase the pressure to ensure the resin can flow adequately and fill the interlayer gaps.

Conduct lamination in a vacuum to maximize bonding results and reduce bubble formation. Monitor the rate of temperature increase and pressure. to ensure uniformity of resin flow time and temperature during heating. Use materials like kraft paper to moderate the heating curve and prevent sudden temperature changes.

Design Optimization:

Reduce the large copper areas in FPC designs. as the bonding strength between large copper areas and resin is relatively weak. Optimize wiring and copper coverage, and consider adding appropriate vents or exhaust paths. especially under large encapsulated areas. to allow gas to escape during heating, thus reducing the buildup of internal pressure. Also, add sufficient space between layers. to ensure that thermal expansion does not cause excessive mechanical stress. These design optimizations enhance overall bonding strength and structural stability.

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Conclusion

By implementing the above strategies, significant reductions in bubbling and delamination. during the production of flexible circuit boards can be achieved. However, these methods require cross-departmental cooperation. including close collaboration with material suppliers, engineering teams. and quality control departments. to ensure that each step meets high standards of execution quality. If anyone has better methods to prevent bubbling and delamination in FPCs. feel free to share your ideas, and let’s discuss more about flexible circuit together.

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