How to use Adhesive Material for Flexible Reinforcing Printed Circuit Board?

 In Materials

Flexible substrates are characterized by being flexible and can be bent freely, but if everything is soft, it will be difficult to handle. A certain degree of thickness and hardness is required, especially for component mounting and connection.

A reinforcing plate is used to harden a part of a flexible substrate like this. Most of the flexible substrates currently in production have some form of reinforcing plate attached.

Adhesive Material for Flex PCB

Any material should be fine for the reinforcing plate, but…

There are no technical restrictions on the selection of reinforcing plate materials as long as they satisfy the purpose of use. You can also freely choose the adhesive used to attach the reinforcing plate.

However, if we outsource the production of flexible substrates to a specialized manufacturer, we will have to choose from among the materials that manufacturers are accustomed to using. Epoxy boards and polyimide sheets are the main products. Other material selections are either custom or user-supplied.

In addition, metal plates such as aluminum and stainless steel have properties that cannot be obtained with ordinary printed circuit board materials, so they are used in large quantities for specific applications.

For this reason, some manufacturers treat aluminum plates and stainless steel plates with specific specifications as equivalent to standard products.

Characteristics of Polyimide and PET

At a time when circuit density was low and lead terminals were used for mounted components, paper phenol and glass epoxy were often used as materials for reinforcing plates. Both are heat-resistant enough for soldering.

First, paper phenol cannot be used with thermosetting adhesives, so only pressure-sensitive adhesives can be used for mounting, and high reliability cannot be expected. If high reliability is required, glass epoxy is attached using a thermosetting adhesive.

Recently, however, the number of substrates on which minute chip parts are mounted is increasing. In such cases, a configuration in which a thick polyimide sheet is fixed with a thermosetting adhesive is becoming common.

Since the insertion part of the FFC connector does not have the possibility of being exposed to high-temperature processes such as soldering, it is possible to use a PET sheet instead of a heat-resistant polyimide sheet. However, PET sheets cannot withstand the process of thermosetting adhesives, so pressure-sensitive adhesives are used.

Diverse Reinforcing Plate Materials

As the use of flexible substrates diversifies, the forms and materials of reinforcing plates other than standard printed circuit board materials are sometimes used.

In addition to various metal plates that are already widely used, various plastic molded products, rubber, ceramics, and even paper, cloth, and wood, which are seemingly unrelated to electronics, can be used as reinforcing plate materials.

Reinforcing Plate Using Metal

The metal reinforcing plate material has the following characteristics that are different from normal reinforcing plate materials.

Conductivity: Can be used as a ground circuit.

Moldability: It can be used for forming three-dimensional structures, fixing with screws, and as part of a housing.

Thermal conductivity: Can be used as a heat sink.

Heat resistance: Compared to organic materials, it is stable even at much higher temperatures and does not undergo thermal degradation even at high temperatures.

Aluminum and stainless steel plates, which are chemically stable and have excellent corrosion resistance, are often used as materials for metal reinforcing plates. In addition, special alloys are sometimes used to provide springiness.

Metal reinforcing plates for Flexible Substrates

Metal reinforcing plates are also used in flexible substrates for automobile powertrain modules, paying attention to their thermal conductivity. In this case, almost the entire surface of the flexible board is covered with an aluminum reinforcing plate, and the metal reinforcing plate acts as a heat sink to remove heat.

In addition, it becomes a carrier when mounting components, and a part of the housing after three-dimensional molding. In addition, the wiring of the magnetic head of a hard disk drive (HDD) is part of a structure called a head suspension, which is a stainless steel reinforcing plate. In this case, we take advantage of the springiness of stainless steel.

Adhesive for Reinforcing Plate

Adhesives that fix the reinforcing plate to the flexible substrate can be broadly classified into two types: pressure-sensitive adhesives and thermosetting adhesives. They have different processing methods and different performances.

A pressure-sensitive adhesive is a so-called adhesive tape-like material that can be attached to an adherend by a simple procedure, but due to the creep phenomenon, it gradually peels off or shifts if tensile stress is applied continuously or Even if it is forcibly peeled off, if the peeled surfaces are put together and pressure is applied, it will adhere again.

Thermosetting Adhesives

On the other hand, thermosetting adhesives, as typified by epoxy resin, are sandwiched between adherents in a semi-cured state, and when heated and pressurized, the cross-linking reaction of the resin molecules progresses and hardens.

Once the curing reaction is completed, it cannot be easily peeled off, but if it is forcibly peeled off, the adhesive interface will be destroyed, and even if the fractured surfaces are brought together, they will not adhere again. The following table compares the two.

Pressure Sensitive Adhesive

Pressure-sensitive adhesives are usually supplied in the same condition as double-sided adhesive tape, with a release sheet laminated to one or both sides of the adhesive film. Cut this into an appropriate shape, peel off the release sheet on one side, and attach it to one side of the adherend.

Next, after removing the remaining release sheet, the other adherend is superimposed and pressure is applied. If the reinforcing plate is small, light pressure by hand will suffice, but if the bonding area is large, a pressurizing device such as a roll laminator or hydraulic press is required to ensure uniform pressure.

Thermosetting adhesive

The reinforcing plate is often seen as a minor part of the flexible printed circuit board, but in recent years, the flexible printed circuit boards used in mobile devices have become denser and highly functional. Sexuality is becoming more demanding.

For this reason, high reliability is also required for the reinforcing structure of the connection part, and many configurations are made by bonding polyimide sheets with thermosetting adhesives. Thermosetting adhesives generally withstand the high temperatures of the soldering process.

On the other hand, the process of attaching the reinforcing plate using a thermosetting adhesive is complicated and can be a major factor in increasing the manufacturing cost of flexible printed circuit boards.

Thermosetting Adhesives for Flexible Substrates

Thermosetting adhesives for flexible substrates come in film form and are supplied by manufacturers with release sheets laminated on both sides, similar to pressure-sensitive adhesives. Using a mold, etc., it is subjected to hole processing and external shape processing and then laminated to a reinforcing plate.

In addition, this reinforcing plate is machined with external holes as necessary. In some cases, a dummy board is prepared in parallel by processing. When all the materials are ready, assemble all the materials and apply high temperature and high pressure with a high-temperature press and maintain for 30 minutes or more.

The cushion material used at this time greatly affects the yield of the process. Finally, it is cooled and dismantled, and finally, the reinforcing plate processing process using thermosetting adhesive is completed.

Thermosetting Adhesives Ideal Temperature

If thermosetting adhesives supplied by material manufacturers are stored at room temperature with the resin in a semi-cured state, the cross-linking reaction proceeds rapidly, and performance as an adhesive deteriorates within a few weeks.  It will be lost.

For this reason, thermosetting adhesives are generally stored in a refrigerator, but the pot life guaranteed by the material manufacturer is about 3 months. Unfortunately, even within the warranty period, the performance of thermosetting adhesives can change subtly.

To maintain the highest performance of thermosetting adhesives, it is necessary to have a good understanding of the adhesive performance and adapt the process conditions accordingly.

Some acrylic thermosetting adhesives have a pot life of 6 months or more even when stored at room temperature. However, such thermosetting adhesives require adhesion processing at considerably high temperatures.

Other Adhesive Types

In addition to pressure-sensitive adhesives and thermosetting adhesives, there are other adhesive materials, but they cannot be said to be versatile for flexible substrates. However, you are not restricted from using it if it is of value in a particular application.

For example, there is a silicone RTV type that cures by reacting with moisture in the air. This type does not require heating and pressurization, so it is useful in cases where sealing performance is required.