Polyimide 0.5mil Flexible PCB Board With Yellow Coverlay

 

The Mysterious Stories of Flexible Printed Circuit

 

In design of FPC board, the type and structure of material are very important. It mainly determines the flexibility, electrical characteristics and other mechanical properties, etc. which plays an important role in the price of flexible board. We must specify all the materials in the design drawings. For a better explanation, it is suggested to use a cross-sectional schematic diagram to express the laminar structure.

 

The flexible copper-clad dielectrics and dielectrics film with adhesive that an FPC factory uses shall be conformed to the regulations of IPC-MF-150, IPC-FC-231 and IPC-FC-232 or IPC-FC-241 and IPC-FC- 232. These IPC norms classify all materials according to their natural characteristics. The following points have to be considered to select suitable material from various materials:

 

1. Moisture sensitive properties

2. Flame retardant properties

3. Electrical properties

4. Mechanical properties

5. Thermal impact characteristics

 

 

Designers and FPC factories must choose materials based on cost, performance, and manufacturability.

 

1. Dielectrics

The base materials used for an FPC are different as per different requirements. It can be comprehensively considered from the aspects of the application occasions and costs, etc. The commonly used are polyimide (polyimide, including adhesive and non-adhesive) and polyester.

 

1.1 Polyimide

Polyimide (PI) is the most commonly used thermosetting insulating material in flexible circuit processing. The thickness range of the material is generally 12.5 µm (0.5mil) and 125 µm (5mil). Partial manufacturers can provide thickness of 7mil, the commonly used specifications are 25µm (1mil) and 12.5µm (0.5mil). Polyimide film has excellent flexibility property, good dimensional stability, can work in a wide range of temperature. In addition, it is a flame retardant material with outstanding performance of resisting welding temperature, without any loss of its electrical properties under the welding condition.

 

1.2 Polyester

Polyester is made of polyethylene terephthalate (abbreviated PET). It generally applies to the flexible board with the thickness range of 25μm (1mil) ~ 125μm (5mil). It has the same excellent flexibility and electrical properties with polyimide. However, it is slightly less dimensional-stable than polyimide during the manufacturing process. In addition, its tear resistance is also worse, and it is more sensitive to the welding temperature.

 

 

2. Conductor

The choice of conductor materials for flexible circuit board depends mainly on the material properties under specific application conditions. Especially in dynamic use, the flexible circuit board is folded or stretched constantly, which requires a thin material with long fatigue life. Flexible circuit board conductors usually are copper foil, copper nickel alloy and conductive coatings, etc.

 

2.1 Copper foil

The most commonly used and most economical conductor material in FPC soft board is copper foil. The copper foil is mainly divided into the electro-deposited copper foil (ED copper) and rolled-annealed copper foil (RA copper). Electrolytic copper foil is formed by electroplating. The crystalline state of copper particles is vertical column, it’s easy to form vertical line edge when etching, which is beneficial to the production of fine circuit; but as the columnar structure is prone to fracture, it is easy to fracture when bending frequently.

 

 

Rolled-annealed copper foil is the most widely used copper foil in flexible board manufacturing. Its copper particles crystallization is in a horizontal axial structure, which is more suitable for repeated bending than ED copper. But as the surface of rolled copper is relatively smooth, special treatment is required on the adhesive side.

 

2.2 Other conductors

In addition to copper foil, other conductor materials of flexible circuit boards are also copper nickel alloys (such as Constantan) and conductive coatings. Conductive coating is a kind of paste made of conductive material (such as silver, carbon, etc.) mixed polymer adhesive (such as resin). The conductive coating is printed on the surface of the dielectric and then covered. For example, silver paste, if the coverage of insulation is good, its conductivity is also very good. Compared with copper foil, conductive coating has lower electrical performance and higher impedance coefficient, so it is not suitable to be used as conductor in some flexible applications.

 

 

3.Adhesive

It is also important to choose the proper adhesive to bond the conductor and the dielectric in the design of the flexible board. It must ensure there is no adhesive failure or excessively adhesive spilling in processing of FPC. The commonly used adhesives for flexible boards are crylic acid (acrylic), modified epoxy, Phenolic Butyrals, reinforce adhesive, pressure sensitive adhesive and so on.

 

3.1 Acrylic and modified Acrylic

Acrylic adhesives and modified acrylic adhesives are thermosetting materials. The material thickness is generally 12.5μm (0.5mil) to 100μm (4mil), the commonly used is 0.5mil and 1mil. It is widely used in applications of high temperature flexibility (such as the requirement of lead-tin welding operation) to ensure no adhesive failure or blistering in these applications. It also has excellent chemical resistance properties which can resist the effects of chemicals and solvents during processing. Unlike traditional acrylics, the modified acrylic has some properties similar to those of the thermoplastic material. It uses the method of local lateral coupling to improve the material. When the temperature is greater than its glass transition temperature (Tg), the adhesive will stick to the copper or dielectric. The adhesive can be re-bonded as needed due to the local lateral coupling structure.

 

 

3.2 Modified Epoxy Adhesives

The modified epoxy adhesive has low thermal expansion coefficient, it is often applied to multilayer flexible board or rigid-flex board. Epoxy is a thermosetting material, in which other polymers are added to improve the flexibility of epoxy resin adhesive. The modified epoxy adhesive has excellent property of Z axle expansion coefficient, high adhesion force, low moisture absorption rate, and resistance to chemical reaction characteristics.

 

3.3 Phenolic Butyral Adhesives

Phenolic Butyrals, like epoxy resins, also have thermosetting properties. In addition, its flexible properties are enhanced, which is more suitable for dynamic flexible applications. However, it cannot bond polyimide dielectric and epoxy adhesive.

 

3.4 Reinforce Adhesives

Reinforce adhesives is formed by injecting epoxy or polyimide resin in the glass fiber. It is most commonly used for interlayer bonding of multi-layer FPC or rigid-flex board.

 

Fiber glass injected with epoxy, also known as prepreg, it can be used as an adhesive or as a base film in rigid-flex board. It differs from the modified acrylic in that it significantly improves Z-axis stability in multi-layer FPCs because of its low thermal expansion coefficient (CTE) and high glass transition temperature (Tg).

 

The fiber glass injected with polyimide resin further increased the Z-axis stability of the metalized holes of the rigid-flex board. Its CTE and Tg are better than that injected with epoxy. But it is more expensive and has a shorter life cycle.

 

3.5 Pressure Sensitive Adhesive

Pressure sensitive adhesives (PSA) may be the easiest and cheapest adhesive in FPC processing. As its name implies, pressure sensitive adhesives do not need special lamination processes and can be pasted to the dielectric surface manually. Because of its sensitivity to temperature and various chemicals, so it cannot be used to glue dielectric and copper foil. Therefore, the main purpose of the pressure sensitive adhesive is to bond the stiffener or stick the rigid board to the flex board.

 

 

4.Non-adhesive Laminates

 

With the development of high-density FPC, the requirements for reliability and dimensional stability are also getting higher and higher. Some leading material suppliers, such as Shengyi, etc., have provided a material called non-adhesive laminates. Non-adhesive laminates solved the adhesive-related issues (such as it is prone to have uneven thickness of adhesive, excess adhesives, etc. in laminating) in the production process, and the thickness is thinner.

 

 

5.Cover Layer

Cover layer is generally a lamination body of dielectric film and adhesive, or a coating of the flexible dielectric. Non-conductor film or coating can optionally be applied to the FPC surface to protect against pollution, moisture, scratches and so on. Common protective layers are cover film and solder mask.

 

5.1 Cover Film

Cover film is a lamination body of dielectric film and adhesive. The adhesive it uses is the same as described above, with the thickness of generally 25μm. Of course, there is also non-adhesive cover film material. The dielectric film is the same with base material dielectric, there are mainly the following two types:

 

A. Polyimide

The thickness range of dielectric is generally 25μm, 50μm ~ 125μm. Its characteristics is the same as those described in base dielectric, which are mainly good flexibility, high temperature resistance

 

B. Polyester

The thickness range of dielectric is generally 25μm / 50μm / 75μm. Its characteristics is the same as those described in base dielectric, which are mainly relative cheap, good flexibility and curvature, while poor high temperature resistance.

 

5.2 Solder Mask ink

In some specific applications, the flexible circuit board can use solder mask ink as the protection, insulation layer of conductor, just like the ordinary rigid PCB board. There are various colors of ink, suitable for many application environments. For example, the flexible board connected to the camera uses black ink to avoid reflections. In addition, the price of ink is cheap, the mode of printing can be used for processing, total cost is very low. But its flexibility is worse than cover film.

 

5.3 The difference of cover film and solder mask ink

a. The advantages of cover film: good bending performance; thicker thickness, high protection, insulating strength.

Disadvantages: the cover film with adhesive cover will spill out adhesives when laminated, unsuitable for small pads. The window opening on pad of solder mask is generally using the modes of drilling; for the realization of right angles and square opening on pad of solder mask, it need to use punching mold or laser cutting, which is more difficult and the total cost is more expensive.

 

b. Advantages of solder mask inks: thin thickness, suitable for applications of thin and flexible requirements; various colors; The processing is simple by use of printing. There is no problem of excessive adhesive, which is suitable for fine spacing pad. The total cost is cheap (1/4 of the PI cover film or less).

Disadvantages: the insulating strength is not as high as PI cover film because it is relatively thinner. Bending performance is poor, which is generally less than 10,000 times, so it is less suitable for the occasions of high dynamic requirements.

 

Specification of standard Polyimide and Adhesive (Shengyi)

Specifications	PI film Thickness (um)	Adhesive Thickness (um)
SF302C 0515	12.5	15
SF302C 0520	12.5	20
SF302C 0525	12.5	25
SF302C 1025	25	25
SF302C 1030	25	30

 

6.Stiffener

In many applications where there are components soldered, the flexible boards require external stiffeners (Stiffener, also known as the reinforcing board) for external support. The stiffener materials are PI or Polyester film, glass fiber, polymer materials, steel foil, aluminum shim and so on.

 

6.1 PI or Polyester

PI and polyester films are commonly used stiffener materials for flexible circuit board. The commonly used thickness is 125μm (5mil), some hardness can be obtained.

 

 

6.2 Glass fibers

Glass fibers (such as FR-4), which are also commonly used materials for stiffeners. The fiber glass stiffener has a higher hardness than that of PI or Polyester, used where the requirements of harness is higher. The thickness range is typically 125μm (5mil) to 3.175mm (125mils). However, its processing is relatively difficult than PI, and may not be a standing material for some FPC factories.

 

6.3 Polymer

Polymer, such as plastic, etc. is also used as stiffeners.

Its water absorption is low, with high pressure and high temperature resistance.

 

6.4 Steel foil, aluminum shim

The support hardness of steel foil, aluminum shim is high, the heat can also be dissipated. The hardness or heat dissipation in the design is the main concern.