Introduction to Flexible Printed Circuit (FPC)

1. Introduction to Flexible Printed Circuit (FPC) A printed circuit board made of flexible substrates has the advantages of small size, lightweight, 3D assembly capability, and dynamic flexing.

2. Basic Materials 2.1 Copper Clad Laminate Composed of copper foil + adhesive + substrate. There is also an adhesive-free substrate, which consists only of copper foil + substrate. This type is more expensive and is currently less commonly used unless there are special requirements.

2.1.1 Copper Foil Copper foil is classified into two types: Rolled Annealed Copper Foil (RA Copper Foil) and Electro Deposited Copper Foil (ED Copper Foil). In terms of characteristics, RA copper has better mechanical properties and is generally preferred for applications requiring flexibility.

The thickness of copper foil is categorized into three standard specifications:

. 1/2oz (0.7mil) . 1oz . 2oz Most applications use 1oz copper foil.

Ounce (oz) Conversion Reference

. Weight unit: 1oz = 28.35g . 1 ounce = 16 drams . 16 ounces = 1 pound (lb)

Different Types of Ounces

. Troy Ounce (oz.tr / oz.t): Used for measuring precious metals such as gold and silver. 1 oz = 31.1035g, 12 oz = 1 lb. . Apothecary Ounce (ap oz): Used in pharmaceuticals. 1 oz = 31.1030g. . Fluid Ounce (fl oz): Used for liquid measurements. . 1 UK fluid ounce = 28.41 ml . 1 US fluid ounce = 29.57 ml

2.1.2 Substrate

Substrate materials are classified into two types:

. PI (Polyimide) Film: More expensive but has better flame resistance. . PET (Polyester) Film: Less expensive but not heat-resistant.

For applications requiring soldering, PI material is commonly used. Standard thickness options include 1mil and 2mil.

2.1.3 Adhesive

There are two common types of adhesives:

Acrylic Adhesive Epoxy Adhesive (most commonly used) Adhesive thickness ranges from 0.4mil to 1mil, with 1mil being the most commonly used.

2.2 Coverlay The coverlay consists of a substrate + adhesive. The substrate is classified into PI and PET, matching the material of the copper-clad substrate. The adhesive type of the coverlay is the same as that of the copper-clad substrate. Coverlay thickness ranges from 0.5mil to 1.4mil.

2.3 Stiffener A stiffener is a rigid material added to specific areas of the flexible circuit board to reinforce certain regions, such as for soldering components or structural support. 2.3.1 Stiffener Materials PI and PET stiffeners FR-4 (Epoxy-based material)

FR-4 Material Overview The term FR-4 is commonly spoken as “FR-4,” but its formal designation in written documents is FR-4.

FR-4 refers to a flame-retardant material grade, meaning that the resin material must be capable of self-extinguishing when exposed to flame. It is not a specific material but a classification standard.

Currently, there are many types of FR-4 materials used in printed circuit boards (PCBs), but most are composed of Tetra-Function Epoxy Resin combined with fillers and glass fiber to create a composite material.

Common Names for FR-4 Epoxy Glass Cloth Laminates: FR-4 epoxy glass cloth, insulation board, epoxy board, epoxy resin board, brominated epoxy resin board, FR-4 glass fiber board, FR-4 stiffener, FPC stiffener, flexible circuit stiffener, FR-4 epoxy resin board, flame-retardant insulation board, laminated board, PCB drilling pad, and many others.

Technical Features and Applications: FR-4 is widely used in high-performance electronic insulation applications, such as:

. FPC stiffener . PCB drilling pad . Glass fiber spacers . Potentiometer carbon film printed glass fiber boards . Precision planetary gears (for wafer grinding) . Precision test boards . Electrical and electronic insulation components (e.g., transformer insulation boards, motor insulation parts, insulating spacers, and grinding gears)

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FR-4 Epoxy Glass Cloth Laminates are available in various colors: . Yellow FR-4 . White FR-4 . Black FR-4 . Blue FR-4

FR-4 is the most commonly used base material in PCBs. Classification of PCB Substrate Materials PCB substrate materials are categorized based on their reinforcement materials:

1. FR-4: Glass fabric-based substrate
2. FR-1, FR-2: Paper-based substrate
3. CEM Series: Composite substrate
4. Special Material Substrate: Includes ceramic-based and metal-based materials

FR-4 is produced by impregnating specialized electronic glass fabric with epoxy phenolic resin and then subjecting it to high temperature and high-pressure lamination.