FPC PCB Manufacturing: How to Choose the Right FPC Parameters for Optimal Performance

What Are FPC Parameters?

FPC flexible printed circuit boards parameters refer to the specific design and material choices that define the performance and functionality of a flexible printed circuit. These parameters include material selection, thickness, layer count, bend radius, and more. Properly selecting these parameters ensures that the FPC meets the requirements of its intended application.

Key Factors to Consider When Choosing FPC Parameters

Board Quantity/Board Size

Board size refers to the dimensions of the board. The maximum size for JLC FPC boards is 234X490mm (limit 250X500mm), while the minimum size is not restricted. However, for sizes smaller than 20X20mm, panelization is recommended.

Number of Layers

The number of layers is generally determined during design, so you can directly select the required number of layers based on design specifications.

FPC boards can be classified into single-layer, double-sided, and multi-layer boards.

• Single-layer boards are the simplest flexible boards and are commonly used in industrial control and electronic instruments.

• Double-sided boards differ from single-layer FPCs by incorporating vias to connect two copper layers, forming electrical pathways. They are typically used in mobile phones, automotive dashboards, and similar products.

• Multi-layer boards consist of multiple single or double-sided FPCs laminated together. Through drilling and plating, they form metallized vias to create electrical pathways between layers. These are commonly used in high-end consumer electronics like smartphones.

Common Processes

Solder Mask Color

• FPC solder masks come in yellow, black, and white. Yellow is recommended.

• Yellow film is suitable for most products, such as various types of flexible ribbon cables.

• Black film is commonly used in high-end applications or products requiring light absorption, such as automotive, mobile phones, and LED displays.

• White film has an additional white coating and is often used in reflective products like lighting equipment, LED displays, and medical beauty devices.

Board Thickness

Board thickness refers to the thickness of the FPC flexible board itself, excluding reinforcement thickness.

If the measured area has no copper or coverlay, the final thickness may be reduced.

If a white coverlay is used, the thickness increases by 18um for single-sided boards and 36um for double-sided boards.

Copper Foil Thickness

This refers to the thickness of the copper foil in the FPC circuit layer, which should match the board thickness.

• Available options for single-sided boards: 18um (0.5oz), 35um (1oz)

• Available options for double-sided boards: 12um (0.33oz), 18um (0.5oz), 35um (1oz)

Minimum Line Width/Spacing

The smaller the width/spacing, the higher the difficulty. The general recommendation is 3/3mil or above.

JLC's limit is around 2/2mil, but it is recommended to avoid designing at this limit.

Minimum Via/Pad

The via outer diameter must be at least 0.2mm larger than the inner diameter, with a recommended margin of 0.25mm or more.

In general, the smaller the via, the more expensive it is. Recommended via inner diameter: 0.3mm, outer diameter: 0.55mm.

Solder Mask Covering

FPC uses a coverlay as the solder mask layer. There are two main types of solder mask coverage: via opening and via tenting.

• Via opening: The via pad is exposed in the final board. However, exposed vias in FPC may cause oxidation and copper fracture.

• Via tenting: The via pad is covered with solder mask to protect the copper from oxidation and to prevent copper fracture during bending. JLC defaults to via tenting. If via opening is required, it must be specifically noted when placing an order.

Minimum Solder Mask Bridge Width

The minimum solder mask bridge width is 0.5mm. That is, pad spacing must be ≥0.5mm to retain the bridge. If the spacing is smaller, JLC will default to exposing the vias.

Reinforcement

There are five main types of reinforcement: PI, FR4, steel sheet, 3M double-sided adhesive, and electromagnetic shielding film.

• PI reinforcement is commonly used for gold finger insertion products.

• FR4 is used for reinforcing component hole areas.

• Steel sheet is more expensive but has excellent flatness and does not deform, making it suitable for products requiring chip mounting (since steel sheets have weak magnetism, they are not recommended for products like Hall sensors).

• 3M adhesive is generally used for fixing FPC boards during assembly.

• Electromagnetic shielding film is used to solve EMC issues. It is generally recommended to add grounding openings in the solder mask layer to connect the shielding film to the ground copper, enhancing shielding effectiveness. (Note: Prototype verification is required first, as the shielding film significantly impacts impedance. Improper design may lead to signal abnormalities.)

Total reinforcement thickness = FPC board thickness + reinforcement thickness, but it is not a direct sum. Solder mask thickness and whether the backside of gold fingers has copper must also be considered.

If you don't know how to calculate reinforcement thickness, click the link below to use the Reinforcement Thickness Calculator:  https://jlcpcb.com/gold-fingers-PI-thickness

Panelization

The maximum panel size is 234X490mm.

Recommended spacing: 2mm; if steel sheet reinforcement is used, spacing should be 3mm.

Important! Many FPC board outlines are irregularly shaped. Improper panelization may reduce material utilization, increase costs, and complicate production. If you are unsure about panelization, JLC engineers can assist.

In addition to the above standard parameters, FPC production also involves gold fingers, impedance, half-cut holes, and other processes.

Choosing the right FPC parameters is essential for manufacturing high-quality, reliable, and cost-effective flexible printed circuits. By considering factors such as material selection, thickness, bend radius, and thermal performance, you can design FPCs that meet the demands of your application. Whether you're working on consumer electronics, automotive systems, or medical devices, understanding FPC parameters will help you achieve optimal results.