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45 FPC Design Skills with Detailed graphics – Engineers Must Read

What is FPC Board?

FPC Flexible Circuit Boards are circuit boards made from a flexible substrate that is not only lightweight, but also capable of bending and folding. This design makes it useful in electronic devices with tight spaces or irregular shapes. Imagine the future of smartwatches, folding screen phones, and even wearable devices that could be thinner, lighter, more portable, and more durable thanks to FPC flexible circuit boards.

Today, from the beginner’s point of view, to bring you a comprehensive understanding of the 45 design specifications of FPC

1. Circuit design

1. large-area copper surface oxidation problem: due to the design of large-area copper surface, it is difficult to exclude the air when pressing the laminating film, and the moisture in the air under high temperature and high pressure reacts with the copper surface to oxidize, resulting in poor appearance. Although it does not affect the function, in order to avoid this problem, it is recommended to design the copper surface into a grid, or increase the solder resist window on the large copper surface.

2. Avoid designing independent pads: As shown in the figure, the line pads are designed to be independent and overlapped on both sides, and the pads are easy to come off due to the FPC intermediate substrate of only 25um. It is recommended to increase the copper cladding, and add connecting lines at the corners of the pads to connect with the copper cladding. At the same time, the top and bottom pads need to be staggered to increase the bonding force.

3. Pad off problem: Connector holder pads are easy to fall off if they are designed to be independent, it is recommended to use the pressure PAD design.

4. Large Exposed Copper Design: It is generally not recommended to design large exposed copper areas to avoid wrinkles and unevenness problems.

5. FPC board soldermask design: soft board using cover film as a soldermask, cover film need to open the window and then laminated, pad to line spacing needs to be 0.2mm or more, the width of the soldermask bridge needs to be greater than 0.5mm. if the pad spacing is less than 0.5mm, we recommend that the customer open the window, accept the production of exposed wire.

6. Anti-tear design: As shown in the picture, the row of lines is sparse and easy to tear at the corners. It is recommended to add anti-tear copper strips on the edge of the board or add mesh copper on the back.

7. Circuit grid design: Grid lines are laid at a 45-degree angle as far as possible, which is more favorable for signal transmission. Line width and line spacing is recommended to be 0.2/0.2mm.

2. FPC Shape and drilling design

  1. Distance between through-hole and plate frame line: through-hole from the plate frame line should be 0.5mm minimum, less than 0.5mm need to be changed to U-shaped hole.

2. Open Distance between the hole and the anti-soldering : the hole should be kept at least 0.2mm away from the anti-soldering to prevent the hole from exposing copper.

3. FPC board is not recommended to design the hole in the Pad, FPC is unable to do the resin plug hole, do the hole in the disk will have the risk of tin leakage.

3. Soldermask Design

  1. Connector chassis dislodgement protection: FPC connector chassis is easy to dislodge, it is recommended to use pressure PAD design.

2. Soldermask in the middle of the chip: A bridge connection is required in the middle of the chip to retain the soldermask in the middle.

3. Open desing of Gold finger pad : Gold finger pads must be soldermask Opening, otherwise it can not be connected to the connector conduction

4. Soldermask use: default use soldermask as soldermask, make sure the soldermask is correct.

5. Solder mask via hole design: In order to prevent the hole copper breakage when the hole bending, FPC holes generally do the default cover oil design. If you need to open the window, you need to make a clear note in the order!

6. Test point design: the test point is designed as an over-hole attribute resulting in no turn out, the test point can not be set as an over-hole attribute or need to add a separate open for the test point.

7. Board edges exposed copper problem: double-sided board board edges have a large area of exposed copper will lead to board edges black, it is recommended that the board edges to increase a circle of covering film.

4. Gold finger design on board edges

  1. Design of insertion and removal of gold fingers: When laser cutting, the edge of the board is carbonized by high temperature, which leads to the problem of micro-shortening between the gold fingers. It is recommended to shrink the gold fingers inward by 0.2mm.

2. Soldering finger Via-hole design: Soldering finger plate pads on the over-hole should not be designed in a row, in order to prevent stress concentration caused by fracture.

3. Soldering gold finger covering film staggered: Soldering gold finger up and down the covering film should be staggered at least 0.3mm, to prevent fracture

4. Soldering finger design: It is recommended to design for the solder resist film pressure PAD effect (i.e., the pad is extended so that the covering film presses the pad more than 0.3mm).

5. Gold finger opening desing : opening is recommended to press the soldering pad more than 0.3mm, to prevent the gold finger PAD and the connection disconnection

6. Reverse finger design: Garrigon does not support Wisps for the time being, reverse fingers need to add pads and holes to realize the layer change.

7. Pad design: FPC use solder resist film can not be like green oil as a solder resist bridge, design IC pads, pads can not have excess copper cladding, so as not to pads become larger, smaller spacing, welding is easy to short circuit.

8. Independent pad design: Gold finger pads should be designed as independent pads. If the finger pads are covered with copper and wires, the copper or wires will be exposed after the window is opened by the solder resist.

9. Goldfinger profile tolerance: Goldfinger shape tolerance is ±0.1mm by default, if you require ±0.05mm, you need to specify when ordering, and you need markout if you accept x-out or not.

Here is Q&A about FPC Manufacturing :

FR4 Stiffener
  1. What is the difference between hot pressing and cold pressing?
    There are two ways to laminate the reinforcement, one is to use 3M tape to directly bond, called cold pressing, the other is to use AD adhesive (thermosetting adhesive) lamination, after high temperature and high pressure to make the reinforcement and FPC tightly combined.
  2. Why thick FR4 reinforcement upgraded to hot pressing process, thin FR4 do not need hot pressing?
    A: 0.1mm and 0.2mm due to the thickness of thin, can be used ordinary fast press hot pressing, so Garrison FPC on-line is the use of hot pressing;
    0.4mm and above the thickness of FR4, due to the board height difference is large, such as the use of ordinary fast press will press the press silicone pad pressure bad, must be used in a vacuum with airbags to press the hot press.
  3. What is the difference between the internal structure of ordinary fast press and vacuum hot press?
    A: Vacuum press is more airbag, when pressing, first extract the air inside the board, and then fill the airbag with pressure gas to realize the pressing. Ordinary quick press machine is directly through the flat steel plate to realize the pressing.
  4. Thick FR4 is upgraded to hot pressing, what do I need to pay attention to on the design?
    A: Try not to make holes in the reinforcement plate corresponding to the pads to prevent the pads from being concave due to the extrusion of the air pockets during press-fit.




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