Guide to avoiding manufacturing problems of PFC in design

FPC production requires the coordination of various technologies and processes, making the manufacturing process complex and challenging. Therefore, compared to ordinary PCBs, FPC circuits have a significantly higher cost per unit area and require more time. However, due to the superior flexibility, thinness, and reliability of FPCs, they offer broader implementation possibilities and new design solutions for equipment and products in numerous fields, making them increasingly popular among solution engineers. This article will explore common pitfalls in FPCB design to help you avoid problems during mass production from the design stage.

Why are FPCs so important, and what are the common types of FPCs?

Flexible printed circuit boards (PCBs) house various electronic components, such as chips, resistors, and capacitors, on their circuit boards. The manufacturing and assembly process of FPCBs is highly complex, involving more than 50 steps and requiring significant investment in various equipment. FPCB manufacturers place great emphasis on the quality and consistency of their PCB manufacturing processes, especially when launching new products.

This is because discovering defects during FPC assembly and manufacturing not only hinders the entire process but also leads to costly redesign and testing cycles, increased warranty claims, and damage to brand reputation when the final product reaches the customer. Below are common types of FPCs.

(1) Single-sided board: After the circuit is formed, a cover film is added to the substrate of the single-sided board.

(2) Double-sided board: After the double-sided circuit is formed, a cover film is added to the substrate of the double-sided board.

(3) Single copper double-layer: When using a single pure copper in the process of circuit forming, different covering films are bonded to the two sides of the surface respectively. At this time, the conductive parts are exposed on both sides, which is called single copper double-layer.

(4) Single + Single (air gap): Combining two single-sided panels, and using a glue-free cutout design in the folding area to achieve the purpose of high flexibility requirements.

(5) Multilayer board: Designed with single-sided or double-sided boards in combination to form three or more circuit layers.

(6) COF (chip on FPC): The driver IC chip and electronic components are directly mounted on the flexible circuit board.

(7) Rigid-flex PCB: Utilizing the flexibility of a flexible PCB and the support of a rigid PCB, a versatile circuit board is created.

FPC manufacturing process

The manufacturing process of FPC is very complex. To produce a high-quality flexible circuit board, a complete and reasonable production process is required, and each step must be strictly executed. Below, we will discuss its manufacturing process.

Cutting	Flexible materials are packaged in a roller manner, and the material is cut into the required size according to the MI size.
drilling	CNC machining is performed on the substrate to drill through holes or positioning holes so that the copper on both sides can be connected after subsequent copper plating.
Black holes/electroplated	When the holes are first drilled, the upper and lower copper layers are not conductive. They need to pass through black holes to form a conductive layer. Then, copper is plated onto the conductive layer using an electrochemical method to make the upper and lower copper layers conductive.
Film application/exposure	A photosensitive film is pressed onto the electroplated board surface, and the circuit pattern is transferred onto the photosensitive film using photolithography.
Developing/Etching/Removing	Development: Developing removes the un-photolithographic dry film, exposing the copper foil areas outside the circuit pattern.  Etching: Using chemical solutions to etch away the exposed copper foil after development.  Film Removal: The dry film on the circuit pattern is removed using sodium hydroxide to expose the final circuit pattern.
Covering film	To protect the circuit patterns and prevent short circuits and oxidation, an insulating layer must be created on the conductors. This insulating layer, typically used on flexible printed circuit boards (FPCBs), is called a cover film. The process involves first creating openings in the cover film to expose the copper pads, then attaching the cover film with the openings to the etched board. Cover films are available in yellow, black, and white.
Sinking Gold	FPC surface treatment generally adopts immersion gold process, which first deposits a layer of nickel on the exposed pads, and then deposits a thin gold layer of 1″ or 2″ to prevent the pads from oxidizing and improve solderability.
character	The characters required by the customer are printed onto the board surface using inkjet printing; then, the ink is hardened onto the board surface by baking to prevent it from peeling off.
test	The continuity of a board is tested using a flying probe tester, primarily to check for open or short circuits.
Apply reinforcement	PI, electromagnetic shielding film, FR4, steel sheet, adhesive backing and other auxiliary materials are applied to the board surface according to customer requirements.
Laser forming	Laser energy is used to cut out the shape of the board, separating unwanted waste materials to obtain the final shape of the board.
Inspection packaging	We conduct comprehensive testing of FPCs according to customer-specific requirements or IPC inspection standards to screen out defects such as appearance defects, so as to meet customer quality standards.

Solving FPC mass production issues during the design phase

FPC drilling, board outline, circuitry, solder mask, and text design are the same as PCB. The only difference is that reinforcement layers are needed on the back of components and at interface locations. However, there are some areas that require special attention, as follows:

1. Shape and drilling design

(1) The minimum distance between the through hole and the plate frame line is 0.5mm. If it is less than 0.5mm, it should be changed to a U-shaped hole (the hole should be connected to the plate frame).

(2) Ensure that the via is at least 0.2mm away from the solder mask opening; otherwise, copper will be exposed at the edge of the via.

(3) It is not recommended to design through-holes in FPCs. FPCs cannot be made with resin-filled through-holes, and making through-holes in FPCs may result in solder leakage.

2. Circuit Design

(1) Oxidation of large copper surface: Because the design is for a large copper surface, it is difficult to remove air during the lamination process. The moisture in the air reacts with the copper surface under high temperature and pressure, resulting in poor appearance, but not affecting the function. To avoid this problem, it is recommended to design a grid copper sheet or add solder mask openings to the large copper surface.

(2) Try not to design independent pads . The pads of FPC are basically independent and overlap on both sides. Since the substrate in the middle of FPC is only 25um, the pads are easy to fall off. It is recommended to add copper cladding and add connecting lines at the four corners of the pads to connect with the copper cladding. The pads on the upper and lower sides need to be staggered to increase the bonding force.

(3) Pad detachment: The connector socket pads are relatively independent and easy to detach. It is recommended to use a pressure pad design.

(4) Avoid designing large areas of exposed copper, otherwise wrinkles will appear.

(5) The flexible circuit board uses a cover film as the solder mask layer. The cover film needs to be opened first and then attached. The distance between the pad and the line needs to be 0.2mm, and the solder mask bridge needs to be 0.5mm or more. That is, the distance between the two pads needs to be 0.5mm or more to ensure the bridge. Otherwise, we can only suggest that the customer open the window and accept the exposed line production.

(6) When the FPC’s wiring design is sparse, it is easy to tear at the corners. It is recommended to add anti-tear copper strips to the edge of the board or add network copper on the back.

(7) The cable mesh should be laid at a 45-degree angle as much as possible, which will be better for signal transmission. The recommended cable width and spacing is 0.2/0.2mm.

3. Gold finger design on the edge of the board

(1) Insertion and removal of fingers: During laser cutting, the edge of the board is carbonized by high temperature, which causes a slight shortness between the gold fingers. It is necessary to shrink the gold fingers inward by 0.2mm (Geyuan Electronics will shrink them inward in a uniform manner. If there are special requirements, please make them out).

(2) Vias on the pads inside the solder finger board should not be arranged in a row to prevent stress concentration on the vias, which can easily lead to breakage.

(3) The upper and lower cover films of the welding gold fingers should be staggered by more than 0.3mm to prevent breakage.

(4) It is recommended to design the solder pads so that the solder mask presses against the pads (i.e., extend the pads so that the cover film presses against the pads by more than 0.3mm).

(5) Gold finger solder mask opening: It is recommended to open the window with a pressure pad of more than 0.3mm to prevent the gold finger PAD from disconnecting from the connection.

(6) Geyuan Electronics does not currently support perforated boards. Reverse fingers require additional pads and vias to achieve layer switching.

(7) Since the solder mask used in FPC cannot be used as a solder mask bridge like green oil, when designing IC pads, there should be no excess copper on the pads ( if the pad design is not reasonable), otherwise the pads will become larger and the spacing will become smaller, making it easy to short circuit during soldering.

(8) The gold finger pads need to be designed as independent pads. If the finger pads have copper cladding and wires, the copper or wires will be exposed after the solder mask is opened.

(9) The default tolerance for the gold finger shape is +/- 0.1 mm. If +/- 0.05 mm is required, it needs to be selected when placing the order.

4. Solder mask design

(1) FPC connector sockets are relatively easy to fall off, so it is recommended to make them into press-pad designs.

(2) A bridge is needed in the middle of the IC to retain the solder mask in the middle.

(3) The gold finger pads must have solder mask openings, otherwise they cannot conduct electricity with the connector.

(4) Soldermask is used as the solder mask by default. You must ensure that the solder mask is applied correctly.

(5) To prevent the copper in the via from breaking when it is bent, FPC vias are generally covered with oil by default. If you want to open the window, you need to make a note when placing an order with Geyuan Electronics.

(6) The test point was designed with via properties, which caused it not to be turned out. The test point cannot be set as via properties or a window can be added separately to the test point.

(7) If there is a large exposed copper gold surface on the edge of the double-sided board, the edge of the board may turn black. It is recommended to add a cover film around the edge of the board.

5. Silkscreen design

(1) For reinforcements with silkscreened characters, you need to select “reinforcement with silkscreened characters” when placing the order to prevent the production line from making mistakes in the process.

(2) Do not design explanatory text inside the board, otherwise it may lead to the next tragedy.

6. Layout design

(1) The whole board is made of steel sheet, which is relatively heavy. FPC is easily stretched and deformed, and it is impossible to attach the sheet. It is recommended that the spacing between the steel sheet reinforced boards be at least 3mm, the groove width be 0.5mm, the connection point width be 1mm, and an additional one should be added every 15mm or so. Order notes: Each sheet should be separated by paper and sandwiched with cardboard at the top and bottom for shipment.

(2) Panel connection: The connection is added to the gold finger, which will cause the front end of the gold finger to be uneven.

(3) If there are too few connection points, the board is easy to fall off. Each PCS should have at least 2 connection points with a connection point width of 0.8mm. The specific width depends on the board size. The larger the board, the more connection points are needed.

(4) If the board is too small and there are too many connection points, it will be difficult to tear the board apart. If SMT is not done, only two 0.3mm connection points are needed per PCS, which will make it easier to tear the board apart by hand.

(5) Low layout utilization will lead to excessively high prices . Therefore , the layout width should be 119mm or 240/250mm as much as possible. It is recommended to choose a third-party layout provider.

(6) If the board size is too small, the laser dust collector will suck the board away. It is recommended that boards smaller than 20*20mm be delivered in a panelized manner, or the panelized production can be handled by the Geyuan Electronics engineering team .

7. Reinforcement Design

Reinforcement refers to adding rigid material to localized areas of an FPC (Flexible Printed Circuit) to facilitate assembly. PI reinforcement is suitable for gold-finger insertion products; FR4 is suitable for lower-end products; steel sheets have good flatness and will not deform, making them suitable for products requiring chip mounting.

(1) It is not recommended to use steel sheets to reinforce the insertion holes, as this may cause a short circuit; in addition, steel sheets have weak magnetism, so Hall effect elements cannot be used; finally, it is not recommended to use steel sheets to insert or remove gold fingers.

(2) When you place an order with us, please be sure to note the total thickness requirement for pluggable gold fingers. The total thickness is usually specified in the connector specification sheet. When selecting the PI reinforcement thickness, you cannot directly calculate it by subtracting the FPC board thickness from the total thickness. You can use the Geyuan Electronics gold finger PI thickness calculator to calculate it.

(3) Reinforcement opening design: This means that the reinforcement should avoid the component holes or pads below. It is best for the customer to design it themselves. Geyuan Electronics generally defaults to avoiding the pads by 0.3mm. If the remaining reinforcement width is less than 2mm after the reinforcement is removed, we will directly extend it (i.e., there is no reinforcement in this entire area). Note that we will not ask the customer for this point. If there are special requirements, please raise them.

(4) It is recommended that the reinforcement height of the gold finger be at least 1.0 mm longer than the gold finger pad to prevent the gold finger from breaking during use.

(5) The two sides of the electromagnetic membrane may be conductive. If the electromagnetic membrane below is not a network, it is recommended to cancel the electromagnetic membrane design.

(6) If there are requirements for the grounding resistance of the electromagnetic film, you need to design the grounding solder mask window yourself. If there are no requirements, Geyuan Electronics will randomly add a solder mask window of more than 1.0mm by default. Note: If the electromagnetic film is not grounded, it may absorb a large amount of electromagnetic waves, which may cause signal problems. You must test it first.

(7) If the steel sheet is attached to the solder pad, it will cause a short circuit.

(8) Reinforcement width: If the reinforcement width of FR4 is too small, it is easy to break and carbonize. It is recommended to replace it with PI or steel sheet reinforcement if the minimum width is less than 5mm. The minimum width of the adhesive backing should also be more than 3mm.

(9) There should be no reinforcement or adhesive around the surface mount pads, otherwise it will be impossible to screen print solder paste (if necessary, the surface mount pads should be placed first and then the reinforcement or adhesive should be applied).

8. Plate Thickness Specification

When you place an order with Geyuan Electronics, the selected board thickness includes the cover film, copper thickness, and board PI thickness. If there are areas without copper or no cover film on the board, the board thickness will be reduced accordingly. Please pay special attention to this during the design process .

A DFM (Design for Manufacturing) checklist for customized FPC projects in batch manufacturing.

To intercept more than 95% of process issues before mass production, it is recommended to complete the following checks before the design is finalized:

DFM rule import	Obtain the process capability limits (such as minimum line width, minimum hole diameter, minimum solder mask bridge width) from PCB/FPC foundries with high manufacturability requirements in advance, and directly restrict these parameters in the rule manager (DRC) of EDA software (such as Altium Designer, Allegro).
Stencil and pad balance	Ensure that the width of the traces leading from the pads at both ends of SMT components (such as 0402 and 0603 resistors and capacitors) is consistent. If one end is connected to a large copper foil while the other end is connected to a thin trace, uneven heat dissipation will cause “tombstoning” defects in batches.
Solder Mask Sliver	For closely spaced chips (such as QFN and TSSOP), a solder mask bridge of at least 5 mil width must be maintained between the pins to prevent short circuits caused by solder bridging during batch soldering.

Custom FPC Mass Production General Validation Process

Having design drawings alone is not enough. To ensure smooth mass production, the following processes must be incorporated into the design phase:

Tolerance Stack-up Analysis	Do not design based solely on nominal dimensions. You must calculate the extreme cases of PCB thickness tolerance (±10%), housing injection molding tolerance, and component height tolerance after stacking to ensure that, in the worst-case scenario, the PFC large inductor or FPC bent component will not be subjected to hard compression against the housing.
Thermal simulation and batch heat dissipation fault tolerance	The PFC rectifier bridge and switching transistors are major heat generators. When designing, the contact area of the heatsink or thermal pad should have a 20% margin to accommodate process deviations caused by uneven application of thermal paste by production line workers.
Design for Testability (DFT)	Dedicated test points must be reserved at both ends of the PFC high-voltage capacitor and on the critical signal lines of the FPC to enable the rapid rejection of defective products through ICT (In-Circuit Testing) and FCT (Functional Testing) on the mass production line. It is strictly forbidden to rely on manual inspection with a multimeter.

Why is F PCB design so important for electronic products ?

Flexible printed circuit board (FPCB ) design plays a crucial role in the development of modern electronic products. Without well-designed FPCBs , electronic devices would struggle to function reliably. Whether it’s a smartphone, medical device, or IoT device, you rely on FPCBs to ensure your device operates as expected.

well-designed FPCB determines the functionality, reliability, and performance of electronic products. It helps you manage the complexity of modern devices, especially those with advanced features such as wireless connectivity. For example, IoT devices often require compact and efficient circuit boards to house multiple components within a small space. Advanced FPCB technology enables miniaturization, allowing you to create smaller, more portable devices.

A highly efficient FPCB design can also help you reduce costs. By optimizing layout and assembly processes, you can minimize material waste and manufacturing costs. This cost-effectiveness is crucial when developing consumer electronics or large industrial products.

Signal integrity is another critical aspect of F PCB design. A well-designed board can avoid issues such as signal distortion and crosstalk, ensuring smooth communication between components. Electromagnetic compatibility (EMC) is equally important. It helps you avoid malfunctions caused by electromagnetic interference and ensures your equipment meets regulatory standards.

Poor FPCB design can lead to performance degradation or even complete device failure. By prioritizing correct design practices, you can avoid these pitfalls and deliver high-quality products. Whether you are developing simple circuits or complex systems, mastering FPCB design is crucial for success in electronics.

Geyuan Electronics integrates FPCB manufacturing and assembly into a single factory workflow.

Geyuan Electronics provides FPC bare board manufacturing and FPCB assembly services. This is especially important for advanced circuit boards, as manufacturing details directly affect SMT, through-holes, and mechanical assembly. VIPPO processes affect BGA soldering; press-fit holes affect connector insertion; flexible circuit boards and rigid-flex circuit boards require proper support; ceramic and copper plates alter heat dissipation; edge connectors and special surface treatments affect the final mechanical interface.

When manufacturing and assembly are coordinated, panel rails, reference markings, via treatments, surface treatments, depaneling, restricted areas, and testing strategies can be reviewed before PCB production release. Items requiring component procurement, SMT/THT placement, programming, or functional testing can proceed through Geyuan Electronics’ one-stop PCB assembly workflow.

in conclusion

Excellent FPCB design not only ensures stable operation but also lays the foundation for the overall success of electronic products. Taking the time to familiarize yourself with these design principles and integrate them into your workflow will allow you to enjoy a smooth and efficient FPCB manufacturing experience.

In summary, the manufacturing and assembly process of FPCBs is highly complex and meticulous, requiring absolute attention to every detail. However, due to the numerous steps and components involved, errors and defects in FPCBs are often unavoidable. Nevertheless, we should still adhere to all necessary safety measures and design considerations, and make every effort to execute design and manufacturing processes according to established standards to ensure that the final mass-produced products meet the required quality standards. If you require the manufacture of large quantities of FPC products, please contact us for free manufacturability analysis and manufacturing services.