Important Considerations for FPC Flexible Circuit Board Installation During the Design Phase
Flexible printed circuit boards (FPCs) are increasingly widely used in smartphones, wearable devices, automotive displays, and medical electronics due to their unique advantages such as flexibility, thinness, and foldability. However, the FPC surface mount technology (SMT) process is far more challenging than traditional rigid PCB surface mount technology. This is mainly because the flexible substrate itself lacks rigid support, making it highly susceptible to warping, deformation, and displacement under stress and heat. If the specific requirements of the surface mount process are not fully considered during the design phase, even if the circuit design itself is sound, serious consequences such as board bending, component lifting, or even complete board scrapping may occur during the SMT production stage. Therefore, understanding the key considerations during the design phase of FPC surface mount technology is crucial for ensuring product manufacturability and final quality. This article summarizes the solutions to problems encountered during the design phase of FPC surface mount technology, based on projects undertaken by the engineering team at Geyuan Electronics over the years.
What is FPC flexible circuit board surface mount technology?
Flexible printed circuit boards (FPCs) are widely used in electronic products such as mobile phones, laptops, and smart wearable devices due to their thinness, flexibility, and resistance to high and low temperatures. FPC surface mount technology (SMD) is a process that precisely solders surface mount components (SMDs) such as resistors, capacitors, and chips onto the pads of an FPC board. This is a crucial step in electronic device assembly, directly affecting the product’s performance and stability.
FPC (Fiber-Chip) Patch Manufacturing Process
| Preparation stage | First, the flexible printed circuit board (FPC) needs to undergo quality inspection to ensure it is free of defects and damage and meets processing requirements. Then, necessary pretreatments, such as cleaning and drying, are performed on the FPC to ensure that surface mount components can be firmly adhered to it. |
| Component preparation | Prepare the necessary electronic components, such as resistors, capacitors, and IC chips, according to production requirements. Check the quality and specifications of the components to ensure they meet production standards. |
| Placement program settings | Set the placement program on the pick-and-place machine, including the position, angle, and sequence of components. Then calibrate the pick-and-place machine to ensure placement accuracy. |
| SMT processing | The FPC board is fixed on the worktable of the pick-and-place machine. The pick-and-place machine precisely places electronic components in the designated positions on the FPC. |
| Welding and curing | The FPC with components mounted is soldered using techniques such as reflow soldering or wave soldering. After soldering, necessary curing treatment is performed to ensure the stability and reliability of the solder joints. |
| Quality Inspection | The completed FPC boards undergo quality inspection, including the quality of the solder joints and the positional accuracy of components. Any defective products are promptly reworked or scrapped. |
| Post-processing | Perform necessary cleaning and drying on qualified FPC boards. Conduct functional tests to ensure the FPC boards are working properly. |
Why is it necessary to address the FPC patch issue during the design phase?
The surface mount technology (SMT) process for Flexible Printed Circuits (FPCs) is a complex system engineering project, requiring meticulous attention to every detail from the design stage. Baking pretreatment addresses the inherent moisture absorption of the material; carrier fixtures and jigs provide support for the flexible substrate; reinforcement design resolves the conflict between localized rigidity and SMT feasibility; pad and circuit design ensures soldering reliability and solder joint lifespan; and panelization and process optimization address production efficiency and yield. These stages are interconnected, and oversights in any one stage can escalate into serious quality issues during mass production. Engineers designing FPCs must break free from the conventional thinking of rigid PCBs, fully understand the unique properties of flexible materials, and consistently apply Design for Manufacturability (DFM) principles. Only in this way can FPCs truly realize their flexibility advantages, rather than becoming a “hidden bomb” in product development.
Considerations for FPC Flexible Circuit Board Installation during the Design Phase
Flexible printed circuit boards (FPCs) are prone to problems such as inaccurate positioning, poor solder paste printing, bubbling, or component detachment during the surface mount technology (SMT) process due to their soft and easily deformable material. Therefore, it is crucial to fully consider subsequent mounting and manufacturing processes during the design phase. The following are key considerations that cannot be ignored:
Baking pretreatment
Pre-baking of FPCs before surface mount technology (SMT) is a crucial step that must be considered during the design phase. Since FPC substrates are mostly made of polyimide (PI), which is highly hygroscopic, if they are directly introduced into the high-temperature reflow soldering environment without sufficient baking, the moisture absorbed inside the substrate will rapidly vaporize and expand at high temperatures, leading to serious defects such as board bursting or pad blistering. Industry-recommended baking conditions are typically 120-150°C for 2-4 hours, with the specific time needing to be adjusted flexibly based on the thickness of the FPC and the storage environment. Design engineers should fully recognize that baking is not an optional process, but rather the first line of defense to ensure SMT quality. After baking, the FPC should be put into SMT production as soon as possible and properly stored in a moisture-proof box to prevent secondary moisture absorption.
Vehicle fixture
The design of the carrier fixture is the most fundamental yet easily overlooked aspect of FPC surface mount technology (SMT) manufacturing. FPCs are inherently flexible and cannot be placed directly on reflow soldering tape or ordinary tracks like rigid PCBs; specialized carriers are essential for rigid support. Current mainstream mounting solutions include magnetic carriers, silicone adhesive carriers, and vacuum suction carriers. During the design phase, engineers need to reserve sufficient optical mark points and positioning holes for the FPC. Typically, four 1mm diameter SMT optical marks are required, with the center of each mark 3.85mm from the board edge, one offset by 5mm to prevent misidentification. The placement of positioning holes is equally important; four 2mm diameter positioning holes are usually added along the process edge, one offset by 5mm to prevent errors. These positioning marks are the basis for visual alignment by the placement machine; improper design will directly lead to solder paste printing misalignment and component placement deviation. The carrier design must also include thermal expansion compensation clearance to ensure that the FPC does not shift due to thermal expansion and contraction during placement and reflow.
Design of reinforcing plate
The design of reinforcement plates is the most problematic aspect of FPC surface mount technology (SMT). Reinforcement plates are used to add support to areas requiring localized rigidity, such as connectors and gold fingers. However, improper reinforcement design can have disastrous consequences for SMT. The core principle is: reinforcement should ideally be designed on the back of the surface mount pads, and avoid designing reinforcement on the component side. Because reinforcement plates have a certain thickness, they create a step with the FPC board surface. If reinforcement is designed on the same side as the surface mount component, it will cause uneven solder paste application during solder paste application, making proper component placement impossible and extremely inconvenient for later repairs. If structural limitations necessitate reinforcement on the component side, it is essential to ensure that the reinforcement avoids the component casing, and the order should clearly specify “reinforcement to be applied after SMT,” while also accepting the use of 3M double-sided tape for adhesion. There are also clear quantitative requirements for the spacing between reinforcements and pads: reinforcements with a thickness of 0.2 mm or less must be more than 3 mm away from the pad; reinforcements with a thickness of 0.25 to 0.3 mm must be more than 10 mm away; and reinforcements with a thickness of 0.3 to 0.5 mm must be more than 20 mm away. For reinforcements thicker than 0.5 mm, it is not recommended to design SMT pads on that side. It is particularly important to note that reinforcements must never be designed between two sets of chip pins, otherwise it will cause the device to float after soldering. For PI reinforcement in the gold finger area, the order of reinforcement placement must be followed: then SMT.
Design and layout of the stiffener
| Reinforcement materials and selection | FPC itself is too soft to be directly put into the SMT production line. Rigid reinforcement (such as steel sheet, FR4 or PI) must be designed on the back of the placement area. |
| Reinforcement avoids solder pads | The edges of reinforcing areas such as steel sheets must be at least 0.3 mm away from the solder pads; otherwise, the steel sheets may be lifted up during placement or thermal expansion during reflow soldering may cause poor solder joints. |
| Reinforcement spacing and thickness | When multiple panels are assembled, the spacing between the steel reinforcement plates should be ≥ 3 mm to avoid interference when separating the panels; the reinforcement material, thickness and adhesive type (such as 3M double-sided tape) should be clearly marked in the design. |
Pad and Circuit Design
The design of the pads and traces directly determines the success rate of surface mount technology (SMT) and the long-term reliability of the solder joints. FPCs use a cover film as the solder mask layer, which is fundamentally different from the green solder mask used on rigid PCBs. The cover film needs to be opened before lamination, therefore a gap of at least 0.2 mm must be maintained between the pads and the traces. When a solder mask bridge needs to be maintained between two pads, the gap between the two pads must be at least 0.5 mm to ensure bridging; otherwise, an open window must be created, and exposed traces must be accepted. Pads on FPCs are particularly prone to detachment, an inherent weakness of flexible substrates. SMD pads should be prioritized in the design, where the cover film presses down on the perimeter of the pad, providing support and preventing detachment. For pads subjected to high stress, such as connector sockets, a “press-pad” design is recommended, extending the pad appropriately so that the cover film presses down on the pad by at least 0.3 mm. Pads on both sides should not overlap as much as possible; otherwise, the 25-micron substrate in the middle will not provide sufficient bonding strength. The diameter of BGA pads should not be less than 0.25 mm; if it is too small, it is very easy for the pads to peel off. In addition, large areas of copper should be avoided on FPCs, because it is difficult for air to escape when laminating large copper surfaces. Moisture in the air will react with the copper surface under high temperature and pressure, resulting in poor appearance. If it cannot be avoided, it is recommended to design a grid copper layer.
Pad and trace structure design
| Avoid isolated pads. | Because the interlayer insulating substrate (PI) of FPC is relatively thin, individual pads are easily peeled off when heated or stressed. During design, copper coverage should be maximized, and connecting lines should be added around each individual pad to anchor it to the copper surface. Furthermore, the pads on the front and back sides should be staggered. |
| Connector pad reinforcement | Connectors on FPCs (such as FPC sockets and FFC receptacles) have high insertion and extraction forces, and the solder pads are very easy to fall off. Therefore, a pressure pad design or the addition of teardrops must be adopted. |
| Large copper surface and grid | Large solid copper surfaces are prone to air residue during the lamination process, which can lead to bubbles or oxidation at high temperatures. For large copper areas, it is recommended to design them as 45° grid copper sheets or to incorporate appropriate openings (windows). |
| Solder Mask Bridge | The spacing between solder pads must ensure that the solder mask bridge is ≥ 0.5 mm; otherwise, solder bridging (short circuit) is likely to occur during production. |
Panel design and process specifications
Panel design and process edge specifications are equally important. FPC panelization does not support V-CUT and stamp hole processes; it can only use connecting bridges for panel separation. The recommended length for connecting bridges is generally 0.7 to 1.0 mm. Connecting bridges in steel-reinforced areas should ideally be 1 mm, and the number of bridges should be increased appropriately to prevent excessive weight of the steel sheets from causing FPC expansion and contraction deformation, affecting subsequent SMT. The spacing between boards is usually 2 mm, but in areas with steel reinforcement, the spacing needs to be increased to 3 mm. A 5 mm width should be left around the process edge, and positioning holes and optical dots should be placed on the process edge. The maximum panel size is generally controlled within 250 mm × 500 mm, and the minimum size is 70 mm × 70 mm. For boards requiring replacement, a defective board optical dot should be added next to each small board. When a small board is scrapped during production, the board manufacturer will blacken the corresponding optical dot, and the pick-and-place machine will automatically skip the board, thus saving costs.
Panelization and positioning hole design
| Optical positioning point (FIDUCIAL) | At least three photolithography positioning points (usually bare copper/immersion gold dots with a diameter of 1.0~1.5 mm) must be designed on the entire FPC board and the diagonal of the panel to provide optical alignment recognition for SMT placement machines. |
| Process edges and connection points (Tab) | The edges of FPCs need to be designed with process edges. It is recommended that there be at least 2 connection points (breakways) on each side between a single FPC and the process edge or panel. The width of the connection points is generally around 0.8 mm. Too few connection points can easily cause the board to fall off during transportation, while too many connection points will make it difficult to separate the boards. |
| Small-sized board assembly | For extremely small FPCs smaller than 20×20 mm, panelization must be mandatory for delivery to prevent them from being blown away or sucked up during pick-and-place machine operation or laser dust removal. |
Via-in-Pad is strictly prohibited.
FPC flexible boards cannot be resin-filled with vias. If vias are drilled on the pads (through-pad), solder paste will leak into the vias during SMT solder paste printing and reflow soldering, directly causing insufficient solder, open solder joints, or short circuits. Vias should be at least 0.2 mm away from solder mask openings.
Considering thermal expansion and contraction and allowance for deformation
The coefficient of thermal expansion of FPC materials at the high temperature of reflow soldering is different from that of ordinary rigid PCBs. When designing, stress relief design should be carried out on the traces around fine-pitch QFP, BGA or large connectors to avoid solder joint cracking due to thermal deformation.
Professional FPC flexible circuit board surface mount solution provider
In the field of FPC flexible circuit board manufacturing and surface mount technology (SMT), Geyuan Electronics, leveraging years of technical expertise and industry experience, provides customers with a one-stop service, from initial design consultation, FPC manufacturing, stencil fabrication, SMT soldering to inspection and rework. The company possesses a full set of professional equipment, including high-precision SMT machines, lead-free reflow ovens, and AOI/X-Ray inspection equipment, and is staffed with an experienced technical team. We strictly control every process: from flexible circuit board pretreatment to component placement, from temperature profile optimization to defective product rework, always prioritizing “precision, efficiency, and reliability” to meet the FPC SMT needs of various fields such as consumer electronics, automotive electronics, and medical devices. Geyuan Electronics adheres to a customer-centric approach, customizing SMT solutions based on different specifications of FPC flexible circuit boards and components, ensuring a stable product yield of over 99.5%, providing customers with high-quality, cost-effective SMT services, and helping customers quickly bring their products to mass production. If you require customized manufacturing of large quantities of FPCs, please contact us for assistance.
Summarize
(FPC) surface mount technology (SMT) is a commonly used process in electronics manufacturing, playing a crucial role in connecting electronic components and realizing circuit functions. Familiarity with FPC surface mount methods and processes helps manufacturers better understand and master the manufacturing process, improving product quality and reliability . More importantly, to obtain high-quality FPCs, potential issues affecting the FPC must be considered in the design phase. This article summarizes problems encountered in our daily projects and outlines how to avoid problems during final surface mount development. If you require a professional manufacturer to mass-produce customized FPC products, please contact us for services.