In the BOM (Bill of Materials) cost structure of commercial display screens, flexible printed circuit boards (FPCAs) may seem to account for only a small proportion, but they are actually the "invisible drivers" of cost fluctuations.
The selection of a single connector or a difference in the manufacturing process of a bending area can cause fluctuations in the unit cost. For LCD display module manufacturers with significant shipment volumes, this cumulative effect cannot be underestimated.
Based on actual production line data and supply chain feedback, we have identified seven key cost leverage points in FPCA design. These details are often overlooked during the R&D phase but can become profit-eating factors during mass production.
The size of the FPCA directly affects the efficiency of board layout. With the same raw material specifications, a larger size means fewer boards can be laid out, thus increasing the cost per unit.
For example, if your module design allows for slight adjustments, reducing the size can significantly improve material utilization.
Regarding tolerances, the outer dimensions are typically controlled within ±0.05mm to ±0.15mm.
Achieving extreme precision, such as ±0.05mm, requires using slow-speed wire EDM machining for the mold, which increases mold costs.
Conversely, a tolerance of ±0.1mm only requires medium-speed wire EDM machining, resulting in a more cost-effective solution.
The connection method between the FPCA flexible circuit board and the display mainboard is another cost variable for FPCA flexible panels.
Currently, TFT-LCD modules mainly use snap-in or plug-in connections, while soldered connections are less common due to their low efficiency and poor stability.
From the perspective of pure FPCA panel cost, snap-in connections are the most expensive, followed by plug-in connections, and soldered connections are the cheapest.
Why are snap-in connections more expensive? They involve more precision components and assembly steps. If your application allows, switching to a plug-in connection can save considerable costs.
We have found in production that, unless there are specific durability requirements, the plug-in connection is the most cost-effective option, reducing overall module costs without sacrificing reliability.
The covering processes for the bending area and component area (component area and connector area) of an FPCA typically include: covering film process and thermosetting green ink process for the FPCA bending area, and covering film window opening process and photosensitive ink process for the FPCA component area.
With the increasingly narrow bezels and thinner LCD panels of current TFT-LCD display modules, the requirements for the rebound stress in the FPCA bending area are becoming more stringent.
The rebound stress in the FPCA bending area is higher when using the cover film process compared to the thermosetting green ink process, which increases the risk of hotspots at the BLU (Backlight Unit) lamp interface.
The cost of using the thermosetting green ink process in the FPCA bending area is higher than that of the cover film process, as it involves an additional screen printing and thermosetting green ink process.
Generally, considering cost, if the display module has a wide bezel and the customer has no special requirements, the cover film process can be prioritized.
The FPCA component area can be mainly divided into the component area and the connector area. The mainstream covering process for the component area is currently the photosensitive ink covering process.
This is mainly because smaller components such as 0201 and 01005 are used, and the opening accuracy of the covering film cannot meet the requirements, nor can the SMT placement accuracy.
The covering process for the connector area can be flexibly selected according to specific project requirements. However, when using the covering film opening process, it is necessary to carefully confirm whether the dimensions of the connector area, such as the covering film opening spacing, the covering film opening PAD width, and the covering film covering PAD width, meet the process capabilities of each FPCA manufacturer.
Generally, the cost of using the covering film opening process is advantageous compared to the photosensitive ink covering process.
If the design of the FPCA connector area meets the process capability requirements of the FPCA manufacturer, the covering film opening process can be prioritized.
There are generally two types of grounding requirements for FPCA steel reinforcement: grounded and ungrounded. Whether the steel reinforcement is grounded and the magnitude of the grounding impedance will affect the price of the FPCA.
If the steel reinforcement requires grounding, the grounding impedance is usually required to be <=5Ω; if the steel reinforcement is not grounded, there are no control requirements for its impedance.
At the same time, the magnitude of the impedance after grounding the steel reinforcement will also affect the cost of the FPCA to a certain extent. If the steel reinforcement grounding impedance requirement is smaller (<2Ω), the steel sheet needs to be nickel-plated, and the exposed copper area and the number of exposed copper points where the FPCA and the steel sheet contact also need to be increased to ensure that the steel sheet grounding impedance meets the requirements.
<5Ω.
To prevent defects such as solder pad loosening and peeling during FPCA component bending, which can affect the functionality and normal use of the TFT-LCD liquid crystal display module, dispensing is usually required in the component area to protect the component solder pads and enhance their strength.
FPCA component dispensing can generally be divided into two types, also known in the industry as: full encapsulation process and partial encapsulation process.
The full encapsulation process covers the entire component area, providing thorough protection but at a higher cost; the partial encapsulation process only covers the component solder pad area, which is more efficient and less expensive, unless dispensing is unavoidable for small components.
Based on our experience with TFT-LCD modules, the partial encapsulation process is the preferred option unless the customer has specific reliability requirements.
The FPCA flexible panel uses an exposed copper design to achieve grounding through conductive materials, enhancing ESD resistance.
However, each exposed copper area on every flexible panel requires electroless nickel immersion gold (ENIG) surface treatment. As the area and number of exposed copper areas increase, the consumption of gold and nickel also increases.
Our recommendation is: while meeting testing standards, minimize the area and number of exposed copper areas on the FPCA panel. This will not only control FPCA costs but also simplify the assembly process of the TFT-LCD module.
To reduce the risk of radio frequency interference in the overall device, an EMI electromagnetic shielding film is typically added to the FPCA flexible panel. The number of layers and the area of the electromagnetic shielding film on the FPCA directly affect the cost of the FPCA.
Considering cost, we recommend avoiding the addition of EMI electromagnetic shielding film whenever possible. If it is necessary to add it, prioritize single-sided coverage with the smallest possible area, as double-sided coverage is more expensive than single-sided, and full coverage is more expensive than partial coverage.
Additionally, it is important to note that the added electromagnetic shielding film must be a single continuous piece. This is to avoid segmented designs and separate application processes, which would increase the application steps and costs.
The design factors of the FPCA flexible panel mentioned above do not exist in isolation; they influence each other and affect the overall cost of the TFT-LCD display module.
Appropriate trade-offs and optimization can significantly reduce the overall cost while maintaining performance.
