**Flexible Circuit Board Plating**
(1) The pretreatment of FPC soft board electroplating is crucial. The copper conductor surface exposed by the FPC during the coating process may become contaminated with adhesive or ink, and high-temperature processes can lead to oxidation and discoloration. To achieve a dense coating with strong adhesion, it’s essential to eliminate any contamination and oxide layers from the conductor’s surface, ensuring it is clean.
However, some contaminants bond tightly to the copper conductors and cannot be entirely removed with mild cleaning agents. Consequently, most surfaces are treated using stronger alkaline abrasives and brushing techniques. The covering adhesives, typically based on epoxy resins, exhibit poor resistance to alkaline substances, which can weaken bonding strength. Although this weakening may not be immediately visible, during the FPC electroplating process, the plating solution might seep in from the edges of the cover layer, potentially causing it to peel off in extreme cases. Ultimately, during soldering, the solder can infiltrate beneath the cover layer.
In summary, the pre-treatment cleaning process significantly influences the fundamental characteristics of flexible printed boards, making it imperative to pay close attention to the processing conditions.
(2) The Thickness of FPC Electroplating
During electroplating, the deposition rate of the electroplated metal is closely linked to the electric field intensity, which varies with the circuit pattern’s shape and the electrode’s spatial relationship. Typically, the narrower the wire, the sharper the terminal, and the closer it is to the electrode, the higher the electric field strength, resulting in a thicker plating layer at that location. In flexible printed board applications, significant differences in wire widths can lead to uneven plating thickness. To mitigate this, a shunt cathode pattern can be placed around the circuit to balance the uneven current distribution during electroplating, ensuring a more uniform coating thickness across all areas. Thus, careful attention to the electrode structure is essential. A balanced approach is proposed here: stringent standards apply to components requiring high coating thickness uniformity, while other areas, such as lead-tin plating for fusion welding and gold plating for metal wire overlaps, can have more relaxed standards. Conversely, lead-tin plating for general anti-corrosion applications has less stringent thickness requirements.
(3) Stains and Dirt in FPC Electroplating
The appearance of the freshly electroplated layer may not initially raise concerns, but shortly afterward, issues like stains, dirt, and discoloration may arise. Often, factory inspections fail to detect these problems, which become apparent during user reception inspections. This issue typically stems from inadequate rinsing, leaving residual plating solution on the surface that reacts chemically over time. Flexible printed boards, due to their softness, are prone to irregularities where solutions can accumulate, leading to color changes. To avoid such issues, thorough rinsing and adequate drying treatment are essential. High-temperature thermal aging tests can verify sufficient rinsing.
Flexible Circuit Board Electroless Plating
When the line conductor intended for electroplating is isolated and cannot serve as an electrode, only electroless plating can be performed. The plating solutions used in electroless plating, known for their strong chemical activity, exemplify this process, particularly electroless gold plating, which uses an alkaline aqueous solution with a high pH. According to Coating Online, this electroplating method can lead to the plating solution seeping beneath the covering layer, especially if the quality control during lamination is inadequate and bonding strength is low. Due to the nature of the plating solution, electroless plating via displacement reactions is susceptible to penetration beneath the covering layer, complicating the attainment of ideal electroplating conditions.
Hot Air Leveling of Flexible Circuit Boards
Hot air leveling originated as a technique for rigid printed board PCB coating with lead and tin but has also been adapted for flexible printed boards (FPC) due to its simplicity. This process involves immersing the board directly and vertically in a molten lead-tin bath and then blowing off excess solder with hot air. These conditions can be particularly harsh for FPCs. To ensure successful immersion without issues, the flexible printed board must be secured between titanium steel screens before being dipped in molten solder, with the surface cleaned and coated with flux beforehand. The demanding conditions of the hot air leveling process may cause solder to penetrate from the edge of the cover layer to underneath it, especially when bonding strength between the cover layer and copper foil is weak. Moreover, since polyimide films easily absorb moisture, any moisture trapped during the hot air leveling process can evaporate rapidly, leading to bubbling or even delamination of the cover layer. Therefore, it is advisable to implement drying and moisture-proof management before the hot air leveling process on the FPC.
(1) The pretreatment of FPC soft board electroplating is crucial. The copper conductor surface exposed by the FPC during the coating process may become contaminated with adhesive or ink, and high-temperature processes can lead to oxidation and discoloration. To achieve a dense coating with strong adhesion, it’s essential to eliminate any contamination and oxide layers from the conductor’s surface, ensuring it is clean.
However, some contaminants bond tightly to the copper conductors and cannot be entirely removed with mild cleaning agents. Consequently, most surfaces are treated using stronger alkaline abrasives and brushing techniques. The covering adhesives, typically based on epoxy resins, exhibit poor resistance to alkaline substances, which can weaken bonding strength. Although this weakening may not be immediately visible, during the FPC electroplating process, the plating solution might seep in from the edges of the cover layer, potentially causing it to peel off in extreme cases. Ultimately, during soldering, the solder can infiltrate beneath the cover layer.
In summary, the pre-treatment cleaning process significantly influences the fundamental characteristics of flexible printed boards, making it imperative to pay close attention to the processing conditions.
(2) The Thickness of FPC Electroplating
During electroplating, the deposition rate of the electroplated metal is closely linked to the electric field intensity, which varies with the circuit pattern’s shape and the electrode’s spatial relationship. Typically, the narrower the wire, the sharper the terminal, and the closer it is to the electrode, the higher the electric field strength, resulting in a thicker plating layer at that location. In flexible printed board applications, significant differences in wire widths can lead to uneven plating thickness. To mitigate this, a shunt cathode pattern can be placed around the circuit to balance the uneven current distribution during electroplating, ensuring a more uniform coating thickness across all areas. Thus, careful attention to the electrode structure is essential. A balanced approach is proposed here: stringent standards apply to components requiring high coating thickness uniformity, while other areas, such as lead-tin plating for fusion welding and gold plating for metal wire overlaps, can have more relaxed standards. Conversely, lead-tin plating for general anti-corrosion applications has less stringent thickness requirements.
(3) Stains and Dirt in FPC Electroplating
The appearance of the freshly electroplated layer may not initially raise concerns, but shortly afterward, issues like stains, dirt, and discoloration may arise. Often, factory inspections fail to detect these problems, which become apparent during user reception inspections. This issue typically stems from inadequate rinsing, leaving residual plating solution on the surface that reacts chemically over time. Flexible printed boards, due to their softness, are prone to irregularities where solutions can accumulate, leading to color changes. To avoid such issues, thorough rinsing and adequate drying treatment are essential. High-temperature thermal aging tests can verify sufficient rinsing.
Flexible Circuit Board Electroless Plating
When the line conductor intended for electroplating is isolated and cannot serve as an electrode, only electroless plating can be performed. The plating solutions used in electroless plating, known for their strong chemical activity, exemplify this process, particularly electroless gold plating, which uses an alkaline aqueous solution with a high pH. According to Coating Online, this electroplating method can lead to the plating solution seeping beneath the covering layer, especially if the quality control during lamination is inadequate and bonding strength is low. Due to the nature of the plating solution, electroless plating via displacement reactions is susceptible to penetration beneath the covering layer, complicating the attainment of ideal electroplating conditions.
Hot Air Leveling of Flexible Circuit Boards
Hot air leveling originated as a technique for rigid printed board PCB coating with lead and tin but has also been adapted for flexible printed boards (FPC) due to its simplicity. This process involves immersing the board directly and vertically in a molten lead-tin bath and then blowing off excess solder with hot air. These conditions can be particularly harsh for FPCs. To ensure successful immersion without issues, the flexible printed board must be secured between titanium steel screens before being dipped in molten solder, with the surface cleaned and coated with flux beforehand. The demanding conditions of the hot air leveling process may cause solder to penetrate from the edge of the cover layer to underneath it, especially when bonding strength between the cover layer and copper foil is weak. Moreover, since polyimide films easily absorb moisture, any moisture trapped during the hot air leveling process can evaporate rapidly, leading to bubbling or even delamination of the cover layer. Therefore, it is advisable to implement drying and moisture-proof management before the hot air leveling process on the FPC.
