There are various types of surface treatments for FPC soft boards. FPC manufacturers should select based on the board’s performance and requirements. Below is a concise analysis of the pros and cons of different surface treatments for FPC soft boards for reference!

1. Advantages of OSP (Organic Protective Film): The process is straightforward, and the surface is exceptionally smooth, making it suitable for lead-free soldering and SMT. It allows for easy rework, convenient production operations, and is compatible with horizontal line production. The board supports multiple processing methods (e.g., OSP+ENIG) and is low-cost and environmentally friendly.

Weaknesses of OSP: There’s a limitation on the number of reflow soldering cycles (the film deteriorates after multiple uses, but generally holds up for two cycles). It’s not suitable for crimping technology or wire binding. Visual inspection and electrical measurements can be challenging. N2 gas protection is necessary for SMT, and SMT rework is not ideal. It also requires stringent storage conditions.

2. Immersion Silver: Immersion silver is an improved surface treatment process. Advantages of Immersion Silver include a simple process, compatibility with lead-free soldering and SMT, and a very smooth surface, making it suitable for fine lines, all at a low cost.


**Weaknesses of Immersion Silver:** Requires high storage conditions and is prone to contamination. The strength of solder joints may face challenges, particularly with microvoiding issues. It is also susceptible to electromigration and Javanni bite corrosion beneath the solder mask. Additionally, electrical measurement can pose difficulties.

**Tin Plating:** Tin plating involves a copper-tin replacement reaction. **Advantages of tinning:** It is suitable for horizontal production lines, fine line processing, and lead-free soldering, making it especially effective for crimping technology. It offers excellent flatness and is appropriate for SMT applications.

**Weaknesses of tin plating:** It demands good storage conditions, ideally limiting storage to six months to manage tin whisker growth. It is not suitable for contact switch designs. The production process has relatively stringent requirements for the solder mask; otherwise, it can lead to mask delamination. Using N2 gas protection is advisable during multiple soldering processes. Electrical testing can also present challenges.

**Immersion Gold (ENIG):** This is a more extensive surface treatment process. Note that the nickel layer consists of a nickel-phosphorus alloy, categorized into high-phosphorus and medium-phosphorus nickel based on phosphorus content, each having distinct applications. **Advantages of Immersion Gold:** It is well-suited for lead-free soldering, providing a very flat surface ideal for SMT. Through holes can also be coated with nickel and gold. It has a long shelf life and does not require stringent storage conditions. It is appropriate for electrical testing and switch contact designs, suitable for aluminum wire bonding and thick plates, exhibiting strong resistance to environmental degradation.

**Gold Plating:** Gold plating can be categorized into “hard gold” and “soft gold.” Hard gold (like gold-cobalt alloy) is typically used for gold fingers (contact connection designs), while soft gold is pure gold. Nickel and gold electroplating is widely utilized on IC substrates (e.g., PBGA), primarily for bonding gold and copper wires. However, additional conductive wires are needed for the electroplated gold finger area. **Advantages of gold plating:** Offers a longer storage time of over 12 months, is suitable for contact switch designs and gold wire bonding, and is effective for electrical testing.

**Weaknesses of gold plating:** It has a higher cost and requires a thicker gold layer. When plating gold fingers, additional design lines are necessary for electrical conductivity. Variability in gold thickness can lead to embrittlement of solder joints during welding, affecting joint strength and surface uniformity. Electroplated nickel gold may not cover the edges of the wire and is not suitable for aluminum wire bonding.

**Nickel-Palladium-Gold (ENEPIG):** This method is gradually gaining traction in the PCB field, previously used in semiconductors. It is suitable for bonding gold and aluminum wires. **Advantages of nickel-palladium-gold:** It is applied to IC carrier boards, making it appropriate for both gold and aluminum wire bonding. It is compatible with lead-free soldering and, unlike ENIG, avoids nickel corrosion (black pad) issues. It is more cost-effective than both ENIG and electro-nickel gold, offering long storage life and compatibility with various surface treatment processes when stored on the board.

**Weakness of nickel-palladium-gold:** The process is complex and challenging to control, and its application history in the PCB field is relatively short.

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This revised version maintains the original meaning while enhancing clarity and flow.