1. ENIG’s protection mechanism: A thick layer of nickel-gold alloy with excellent electrical properties is applied to the copper surface, providing long-term protection for the PCB. Unlike OSP, which serves merely as an anti-rust barrier, ENIG is beneficial for prolonged PCB use and ensures superior electrical performance. Additionally, ENIG offers environmental resistance that other surface treatments may lack.
2. Ni/Au is applied to the copper surface through a chemical process. The typical deposition thickness of the inner Ni layer ranges from 120 to 240 μin (approximately 3 to 6 μm), while the outer Au layer is much thinner, generally between 2 to 4 μin (0.05 to 0.1 μm). Ni acts as a barrier layer between the solder and the copper. During soldering, the Au layer on top quickly melts into the solder, forming a Ni/Sn intermetallic compound with the solder. The purpose of the gold plating is to prevent Ni oxidation or passivation during storage, so the gold layer must be sufficiently dense and not too thin.
1. **Immersion gold:** In this process, the goal is to deposit a thin, continuous gold protective layer. The thickness of the gold should not be excessive, as overly thick gold can make the solder joints very brittle, significantly affecting welding reliability. Like nickel plating, immersion gold requires high temperatures and extended processing times. During the immersion process, a displacement reaction occurs—gold replaces nickel on the nickel surface. Once the displacement reaches a certain level, the reaction automatically stops. Gold has high strength, abrasion resistance, high temperature tolerance, and is resistant to oxidation, which helps protect the nickel from oxidation or passivation, making it suitable for high-strength applications.
2. The PCB surface treated with ENIG is very flat and has excellent coplanarity, making it ideal for contact surfaces such as buttons. Additionally, ENIG offers excellent solderability; the gold quickly dissolves into the molten solder, exposing fresh nickel underneath.
3. **Limitations of ENIG:** The ENIG process is complex and requires strict control of process parameters to achieve good results. A significant issue is that ENIG-treated PCB surfaces can develop black pads during either the ENIG process or soldering, which can severely impact solder joint reliability. The formation of black pads is complex and occurs at the Ni and gold interface, often due to excessive oxidation of the nickel. Excessive gold can also make solder joints brittle and compromise reliability.
4. **Chemical Immersion Silver:** Compared to OSP and electroless nickel/immersion gold, this process is simpler and faster. It maintains good electrical performance and solderability even when exposed to heat, humidity, and contamination, though it will lose its luster over time. Since there is no nickel layer under the silver, immersion silver lacks the physical strength provided by electroless nickel/immersion gold.
5. **Electroplating Nickel Gold:** This method involves electroplating a layer of nickel followed by a layer of gold on the PCB surface. The primary purpose of nickel plating is to prevent diffusion between the gold and copper. There are two types of electroplated nickel gold: soft gold plating (pure gold, which is less shiny) and hard gold plating (smooth, hard, wear-resistant, containing cobalt and other elements, with a brighter surface). Soft gold is mainly used for wire bonding during chip packaging, while hard gold is used for electrical interconnections in non-soldered areas (such as gold fingers).
6. **PCB Hybrid Surface Treatment Technology:** This approach combines two or more surface treatment methods. Common combinations include: Immersion Nickel Gold + Anti-oxidation, Electroplating Nickel Gold + Immersion Nickel Gold, Electroplating Nickel Gold + Hot Air Leveling, and Immersion Nickel Gold + Hot Air Leveling.
7. Among all surface treatment methods, hot air leveling (both lead-free and leaded) is the most common and cost-effective, but it is important to comply with the EU’s RoHS regulations.
8. **RoHS:** RoHS is a mandatory standard established by EU legislation, formally known as “Restriction of Hazardous Substances.” Implemented on July 1, 2006, it standardizes material and process requirements for electronic and electrical PCB products to improve human health and environmental protection. The standard aims to eliminate six substances, including lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ethers, and specifies that lead content must not exceed 0.1%.
