1. Introduction to electroless nickel plating/immersion gold
1. PCB coating thickness on electroless nickel/immersion gold surface:
Ni/Au is plated on the copper surface through chemical methods. The deposition thickness of the inner layer Ni is generally 120 to 240 μin (about 3 to 6 μm), and the deposition thickness of the outer layer of Au is relatively thin, generally 2 to 4 μinch (0.05 to 0.1 μm).
2. Features:
The PCB surface treated by ENIG is very smooth and has good coplanarity, so it is the perfect choice for the button contact surface. Secondly, ENIG has excellent solderability, and gold will quickly blend into the molten solder, thereby exposing fresh Ni. Solder and Ni form Ni/Sn intermetallic compounds.
3. Function:
Nickel plating mainly prevents the diffusion between gold and copper. Ni forms a barrier layer between solder and copper. The purpose of gold plating on the outside is to prevent Ni from oxidation or passivation during storage, so the gold plating must be dense enough and not too thin.
4. Benefit
Another benefit of electroless nickel/immersion gold plating is the strength of nickel. A thickness of only 5 microns can limit Z-direction expansion at high temperatures. In addition, electroless nickel plating/immersion gold can also prevent the dissolution of copper.
2. ENIG treatment process
1. Cleaning: Remove organic or inorganic residues on the copper surface.
2. Etching (Microetch): microscopically roughening the copper surface and further removing oxides
3. Catalyst: Deposit a layer of catalyst film on the copper surface, thereby reducing the active energy of copper, making it easier for Ni to deposit on the copper surface. Palladium and ruthenium are both catalysts that can be used.
4. Electroless nickel plating: The specific process will not be introduced in detail here. Nickel deposits contain 6 to 11% phosphorus. Depending on the actual specific use, nickel may be used as a welding surface or a contact surface, but no matter what, it must be ensured that the nickel has sufficient thickness to protect copper.
5. Immersion gold: The purpose is to deposit a thin and continuous gold protective layer. During the immersion process, a displacement reaction will occur - gold replaces nickel on the nickel surface. Gold prevents nickel from oxidizing or passivating.
6. Cleaning residues: Clean away excess chemical residues or other useless ingredients.
There are mainly 6 chemical tanks involving nearly 100 chemicals, so process control is difficult.
3. Application of ENIG
1. The electroless nickel plating/immersion gold process is different from organic coating. It is mainly used on boards with surface connection functional requirements and long storage period, such as mobile phone key areas, edge connection areas of router casings, and chip processing. The electrical contact area of the device's elastic connection.
2. Due to the flatness problem of hot air leveling and the removal problem of organic coating flux, electroless nickel/immersion gold plating was widely used in the 1990s; later, due to the emergence of black plates and brittle nickel-phosphorus alloys, chemical plating The use of nickel plating/immersion gold processes has decreased.
3. Considering that the solder joint will become brittle when removing the copper-tin intermetallic compound, many problems will occur at the relatively brittle nickel-tin intermetallic compound. Therefore, portable electronic products (such as mobile phones) almost all use copper-tin intermetallic compound solder joints formed by organic coating, immersion silver or immersion tin, while electroless nickel/immersion gold plating is used to form the button area, contact area and EMI shielding area. .
4. It is estimated that about 10%-20% of PCBs currently use the electroless nickel plating/immersion gold process.
4. Limitations of ENIG:
1. Black pad:
The ENIG-treated PCB surface is prone to produce a black pad effect during the ENIG or welding process. It occurs at the interface between Ni and gold and directly manifests as excessive oxidation of Ni.
2. Crisp gold
Too much gold (2--7Wt%) will embrittle the solder joints (brittle gold phenomenon) and affect reliability.
3. IMC
Nickel-tin intermetallic compounds are lower in strength than copper-tin intermetallic compounds, so there will be many problems with the relatively brittle nickel-tin intermetallic compounds.
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