PCB Surface Treatment

Due to the oxidization-prone characteristic of the PCB’s copper surface in air, a surface coating is added to the exposed areas not covered by the solder mask to protect such areas against oxidization. To meet the needs of different methods of subsequent processing, surface finishes of different materials, prices, and protection abilities are developed.

The PCB surface finishes are as follows：

• Bare Copper：

  As the term suggests, it means the copper is exposed without any cover. Advantages: low cost, good solderability, and high smoothness. Disadvantages: no protection and oxidization-prone in the air.

• HASL/HAL：

  A tin layer coated on the surface of the copper foil and smoothened with a hot air knife. Equipment includes horizontal and vertical HASL/HAL.The HASL/HAL is the common surface PCB treatment.
  
  Advantages: good solderability, longer storage time, and a lower cost.
  Disadvantages: since surface smoothness of horizontal HASL/HAL is poor, it easily has a soldering defect during SMT. In addition, bumps cause short circuit easily if the interval between soldering points is too close.
  
  As tin-lead alloy is used for HASL/HAL, recently, many countries advocate environmental friendly; therefore, the lead-free HASL becomes another option.

• Lead-Free HASL：

  The process is the same as the HASL/HAL; however, the difference is no lead inside. In general, the lead-free solder includes SnCuNi and SnAgCu solder.

• Electroless Nickel Immersion Gold (ENIG)：

  The ENIG is a gold plating process without using additional electricity. After coating the electroless nickel on the bare copper surface, the substitution reaction in the tank which gold will be gradually deposited on the nickel layer until the nickel layer is covered by the gold completely. Generally speaking, the thickness of ENIG is around 1~5μ inches, and nickel thickness is around 100–200μ inches.
  
  Advantages: no need extra wiring for electroplating to simplify the PCB design. In addition, the solder area is rather smooth so it seldom occur short circuit caused by solder defects or solder bridge in the BGA.
  Disadvantages: a higher cost and poor soldering strength after SMT.

• Gold Plating：

  Hard gold is often used in nickel gold plating. Nickel and gold are plated on the bare copper PCB by connecting electricity through wire. Usually, gold plated on the PCB edge for plug in/out area, like gold finger. However, some of products will choose full-board gold plating or partial gold plating. In general, the gold plating thickness is around 5~30μ inches.
  Advantages: the ENIG PCB is robust and durable to reduce oxidization after using many times.
  Disadvantages: expensive and unsmooth surface.

• Immersion Silver：

  Immersion silver is an environment-friendly surface finish plated on the copper surface of a PCB to assure good soldering of components. It is the process of depositing a thin layer (6~10μ”) of silver by a chemical reaction between silver ions and the metal copper present on the board. Currently, this surface finish is the best choice due to its outstanding performance and affordable cost. When compared to OSP and immersion gold, silver finish process is a lot simpler and quicker. It can maintain its electrical properties and solderability even after exposure to heat, pollution, humidity, etc. However, it will tarnish because of the absence of nickel underneath the silver. This finish lacks physical strength like immersion gold.

• OSP（Organic Solderability Preservative）：

  OSP is a process for surface treatment of printed circuit board (PCB) copper foil in accordance with RoHS instructions, which uses a water-based organic compound that rust (oxidation or vulcanization, etc.) in normal environment. However, at subsequent welding temperatures, the protective film must be easily removed by the flux, so that the exposed clean copper surface can be immediately combined with the molten solder to form a solid solder spot in a very short time.
  
  Advantages: Flat surface, no Pb, simple process, re-workable and cost effective.
  Disadvantages: No way to measure thickness, not good for PTH, short shelf life, can cause ICT issues, exposed Cu on final assembly and handling sensitive.