SURFACE TREATMENT OF PCB
Due to the oxidization-prone characteristic of the PCB’s copper surface in air, a surface coating is thus 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 common PCB surface finishes are as follows:
As the term suggests, it means the copper is exposed without a cover of any kind.
Low cost, good solderability, and high smoothness are the advantages. No protection and oxidization-prone in the air are the disadvantages.
A tin layer is coated on the surface of the copper foil and smoothened with a hot air knife. Equipment includes horizontal and vertical HASL/HAL machines.
The HASL/HAL is the most common surface PCB treatment. It is characterized by good solderability, longer storage time, and a lower cost. However, as the surface smoothness of horizontal HASL/HAL is lower, soldering defects due to inconsistent solder volume occur easily in SMT. In addition, bumps causing short circuit easily occur if the interval between soldering points is too close.
As tin-lead alloy is used in the solder for HASL/HAL, and countries around the world have been actively advocating environmental protection in recent years, the demand for lead-free process rises, and lead-free HASL becomes another option.
The process is the same as that of the HASL/HAL. The presence or absence of lead in the solder is the main difference. Common lead-free solders include SnCuNi solder and SnAgCu solder.
Electroless Nickel Immersion Gold (ENIG)
The ENIG is a gold plating process without using additional electricity. After coating an electroless nickel layer on the bare copper surface, the substitution reaction in the tank solution can attract gold to deposit on the nickel layer until gold covers the nickel layer completely. As gold is deposited by means of substitution reaction, the thickness of ENIG cannot be compared with that of electroplating. The gold thickness is about 1–5μ inches, and nickel thickness is about 100–200μ inches. No need of additional wiring for electroplating to simplify the PCB design is the advantage of ENIG. In addition, as the area for soldering is rather smooth and even, short circuit causing by soldering defects or solder bridge in areas with more components (e.g. BGA) will be less likely to happen. However, a higher cost and poor soldering strength after SMT are its disadvantages.
Hard gold is often used in nickel gold plating. By connecting electricity through wiring, nickel and gold are plated on the bare copper of the PCB. Partial gold plating is often used on the areas requiring frequent plugging and unplugging on the PCB, such as the “connecting fingers.” However, full-board gold plating or partial onboard gold plating are also found in some products. In general, the gold plating thickness is about 5–30μ inches. Being robust and durable to reduce oxidization from wearing after repeat use are the advantages of gold plating. Wiring is required, unsmooth surface, and an expensive cost are the disadvantages.
Immersion silver is achieved by immersion plating using the potential differences between copper and nickel to substitute the copper particles on the copper surface with silver for surface chemical modification. In general, the plating thickness is about 6–10μ inches. A smooth surface and good solderability are the advantages. A higher cost, oxidization/sulfurization after contacts with air, short waiting time after opening, and short storage time are the disadvantages.
Organic Solderability Preservative (OSP)
An OSP layer is coated on the bare copper on the PCB by chemical means to block contacts between the bare copper on the PCB and air to prevent oxidization. A lower cost, quick processing, and a smooth surface and good solderability of the bare copper are the advantages. Inability for direct ICT and difficulty in visual inspection after processing are the disadvantages.