The most basic purpose of surface treatment is to ensure good solderability or electrical properties. As copper in nature tends to exist as an oxide in air and is unlikely to remain as virgin copper for long periods of time, other treatments for copper are required. Although strong flux can be used to remove most copper oxides in subsequent assemblies, strong flux itself is not easily removed, so the industry generally does not use strong flux.
Now there are many PCB surface treatment processes, the common is hot air leveling, organic coating, chemical nickel / immersion gold, immersion silver and immersion tin these five processes, the following will be introduced one by one.
Hot air levelling (tin spraying)
Hot air leveling, also known as hot air solder leveling (commonly known as spray tin), it is coated with molten tin (lead) solder on the surface of the PCB and heated compressed air (blowing) flat process, so that it forms a layer of both copper oxidation resistance, but also provides a good solderability coating layer. Hot air leveling forms a copper-tin intermetallic compound at the bond between the solder and copper; PCBs are leveled by hot air to sink into the molten solder; the air knife blows the liquid solder flat before it solidifies; the air knife minimises solder mooning and prevents solder bridging on the copper surface.
Organic Solderability Protectant (OSP)
OSP is a RoHS compliant process for the surface treatment of copper foil on printed circuit boards (PCBs). OSP stands for Organic Solderability Preservatives and is also known as Preflux. Simply put, OSP is an organic film that is chemically grown on a clean, bare copper surface.
This film is resistant to oxidation, thermal shock and humidity and protects the copper surface from further rusting (oxidation or sulphurisation etc.) in the normal environment; however, in the subsequent high soldering temperatures, this protective film must be easily and quickly removed by the flux so that the exposed clean copper surface can immediately bond with the molten solder to form a solid solder joint in a very short period of time.
Full board nickel-gold plating
Nickel gold plating is a layer of nickel plated on the PCB surface conductors before plating a layer of gold, nickel plating is mainly to prevent the diffusion of gold and copper between. Nowadays, there are two types of electroplated nickel gold: soft gold plating (pure gold, gold surface does not look bright) and hard gold plating (smooth and hard surface, wear-resistant, containing other elements such as cobalt, gold surface looks brighter). Soft gold is mainly used in chip packaging when hitting the gold line; hard gold is mainly used in non-soldering at the electrical interconnection.
Immersion gold
Immersion gold is a thick, electrically sound layer of nickel-gold alloy wrapped around the copper surface, which protects the PCB for a long time; in addition it has a tolerance to the environment that other surface treatment processes do not have. It also prevents the dissolution of copper, which will benefit lead-free assembly.
Immersion tin
As all current solders are tin based, the tin layer can be matched to any type of solder. The sink tin process creates a flat copper-tin intermetallic compound, a property that makes sink tin as good a solderability as hot-air levelling without the flatness headaches of hot-air levelling; sink tin sheets cannot be stored for long and must be assembled in the order in which they are sunk.
Silver immersion
Between organic coating and electroless nickel/gold plating, the process is relatively simple and fast; silver retains good solderability even when exposed to heat, humidity and contamination, but loses its lustre. Immersion silver does not have the good physical strength of electroless nickel/immersed gold because there is no nickel underneath the silver layer.
Electroless nickel palladium gold
The palladium prevents corrosion due to displacement reactions and prepares the material for gold deposition. The gold is closely covered with palladium, providing a good contact surface.
Hard gold plating
Hard gold plating is used to improve the wear resistance of the product and to increase the number of insertions and removals.
