The current surface finish methods are the following:
1. Hot air solderleveling (HASL)
Theprocess of applying molten tin-lead solder to the PCB surface and leveling(blowing flat) with heated compressed air forms a coating that is resistant tocopper oxidation and provides good solderability. Hot air leveling flat solderand copper at the junction of copper and tin metal compounds, the thickness ofabout 1 ~ 2mil.
2. Organic antioxidant(OSP)
Ona clean bare copper surface, a layer of organic film grows chemically. Thislayer of film with anti-oxidation, thermal shock resistance, moistureresistance, to protect the copper surface in the normal environment no longercontinue to rust (oxidation or sulfide, etc.); at the same time must be in thesubsequent high temperature welding, can be easily assisted quick removal offlux for welding.
3. Chemical nickelplating gold
Wrapa thick layer of copper on the surface, good electrical properties ofnickel-gold alloy and long-term protection of the PCB. Unlike OSP, which isonly used as a rust barrier, it can be useful during long-term PCB use andachieve good electrical performance. In addition it also has other surfacetreatment processes do not have the patience of the environment.
4. Chemical silver
Betweenthe OSP and electroless nickel / gold immersion, the process is relativelysimple and fast. Exposure to heat, humidity and contaminated environments stillprovides good electrical properties and good solderability but is tarnished.Because there is no nickel below the silver layer, silver does not have all thegood physical strength of electroless nickel / gold immersion.
5. Nickel plating
Conductorsin the PCB surface after plating a layer of nickel plating on a layer of gold,nickel plating is mainly to prevent the diffusion between gold and copper. Nowthere are two types of nickel plating: soft gold (gold, gold does not lookbright) and hard gold plated (smooth and hard surface, wear, containing cobaltand other elements, the surface looks more shiny). Soft gold is mainly used inthe chip package hit the gold line; hard gold is mainly used in the non-weldingof the electrical interconnection (such as gold fingers).
6. PCB mixed surfacefinish technology
Choosetwo or more surface treatment methods for surface finish, the common forms are:Nickel gold + anti-oxidation, nickel-plated gold + nickel-plated gold, nickel +gold plated hot air leveling, nickel-plated gold + hot air leveling.
Hotair leveling (lead-free / lead) is the most common and cheapest solution forall surface finishes, but please note the EU RoHS directive.
RoHS:RoHS is a mandatory standard set by the EU legislation, its full name is"Restriction of Hazardous Substances in the use of electrical andelectronic equipment directive" (Restriction of Hazardous Substances). Thestandard has been formally implemented on July 1, 2006, mainly for thestandardization of electrical and electronic products, materials and processstandards to make it more conducive to human health and environmentalprotection. The purpose of this standard is to eliminate 6 substances of lead,mercury, cadmium, hexavalent chromium, polybrominated biphenyls andpolybrominated diphenyl ethers in motor and electronic products, and tostipulate that the content of lead should not exceed 0.1%.