1. Organic Coating (OSP)
2. Electroless nickel plating/gold dipping (chemical gold plating)
Unlike organic coating, the electroless nickel plating/gold plating process is mainly used on boards with surface connection functional requirements and long storage life, such as the key area of mobile phones, the edge connection area of router housing and the electrical contact area of the elastic connection of chip processors. Electroless nickel plating/gold dipping was widely used in the 1990s due to the flatness of hot air leveling and the removal of organic coated flux. Later, the use of electroless nickel plating/gold leaching process declined due to the advent of the black disc, brittle nickel-phosphorus alloy, but today almost every high-tech PCB plant has electroless nickel plating/gold leaching line. Considering that the solder joint becomes brittle when the copper tin intermetallic compound is removed, the relatively brittle nickel tin intermetallic compound will present many problems. Therefore, portable electronic products (such as mobile phones) almost all use organic coated, silver or tin impregnated copper-tin intermetallic solder joints, while electroless nickel/gold plating is used to form key areas, contact areas and EMI shielding areas. It is estimated that 10% to 20% of PCBS currently use the electroless nickel/gold plating process.
3. Dipping silver (sinking silver)
Silver immersion is cheaper than electroless nickel/gold plating, and is a good choice if the PCB has connection functional requirements and needs to reduce costs; Plus silver immersion good flatness and contact, it should be more choose silver immersion process. In communication products, automobiles, computer peripherals in a lot of applications, in high speed signal design also has applications. Silver immersion can also be used in high frequency signals because it has excellent electrical properties unmatched by other surface treatments. EMS recommends the silver immersion process because it is easy to assemble and has good inspectability. But it grows slowly (but does not decline) because of drawbacks such as tarnish and hollow solder joints. It is estimated that 10% to 15% of PCBS currently use the silver leaching process.
4. Dipping tin (sinking tin)
Tin was introduced into the surface treatment process in the last ten years. The appearance of the process is the result of the requirement of production automation. The tin immersion does not introduce any new elements into the weld, especially suitable for communication back board. Tin will lose its solderability outside the board's storage life, so better storage conditions are needed for leaching tin. In addition, tin leaching process is restricted because of carcinogenic substances. It is estimated that about 5-10% of PCBS currently use the tin leaching process.
5. Concluding remarks
With more and more high customer requirements, environmental requirements are more and more strict, surface treatment process is more and more, in the end the choice of the kind of development prospects, more universal surface treatment process, it seems a little dazzling, confusing. It is impossible to predict exactly where PCB surface treatment technology will go in the future. Anyway, to meet customer requirements and protect the environment must be done first!
In vacuum packaging, good storage environment: the temperature is not higher than 25°, the relative humidity is not greater than 65%, indoor and non-corrosive gas environment, surface treatment method and storage period:
OSP (Organic Preservative flux) for six months
HASL (Hot Air Leveling - Spray tin) for six months
Immersion gold (Chemical Gold) for 12 months
Immersion tin (chemical Immersion Tin) for 12 months
Immersion silver (Chemical Immersion Silver) for 12 months
Electrolytic gold (electric plating) for 18 months
After the vacuum package is opened, the OSP lasts 24 hours and the other 48 hours. In fact, the storage period, as long as the solder OK can be used, does not have to limit a time!