The fundamental objective of surface treatment is to ensure excellent solderability and electrical properties. Natural copper tends to oxidize when exposed to air, necessitating additional treatments. While strong flux can remove most copper oxides during subsequent assembly, its own residues are challenging to eliminate, leading to its limited use in the industry.
Various surface treatment processes exist for PCBs, including hot air leveling, organic coating, electroless nickel/immersion gold, immersion silver, and immersion tin, each of which will be detailed below.
1. **Organic Solderability Preservative (OSP)**
OSP is a surface treatment process for PCB copper foils that complies with the RoHS directive. OSP, short for Organic Solderability Preservative, is also known as Copper Protector or Preflux in English. Essentially, OSP involves chemically growing an organic film on the clean copper surface.
This protective film provides anti-oxidation, thermal shock resistance, and moisture resistance, shielding the copper from environmental corrosion (oxidation or sulfidation). During subsequent soldering at high temperatures, this organic film quickly dissolves under flux, exposing the clean copper surface. This enables rapid formation of strong solder joints with molten solder.
2. Hot air leveling (spray tin)
Hot air leveling, also known as hot air solder leveling (commonly referred to as spray tin), is a process where molten tin (lead) solder is coated onto the PCB surface and flattened (blown) using heated compressed air. This forms a layer that resists copper oxidation and ensures good solderability. During hot air leveling, a copper-tin intermetallic compound forms at the joint. The PCB must be immersed in molten solder during leveling; an air knife blows the liquid solder just before it solidifies. This minimizes solder meniscus on the copper surface and prevents solder bridging.
3. Entire board nickel-gold plating
Nickel-gold plating involves depositing nickel followed by gold onto the PCB surface. Nickel plating primarily prevents diffusion between gold and copper. There are two types of electroplated nickel gold: soft gold (pure gold, non-bright appearance) and hard gold (smooth, hard, wear-resistant with cobalt and other elements, bright gold appearance). Soft gold is used for gold wire during chip packaging, while hard gold is used for electrical interconnection in non-soldered areas.
4. Tin sinking
Since all current solders are tin-based, tin sinking can match any type of solder. The tin sinking process forms a flat copper-tin intermetallic compound, offering excellent solderability like hot air leveling but without its flatness challenges. Tin plating should not be stored for extended periods; assembly must follow tin sinking promptly.
5. Immersion silver
Immersion silver is a process between organic coating and electroless nickel/immersion gold. It offers a simple and fast PCB process; despite exposure to heat, humidity, and pollution, silver maintains good solderability although it loses its shine. Unlike electroless nickel/immersion gold, immersion silver lacks nickel beneath the silver layer, thus lacking comparable physical strength.
