The most common surface treatments

To protect our living environment requirement, human continue to improve it. At present, the environment of PCB production is especially true. Lead free and halogen-free will effected PCB development in many ways. Although PCB surface treatment seems little change at this moment, we should focus on that: long term slowly change will result in big change. In the case of increasingly high environmental protection, PCB surface treatment process will certainly change in future.

There are many different surface treatments, the normal are: HAL (hot air level), OSP (Organic Solderability Preservatives), ENIG (Electro less Nickel/Immersion Gold), Immersion silver, immersion tin, I will introduce individually.

1. HAL(hot air level)
Hot air level (HAL), it also called hot air solder level (HASL), the technology is: coating tin (lead) solder on PCB surface, and level it by heating compressed air, it will forming one layer which can anti-oxidation and good soldering.

2. Organic Solderability Preservatives(OSP)
OSP is one of surface treatment which comply with ROHS standard. OSP means Organic Solderability Preservatives, it also calls Preflux. Simply speaking, by chemical method, it will send up one organic film on clean bare copper surface. For this film, it can anti-oxidation, heat shock/humidity resistance, it protect copper surface away from rusty. In the following soldering with high temperature, this film will easily clean by flux, and also the bare copper will combine with tin solder shortly.

3. Ni/Au Plating
Ni/Au plating: plating one layer Ni and then one layer gold on surface of PCB conductor. The main purpose of plating nickel is preventing diffuse between gold and copper. There are two different gold plating at present, plating soft gold (pure gold, the surface not very bright) and plating hard gold (the surface is smooth and hard, wear, which include Co and other elements that will make surface brightly). Soft gold usually used for chip mounting, and hard gold used for electrical interconnection on non-soldering area.

4. ENIG(Immersion gold)
About ENIG surface treatment, which is coating one thicker and good electrical nickel and gold alloy, it can protect PCB with long period. And also the tolerance to environment, that is other surface treatments cannot reach. Moreover, ENIG can prevent copper dissolution, which benefit for lead free assembly.

5. Immersion tin
At this moment, tin is all solder’s base, so tin layer can match with all any solders. Through immersion tin technology, it can form flat copper tin compound, which have same Solderability characters with hot air level, and free of flatness problem. Boards with immersion tin, it can’t stock for a long period.

6. Immersion silver
It is a simple and fast technology, it is between OSP and ENIG, even exposure on hot, moisture and contaminated environment, the silver still keep well Solderability, just tarnish. But immersion silver haven’t character like ENIG with good physical strength, there isn’t nickel below silver layer.

7. ENEPIG

Compared with ENIG, there is a Pd layer between nickel and gold layer. Pd can prevent corrosion, it is helpful for immersion gold. Gold can combine with Pd and provide good contact area.