Comparison of several PCB surface finish types

PCB surface finish is a coating between a component and a bare board PCB. It is applied for two basic reasons: to ensure solderability, and to protect exposed copper circuitry. As there are many types of surface finishes, selecting the right one is no easy task, especially as surface mounts have become more complex and regulations such as RoHS and WEEE have changed industry standards.

1.Organic Solderability Preservatives (OSP):

Unlike other surface treatment processes, OSP acts as a barrier between copper and air. In simple terms, OSP is a chemical process to grow a layer of organic skin film on the clean bare copper surface, which has anti-oxidation, heat shock and moisture resistance, to protect the copper surface from rust (oxidation or vulcanization, etc.) in the normal environment. At the same time, it must be easily removed by the flux in subsequent welding temperatures for welding. The organic coating process is simple, low cost, make it widely used in the industry. The early organic coating molecules are the anti-rust imidazole and benzotriazole, and the most recent molecules are mainly benzimidazole. To ensure that multiple reflows can be performed, it might not work if there was only a single layer of organic coating on the copper surface, that’s to say, there must be many organic layers, which is why the copper solution is usually added to the chemical bath. After coating the first layer, the coating layer adsorbs copper, and then, the second layer of organic coating molecules bind to copper until twenty or even hundreds of organic coating molecules concentrate on the copper surface.

Its general process is: degrease → micro-etching → pickling → pure water cleaning → organic coating → cleaning, which is much easier compared with other treatment processes.

2.HASL (Hot air solder leveling)/HASL lead Free:

Hot air solder leveling is the process of coating molten tin-lead solder on the surface of PCB and heating compressed air leveling (blowing), so that it forms a layer of both anti-copper oxidation and good weldability of the coating. PCB shall be immersed in the molten solder during hot air solder leveling. The air knife shall flush the liquid solder before the solder solidifies, and it can minimize the crescent shape of the solder on the copper surface and prevent the solder bridge.

Hot air rectification is divided into two types: vertical type and horizontal type. Generally speaking, the horizontal type is better, and horizontal type hot air leveling coating is mainly uniform. The general process of automatic production is as follows: micro-corrosion → preheating →coating flux → spraying tin → cleaning.

3.Electroless Nickel/Immersion Gold(ENIG):

Unlike OSP, ENIG only is a thick nickel gold alloy with excellent electrical properties on the copper surface and can protect PCB for a long time and acts as a rust barrier, which can be useful and achieve good electrical performance in the long term use of PCB. What’s more, it also has environmental tolerance than other surface treatment processes do not. Nickel plating is because gold and copper diffuse to each other, and the nickel layer can prevent the diffusion between them. Without the nickel layer, gold will spread to the copper within hours. Another benefit of electroless nickel/impregnation is the strength of nickel, which is only 5um thick to control Z expansion at high temperatures. Besides, electroless nickel/gold plating can also prevent copper from dissolving, which will be beneficial for lead-free welding.

The general process is pickling cleaning → micro-etching → pre leaching → activation → electroless nickel plating → chemical leaching. There are 6 chemical tanks in the process, involving nearly 100 chemicals, which makes it relatively complex.

4.Immersion Silver:

The difficulty of the process of silver impregnation is between OSP and electroless nickel impregnation. Silver impregnation does not put heavy armor on PCB, which provides excellent electrical properties and maintains good weldability even when exposed to heat, humidity and pollution, but loses its luster. Since there is no nickel below the silver layer, silver immersion does not have all the good physical strength required for ENIG. Silver impregnation is a displacement reaction that is almost submicron - grade pure silver coating. Sometimes the process also contains organic matter, mainly to prevent the corrosion of silver and to eliminate the silver migration problem. In general, it is difficult to measure the organics in this thin layer, and the analysis shows that the weight of the organism is less than 1%.

5.Immersion Tin:

As all solder is based on the tin at present, the solder layer can match any type of solder. However, the previous PCB impregnation process is prone to tin whisker, and the transfer of tin whisker and tin during welding will bring some unavoidable problems, thus limiting the use of the impregnation process. After adding organic additives to the solution, the tin layer structure is granular, which overcomes the previous problems and has good thermal stability and weldability.

The process of tin impregnation can form a flat copper-tin intergeneric compound which haves good solderability like that of hot air leveling but does not have the flatness problem. There is also no diffusion problem between chemically plated nickel and impregnation metals but just can't soak tin board to store too long.

Comparison of cost and solderability

Cost: Electroplating nickel gold > ENIG > Immersion silver> Immersion tin > HASL > OSP.

Actual solderability: Electroplating nickel gold> HASL > OSP > ENIG >Immersion silver >Immersion tin

Characteristics of common surface treatment methods

physical property Sn-Pb HASL Immersion silver Immersion Tin OSP Immersion gold (ENIG)

Shelf life (month) 18 12 6 6 24

Reflow times 4 5 5 4 4

Cost medium medium medium low high

Process Complexity high medium medium low high

Process temperature 240°C 50°C 70°C 40°C 80°C

Thickness range (um) 1-25 0.05-0.20 0.8-1.2 0.2-0.5 0.05-0.2 Au

3-5 Ni

Flux compatibility Good Good Good general Good

Environmental protection No Yes Yes Yes Yes

Others Thickness Nonuniformity Easily scratched Short storage Sensitive to environment Easily-broken interface