The surface finish of PCB board may be an organic substance or a metallic substance. Comparing the two types and all the existing options, you can quickly see the corresponding advantages and disadvantages. In general, when determining the most suitable surface treatment, the decisive factors are the end use of the product, the assembly process and the design of the PCB itself. The following will briefly introduce some of the most commonly used surface finish type.

Hot Air Solder Leveling (HASL)

HASL Surface finish

Hot air leveling, also known as hot air solder leveling, is a process of coating molten tin-lead solder on the PCB surface and leveling (blow-out) with heated compressed air to form a layer that resists copper oxidation and provides good solderability. When the hot air is leveled, the solder and copper form a copper-tin metal compound at the joint, and its thickness is about 1 to 2 mils.

The PCB is immersed in molten solder when hot-air leveling. The air knife blows the liquid solder flat before the solder solidifies, and can minimize the meniscus shape of the solder on the copper surface and prevent solder bridging.

Hot air leveling is divided into two types: vertical and horizontal. Generally, the horizontal type is considered to be better. The horizontal hot air leveling coating is relatively uniform, which can realize automatic production.

The general process is: micro-etching-> preheating-> coating flux-> spraying tin-> cleaning.

Advantages

1. Excellent solderability

2. Cheap / low cost

3. Allow long processing period

4.The industry has a long history / universal surface treatment

5. Shelf life is at least 12 months

6. Multiple thermal deviations

Disadvantages

1. There is a difference in thickness / surface shape between the large and small solder pads

2.Not applicable to SMD and BGA with pin pitch <20 mil

3. Micro-pitch formation of bridges

4. Not ideal for HDI products

LF HASL-Lead-free hot air solder leveling

Advantages:

1. Excellent solderability

2. Relatively cheap

3. Allow long processing period

4.The industry has a long history / universal surface treatment

5. Shelf life is at least 12 months

6. Multiple thermal deviations

Disadvantages:

1. There is a difference in thickness / surface shape between small and large solder pads, but the difference is lower than SnPb

2. High processing temperature, 260-270 degrees Celsius

3. Not suitable for SMD and BGA with pin pitch <20 mil

4. Micro-pitch formation of bridges

5. Not ideal for HDI products

In fact, HASL and LF HASL are roughly the same. The surface color and flatness of the pads presented have the same effect on the soldering quality.

OSP (Organic Solderability Preservative)

OSP surface

OSP differs from other surface treatment processes in that it functions as a barrier layer between copper and air; in short, OSP is a chemically grown organic film on a clean bare copper surface. This film has anti-oxidation, thermal shock resistance, and humidity resistance to protect the copper surface from rusting (oxidation or sulfurization) in the normal environment. At the same time, it must be easily assisted at the subsequent welding temperature. The flux is quickly removed for soldering.

The organic coating process is simple and low cost, which makes it widely used in the industry. The early organic coating molecules were imidazole and benzotriazole, which played a role of rust prevention, and the latest molecules were mainly benzimidazole. In order to ensure that multiple reflows can be performed, it is not possible to have only one organic coating on the copper surface. There must be many layers. This is why copper baths are usually added in chemical tanks. After coating the first layer, the coating layer adsorbs copper; then, the organic coating molecules of the second layer are combined with copper until twenty or even hundreds of organic coating molecules are aggregated on the copper surface.

The general process is: degreasing-> micro-etching-> pickling-> pure water cleaning-> organic coating-> cleaning. The process control is relatively easy compared to other methods.

OSP (Organic Solderable Protective Layer) (usual thickness 0.15 – 0.65 μm)

Advantages:

1. Excellent flatness

2.Suitable for fine pitch / BGA / smaller components

3. Cheap / low cost

4. Can be reworked

5.Clean and environmentally friendly technology

Disadvantages

1. Sensitive to operation – gloves must be used and avoid scratching

2. Short processing period during assembly phase

3. Limited thermal cycling, not the first choice for multiple welding processes (> 2/3)

4. Limited shelf life – not suitable for certain modes of transport and long-term storage

5. Difficult to test

6. Misprinted solder paste may have an adverse effect on the OSP coating

7. Baking may be adversely affected before use

Chemical Gold

Chemical gold

Electroless nickel plating / gold immersion is a thick layer of nickel-gold alloy with good electrical properties and can protect the PCB for a long time. Unlike OSP, which is only used as a rust-proof barrier layer, it can be useful and achieve good electrical properties during long-term use of the PCB. In addition, it also has environmental tolerance not available in other surface treatment processes.

The reason for nickel plating is that gold and copper will diffuse to each other, and the nickel layer can prevent the diffusion between them. Without the barrier of the nickel layer, gold will diffuse into the copper in a few hours. Another advantage of electroless nickel plating / immersion gold is the strength of nickel. Only 5um thickness of nickel can control the Z-direction expansion at high temperature. In addition, electroless nickel / immersion gold can prevent the dissolution of copper, which will be beneficial for lead-free soldering.

The general process is: de-acid cleaning-> micro-etching-> pre-immersion-> activation-> chemical nickel plating-> chemical immersion gold; there are 6 chemical tanks in the process, involving nearly a hundred species Chemicals, the process is more complicated.

ENIG-Immersion Gold / Electroless Nickel Plating (usually 3-6 μm (nickel) / 0.05 – 0.125 μm (gold)

Advantages

1. Chemical immersion, excellent flatness

2.Suitable for fine pitch / BGA / smaller components

3. Process tested and tested

4. Long shelf life

Disadvantages

1. Costly surface treatment

2.BGA has black pad problem

3. Aggressive to solder masks-especially larger solder masks

4. Avoid BGA defined by solder mask

5. Should not be plugged on one side

Immersion Silver

The silver immersion process is between OSP and electroless nickel / gold plating, and the process is simple and fast. Immersion silver is not a thick armor for PCB. Even if exposed to heat, humidity and pollution, it can still provide good electrical properties and maintain good solderability, but it will lose its luster. Because there is no nickel under the silver layer, immersion silver does not have all the good physical strength of electroless nickel / immersion gold.

Immersion silver is a displacement reaction, it is almost submicron pure silver coating. Sometimes the silver immersion process also contains some organics, mainly to prevent silver corrosion and eliminate silver migration problems. It is generally difficult to measure this thin layer of organics. Analysis shows that the weight of the organism is less than 1%.

Since all solders are based on tin, any type of solder can match the tin layer. From this point of view, the immersion tin process has great development prospects. However, tin whiskers tended to occur after the previous PCBs were immersed in tin. The solder whisker and tin migration during the soldering process will bring reliability problems, so the use of tin immersion is limited. Organic additives were added to the tin immersion solution to make the tin layer structure a granular structure, which overcomes the previous problems, and also has good thermal stability and solderability.

The immersion tin process can form a flat copper-tin intermetallic compound. This feature makes the immersion tin have the same good solderability as hot air leveling without the troublesome flatness problems of hot air leveling; nor does it have electroless nickel plating / immersion Diffusion issues between gold and metal; just the tin immersion board cannot be stored for too long.
Three. The thickness of immersion silver (0.12 – 0.40 μm)

Immersion silverAdvantages

1. Chemical immersion, excellent flatness

2.Suitable for fine pitch / BGA / smaller components

3.Medium cost range for lead-free surface treatment

4. Can be reworked

5. Medium shelf life with proper packaging

Disadvantages

1. Must worn very sensitive to handling / discoloration of silver surface / appearance problems-gloves.

2. Special packaging required-If the panel is not used up after the package is opened, it must be resealed quickly.

3. Short processing period during assembly phase

4. It is not recommended to use peelable glue

5. Should not be plugged on one side

6. Fewer suppliers provide this surface treatment process

There are also gold plating and flash gold processes. Used very little, customers will require different surface processes based on the difficulty of their substrates. The most widespread is still HASL. If the pads are required to be very flat and cost savings are required, then OSP is used. High-precision products, substrates with BGA and other chips are generally ENIG.

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