Classification of PCB surface treatment
01 What is PCB surface treatment process?
There are no copper surfaces covered by solder mask on the PCB, such as component pads, gold fingers, mechanical holes, etc. Without a protective coating, the copper surface is easily oxidized, affecting the soldering between bare copper and components in the solderable area of the PCB.
As shown in the figure below, the surface treatment is located on the outermost layer of the PCB, above the copper layer, and acts as a "coating" on the copper surface.
The main function of surface treatment is to protect the exposed copper surface from oxidation of the circuit, thereby providing a solderable surface for soldering.
02 PCB surface treatment process classification
PCB surface treatment processes are divided into the following categories:
Hot Air Solder Leveling (HASL)
Hot Air Solder Level (HASL), commonly known as tin spraying, is divided into lead-free tin spraying and lead-based tin spraying. It is the most commonly used and relatively cheap surface treatment process.
The storage life of PCB can reach 12 months, the process temperature is 250℃, and the surface treatment thickness range is 1-40um.
In the spray tin process, the circuit board is immersed in molten solder (tin/lead), allowing the solder to cover the exposed copper surface on the PCB. As the PCB leaves the molten solder, high-pressure hot air is blown across the surface via an air knife, smoothing the solder deposit and removing excess solder.
The process of tin spraying process needs to be mastered: welding temperature, air knife air temperature, air knife pressure, dip soldering time, lifting speed, etc. To ensure that the PCB is completely immersed in the molten solder, the air knife can blow up the solder before it solidifies. The pressure of the air knife can minimize the meniscus on the copper surface and prevent solder bridging.
Immersion Tin (Immersion Tin) (ImSn)
Immersion Tin (ImSn) is a metal finish deposited through a chemical replacement reaction. It is applied directly to the base metal (i.e. copper) of the circuit board and can meet the PCB surface flatness requirements of small-pitch components. Require.
Immersed tin can protect the underlying copper from oxidation during the shelf life of 3-6 months. Since all solders are tin-based, the immersed tin layer can match any type of solder. After adding organic additives to the tin immersion liquid, the tin layer structure becomes granular, which overcomes the problems caused by tin whiskers and tin migration, and also has good thermal stability and solderability.
The immersion tin process temperature is 50℃, and the surface treatment thickness is 0.8-1.2um. Especially suitable for PCBs connected by crimping, such as communication backplanes.
Electroless nickel gold (immersion gold) (ENIG)
Electroless Nickel Immersion Gold (ENIG) can meet the PCB requirements for surface flatness and lead-free processing of small-pitch devices (BGA and μBGA).
ENIG consists of two metal coatings, with nickel deposited on the copper surface through a chemical process and then gold atoms coated through a displacement reaction. The thickness of nickel is 3~6μm, and the thickness of gold is 0.05~0.1μm. The nickel acts as a barrier to the copper and is the surface to which the components are actually soldered, and the gold serves to prevent the nickel from oxidizing during storage, giving it a shelf life of about a year and ensuring excellent surface flatness.
The immersion gold process is widely used in high-density boards, conventional hard boards and soft boards. It has high reliability and supports the use of aluminum wires for wire bonding. Widely used in consumer, communications/computing, aerospace and medical industries.
Organic Solderability Preservative (OSP)
Organic solderability preservatives (OSP) apply a very thin protective layer of material on exposed copper to protect the copper surface from oxidation.
The organic film has properties such as antioxidant, thermal shock resistance, and moisture resistance, and can protect the copper surface from being oxidized or sulfurized in normal environments. In the post-high-temperature soldering process, the organic film will be easily and quickly removed by flux, allowing the exposed clean copper surface to immediately combine with the molten solder to form a strong solder joint in a very short time.
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OSP is a water-based organic compound that selectively binds to copper to protect it before soldering. Very environmentally friendly compared to other lead-free surface treatment processes that can be toxic or consume more energy.
Immersion silver (chemical silver) (ImAg)
Immersion silver (ImAg) directly coats copper with a layer of pure silver by immersing PCB in a silver ion bath through a displacement reaction. Silver has stable chemical properties. PCBs processed by the immersion silver process can still maintain good electrical properties and solderability even if they are exposed to heat, moisture and pollution, even if the surface loses luster.
Sometimes immersed silver is used in combination with an OSP coating to prevent the silver from reacting with sulfides in the environment. Immersed silver can replace immersed gold for most applications. If you do not want to introduce magnetic materials (nickel) into the PCB, you can choose to use immersion silver.
The surface thickness of immersion silver is 0.12~0.40μm, and the shelf life is 6 to 12 months. The immersion silver process is sensitive to the cleanliness of the surface during processing, and it is necessary to ensure that the entire production process will not cause surface pollution of immersion silver. The immersion silver process is suitable for applications such as PCBs, membrane switches and aluminum wire bonding that require EMI shielding.
Electroless Nickel Plating Electroless Palladium Immersion Gold (ENEPIG)
Electroless Nickel Plating Electroless Palladium Immersion Gold (ENEPIG) Compared with ENIG, ENEPIG has an extra layer of palladium between nickel and gold, which further protects the nickel layer from corrosion and prevents black pads that may appear in ENIG surface treatment, thus improving the surface finish have more advantages. The deposition thickness of nickel is about 3~6μm, the thickness of palladium is about 0.1~0.5μm, and the thickness of gold is 0.02~0.1μm. Although the thickness of the gold layer is thinner than ENIG, it is more expensive.
Copper-nickel-palladium-gold layer structure for direct wire bonding to plating. The final gold layer is very thin and soft, and excessive mechanical damage or deep scratches may expose the palladium layer
Electrolytic Nickel/Gold
Electroplated nickel gold is divided into "hard gold" and "soft gold".
Hard gold has a lower purity (99.6%) and is often used in gold fingers (PCB edge connectors), PCB contacts or other hard wear areas. The thickness of the gold can vary according to requirements.
Soft gold is purer (99.9%) and is commonly used for wire bonding.
Hard electrolytic gold
Hard gold is a gold alloy containing cobalt, nickel or iron complexes. Use low stress nickel between gold plating and copper. Hard gold is suitable for components that are used frequently and are likely to wear out, such as carrier boards, golden fingers, and button pads.
The thickness of the hard gold surface treatment will vary depending on the application, IPC recommends a maximum solderable thickness of 17.8μin, IPC Class 1 and 2 applications recommend 25μin gold and 100μin nickel, and IPC Class 3 applications recommend 50μin gold and 100μin nickel.
Soft electrolytic gold
Mainly used for PCBs that require wire bonding and high solderability requirements. Compared with hard gold, soft gold welding joints are stronger.
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