Hot air leveling, also known as hot air solder leveling,is a procedure in which molten tin-lead solder is covered on the surface of a PCB and leveled with heated compressed air to form a coating that resists copper oxidation and provides better solderability. A copper-tin metal compound obtained from the junction of solder and copper in hot air leveling to a thickness of approximately 1 to 2 mils.
Coat the surface of the PCB with molten tin-lead (lead and lead-free) solder, heat compressed air to level it (blow) to form a coating that is resistant to copper oxidation and has good solderability.
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The general process of automated production is: micro-corrosion → preheating → fluxing → spraying tin → cleaning.
During the hot-air solder leveling process, the PCB is immersed in the molten solder. The air knife should flush the liquid solder before it solidifies to minimize solder falciform on the copper surface and prevent solder bridges.
There are two kinds of hot air levelling: vertical and horizontal. Ordinarily the horizontal type is better, and the coating of the horizontal hot air leveling is mainly smoothly.
Advantages: low cost, solid structure and excellent solderability, long shelf life, reworkable;
Disadvantages: uneven surface, prone to solder bridging, not suitable for fine-pitch components, not suitable for HDI PCB, not suitable for electroplated through holes (PTH), poor wettability;
OSP is different from other surface treatment procedures: its role is to act as a baffle layer between copper and air; General speaking, OSP is a chemistry growth of a chemical-free thin film on a bare copper ground. This layer of film has thermal shock resistance, antioxidant, and moistureproof for protecting the copper surface from rust. in a normal environment; at the same time, it must be easily removed in the subsequent welding high temperature with the scaling powder so as to do facilitate soldering.
The organic coating procedure is simple and cheap in price, with extensive use of it in the industry. The early organic coating elements are colorless crystalline and benzotriazoles with rust-proof, and the latest elements are primarily benzimidazoles. To ensure that multiple flows are possible, only one organic coating is not right on the copper surface. There must be many layers. This is why copper liquid is usually needed to chemical bathing. After the first layer is applied, the coating adsorbs the copper; then the organic coating molecules of the second layer attach to the copper until more or even hundreds of organic coating molecules accumulated on the copper surface.
The surface finish of OSP is also non-conductive. It is RoHS Conformance and has good solderability.
The general process flow is: degreasing→micro-etching→pickling→pure water cleaning→organic coating→cleaning. which is much easier than other treatment processes.
Advantages: low cost, simple manufacturing process, lead-free, smooth surface, suitable for SMT assembly, and repairable;
Disadvantages: highly sensitive, not suitable for PTH, difficult to measure thickness, limited shelf life, non-conductive
Electroless nickel/immersion gold, or ENIG for short, also known as nickel-gold, immersion nickel-gold or electroless nickel-gold plating, is to replace palladium on the copper surface by chemical reaction, and then chemically coat a layer of nickel-phosphorus alloy on the palladium core, and then replace it by replacing palladium on the copper surface. The reaction coats the nickel with a layer of gold. There are two processes for the immersion gold of nickel and gold, the substitution semi-replacement and the semi-reduction mixed plating.
Electroless nickel gold is mainly used for surface treatment of circuit boards. Used to protect the copper not being oxidized or corroded on the surface of the PCB board. It can also be used for soldering and contacts ,like buttons, gold fingers on memory sticks, etc.
ENIG surface treatment is a nickel-containing double-layer metal coating, which can protect the PCB for a long time and act as an anti-rust barrier. It can play a role in the long-term use of the PCB and obtain good electrical properties. At the same time, it also has feature of environmental resistance but other surface treatments without it. It can be used as a protective barrier for copper circuits and the surface of soldering components.
The general process flow is pickling→micro-etching→pre-dipping→activation→electroless nickel plating→chemical immersion.
Advantages: Hard and durable, with longer shelf life,excellent weldability, lead-free and RoHS certification, smooth surface, better anti-corrosion and simple procedure mechanism,as well as high temperature resistance.
Disadvantages: with connection errors and cracks on the surface, a little difficult to rework,higher-cost.
Immersion silver is the surface finish of the immersion silver plated on the board. The characteristics of the immersion silver process are between OSP and ENIG. Immersion silver not wearing heavy armor to the PCB. Even if uncovered to high temperature, humidness and contamination, it can provide excellent electrical functioning and maintain excellent solderability, but it will lose its lustre. Immersion silver does not have all the better physical force required by ENIG as there is no nickel below the silver layer.
Immersion silver is a hypallage response, which is almost a sub-micron coating of pure silver. But the immersion silver procedure also involves some organic material, mainly to avoid silver erosion and get rid of the problem of silver migration. It is always difficult to estimate this thin layer of organic material. The survey displayed that the the organism weight was no more than 1%.
Silver is the most electroconductive material at the moment. Immersion silver is fit for riskless PCB assembly procedures, especially for high-speed print circuit board appliances.
Advantages: excellent solderability, high-cost performance, flatness, high stability, and high conductivity;
Disadvantages: silver chips, loss of luster, creep corrosion;
Immersion tin refers to plating a layer of immersion tin on copper circuits. The immersion tin procedures can form flattened copper-tin chemical compounds with excellent solderability, such as hot air leveling(HAL), but there is no flatness problem. there is also no electroless nickel plating/dipping Diffusion between gold and metal; it’s just that the immersion tin plate can’t be stored for too long.
This surface finish is ideal for SMT applications, BGA and fine pitch parts.
Since all solders with material tin, the tin layer can be matched with any kind of solder. Looking from this angle, the immersion tin process is very promising. However, in the past, tin beard were available after the immersion tin procedure, and the moving of tin beard and tin during the soldering procedure would result in reliability problems, thus limiting the use of the tin immersion procedure. Later, organic addition agent were added to the tin dipping solution for making the tin layer granular structure, which conquers the previous problems and has excellent solderability and thermal stability.
Advantages: lead-free and RoHS compliant, smooth surface, high cost performance, high reliability;
Disadvantages: high sensitivity during processing, exposed tin is easy to corrode, and the shelf life is short (about six months), expensive, prone to surface cracks and connection errors, not easy to rework.
Tin plating is a common coating applied to a large variety of copper products including busbars, electrical terminals, battery connectors or any other copper component used in the passing of current. With the electrification vehicles, power equipment and interconnectivity of the internet of things, the need for conductive coatings such as tin plating is growing due to its low cost, conductive and solderability properties.
One of the most common uses of tin is for copper plating of bus bars used in transferring electrical power. This article focuses on the application of tin plating on copper busbars and what you need to know when specifying tin plating. In addition, the various properties of tin plating are covered as well as how those properties are affected by bright, matte or even tin/lead alloy plating of tin.
Copper bus bars are used to distribute high amounts of current and used for mounting components and dissipating heat in various electrical and electronic applications. The main purpose of a busbar is to carry electricity and distribute it. Typically, C110 copper – a general purpose copper per ASTM B152 – is used in bus bar applications due to it having a very high conductivity and offering excellent formability. These properties make this copper grade a cost-effective choice for electronics, power equipment and automotive applications.
Copper Busbars are plated with bright or matte tin to further enhance the corrosion protection, electrical conductivity and solderability of the copper bus bar. Tin has many favorable properties including excellent conductivity, solderability, corrosion protection and it provides an optimal surface for electrical and heat transfer. Unlike raw copper bus, the tin coating provides a soft and ductile barrier which does not easily oxidize when exposed to oxygen or other elements. A raw copper bus will oxidize and lose conductivity very rapidly as compared to a tin coating. Although tin will eventually lose some conductivity tin forms a relatively thin oxide is still reasonably conductive.
Tin-plated copper busbars are used in a variety of industries and applications, including:
Electrical power transmission and distribution: Tin-plated copper busbars are commonly used in electrical power systems to transmit high-voltage electrical current. The tin plating helps to protect the underlying copper from corrosion, improving the reliability and lifespan of the busbars.
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Manufacturing: Tin-plated copper busbars are used in manufacturing environments to provide power to machinery, tools, and other equipment. The tin plating greatly improves the soldering process to ensure a reliable joint with low voltage drop.
Construction: Tin-plated copper busbars are used in the construction industry to provide power to buildings and other structures within power panels. Tin plating helps to extend the life of the busbars and breaker components as well as improve their appearance.
Transportation: Tin-plated copper busbars are used in the transportation industry to power electric vehicles, such as electric buses, trains, and trams. The tin plating can help to improve the electrical conductivity and reliability of the busbars.
Renewable energy: Tin-plated copper busbars are used in renewable energy systems, such as solar panels and wind turbines, to transmit electrical current from the generation source to the point of use or point of storage (battery banks). The tin plating can help to improve the electrical conductivity and corrosion resistance of the busbars especially in humid environments.
Overall, tin-plated copper busbars are used in a variety of industries and applications due to a range of properties that are favorable to reliable transfer of electrical current and heat transfer over the lifespan of the product.
Busbars can be used in a wide variety of applications. When specifying a tin coating for a busbar application, there are a few key characteristics to consider. There are several types of tin that can be used to plate copper busbars, including:
Electrolytic Bright Tin: Bright tin should be specified for copper bus bars or contacts that require improved electrical conductivity, corrosion protection and lubricity. Bright tin is a lustrous deposit that offers improved cosmetic appeal as well as these improved functional characteristics. However, bright tin should not be used in soldering applications as the brightener systems used to create the bright deposit will co-deposited organic elements into the tin deposit. These organic brighteners can cause de-wetting or even charring or blackening of the solder joint which can impede the durability of the solder joint especially when using mild fluxes.
Electrolytic Matte Tin (Solderable Tin): Matte or solderable tin should be specified for copper bus bars or contacts that require soldering, improved electrical conductivity, and corrosion protection with an industrial non-reflective finish. Due to the coarse grain structure, matte tin can result in higher initial insertion forces of mating contacts. A un-brightened nickel underplate is recommended prior to the matte tin to minimize diffusion of base material elements such as copper or zinc (for brass components) into the tin deposit. The nickel underplate provides an excellent base to solder to and ensures the longest possible shelf life of a solderable, matte tin deposit.
Matte tin is less aesthetically pleasing compared to bright tin but will provide a functional finish for soldering since it is free of any intentionally added organic compounds. The dull finish of matte tin can be burnished by part-to-part contact and as such care should be taken in packaging of larger parts to minimize contact if a uniform, dull finish is desired. Larger of heavier barrel plated parts will naturally have burnishing marks on the surface.
Hot-dip tin: Hot-dip tin is applied by immersing the part in a bath of molten tin. This process produces a heavy deposit of tin often 0.001” or more. It also can be used to coat complex or irregularly shaped parts since the tin deposits wherever the part is wetted during immersion in the molten bath. Hot-dip tin will result in buildup or pooling in corners, threads and ID features of parts and as such, should not be used on parts with tight tolerances.
Tin/Lead: Tin/lead plating is an alloy of tin and lead that can range from 5/95 (5% tin to 95% lead) to 95/5 (95% tin to 95% lead) tin/lead. The specific alloy will affect coating properties such as corrosion & chemical resistance, melting point and solderability. Tin/lead plating of copper bus bars or contacts is often used when tin/lead solder is specified. The addition of a small percentage of lead within the tin prevents the formation of tin whiskers which is desirable for critical electronic applications where whiskers can cause a short circuit path. The addition of lead greatly improves the chemical resistance of the deposit to mineral acids such as sulfuric acid making tin/lead plating of copper bus bars popular for lead/acid battery applications.
Ultimately, the best type of tin to use on a copper bus bar will depend on the specific requirements of the application, including factors such as corrosion & chemical resistance, electrical conductivity, soldering characteristics and cosmetics. It is important to carefully consider these factors and consult with a plating specialist to determine the best type of tin for a particular application.
Tin plating is a versatile and functional plating for copper busbars and electronic applications due to its low cost, corrosion protection, conductivity, solderability, heat-transfer and anti-galling characteristics. A brief summary of these benefits is provided below:
Conclusion
With the on-going developments within the electronics, electric vehicle, and power distribution industries, the need for a cost-effective and conductive metal is growing in demand. Tin plating provides many desirable attributes for these industries including improved conductivity, lubricity, solderability, heat transfer and corrosion resistance. Tin plating of copper bus bars is commonly used to provide a cost-effective, conductive coating to ensure reliable current transfer over the life of the product. Tin plating is offered in both matte and bright formulations and can be alloyed with elements such as lead to improve solderability and chemical resistance. Tin plating can be preceded by an underplate of copper or nickel to optimize the function, solderability and longevity of the plated bus, contact or terminal.
Advanced Plating Technologies (APT) has a team of dedicated engineers and technical sales members who can assist with any application, specification or general questions for your tin plating needs. APT has 75 years of experience plating tin and tin alloys for various power industries and can help assist with most company specifications.
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