Tin has probably been a part of your life since the time you first saw the Tin Man in the Wizard of Oz as a child. Of course, the use of tin extends well beyond the creation of a fictional movie character — it has many practical applications in our daily lives. We’re all familiar with tin cans that are used for holding many types of food and beverages; these containers are actually manufactured from sheet steel that is coated with a thin layer of tin known as tinplate.
Today, roughly half of all tin produced is used in soldering applications. Tin is also used in the production of pewter, bronze and phosphorous bronze. Tin salts are sometimes sprayed onto windshields and window glass to provide an electrically conductive coating. The window glass itself is often made by floating molten glass onto molten tin, which produces a flat surface. The metal used to manufacture bells is often a combination of bronze and tin. Additionally, tin and tin alloys offer tremendous value for electroplating, which is the process of depositing a coating of metal onto the surface of a material via an electrical current.
Fast Facts About Tin
Tin is a soft, malleable, silvery-white metal that is available in abundance throughout many parts of the world. The extraction of tin dates back to around 3000 B.C. during the Bronze Age. Bronze is a yellowish-brown alloy of copper and tin that typically consists of about one-third tin. The earliest bronze objects were found to contain a small tin component.
The Chinese began to engage in the practice of tin mining around 700 B.C. Today, tin can be found in China, Thailand and Indonesia, and it is also mined in Brazil, Peru and Bolivia. Tin is obtained by carbothermic reduction of the oxide ore, which is produced by heating the ore in a furnace.
Other key tin facts:
- Tin is the 49th most abundant element in the Earth’s crust.
- Tin is listed on the Periodic Table of Elements under the atomic symbol “Sn” and the atomic number of 50.
- Tin is not a native element, meaning it must be extracted from ores as opposed to being found in a natural state.
- Tin can be extracted from various ores, but the most common ore is Cassiterite (SnO2).
- While tin in its metallic form is non-toxic, certain tin compounds can be poisonous.
- The small amount of tin in the U.S. is primarily found in Alaska and California.
- Tin’s crystalline composition results in a distinctive screaming sound when the metal is bent, which is known as the “tin cry”.
The Benefits of Tin in the Electroplating Process
Why is tin such a popular choice for electroplating? Perhaps the biggest reason is that tin plating — or “tinning” — is an extremely cost-effective process. Because tin is so readily available, it is much less expensive than pricier metals such as gold, platinum or palladium. Tin also offers excellent solderability, as well as superior protection against corrosion.
Tin plating can produce a whitish-gray color that is preferable when a dull or matte appearance is desired. It can also produce a shiny, metallic look when a bit more luster is preferred. Tin offers a decent level of conductivity, making tin plating useful in the manufacturing of various electronic components. Tin is also FDA approved for use in the food service industry.
Industries That Make Use of Tin Plating
The benefits listed above make tin the metal of choice for plating applications in a wide range of industries including:
- Aerospace
- Food Service
- Electronics
- Telecommunications
- Jewelry Manufacturing
Sharretts Plating services many of these industries, contact us today for a free quote!
Basic Tin Plating Processes
There are three basic types of tin plating, each of which relies on the deposition of an electrolytic tin solution onto the surface of a metal object:
- Barrel plating: Barrel plating is normally used for plating smaller parts and entails placing the objects in a specially constructed vessel, commonly referred to as a barrel. The barrel slowly rotates while immersed in the electrolytic plating solution. Barrel plating of tin is extremely cost-effective, although it takes a relatively long time to complete the plating process.
- Rack plating: Rack plating is the preferred option for plating tin on larger or more delicate parts that may not be suitable for the barrel plating process. With rack plating, the objects are hung on a rack and immersed in the plating solution. Although rack plating is more labor-intensive and therefore more expensive than barrel plating, it offers greater control over plating thickness and can be more effective in reaching cavities deep within the object.
- Vibratory plating: Also used for delicate parts, vibratory plating involves placing the parts in a basket equipped with metal buttons that also contains the electrolytic plating solution. A generator is used to produce a vibrating action that causes the parts to move and make contact with the metal buttons. Vibratory plating is typically the most expensive form of tin plating and requires a special drying process that may cause the parts to bend.
Tin Plating Process Elements
Tin can be electrodeposited onto just about any type of metal. Let’s take a closer look at the specific components of an effective tin plating process:
1. Cleaning: It is vital to purify the substrate — the part that receives the tin coating — prior to immersion into the plating bath. Cleaning removes oil, grease and other surface contaminants that can reduce the effectiveness of the plating process.
Cleaning is a multi-step process that can vary somewhat depending on the composition of the substrate and the amount of grime and debris that it contains, as well as the types of cleaning equipment available for use. In general, the cleaning process includes:
- Grit blasting: This is the process of using pressurized air to project media such as crushed glass, aluminum oxide, silicon carbide, steel, corn cob or walnut shells to remove foreign matter from a surface.
- Boiling: Boiling the substrate in water can be an effective method of removing grease and oil, without having to resort to using chemical additives.
- Electrolytic degreasing: Immersing the substrate in an electrolytic solution will remove grease and oil that accumulates in cracks, crevices and other hard-to-reach surface areas.
- Rinsing: Rinsing the substrate in water after electrolytic degreasing removes any remaining cleaning solution and surface debris.
2. Preparing the Plating Bath
The next step is to prepare an electrolytic solution, also known as a plating bath. Electrolytic tin plating baths can be comprised of acid tin, alkaline tin or methyl sulphonic acid solutions. A plating bath includes tin that is dissolved to form positively charged ions that are suspended in the solution, as well as other chemical additives. The bath serves as the conductive medium during electrodeposition.
Acid baths tend to be used with greater frequency, because they result in a higher deposition rate. However, while acid baths generally provide a uniform coating, they do not always reach holes or other surface irregularities with a high level of consistency.
3. The Electrodeposition Procedure
Once the substrate has been clean and immersed in the electrolyte bath, it is ready for electrodeposition of the tin coating. The object is typically placed in the center of the specially designed tank that contains the electrolytic solution. The object serves as the cathode, which is the negatively charged electrode in the electrical circuit. The anodes, which are the positively charged electrodes in the circuit, are placed near the edge of the plating tank.
The next step is to introduce a low-voltage DC current into the plating bath. A device known as a rectifier is used to convert AC power to the DC current. The introduction of the electrical current ultimately causes positively charged ions at the anode to flow through the electrolyte in the plating bath toward the negatively charged cathode (substrate), where they are electrodeposited onto the surface. The current then flows back toward the anode to complete the circuit.
4. Post Electroplating Process
Post-treatment is usually not required at the conclusion of the tin plating process. Passivation — which is the application of a light coating of protective material — may be used in special plating applications to provide additional corrosion protection or enhance the reactive properties of the tin. Heat treatment may also be used to prevent hydrogen embrittlement, which is a weakening of metal caused by exposure to hydrogen.
Common Tin Plating Issues
A number of issues can occur during the tin plating process that can negatively impact the final outcome. These include:
- Tin “Whiskers”: Small, sharp protrusions known as whiskers can form on the surface of the pure-tin-plated objects long after the conclusion of the plating process. These microscopic metal fibers are not visible to the naked eye, but they can cause significant damage to the finished product.Because they are electrically conductive, whiskers can cause short circuits in electronic components. Tin whiskers have even been known to lead to the failure of computer systems and satellites, as well as disruptions in the operations of nuclear power plants.It is not known what causes tin whiskers to form, and there is no proven method for totally preventing their occurrence. There has been some success in limiting the formation of tin whiskers through the modification of the crystalline structure of the tin deposit, although this method is far from fool-proof.
- Lack of uniform thickness: In some cases, the tin may not be deposited uniformly on the plated object. Sometimes, the shape and contour of the object makes it difficult to achieve the desired thickness, which is typically in the range of ten to 20 microns.When plating on metal objects with sharp corners, the tin may be deposited with greater thickness on the outer corners and reduced thickness in recessed areas. This can often be remedied by repositioning the anodes and modifying the density of the DC current.
- Perishable solderability: While tin-plated metals are known for their excellent solderability, this characteristic will diminish over time. The soldering lifespan can be extended through proper deposit specification, appropriate substrate preparation and proper packaging of the plated components. Sealing plated products in nitrogen-filled bags has been known to result in a ten-fold increase of solderability shelf-life.
Tin Alloy Plating
One way to enhance the tin plating process is by co-depositing (alloying) tin with another metal or several metals. Commonly applied tin alloys include:
- Tin-lead: Provides corrosion resistance and excellent solderability and can produce a soft, ductile finish while helping prevent tin whiskers.
- Tin-copper: Improves the overall strength of the coating, but it may also make it more brittle. It can also lead to insufficient wetting for soldering applications and promote the development of tin whiskers.
- Lead-tin-copper: This combination is often used for friction reduction on sliding engine bearings.
- Tin-silver: Improves overall mechanical strength and increases maximum service temperatures, but the silver component can make this alloy cost-prohibitive for many companies.
- Tin-zinc: Offers a high melting point and superior fatigue strength, but results in poor wettability and limited protection against corrosion.
- Tin-bismuth: Ideally suited for low-temperature plating applications, this alloy also offers good wettability and can limit whisker formation. However, it may not be compatible with objects containing lead, and the low melting point makes it unsuitable for most high-temperature plating processes.
Consider a Tin-Lead Alloy for Tin Whisker Reduction
If tin whiskers are a concern in your manufacturing environment, then you should strongly consider a tin-lead plating alloy. Unlike pure tin, as previously mentioned, tin-lead can be effective at preventing formation of whiskers, making it an excellent choice in the manufacturing of electronic components such as printed circuit boards, connectors and semiconductors. Because both metals feature a high hydrogen overvoltage, the deposition of a tin-lead alloy can be produced via strong acid solutions without the addition of complexing agents.
In addition to whisker reduction, a tin-alloy can offer greater protection against corrosion than pure tin alone. Tin-lead also provides excellent solderability and can produce a softer, more ductile finish. Because of its ductility, tin-lead can prevent damage to the underlying metal during rigorous manufacturing processes such as stamping.
Sharretts Plating Can Handle All of Your Tinning Needs
Sharretts Plating Company has been in business since 1925. Over the course of nine decades, we have developed and perfected an effective and affordable tin plating process that can be customized to your specific operating requirements. In addition to pure tin plating, we also offer an innovative tin-lead plating process that can significantly reduce the formation of tin whiskers on your electronic components.
Contact us today to learn more about our tin plating process and how it can be tailored to your operation. We’ll also be happy to schedule a tin plating consultation and provide a free, no-obligation quote.
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