Silver Tarnish and Its Properties

Silver Tarnish (Left) vs No Tarnish (Right)

Silver plating is often used for cosmetic applications and is found on items such as silverware and jewelry. While silver provides value and an aesthetic appearance to these items, it is also used in multiple sub sectors of manufacturing – Power Transmission, Medical, Aerospace, Electronics, Electric Vehicle and many more. The reasons silver plating is used is vast: ductility, electrical and thermal conductivity, solderability, high temperature lubricity, as well as excellent optical reflectivity. Although there are many positive attributes to silver plating, silver tarnish is one is a common occurrence when the proper steps are not taken.

Silver tarnish occurs when the silver plating is exposed to air or water containing an oxidant and a source of sulfur. This chemical reaction is what forms the silver sulfide on the surface of the part and can turn the white luster finish to a yellow or sometimes a black or brown. Aside from an unpleasant appearance, the tarnish can continue to increase over time.  Although the cosmetic aspect of silver tarnish is not typically a desired outcome, silver sulfide is still conductive, which is different than other metal oxides that form over time.  However, industries typically desire a more pleasing cosmetic outcome and Advanced Plating Technologies can help avoid silver tarnish through post-plate processes as well as protective packaging.

Two commonly used silver plating specifications, ASTM B700 and AMS 2410, both contain call outs for supplementary anti-tarnish applications. It is crucial to include the instructions on a print or plating call out to ensure that a post-plate anti-tarnish application is provided. Advanced Plating Technologies has a wide vendor base that can locate any specific anti-tarnish application that a job may require. Additionally, packaging options can be put in place to help avoid silver tarnish. These packaging options can be as simple as including desiccant packs with the plated parts to nitrogen bagging. Nitrogen bagging fills the bag of parts with nitrogen and removes the oxygen which plays a key role in forming oxides.

While there are many ways to avoid silver tarnish, two good questions to ask before looking at anti-tarnish applications is how long the parts will be stored and what type of environment the parts will be stored in. By answering these questions APT can ensure that the proper post-plate tarnish application is selected.

Have Questions? Contact us at sales@advancedplatingtech.com for more information.

By: E. Probasco, Plant Manager

The post How to Avoid Silver Tarnish appeared first on Advanced Plating Technologies.

By E Probasco

Wire bonding is a method in which connections are made between components and/or the leads of a lead frame with extremely fine wires. The wires are typically aluminum or gold, but also include copper and silver. There are three common types of bonding, thermocompression, thermosonic, and ultrasonic. Thermocompression uses force, time, and heat to join metals together, while thermosonic uses force, times, heat, and sonics, and ultrasonic bonding uses force, time, and sonics. In any of these instances the most important aspect of bonding is to form a good metallurgical bond with the wire and the metal substrate.

There are many types of plating finishes that are popular for use in wire bonding applications. High purity gold plating such as Type III to ASTM B488 of Mil-G-45204 is a tried and true staple for wire bonding finishes. It offers long term reliability in most applications including telecommunications and automotive markets and is typically the preferred option in high reliability aerospace and defense applications. When used in aluminum wire bonding, gold-aluminum intermetallic may form during the bonding process which can negatively affect the integrity of the bond. This doesn’t mean that pure gold plating is a bad choice for wire bonding, it just means that it is very important to select the proper plating finish for the end wire bonding application.

High purity silver plating such as Type I silver per ASTM B700 is gaining popularity as a wire bonding finish. It has been used in both gold and aluminum wire bonding. However, an oxide/sulfide layer can form if the silver surface is not protected and packaged correctly, leading to issues bonding to the surface. This is due to the fact that silver will form sulfur compounds (tarnish) on the surface if exposed to sulfur-bearing materials such as common packaging materials including plastics and cardboard. Advanced Plating Technologies can help specify sulfur-free materials combined with sealed nitrogen filled packaging to complement silver for wire bonding applications.

No matter which type of finish or type of wire bonding is used, the following factors are important in the plating process.

• Purity of the deposit – Purity of the gold plating layer is extremely important in successful wire bonding, making sure the deposit is pure and that metallic impurities are under maximum allowable levels. This may seem intuitive in regards to bonding performance. However, it is just as important for the immediate underplate used prior to the gold or silver plating to be clean and free of impurities and large levels of co-deposited brighteners. Nickel is typically used as a diffusion barrier when gold or silver plating copper alloys commonly used in lead frames. APT typically recommends the use of a high purity sulfamate nickel as an excellent underplate for wire bonding applications.
• Final Rinsing – A very important factor in a successful wire bonding plating finish is the final rinsing of the parts. Good rinsing to remove any residual plating solution is extremely important as salts from plating solutions can dry on the surface of parts and cause issues with the bonding process. Also it is quite typical to use a hot deionized rinse as a final rinse. This leaves a very clean surface and doesn’t leave cationic and anionic contaminates that can exist in standard city supplied rinse water sources.
• Handling, packaging, and storage of plated product are often overlooked key factors to bonding success. Keeping parts clean and dry when removed from storage is crucial. If plated parts are going through a secondary machining/forming operation, it is important that parts are cleaned in such a manner that any oils or lubricants used in that process are not left on the surface of the plating, which would cause issues in the bonding process and make root cause analysis of where the issue is occurring more difficult. It is common practice to use wire bonding testing samples to accompany orders so that prior to any other post plating operations that may take place, wire bondability as plated can be verified. If there is an issue on the sample coupon then root cause of the failure can be localized to the plating. If they pass and then parts fail downstream after secondary operations, then the root cause can be focused on the handling and cleaning post plating. This practice can ultimately save costs associated with wasted added value operations and downtime.

Wire bonding plating finishes often causes complications within supply chain if not properly specified, processed, packaged and stored. However, as long as the supply chain is clear on the type of bonding application that is being used, agreed upon having the best finish (plating scheme) for that application, including best plating practices in regards to purity and cleanliness of the deposit, and every effort is taken to ensure the proper handling and storage of parts prior to the wire bonding operation, then a successful and robust process can easily be achieved.

The post Application Considerations For Gold and Silver Wire Bonding Finishes appeared first on Advanced Plating Technologies.

Silver plating offers a range of physical and electrical properties that make it extremely useful for industrial applications. Due to the current increases in the cost of other precious metals such as gold, palladium and rhodium, silver is a cost-effective alternative precious metal for various electrical, thermal and mechanical applications. For these reasons, silver has continued to grow as the finish of choice for various product designs and applications.

Silver has extremely high electrical and thermal conductivity – better than that of gold – which has solidified its use in high current power transmission and lower current power connector applications. Silver also is an excellent high temperature lubricant and can be utilized for various high temperature anti-galling applications within the bearing or fastener industries.

The use of silver as an interconnect and electronics finish has been catalyzed by the increased cost of gold and other precious metals. In addition, the reliability of RoHS compliant pure tins has become a design concern due to the risk of tin whisker formation. Silver offers a whisker-free alternative to tin that is fully RoHS compliant.

One of the perceived downsides of silver as a plating finish is its propensity to tarnish. Silver tarnish is not an oxide but is rather silver reacting to the atmosphere containing sulfur or hydrogen sulfide to forming silver sulfide or silver nitrate. The tarnish that forms is somewhat conductive and can be readily removed. In sliding contact applications, silver tarnish is removed by the inherent wiping action of the contact design.

In solderability applications, a light tarnish will not typically affect solderability, however in case where tarnish is greater; a more active flux can be chosen to overcome any potential issues. The use of post treatment anti-tarnishes and sealed nitrogen-filled packing can help extend shelf life and prevent tarnishing from occurring.

Silver plating services have gained popularity in the lead frame industry as well as in LED applications where silver’s optical reflectivity is utilized. Silver is anti-microbial and has been used in the medical, consumer, and water treatment industries for these natural germicidal properties.

Various types of silver plating services exist from bright, semi-bright, and matte finishes at varying purity levels up to 99.9%. Most specifications contain callouts denoting if the use of an under plate or anti-tarnish is required. The most common specifications called out for silver plating are QQ-S-365 and ASTM B700. In addition to these specifications, Advanced Plating Technologies provides silver plating services to fulfill the most demanding applications including aerospace specifications including AMS 2410, AMS 2411 and AMS 2412 as well as most company specific or corporate specifications.

The engineering properties of silver make it an outstanding finish at a much lower cost than many precious metal finishes. The perceived deleterious effects of tarnish can often be mitigated for many design applications making it an excellent choice for many design applications. A member of Advanced Plating Technologies technical sales staff can assist with designing a finish the meets the specific design requirements of your application.

Blog Authored by Eric Probasco. , Plant Manager

The post SILVER PLATING –An Engineering Finish Gaining in Popularity Within Various Industries appeared first on Advanced Plating Technologies.