Gold has and continues to be a principle finish for electrical components especially with the continuing miniaturization of electronics. One of the primary benefits of gold plating services is a finish that is both conductive and receptive to soldering. When soldering gold plated components there are a variety of important considerations when specified the surface finish. The primary considerations are thickness, purity and the proper selection of an underplate.
Gold plating thickness is a critical, and often misunderstood, tenant of gold soldering. In gold soldering the physical bond is made between the underlying nickel layer and the solder itself, with the gold layer serving as barrier to help maintain the solderability of the nickel layer. Typical gold thickness for solderability is in the range of 10uin to 30uin as it provides adequate protection against oxidation to preserve wetting while keeping the cost of the finish as competitive as possible.
When soldering, gold dissolves into the solder through solid state diffusion. With heavier gold deposits, more gold alloys within the solder joint. In the diffusion process the gold reacts with the solder creating a gold intermetallic amalgam. If the gold in the solder exceeds 3% by mass, the solder joint can become embrittled causing joint failure, especially in dynamically or thermally stressed joints. The level of impurity and thickness of gold are directly related, thus thickness of the gold must be balanced between corrosion/oxidation protection, contact cycle life and solderability. (Soldering to Gold – A practical Guide).
Silver Plating of Copper or Copper Alloys – Silver Properties
Silver plating of copper or copper alloys is a highly functional finish for transferring heat and electricity utilized across a wide breath of industries. Silver has been applied since late 1800s onelectrical switchgear and other components that pass electrical current. In recent years silver plating of copper electronic components including connectors and terminals has grown rapidly within the electronic, automotive and electric vehicle (EV) markets. Silver plating has many unique properties that make it desirable for these applications. The primary reason is that silver has the highest electrical and thermal conductivity of any metal, which facilitates the efficient transmission of electricity and heat. In addition, silver is a relatively soft metal which allows the silver deposit to compress and form around a mating connecter filling small voids and micro-roughness. This increases the effective contact area resulting in less overall connector resistance.
Figure 1: Conductivity of Silver as Compared to Copper and Other Metals
Silver has excellent lubricity and resists galling in switching, sliding or rotary applications. However, high pressure wear surfaces such as blade-style stab connectors can be susceptible to silverwear. In applications such as this, a higher deposit thickness of silver is recommended as well as the use of a nickel or electroless nickel underplate. Thinner silver plating without a nickel underplate is best used on static joints or low duty cycle connectors that are mated and unmated relatively infrequently.
Stainless steel is an inherently corrosion resistant material, however when stainless steel is machined, formed or fabricated free iron can be introduced to the surface that can corrode independent of the base material. Proper passivation of stainless steel with an oxidizing acid such as nitric or citric acid removes this free iron and promotes the growth of a thin, dense protective oxide layer which maximizes the corrosion resistance of the stainless steel. Depending on the type of stainless steel and end application certain passivation processes may perform better at passivating than others. In this article we will compare nitric vs citric acid passivation which are the two primary chemistries specified in ASTM A967 and AMS 2700.
Nitric Acid Passivation
When comparing nitric vs citric passivation, the most common method used throughout industry is nitric acid passivation. The Nitric acid passivation processes was the original passivation processed specified in QQ-P-35, the first military specification covering passivation, revision A being released in the 1960s. Nitric acid passivation offers a range of options to customize the oxidizing potential of the acid to suit a specific grade of stainless steel. The various methods and types of nitric acid passivation include several heated options as well as options that include a sodium dichromate.
Proper specification of gold plating thickness for connector and contact applications is a key design consideration. Gold plating is an exceptional finish for connectors of that demand both high reliability and durability; however, the thickness of the gold plating will impact the durability and ultimate cycle life of the connector. Gold plated connectors have low contact resistance which is suitable for applications with low signal voltages and current in the millivolt and milliamp range. Because gold is a noble metal, it does not readily react with chemicals in most environments, meaning that gold plated connectors will retain their conductivity over time provide the thickness of the gold provides a sufficient barrier to the substrate from the environment.
Silver Plating of Stainless Steel – Silver Properties
Silver plating on stainless steel and other high temperature alloys such as Inconel®, Nitronic® and Hastelloy® is a common silver plating service for nuts, fasteners, slip-rings, thrust-washers, bushings and other bearing surfaces that benefit from the lubricity of silver at high temperatures allowing parts to exhibit anti-galling and anti-seizing properties. Silver is a unique precious metal that exhibits many desirable properties for utilization across a broad range of engineered applications. Of all metals, silver has the highest thermal conductivity, electrical conductivity, and optical reflectivity in the visible portion of the electromagnetic spectrum; silver has outstanding temperature resistance with a melting point of 962° C (1763° F). Additionally, silver is a soft, ductile metal with good embeddability that performs well under high torque and loads. Silver also provides excellent solderability and brazing characteristics for joining applications of stainless steel and other high temperature alloys. The unique combination of lubricity, high temperature resistance and thermal conductivity make silver plating on stainless steel and other high temperature alloys an outstanding combination for high temperature fastening or bearing applications where heat transfer high temperature lubricity are the principle design considerations.
Hard Gold Plating vs Soft Gold Plating – Which is Right for My Application?
When specifying gold plating services for an application, the question of hard gold plating versus soft gold plating is common design topic. Hard gold plating is a gold electrodeposit that has been alloyed with another element to alter the grain structure of the gold to achieve a harder deposit with a more refined grain structure. The most common alloying elements used in hard gold plating are cobalt, nickel or iron. Soft gold plating is the highest purity gold electrodeposit that essentially is pure gold without the addition of any alloying elements. Soft gold plating produces a more coarse grain structure that is free of any signficant codeposits.
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, Oil & Gas, Medical, Telecommunications, Aerospace, Electronics 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.
Powder coating is a surface finishing option that applies a relatively thin film to provide excellent corrosion protection and chemical resistance in a highly cosmetic manner. While parts are often designed with specific colors, gloss, and textures – the types of powder coating are often overlooked, yet a critical component to every powder coating job.
Powder coatings are applied in a variety of types. Each resin system has specific attributes that are able to better suit needs of specific environments. Some of the most popular types of powder coating include: Epoxy Powder Coatings; Polyester Powder Coatings; Hybrid Powder Coatings.
Electroless Nickel Phosphorus Content – Low, Medium & High
Electroless Nickel plating has become a very popular surface finish option offered by a wide range of suppliers, often with varying amounts of phosphorus content in the reducing agent. These variations are often referred to as Low Phosphorus, Medium Phosphorus, and High Phosphorus. Low Phosphorus usually has between 1-4% phosphorus in the chemical deposit, while Medium Phosphorus has between 5-9% phosphorus. Anything greater than 9% phosphorus is referred to as High Phosphorus. The variance of this phosphorus content in the Electroless Nickel plating deposit has significant impact on the mechanical properties of the deposit and the applications that deposit can be applied to. Continue reading →
In ordering a surface coating system to enhance the performance of an article in service, the use of standard finishing specifications is the established procedure employed by most manufacturing entities. The specifications most commonly referenced are ASTM, MIL Specs, AMS and unique corporate specifications.
Surface finishing specifications identify a number of parameters which evaluate the ability of the surface coating to perform its intended function. The most common coating requirements used to qualify a coating system are: Continue reading →