Industrial Gold Plating – What You Need to Know

 

Industrial Gold Plating Overview

Hard Gold Plating on interconnect pins to provide durable, conductive parts

Industrial gold plating is utilized across a range of industries for both the conductive and noble properties that gold provides. Gold is the third most conductive metal behind silver and copper but unlike these metals, gold’s contact resistance is extremely consistent since gold does not oxidize or tarnish. This makes gold an excellent choice for low-voltage applications such as signal transmission when small changes in resistance can be problematic. In addition, gold provides excellent barrier corrosion resistance in many industrial applications since it does not corrode.

Functional industrial gold plating is plated as soft gold with 99.9% purity (Type III) as well as hardened 99.0-99.7% pure (Type I or II) deposits alloyed with nickel or cobalt.   Unlike decorative gold applications which often use a gold flash of less than 0.00001” inches (0.25um), industrial gold deposits are normally thicker to provide improved function and durability.  Industrial gold is commonly plated in thickness typically ranging between 0.00001 inches (0.25 micrometers) to 0.0001 inches (2.5 micrometers) and is commonly plated over underlayers of nickel and/or copper.

Table 1: Common Gold Plating Thickness for Functional Gold Use

Common Thickness of Gold Relevant ASTM B488 Class Relevant MIL-G-45204 Class Applications
10µin

0.25µin

Class 0.25 N/A Suitable for static connections in controlled environments without cyclical use. Good for solderability and wire bonding (10-20µin)
30µin

0.75µin

Class 0.75 Class 0 Good for connections that may be soldered. Contacts can be exposed to moderate environments and wear cycles but not high cycle or chemical attack.
50µin

1.25µin

Class 1.25 Class 1 Suitable for connections that may be soldered, exposed to more aggressive environments, offers superior protection against wear in moderate to high cycles.
100µin

2.5µin

Class 2.5 Class 2 Not recommended for connections that may be soldered, offers excellent protection against corrosive environments and durability high wear applications

Considerations when Specifying Industrial Gold Plating – Gold Purity and Hardness

Industrial gold plating is divided into two primary categories – hard and soft gold plating – based on the hardness of the deposit.  Soft gold is the highest purity (99.9% minimum) and is used where the noble properties of gold take priority over wear resistance.  Soft gold lives up to its namesake with a maximum hardness of 90 Knoop; whereas hard gold plating has reduced purity (99.0-99.7% Pure) but can have hardnesses of up to 200 Knoop due to the addition of alloying elements such as nickel and cobalt.  Common industrial uses for soft gold include wire bonding or soldering of electronics, lapping electrical contacts with low contact pressures (< 10 Pa), infrared reflectors and high corrosion resistance contacts or electrodes.

Soft Gold Plating for medical applications due to Gold’s biocompatibility

Soft gold is deposited without any alloying elements, allowing for the natural grain structure of gold, alongside gold’s natural low porosity, to provide a corrosion resistant barrier coating over the substrate. Soft gold is often seen in connector applications that are designed to remain connected for long periods of time.  Soft Gold is also an excellent reflector of near-infrared (NIR) and infrared (IR) radiation and making it an excellent choice for thermal shielding in extremely low temperature quantum computing applications. The properties of soft and hard gold can also be combined into a “duplex” coating, where the hard gold outer layer provides wear resistance, while the soft gold underplate increases the corrosion resistance considerably due to the offsetting of pores between the hard and soft gold plating layers.

Figure 1: Visualization of Grain Structure for Barrier Corrosion Protection

Hard Gold Plating for a conductive, rotational application

Hard gold is not truly hard in terms of other metals such as nickel or titanium; however, its hardness is relative compared to the very soft nature of pure gold.  The increase in hardness of hard gold affords greatly improved wear resistance where sliding wear or repeated contact can abrade gold from the surface.  Hard gold is commonly used in electrical terminals and contacts where higher contact pressures (> 10 Pa) and repeated engagement occurs such as male/female connectors or pogo pin contacts.  In addition, hard gold plating provides a lubricious surface that is not prone to fretting and resists galling. Hard gold can be utilized for joining applications such as soldering even with a mild rosin-only flux; however hard gold is not recommended for wire bonding due to its alloying elements.

 

Figure 2: Impact of Nickel Underplate on Hard Gold for Wear Protection at Different Contact Pressures and Underplate Thicknesses

Comparison of Industrial Gold Plating to Other Conductive Finishes

In industrial uses, gold plating is often compared to other plated conductive finishes, most commonly, silver, copper and tin. Silver plating, like gold, is often used in connector applications.  Like gold, industrial silver plating provides excellent conductivity and lubricity – even at high temperatures.  However, silver readily reacts with sulfur forming a blue or black tarnish of silver sulfide. Although silver sulfide is relatively conductive, it still will increase contact resistance which can be problematic in sensitive low-voltage or signal transmission applications.  While numerous anti-tarnish treatments exist, they only mitigate and do not eliminate silver tarnish from forming.  Due to gold’s nonreactive nature, it will never tarnish or corrode, which makes gold desirable for applications sensitive to slight changes in contact resistance.

Copper plating, like silver and gold plating, is a highly conductive metal.  In fact, copper is second only to silver in conductivity.  Since copper is not a precious metal, it can be plated to a much higher thickness than silver and gold without as great of a cost impact.  This is very beneficial for promoting corrosion resistance when used as an underplate since copper is a relatively noble metal that provides very good barrier corrosion protection. The downside of copper as a final finish is that copper reacts readily with sulfur, oxygen or even chlorine to form copper oxide (CuO or Cu2O), copper sulfide (CuS), copper sulfate (CuSO4) or various halides.  Any of these compounds have a major impact on the conductivity of copper and will increase contact resistance and create hot-spots in conductors that can lead to thermal run-away events (fires). For this reason, copper plating is typically used as a “supporting character” in conductive finish stack-ups to help promote overall conductivity and corrosion resistance with the ultimate or topcoat being an alternative finish such as gold, silver, nickel or tin.

Tin plating is often referred to as poor-man’s silver since it provides many of the similar properties as silver but just not as well.  Tin has reduced conductivity and corrosion resistance over silver or gold plating, but it performs adequately for many industrial applications.  Matte tin plating provides good solderability but unlike industrial gold plating, the solderability of tin degrades over time due to the formation of tin oxides. However, tin does maintain solderability longer than nickel plating and the solderability of tin can be extended with proper packaging techniques.

Unlike gold plating, tin has a low melting point and as such, the service environment should be considered; nickel is preferred for a conductive finish in higher temperature applications where a precious metal is not needed.  Tin or nickel plating are most commonly used for plating of larger conductors such as bus bars and connector terminals where the application has some forgiveness for contact resistance increasing over time. By comparison industrial gold plating is used for critical conductor applications where increases in contact resistance over time pose a design concern.

There are many considerations to account for when specifying a finish for a component. The technical sales and engineering staff at Advanced Plating Technologies (APT) can help with specifying the right plating or plating stack-up for your tin or nickel plating application.  APT has over 75 years of experience plating tin and nickel across a range of industries and can assist with proper test plans and packaging methods to ensure deposit properties are maintained and protected.

A member of our engineering group can be contacted at [email protected] or 414.271.8138.

Blog Authored by Zach Hatseras, Estimating Engineer; Technical Editing by Matt Lindstedt, President – Advanced Plating Technologies

References:

  • Bulwith, Ronald A “Soldering to Gold- A Practical Guide” chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://advancedplatingtech.com/wp-content/uploads/2016/04/Soldering-to-Gold-A-Practical-Guide.pdf
  • Dr. Bob Mroczkowski Founder at connNtext associates Dr. Bob Mroczkowski was one of the connector world’s most significant innovators and educators. “Connector Degradation Mechanisms-Corrosion Part II.” Connector and Cable Assembly Supplier, 31 Oct. 2019, connectorsupplier.com/connector-degradation-mechanisms-corrosion-part-ii/.
  • Scardinio, Dominic. “How to Prevent Corrosion of Gold Plated Contacts or Terminals.” Advanced Plating Technologies, 21 July 2023, advancedplatingtech.com/blog/prevent-corrosion-of-gold-plated-contacts-terminals/.
  • Zednicek, Antonin. “Nickel Underplates and Noble Metal Finish Wear.” Passive Components Blog, 7 Sept. 2021, passive-components.eu/nickel-underplates-and-noble-metal-finish-wear/.

Tin vs Nickel Plating: Choosing the Right Finish for My Electrical Application

Similarities and Differences Between Nickel and Tin Plating

Tin and nickel plating are both conductive finishes that are typically used in electrical or power applications such as plating of copper or aluminum terminals and bus bars.  Both tin and nickel provide improved corrosion protection, conductivity and are used to facilitate common joining methods such as soldering, brazing or ultrasonic welding.  From a material property standpoint, tin is a much softer, more ductile metal than nickel.  These core elemental differences separate the two metals functionally and differentiate when each should be specified.  In addition, tin and nickel are each plated with different variants including matte or bright tin, sulfate (Watts) or sulfamate electrolytic nickel as well as an entire family of alloy processes such as tin/lead or electroless nickel deposits of nickel phosphorous or nickel boron.

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Tin Plating of Copper Busbars – What You Need to Know

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.

tin plating of copper busbars 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. Continue reading

Advantages of Powder Coating Over Wet Spray Painting-What You Need to Know

Protection of metal fabricated components is critical to the long-term durability of the final product. Surface finishing through powder coating or wet painting are both common solutions to protect various components. Both powder coating and wet paint systems contain similar resins, additives, and pigments; however, there are marked differences between these two painting systems. Most notably, wet paint systems require the use of solvents to suspend the mixture in the fluid form.

Powder coating is applied as a dry powder without solvents, however, the powder paint must be baked or cured on the surface of the part. This primary difference in application method results in several advantages and disadvantages between these two coating options. Continue reading

How to Prevent Corrosion of Gold Plated Contacts or Terminals

One of the greatest shortcomings for the longevity of copper, brass, or even stainless steel contacts is corrosion of the base material. This issue is a greater problem in harsh or extended duty contacts which is why gold plating is preferred for these applications.

Gold plating deposits a noble gold layer that will not corrode or form oxides or compounds even at elevated temperatures or when exposed to highly corrosive environments. An added benefit is that gold is a better electrical and thermal conductor than the many of the base materials contacts are fabricated from. Continue reading

A Discussion of Metal Finishing & Environmental Stewardship

Metal Finishing Environmental Stewardship Benefits More Than Just the Environment

Advanced Plating Technologies has always taken a clear stance on the environment.  Since the beginning of the clean water act in 1972, APT has been committed to not only meeting but exceeding all local, state and federal environmental laws.  This record speaks a silent assurance that many “low cost” metal finishing suppliers cannot.  In fact, metal finishing companies with substandard environmental systems is a liability not only for their continued operation but for potential litigation down the road against themselves and their customers.

Surface Finishing Waste Treatment Facility

APT’s Modern Waste Treatment Facility

The metal finishing industry continues to be one of the most highly regulated industries in the country with new regulations and requirements added each year.  New regulations on PFOA and PFOS compounds and additions to the Toxic Substances Control Act (TSCA) are just a few of the latest impending regulatory restrictions. For this reason, APT continues to invest in improved treatment technologies and self-imposed compliance standards below permit levels as an example within the Industry.  APT’s leadership has been recognized with numerous local, state and national environmental awards from the Milwaukee Metropolitan Sewage District (MMSC), Wisconsin State Department of Natural Resources (DNR) and the United States Environmental Protection Agency (EPA).

The history of environmental compliance within the metal finishing industry is marked by various landmark laws that have far-reaching implications counter to what many would consider sound environmental practices.  In 1980 the Resource Conservation and Recovery Act (RCRA) mandated that the F006 wastewater treatment sludge produced by electroplaters shall be listed as hazardous waste categorically. This designation was applied without testing the waste product for its actual chemically hazardous profile using the TCLP methodology.  Thus, the applied arbitrary designation limits the ability of the waste to be recycled to recover the valuable metals contained therein. Since then, this legislation has come under increased criticism from many in the industry as it results in the waste of thousands of tons of valuable metals each year. Continue reading

Benefits of Tacti-black® Electroless Nickel Over Traditional Firearm Finishes

When it comes to selecting a coating for your firearm or firearm components there are several key features you need to consider. When going through options you need to ensure your firearm finish can meet several key characteristics such as the ability to hold tight tolerances, provide enhanced lubricity, wear resistance, corrosion protection, and, of course, provide a uniform and consistent black appearance.

Advanced Plating Technologies has worked hand-in-hand with numerous firearm OEMs throughout the years to provide a superior black finish that meets all the above requirements. Tacti-black® HP+ was developed in response to feedback received from numerous firearm OEMs that needed a finish that could meet all of the above requirements on a range of materials including steel, stainless steel, and aluminum.


Why Choose Tacti-Black®?

 

A Firearm Finish for Any Base Material tactical black firearm finish for Lower Receiver

Unlike common firearm coatings such as black oxide, nitriding, anodizing, and QPQ, Tacti-black® HP+ is “material blind” meaning it can be plated onto any metal material. This tactical firearm finish is perfect for a large array of firearm components like custom lower receivers, trigger sears, hammers, and stamped magazines.

APT can provide this proprietary tactical firearm finish to most any metallic substrate from CNC firearm components to 3D printed and MIM alloy components with a density of over 90%.


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Solderable Gold Plating of Electrical Contacts

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.

 

Plating Thickness

gold plating

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).

 

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Nitric vs Citric Acid Passivation

Nitric vs Citric Passivation Methods

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

Passivation of stainless steel micro parts

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.

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How to Avoid Silver Tarnish

Silver Tarnish and Its Properties

Silver Tarnish vs No Tarnish

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.

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