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

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 sales@advancedplatingtech.com 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/.

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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.

Corrosion & Oxidation of Contacts or Terminals

Most gold connectors and contacts are not made from pure gold but are rather fabricated from a base metal such as C172 or C173 beryllium copper, C510 phosphor bronze, C360 leaded brass or austenitic stainless steel. This is for obvious cost reasons but also to provide a foundation material with the strength and spring temper that the design requires. Since these base materials are not precious metals, they are subject to corrosion which forms compounds on the surface such as oxides, sulfates or sulfides.

Since these compounds are not conductive metals, they interfere with the transfer of current or signals through the conductor. Corrosion and the formation of these compounds occur mainly when the base metal is exposed to the atmosphere or galvanic corrosive effects from contact with other metals.

Copper and Copper Alloys (e.g. C101 Oxygen Free Copper, C110 Copper, C172 or C173 Beryllium Copper, C510 Phosphor Bronze) corrodes when exposed to elements in the environment including oxygen and sulfur. Copper corrosion products such as cupric or cuprous oxide as well as copper carbonate, sulfide or sulfate can appear brown, blue or green (patina) depending on the compound formed and the pH of the environment.

The Statue of Liberty is a great example of the patina that is formed when copper is exposed to the acidic environment of New York’s acid rain. The compounds that form on copper becomes a protective film that slow the rate of future corrosion. The negative effect of this is that the copper compounds formed are not conductive and degrade the performance of the electrical contact or connector.

Brass (e.g C260 Brass, C360 Free Machining Brass) being an alloy of copper and zinc, oxidizes and corrodes very similarly to copper. It also corrodes because of the high amount of zinc that is alloyed with the copper. The addition of the zinc makes the brass less corrosion resistant than copper and more susceptible to both corrosion and loss of zinc or dezincification. In either scenario the brass will become less conductive and will not be a good electrical connector.

Stainless Steel (e.g. 304 or 316 Stainless Steel or 303 or 416 Free Machining Stainless Steel) is often thought of as being free from corrosion. However, stainless steels are very susceptible to attack from chlorides found in bleach or salt water and can form traditional rust readily in these environments.

In addition, in less aggressive environments, stainless steel naturally forms a thin nickel/chrome oxide on the surface that protects the steel from further oxidation. However, this film like all oxides is less conductive and will increase the contact resistance of the terminal or contact.

Underplate Role in Minimizing Corrosion of Gold Plated Contacts

Gold plating will not corrode but having a layer of gold on the parts will not stop the substrate from corroding or oxidizing. This is due to the fact that thin layers of plated metals have small pores which allow for corrosion to propagate through.

To minimize gold plating corrosion through the plating there are several design factors that must be taken into consideration. The first is the thickness of the plating, in most scenarios the thicker the gold plating the better the corrosion resistance will be. But the second and more important factor is the porosity of the plating.

Although the thickness of gold plating is key to making contacts have more corrosion protection, there are limitations to the gold thickness primarily due to cost. In addition a higher gold thickness, nickel plating is used to increase overall plating thickness and make the parts more corrosion resistant.  It is the combination of gold plating and nickel plating that can help make gold plated electrical contacts and connectors corrosion resistant.

Plating Porosity is a major factor in corrosion resistance of any gold plated parts. At thin thicknesses below 0.0005” per side, most plated metals have an intrinsic porosity that allows corrosive products to reach the substrate and likewise, corrosion products to propagate back to the surface.

Each plated layer has a unique porosity similar to a fingerprint, as more layers are added it is less likely for the pores to reach the base material.  This is due to the fact that pores are concentrated at the grain boundaries and each plated layer has a unique grains structure. Adding layers adds the complexity of the path from the substrate to the surface for a continuous pore to exist.

Figure 1: Example of Grain Structure in Copper- Pores Are Concentrated at the Grain Boundaries

A tangible example of this phenomena is pouring water through a screen.  The water will not pour thru the screen as fast as pouring water straight from a pitcher. Likewise, as more screens are added, the water will flow more slowly. The rate of the water flow through the screens can be thought of the rate at how quickly corrosion will occur in plated layers.

As the thickness of the plated layers are increased, the pore size and density in the screen becomes smaller and smaller. By plating thicker layers to reduce the pore size and adding more layers (screens) eventually, the water flow can be stopped. This condition is what is referred to as a pore-free barrier coating.

Nickel Plating is a common underplate for gold plated contacts because it is a hard metal that is load-bearing, which is useful when both the base material and the final plating layer are soft metals (such as a copper part and gold plating).

Nickel plating prevents parts from corroding by becoming a barrier between the substrate and the environment and it works by physically stopping the moisture and air from getting to the copper, brass, or steel. Nickel also can work as a diffusion barrier to prevent the copper from migrating through the gold to the surface which is undesirable since the copper could then oxidize or corrode on the surface.

ASTM B488, Standard Specification for Electrodeposited Coatings of Gold for Engineering Uses, summarizes the reasons for utilizing a nickel underplate in Appendix X6 which is provided in Figure 2 below:

Figure 2: ASTM B488 Appendix X6 Summarizes Reasons for Using Nickel as an Underplate to Gold

Copper Plating is an additional layer that can be added to gold and nickel plated contacts in order to increase the overall corrosion resistance. Figure 2 illustrates the porosity of a part that has a gold over nickel plating (top) and one with gold over nickel over a copper subplate (bottom). The reduction in pore density is significant which directly correlates to better corrosion resistance.

Figure 2: Pore Density of a Gold and Nickel Plated Contact with a Copper Underplate (lower plot) and Without a Copper Underplate (upper plot)

The reduction in porosity is due to the additional layer of plating pores that would need to line up in order for the moisture to reach the substrate. However, copper is not recommended to be the penultimate layer before the final gold since it can both oxidize through the gold as well as form a gold/copper eutectic through solid state diffusion. This will not only reduce the effective gold thickness but will also form a weak eutectic that can reduce adhesion and conductivity.

Gold Plating Specifics to Optimize Corrosion Resistance of Contacts

Gold Plating is the final deposit in connectors and contacts due to its outstanding electrical properties and noble properties. Because of gold’s inert nature it will not corrode or oxidize. However, the above noted underplate porosity and thickness characteristics need to be taken into consideration to ensure the gold plated contact is sufficiently robust for the intended application.

Duplex Gold Plating is a special type of plating gold that combines two different types of gold – soft gold (99.9% pure) and hard gold (99.0-99.7% pure). This is done to decrease the effective porosity in overall gold plated layers since the two gold layers have unique grain structures which do not align. Which gold layer should be the final or ultimate layer is a function of the part use as summarized below:

·        Static Connectors: Gold plated static connectors are connectors that, when installed, make a connection with something that is intended to stay in contact with it. A few examples of this are grounding nuts or studs, fixed contacts or solder pads. Due to the small amount of movement the hardness of the gold plating is not as important and soft gold is the preferred final or ultimate layer.

·        Dynamic Connectors: Gold plated connectors that make repeated connection with something are considered dynamic. A few examples of this are male/female pins/sockets and battery contacts. Since the wear of these connectors is of principle concern, hard gold is the preferred final or ultimate layer. As such, with most dynamic connector duplex gold applications, the hard gold should be the outermost layer.

Gold Thickness is a key characteristic in the corrosion resistance of gold plated connectors. Since the cost of gold plating is directly proportional to gold thickness, the target thickness should be carefully considered and evaluated.

Figure 4 below details four common connector thicknesses and common applications for each. This also illustrates how higher thickness correlates to improved corrosion performance. As noted in the table, a thin layer of gold is sufficient for static connections in controlled environments. However, as the wear resistance and/or corrosion requirements of the application increase, so must the gold thickness as well as the thickness of the respective underplates.

Figure 4 below: Thickness of Gold Plating Based on End Use and Environment

Conclusion

Gold plating offers many advantages for connectors and terminals including excellent electrical and thermal conductivity and reliable contact resistance for electronic applications and more. However, gold plated connectors need to be designed properly to meet the corrosion resistance of the intended application or the parts will not perform reliably.

All base materials used for making contacts or terminals will form compounds or oxides which can migrate to the surface of the gold plated layer increasing the contact resistance. By using multiple plated layers including copper, nickel, gold and even duplex gold plating the overall porosity of the deposit is decreased thereby increasing corrosion resistance.  Increasing the thickness of each of the plated layers helps reduce the pores in the plating of the parts which makes it harder for the atmosphere and moisture to get to the substrate.

The above information is provided as a general guide for engineering a finish for a specific contact or connector. There are many additional considerations specific to each plating application that are beyond the scope of this article.

Advanced Plating Technologies an ISO 9001:2015 gold plating company offers extensive surface engineering support for gold plating services or other applications. Reverse engineering of existing or failed applications and components is available to provide design assistance. Feel free to contact a member of APT’s technical sales team for further assistance at sales@advancedplatingtech.com or 414.271.8138.

Blog authored by Dominic Scardino, Estimating Engineer

References:

1. Reid, F. H., & Goldie, W. (1987). Gold Plating Technology (Reprint ed.). Amer Electroplaters Soc.
2. (2021, September 28). Protecting electrical connectors from water ingress with Nyogel 760G. Newgate Simms Tech Support. https://support.newgatesimms.com/protecting-electrical-connectors-from-water-ingress-with-nyogel-760g/
3. Ignition Switch, Repairing electrical parts, MGA. (2014). MGAGuru. http://mgaguru.com/mgtech/electric/et126.htm
4. (2018, June 11). How Much More Chromium Does D2 Need to be Stainless? Knife Steel Nerds. https://knifesteelnerds.com/2018/06/11/how-much-more-chromium-does-d2-need-to-be-stainless/
5. Grain size analysis in copper. (2019, July 26). Clemex. https://clemex.com/analysis/grain-size-analysis-2/

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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 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).

Purity

Purity of both the gold and underplate layers is critical to achieve the best solderability. For gold plating there is an importance to minimize organic impurities through proper tank maintenance. Organic impurities that are imparted into the plating layer can interfere with the soldering and can cause dewetting or voids in the solder.  Soft gold of 99.9% purity is typically the preferred gold for bonding or soldering applications.  However, nickel or cobalt hardened gold can solder well and provide improved wear resistance on contact surfaces.  However, the purity of the hard gold needs to be preserved through proper analytical work and tank maintenance.

Nickel purity is critical as this layer is the functional bonding layer. For nickel soldering the higher purity nickel the better the soldering. Often plating companies use an organically brightened nickel layer such as a brightened Watts or sulfate-based nickel to give a bright finish at the expense of solderability. Advanced Plating Technologies offers an engineered sulfamate nickel-plating layer recommended as an underplate for gold plating including soldering applications.  This nickel system is free of codeposited organics that can outgas or volatilize during soldering thereby causing voids in the solder joint.

Another common underplate for gold plating is electroless nickel. While there are many advantages to electroless nickel plating including hold tight tolerances, large deposit range, corrosion protection, lubricity – one issue is that phosphorus is deposited in conjunction with nickel on the surface. The phosphorous acts as an impurity in soldering and can impedes soldering. When specifying electroless nickel deposits a medium phosphorous electroless nickel can give you the balance of the positives of electroless nickel while preserving solderability.

Solder Process

When designing a solder process for gold plated parts it is important to remember that the solder joint forms between the solder and the nickel. Therefore, when performing multiple solder operations or reflow soldering, additional liquidous time must be allotted for to provide ample time for the solder to bond to the nickel.  In addition, typically a rosin-only flux is needed when soldering to gold plating.  However, for very thin gold deposits that are significantly aged, a rosin mildly activated (RMA) flux can help with removing any nickel oxides that may have propagated through the gold layer to the surface.

Conclusion

Soldering to gold plating is becoming more and more prevalent with the rise in high-end microelectronics. More than ever it is important to understand the process of soldering to gold and designing surface finishes to provide the most robust, cost effective finish. The proper gold finish must balance corrosion and wear performance with solderability to ensure the best possible design.  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 William Troske., Process & Estimating Engineer  

Edited by Matt Lindstedt, P.E. , President

References:

1. Ronald A. Bulwith ; Soldering to Gold – A Practical Guide

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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.

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Gold Plating of Micro Components

Gold Plated Interconnect Crimp Tubes

Gold plating of micro-components such as those used within the medical and interconnect industries presents unique plating challenges. Many micro-component surface treatments, whether being gold plated, silver plated or even passivated corrosion-resistant alloys require specialized process tooling and processing techniques to ensure plating finishes are uniform across all functional surfaces. The cleaning, processing, rinsing and drying processes must all be engineered around the characteristics of the micro component.

Prior to developing micro-compatible processes, plating techniques were limited to significantly larger components on a more macro-scale. Opportunities to expand services within the medical and interconnect industries was the business driver that pushed Advanced Plating Technologies (APT) to focus on process development for substantially smaller micro-components.

Specialized Techniques in Gold Plating

Our processes-specific development of cleaning, plating and rinsing techniques for micro parts came from our work within industries such as the medical industry. Numerous applications required our engineering group to search out new tools and develop methods and techniques to handle smaller and smaller parts. Most of the techniques we have developed have been from good old fashioned R&D coupled with some trial and error. We also have a very good cadre of preferred vendors and suppliers with technical resources that help amplify our own in-house resources. We often select our vendors based upon their technical resources and ability to help our company push techniques and processes further.

The challenges in gold plating micro-components often lies in the boundary-layer effect of fluids. Any fluid that comes into contact with a solid has a boundary layer in which the liquid wants to remain in contact and static with the solid surface. As the parts get smaller and smaller, there is a higher ratio of surface area to the overall volume of the part which results in a stronger boundary-layer effect per piece. In addition, many micro parts used in the medical and telecom parts are tubular components such as eyelet funnels or female interconnect sockets. These components act as micro-straws that want to hold solution at the surface rather than rinse it.

Micro Precision Passivated Eyelet Funnels

Plating processes employ numerous aqueous solutions including hot alkaline cleaners, various acids used for deoxidizing and activating as well as acidic, alkaline and cyanide-bearing plating solutions. Nearly all process solutions have specific gravity greater than 1.0 meaning that they are more viscous than water with an increased surface tension. This characteristic further amplifies the boundary layer effect within the processing solutions. If the preceding solutions are not adequately removed from the surface of the parts, it can cause failure in the subsequent step.

The engineering group at APT recognized that overcoming the boundary layer effect was key in developing successful plating processes for micro components. Use of ultrasonic generators, proprietary mechanical agitation techniques, thermal cycling during processing and extensive specialized hot deionized rinses are all integral to APT’s successful processing techniques. In addition, APT has invested heavily in inspection equipment required to inspect deposits such as gold plating of micro parts. X-ray spectroscopy with ultra-fine collimators that allow focusing of the x-ray beam as small as 0.004 inches are a good example of the type of equipment used in the inspection of gold plated micro parts.

Contact a member of our Technical Sales Team for more information on APT’s Gold Plating Services.

By M. Lindstedt

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By M.  Lindstedt

Gold Plating Services – Delicate and Micro Parts
Gold plating services within the medical, electronic or telecommunications markets often involves the application of gold deposits on very small or micro parts.  In these industries, there is an on-going emphasis on Swiss and micro machining of smaller and smaller components to meet design requirements.  In turn, the metal finishing supplier needs to be able process increasing smaller parts when processing these components. The technique of gold plating delicate and micro parts involves more than just the gold plating process.  The metal finisher needs to be able to clean, rinse, dry and inspect the deposits on these difficult components as well.  As such, the job shop needs to utilize not only the proper process method but have adequate line and inspection equipment to properly plate delicate and micro parts in gold.

Gold Plating – Ultrasonic Cleaning of Parts
Ultrasonic cleaning is a process that utilizes sound energy to create cavitation of microscopic bubbles on the surface of a part.  This mechanical energy combined with the thermal and chemical cleaning effect of ultrasonic cleaning solution makes for an extremely powerful cleaning tool.  Ultrasonic cleaning prior to gold plating of delicate parts – especially those with small ID holes and features – is critical to remove cutting oils, shop dirt and debris from the surface of the parts.  AP utilizes various ultrasonic cleaning techniques generated from both piezoelectric and magnetostrictive generators for a full range of cleaning applications.

Gold Plating – Proper Process Tooling
The process method and tooling needs to match the part to be gold plated.  Several methods to plate micro parts can be used:

• Traditional Barrel – A traditional plating barrel is essentially a horizontal chamber that rotates during plating.  The hole size (perforation) of the plating barrel needs to be small enough that parts will not be caught in the barrel wall.  Barrel perforations as small as 0.010” can be utilized for gold plating of very small diameter components.
• MPM Barrel – Minimum Part Movement barrel technology is a proprietary barrel plating technology which minimizes the ability of the parts to tumble and interlock over one another.   Advanced Plating Technologies helped pioneer this plating technology with Hardwood Line Manufacturing Co.  – the premier hardware manufacturer in the plating industry.  MPM allows for barrel plating of long/narrow pins and components often found in gold plating within the telecom or interconnect industries.
  

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• Vibratory Barrel – Vibratory plating originated in Switzerland with roots in the Swiss watch industry.   A vibratory barrel utilizes mechanical vibratory energy to move parts over an electrode bed during plating.  The vibratory energy also helps transfer plating solutions into the ID of small inner diameters during plating making this gold plating method an excellent choice for interconnect components such as female sockets where the functional area is in the ID of the part.

Gold Plating – Rinsing, Drying and Packaging Proper rinsing is a key tenat of gold plating services for applications in joinability including wire bonding, soldering and laser welding.  A sound gold deposit can fail to solder or bond if the surface is contaminated due to poor rinsing.  As such, proper rinsing of gold plated parts – especially small and micro parts – is a critical step in the process. Use of multiple high-quality water counter flow rinses, ultrasonic deionized water rinses and a final spray rinse of virgin deionized water are all techniques used by Advanced Plating Technologies to ensure the highest cleanliness of the gold plated deposit.  Drying in a clean, stainless steel oven in a controlled environment ensures that the residual water fully evaporates from the surface without contamination from the oven itself. Packaging of gold plated parts should be selected to protect the purity of the deposit during storage. Nitrogen bag packaging is a specialized packaging method in which the air within a heat sealed bag is extracted and back-filled with pure nitrogen prior to being sealed.  This packaging method protects the parts in an inert nitrogen atmosphere until which time they are used.  Sealed nitrogen packaging is the preferred packaging method by Advanced Plating Technologies as it prevents oxidation of underplates such as nickel from propagating to the surface of the gold deposit greatly extending the solderability or wire bonding shelf-life.  This is especially critical when dealing with thin gold deposits below 0.00003” per side where oxidation through the gold can occur more readily.

Gold Plating – Inspection Inspecting the thickness of a micro gold plated component with x-ray fluorescence per ASTM B568 requires an x-ray with a collimator of a small enough beam size to focus on the features of the part.  Advanced Plating Technologies utilizes x-rays manufactured by Oxford, Fischer and CMI with collimators as small as 0.004” to enable the inspection of extremely fine components.  Deposit thickness readings to the micro-inch level (0.000001 inch) are common and accurate with these tools.  Multilayer standards are also available in modern x-rays to inspect the thickness of multiple deposits such as gold plating over nickel without having to use coupons or pull parts after each step in the process.   Adhesion testing and inspection under magnification is often required when gold plating micro parts.  Commonly crimp and/or bake and quench tests per ASTM B571 are performed and inspected under 10-100x magnification to properly evaluate the deposit.  Without high-level magnification, minute failures or issues with the gold plated deposit can be missed and overlooked.

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