Electroplating is depositing a metal layer onto a base metal to achieve a final finish that meets a customer’s requirements. The thickness of the electroplated deposit is based on the engineers’ specifications. Some components require high thickness electroplating and others low thickness electroplating. The thickness specifications are decided based on desired part performance and function. Functions include electrical conductivity, corrosion resistance, thermal conductivity, cosmetic appearance, and durability. Electroplating can be performed on virtually any kind of metal with very few limitations. While most engineers use low thickness for their components, other engineers desire very heavy build plating for their specific applications. This article will discuss the measure of thickness for electroplating and some of the purposes and uses of high thickness electroplating.
Thickness Measurements
In the electroplating industry, thickness is measured on the scale of microinches. One microinch [µin] is equivalent to 1 millionth of an inch. In mathematical terms, it would be defined as .000001″. To measure the thickness of the deposit, an X-ray fluorescence (XRF) is typically used. The x-ray will precisely measure the amount of plating in millionths of an inch. Generally, plating thicknesses are requested in the range of 30 microinches to 500 microinches. Precious metals like Gold and Rhodium are most often requested at lower thicknesses because of the cost associated with precious metals. Specifications of 30 to 100 microinches are a typical request for finishes like Gold and Rhodium. Non-precious metals like Nickel and Copper are often requested at much higher thicknesses because of the low cost and other factors; discussed later in this article. To find the average thickness, very high thicknesses must be read using a micrometer pre and post-plating. This method is used when the XRF cannot read through the very high buildup. High electroplated thickness is considered anything over .001″ or one-thousandth of an inch (25µ).
Process Challenges
Applying a high plating thickness can be a slow process because of the difficulty and unique challenges associated with the heavy build-up of plating deposits. Electroless nickel has a constant plate rate of about ten microinches per minute, depending on the bath operating parameters and chemistries. Due to Electroless Nickel being an electroless process, meaning it does not need continuous power or anodes to plate, the deposit is generally uniform with some thickness variations along the edges. High thicknesses can be more problematic if not correctly engineered on electrolytic deposits, using anodes and cathodes with consistent current. The use of current can create high current density areas on the part. These areas are generally along the edges and ends of the part or any sharp features. In plating, using amps per square foot to determine the current is used. The part’s surface area is calculated, and the square footage of the part is multiplied by the pre-determined amps per square foot of the plating bath the technician is going to use. On high thickness electrolytic plating, the amps per square foot are often significantly reduced. The reduction of amps allows for a more even distribution of power and, in turn, a more even deposit of plating. The downside to this technique is that it takes significantly more time to achieve the thickness. Several hours to several days can often be the duration to achieve a thickness of multiple thousandths of an inch (0.001″-0.008″). In addition to the current reduction, often robber bars are introduced to “rob” the current and create high current density areas in areas away from the parts themselves. When a high build-up is requested, this creates a more even distribution of plating on the parts. Anode placement is also an essential factor when plating high thicknesses. The anodes are the part of the plating bath that distributes or “throws” the metal onto the part being plated, also called the cathode. The anode screens or bars must be evenly distributed outside the parts being plated. All of these methods are critical for high build-up plating that is evenly distributed.
Another unique challenge to high thicknesses is the formation of pits. The formation of pits occurs when a defect is found in the base plating layer, or there are small contaminants in the plating chemistry. As the plating thickness increases, the pit becomes more prominent and expands. A thorough cleaning of the part and filtering of the plating bath must be performed as well as a lowering of the amps per square foot to ensure a clean and uniform final finish. Cracking in thick finishes can also be an issue in parts plated with hard metals like Nickel. The flexibility of the part as well as the plating layer used must be taken into consideration to prevent cracking of thick deposits put under strain. The higher the deposit, the more stress the finish is put under. Stress-reducing chemicals are often added to the plating baths to help prevent cracking in thick deposits. The type of metal used in the application is essential based on the final use of the plated part. Soft metals like Silver, Tin, and Gold are more easily bent without cracking and are commonly used for high deposit plating at ProPlate®.
Benefits and Unique Uses
High thicknesses of electroplating can be deposited with hard metal or soft metal. In a hard metal deposit like Nickel, the application, as previously stated, is generally corrosion-resistant. Here at ProPlate, we have uniquely plated high thicknesses with precious metals like silver and soft gold. The specific purpose for these requests is often an engineered solution for a specific application. On some engineered parts, the use of the part is to act as a seal or a gasket. Often these components have threads that need a very tight seal which plating can offer when performed correctly. Soft materials such as Silver and soft Gold are used to plate the threads of these seals selectively. The deposited thicknesses as high as .008 (8,000 microinches) are used to create a gasket or a seal on the threads in order to seal the component for its final downstream assembly completely. This thick plating prevents liquids or gasses from escaping in high-pressure applications from the seals and provides corrosion protection from harsh environments.
The thickness of a plating finish greatly affects the life of the part. Galling is an example of an issue in high friction environments or applications with low lubrication. Galling is a process in which friction between two surfaces causes base material to degrade and wear out. A heavy build-up of plating on high friction areas can act as a barrier and prevent or minimize galling of the base material.
Carbon steel is another material that demonstrates the use and benefits of a high plating thickness. On a steel substrate, rust can be a problem. Rust has the ability to develop through even low Nickel-plated finishes. The higher the thickness of the electroplating deposit on a steel part, the less likely rust will develop. Also, high plated Electroless Nickel finishes will last very long and increase the part’s durability. A thick finish also protects the base material from physical damage as well as environmental damage. Water, salt, heat, and chemicals are a few examples of what high-thickness plating can protect the components from. A controlled experiment found that thickness under 0.0005″ would not protect aluminum from acids like hydrochloric and sulfuric even at low concentrations. A plating thickness of 500 (0.0005″) microinches and above had to be applied to prevent the undermining of the Nickel and allow the abrasive acid to attack the aluminum.
Silver-plated bipolar forceps are another plated solution of a heavy build-up Silver finish in which thermal conductivity and electrical conductivity are the desired outcome of the plating. The forceps are used in the medical field with a high electrical current application where the electricity is passed through the tip of the forceps. The high current is intended to coagulate or make solid tissue in surgical processes, and the high thermal conductivity of the Silver plating is what prevents tissue from sticking to the forceps during the procedure. The engineers desired 7000 to 8000 micro inches of Silver plating on only the tips of these forceps where the part would come in contact with the body, as this provides a greater thermal mass due to the large cross-sectional area of the Silver plating deposit.
Another example of a heavy-build-up plating application is ProPlate’s Vizi-Band® technology. Guidewires, stents, and catheter-based components are very commonly used in minimally invasive surgical procedures in the medical field. Often these devices are difficult to see under the x-ray during a medical procedure if not enhanced with RO markers. Gold and Platinum coatings have been highly visible during procedures as they have high density compared to common base materials such as Nitinol and stainless steel. For this reason, desired areas of the medical component are plated with a high build-up coating of pure Au or Pt to assist the physicians with RO visibility during surgeries for device location and orientation purposes. Often the gold build-up is between 250 to 900 microinches, which is a lower profile than traditional machined marker bands and can be applied as high as 8,000 microinches for unique applications. Platinum deposits are typically between 225-800 microinches and typically do not exceed 1,000 microinches due to it being a higher stress deposit than Gold. These marker bands are an innovative solution to the visibility issues associated with traditional marker bands used in catheter-based applications. These are just a few examples of the many uses for high thickness electroplating. Although challenging, the benefits are great and the possible applications for different engineered parts are endless.
- Electrical conductivity
- Corrosion resistance
- Thermal conductivity
- Cosmetic appearance
- Long term durability
- Surgical visibility
- AG/AU Biocompatibility
Learn more information about ProPlate’s Vizi-Band® technology
