Electroplating is a type of metal electrodeposition process. It involves the discharge reduction of simple metal ions or complex ions via electrochemical methods on the surface of a solid (conductor or semiconductor), resulting in the adherence of metal atoms to the electrode surface to form a metallic layer.
Electroplating alters the surface properties of solids, thereby changing their appearance, enhancing corrosion resistance, wear resistance, and hardness, and imparting special optical, electrical, magnetic, and thermal surface properties.
III. Introduction to Terminal Electroplating
Most electronic connectors and terminals undergo surface treatment, typically electroplating, for two main reasons: first, to protect the terminal spring material from corrosion; second, to optimize the terminal surface performance, establishing and maintaining the contact interface between terminals, especially in terms of film layer control. In other words, to facilitate metal-to-metal contact.
Most connector springs are made of copper alloy, which can corrode in the operating environment, such as through oxidation and sulfidation. Terminal electroplating isolates the spring from the environment, preventing corrosion. The electroplating materials must be corrosion-resistant, at least within their application environment.
The optimization of terminal surface properties can be achieved in two ways. One is through connector design, establishing and maintaining a stable contact interface. The other is by establishing metallic contact, requiring that any surface film is either non-existent or ruptures upon insertion. The distinction between having no film and film rupture differentiates precious metal plating from non-precious metal plating.
Precious metal plating, such as gold, palladium, and their alloys, are inert and have no inherent film layer. Therefore, for these surface treatments, metallic contact is “automatic.” The consideration then becomes how to maintain the terminal surface’s nobility, protecting it from external factors like contamination, substrate diffusion, and terminal corrosion.
Non-precious metal plating, particularly tin and lead and their alloys, is covered with an oxide film that easily ruptures upon insertion, thereby establishing a metallic contact area.
(1) Precious Metal Terminal Plating
Precious metal terminal plating involves covering the underlying surface, typically nickel, with a precious metal. Standard connector plating thicknesses are 15-50 microns for gold and 50-100 microns for nickel. The most commonly used precious metals for plating are gold, palladium, and their alloys.
Gold is the ideal plating material due to its excellent conductivity and thermal properties, as well as its resistance to corrosion in any environment. Because of these advantages, gold plating is predominantly used in connectors for applications requiring high reliability, although gold is costly.
Palladium, also a precious metal, has higher resistance, lower thermal transfer, and less corrosion resistance compared to gold, but it offers superior wear resistance. Palladium-nickel alloys (80-20 ratio) are generally used in the terminal posts of connectors.
When designing precious metal plating, the following factors need to be considered:
During the plating process, gold nucleates on numerous exposed surface contaminants. These nuclei continue to grow and spread across the surface, eventually colliding and completely covering the surface, resulting in a porous plating.
The porosity of a gold layer is related to its thickness. Below 15 microns, porosity increases rapidly, whereas above 50 microns, porosity remains low and the rate of decrease is negligible. This explains why the thickness of precious metal plating is typically within the 15-50 micron range.
Porosity is also related to substrate defects such as inclusions, lamination, stamp marks, improper cleaning after stamping, and incorrect lubrication.
Wear on the plated surface can lead to exposure of the base material. The wear or longevity of the plated surface depends on two surface treatment characteristics: the coefficient of friction and hardness. As hardness increases and the coefficient of friction decreases, the lifespan of the surface treatment improves.
Electroplated gold is usually hard gold, containing hardening activators such as cobalt (Co), the most common hardening agent, which enhances the wear resistance of gold. Choosing palladium-nickel plating can significantly improve the wear resistance and longevity of the precious metal coating.
Typically, a 3-micron layer of gold is applied over a 20-30 micron palladium-nickel alloy, providing both good conductivity and high durability. Furthermore, a nickel underlayer is often used to further extend the life.
c. Nickel Underlayer
The nickel underlayer is a primary consideration in precious metal plating, providing several important functions to ensure the integrity of the terminal contact interface.
By offering an effective barrier layer through a positively oxidized surface, nickel reduces the potential for porosity corrosion and provides a hard support layer beneath the precious metal plating, thus enhancing the lifespan of the plating. What is the suitable thickness for this layer?
The thicker the nickel underlayer, the lower the wear, but in terms of cost and controlling surface roughness, a thickness of 50-100 microns is generally chosen.
(2) Non-Precious Metal Plating
Non-precious metal plating differs from that of precious metals as it always involves a certain number of surface film layers. For connectors, which aim to provide and maintain a metallic contact interface, the presence of these films must be taken into account.
Typically, for non-precious metal coatings, a high contact force is required to break the film, thereby ensuring the integrity of the terminal contact interface. The scrubbing action is also crucial for terminal surfaces with film layers.
There are three types of non-gold surface treatments in terminal plating: tin (tin-lead alloy), silver, and nickel. Tin is the most commonly used, silver excels in high-current applications, and nickel is reserved for high-temperature environments.
a. Tin Surface Treatment
Tin also refers to tin-lead alloys, especially the tin 93-lead 3 alloy.
The use of tin surface treatment stems from the fact that tin’s oxide film is easily disrupted. A tin coating will have a layer of hard, thin, and brittle oxide film on its surface. Underneath the oxide layer is soft tin. When a positive force acts on the film, the oxide of tin, being thin, cannot withstand the load and, due to its brittleness, cracks easily.
Under such conditions, the load is transferred to the tin layer, which, being soft and malleable, flows easily under pressure. As the tin flows, the cracks in the oxide widen. Through these cracks and the interlayer, tin squeezes out to the surface, providing metallic contact. In tin-lead alloys, the role of lead is to reduce the formation of tin whiskers.
Tin whiskers form as monocrystalline strands on the surface of plated tin under stress, potentially causing short circuits between terminals. Adding 2% or more lead can reduce the formation of whiskers. Another common tin-lead alloy ratio is 60:40, similar to the solder composition ratio (63:37), mainly used in connectors that require soldering.
However, recent legislation increasingly demands the reduction of lead content in electronic and electrical products, leading to a rise in demand for lead-free platings, such as pure tin, tin/copper, and tin/silver. The growth of tin whiskers can be slowed by plating a nickel layer between the copper and tin layers or using a matte, non-glossy tin surface.
b. Silver Surface Plating
Silver is considered a non-precious metal surface treatment because it reacts with sulfur and chlorine to form a sulfide film. This sulfide film acts as a semiconductor and can exhibit diode-like characteristics.
Silver is also soft, comparable to soft gold. Since the sulfide is not easily disrupted, silver is free from fretting corrosion. With excellent electrical and thermal conductivity, silver does not melt under high currents, making it an excellent choice for high-current terminal surface treatments.
(3) Terminal Lubrication
Lubrication plays different roles depending on the terminal surface treatment, serving primarily two functions: reducing the coefficient of friction and providing environmental isolation.
Reducing the coefficient of friction has two benefits: firstly, it lowers the insertion force required for connectors; secondly, it extends the connector lifespan by minimizing wear. Terminal lubrication can form a protective layer that prevents or slows down environmental degradation of the contact interface, thus providing isolation.
Typically, for precious metal surface treatments, lubrication is used to reduce friction and enhance connector longevity. In the case of tin surface treatments, it provides environmental isolation to prevent fretting corrosion. Although lubricants can be applied in the post-plating process, it’s merely an additional measure.
For connectors that need to be soldered onto PCB boards, the soldering cleaning process may remove the lubricant. Lubricants attract dust, which, in a dusty environment, can lead to increased electrical resistance and reduced lifespan. Lastly, the temperature resistance of lubricants may also limit their application.
(4) Summary of Terminal Surface Treatments
Precious metal plating is assumed to be layered over a 50-micron nickel base. Gold is the most common material, with thickness varying according to lifespan requirements, yet it is susceptible to porosity issues.
Palladium is not recommended for situations requiring solderability protection. Silver is sensitive to tarnishing and migration, mainly used in power connectors, but its lifespan can be significantly improved through lubrication. Tin boasts excellent environmental stability, but mechanical stability must be ensured.
IV. The Ten Ironclad Rules of Tin Plating for Terminals
Tin or tin alloy materials are among the finest choices for terminal electroplating, offering a cost-effective solution with low contact resistance and excellent solderability. These materials meet the performance requirements of various applications, making them an ideal substitute for gold and other precious metals in plating.
Below are ten steadfast rules, although as new applications emerge, more principles await discovery.
- When using tin-plated materials, ensure that the male and female terminals maintain good mechanical stability once connected.
Avoid using tin-plated terminals in vibrating environments. Differential thermal expansion (DTE) among terminal metals can lead to fretting corrosion within a range of 10 to 200 micrometers, damaging the plating, exposing the base material, and significantly increasing contact resistance due to oxidation.
- To maintain stable contact between tin-plated terminals, apply a minimum force of 100 grams in the axial direction.
- Tin-plated terminals require lubrication.
Following the second rule, with increased axial pressure, proper lubrication becomes necessary to reduce friction. Both male and female terminals should be lubricated, or at the very least, one end.
- Tin-plated materials are not recommended for continuous high-temperature environments.
High temperatures accelerate the formation of intermetallic compounds between copper and tin, resulting in brittle and hard interlayers that affect functionality. A nickel plating layer is recommended as an intermediary since nickel-tin intermetallic compounds grow more slowly.
- Various tin-plating processes have minimal impact on electrical performance.
Bright tin plating is aesthetically pleasing; matte tin must maintain a clean surface to not affect solderability. Brass tin plating should include a nickel underlayer to prevent the loss of zinc from the base material, which would degrade solderability.
- Aim for a tin plating thickness between 100 and 300 microinches.
Thicknesses below 100 microinches are typically used for cost-sensitive products with lower solderability requirements.
- The combination of tin-plated and gold-plated terminals is not recommended.
This practice leads to increased oxidation and corrosion. Tin migrates to the gold surface, eventually causing an accumulation of tin oxides on the harder gold substrate. It is harder to disrupt tin oxide on gold than to penetrate the oxide layer directly on tin. However, the fretting corrosion between tin-plated and silver-plated is similar to that of terminals plated with tin on both ends.
- It is advisable to mate tin-plated terminals two or three times initially.
This procedure removes the oxide layer on the tin plating, ensuring reliable metal-to-metal contact. This is also recommended for ZIF (Zero Insertion Force) terminals.
- Tin or tin alloy plated terminals are not suitable for applications with frequent circuit interruptions.
Due to the low melting point of tin, it is not advisable to use these materials in situations prone to arcing, such as contact points.
- Tin-plated terminals are best used in dry circuits and applications with moderate requirements.
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