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.
The table below provides a high-level overview of a few of the fundamental properties of tin, electrolytic nickel and common electroless nickel deposits:
Table 1: A Comparison of Tin and Nickel Properties
Property
Tin
Electrolytic Nickel
Medium Phosphorous Electroless Nickel
High Phosphorous Electroless Nickel
Electrical Resistivity [nΩ*m]
115
70
550
925
Percent Elongation [%]
49.50%
30.00%
0.5-1%
1-2.5%
Hardness [As Noted]
51 Hb
300 HV
600 HV
530 HV
Melting Point [°C]
232
1455
1000
800
Thermal Conductivity [W/(cm*K)]
0.67
0.91
0.05
0.08
When Tin Plating Is Preferred Over Nickel Plating
Tin is typically plated in the pure form as either an unbrightened process (matte tin) or a chemically brightened process (bright tin) in which organic brighteners are added to refine the grain structure of the metal resulting in a bright process. Tin is also commonly alloyed with lead for applications within the lead acid battery market, critical soldering applications or bearing applications. The primary advantages of tin include its ductility for applications that must flex or bend as well as for solderable applications. In addition, since tin is a soft metal, it is often preferred for lapping bus-bar connections to improve the long-term conductivity of the connection points. Tin is also resistant to galling and can be used as a conductive, metallic lubricant on sliding contacts or threads.
The Ductility Factor – Advantage Tin
Tin is an extremely ductile metal with a percent elongation of 49.5%; as shown in Figure 1, tin is near the top of the list in relative ductility of metals, especially of non-precious metals. This property makes tin ideal for conductive applications that must flex as well as for applications in which components are formed post-plate such as bus bars or fuse caps that have secondary forming operations after plating.
Being a soft metal, tin easily conforms to mating surfaces under compressive loads. This properly makes tin ideal for maximizing metal-on-metal contact in lapping connections such as bus bars, especially when plated to higher thicknesses. This property minimizes the airgap between the joined components which in-turn reduces oxidation and corrosion within the bolted joint. This can improve conductivity over time of tin plated bus bars as compared to nickel plated bus bars.
Soldering and Solderability – Advantage Tin
Soldering is a common method of joining electrical components. The basic process involves melting a metal solder onto the surface of two components. The solder flows or “wets” the two components and as it cools, forms a solid joint that has very good electrical and thermal conductivity. When soldering to tin plating the tin deposit melts and becomes part of the joint with the molten solder. The bond is made with the base metal or underplate beneath the tin. This contrasts with soldering to nickel plating in which the solder joins directly to the nickel without melting or flowing the deposit.
This difference makes nickel plating much more susceptible to dewetting due to oxides forming over time. The solderability of both tin and nickel deposits is perishable and reduces over time due to the natural oxidation of the deposits. Storage conditions greatly impact how quickly this degradation occurs. However, since tin melts to join with the solder, tin deposits solder more consistently as they age, especially with rosin-only (R) fluxes. Nickel deposits will typically require acid activated fluxes (RA or RMA fluxes) to remove the oxide layer as they age. Since acid activated fluxes leave a corrosive film on the surface, they are generally not preferred in most applications. Matte tin deposits are recommended for soldering applications since they are free of organic brighteners that can impede consistent wetting of the surface.
When Nickel Plating is Preferred over Tin Plating
High Hardness, High Melting Point and Diffusion Barrier – Advantages of Nickel Plating
One of the greatest advantages of nickel over tin is its improved hardness and high melting point. As noted in Table 1 above, electrolytic nickel has an average hardness of 300 HV and a melting point of 1455 °C. Electroless nickel has hardness of 600 HV with a melting point around 1000 °C. These properties make nickel a great finish for high temperature and high wear applications. Many switches, contacts, fuse stabs and terminal pins are nickel plated when they must endure high contact loads and pressures especially for switching applications with thousands of cycles.
The face centered cubic (FCC) structure of nickel it is superior to the body centered tetragonal (BCT) structure of tin at preventing solid state diffusion or migration of base elements. When tin is plated directly over brass, zinc readily migrates through solid state diffusion tin the tin deposit. This forms an intermetallic boundary layer that degrades the electrical performance of the tin over time. This reaction is accelerated with temperature making tin less preferred within higher operating temperature electrical applications.
Tin Whiskers and Tin Pest – Areas of Caution when Specifying Tin Plating
Tin also suffers from several unique degradation mechanisms including tin pest and tin whiskers. Tin pest is an autocatalytic phase change of tin from the white beta phase into a brittle grey alpha phase that is not conductive. This transition is slow to occur at normal temperatures but at very low temperatures of -30 °C, the transition can initiate and occur rapidly degrading the tin properties.
Tin whiskers form to relieve stress within the tin deposit. The whiskers are thin (2.5um diameter) conductive filaments of tin that grow outward from the surface and can extend up to 25mm from the surface2. Whiskers can grow within a period of months or may take years after plating. They are especially problematic in tightly packaged electronic applications in which whiskers can form short-circuits with a current carrying capacity of up to 10mA. The driving mechanism for whiskers of tin is to relieve stress within the deposit. As such, organically brightened tin deposits are more susceptible than matte tin deposits. To avoid tin whiskers the following steps can be taken:
Alloy the tin with lead, bismuth or antimony. Tin/lead deposits with 10% or more lead have been proven very effective at eliminating whiskers.
Anneal tin deposits after plating to reduce internal stress
Plating matte deposits in lieu of bright deposits of tin
Use a nickel underplate prior to tin plating
Increase the thickness of the tin deposit to avoid epitaxial effects of stress
Electroless Nickel Plating – Finish that Can be Engineered for an Application
One of the advantages of nickel plating over tin plating is the diversity coating properties that can be customized if electroless nickel plating is specified. Electroless nickel deposits are plated from a chemical reduction of nickel from a hypophosphite solution in which the deposit is an alloy of nickel and phosphorous. The percentage of phosphorous can range from low phosphorous deposits (1-6% P), to medium phosphorous deposits (6-10% P) to high phosphorous (11-14% P).
Since the mechanism for electroless nickel plating is a chemical reduction rather than an externally applied voltage, electroless nickel deposits plate very uniformly everywhere the part is wetted by the solution provided the solution can be continuously replenished (agitated) at the surface. The uniformity of electroless nickel is a fundamental advantage that allows for tighter manufacturing tolerances as well as heavier deposits that provide improved wear and corrosion resistance. Figure 4 provides an illustration of how electroless nickel plates very uniformly without buildup on edges or corners as occurs with electrolytic plating.
As shown in Table 1 above, the electrical and mechanical properties of electroless nickel plating vary as a function of the percentage of phosphorous within the deposit. Figure 5 is taken from Appendix X5.1 of ASTM B733 and further illustrates how the hardness, strength and even magnetism of nickel phosphorous coatings vary with increasing phosphorous content.
It is the ability to plate very uniform deposits and match the properties of the deposit to the application that make electroless nickel plating superior to traditional electrolytic nickel and tin in many applications. The cost of electroless nickel is higher than electrolytic plating with high and low phosphorous nickel being a higher price point than medium phosphorous electroless nickel.
Ultrasonic Welding (USW) – Advantage Nickel Plating over Tin Plating
Ultrasonic welding is a joining process that is growing in popularity in manufacturing specifically within the Electric Vehicle (EV) market. USW provides a mechanism to join dissimilar metals in a very reliable manner that reduces contact resistance over traditional crimped or lapped joints. In addition, USW provides a very durable joint that will not increase in contact resistance over time even when exposed to corrosive or thermal cycles. USW is especially popular for joining aluminum conductors to plated copper terminals to utilize the light weight of the aluminum conductors with the improved electrical performance of the plated copper terminals.
USW is performed by pressing two metals together in a mandrel and using ultrasonic generators to vibrate the metals against one another. The energy and pressure cause the metals to diffuse into one another at temperatures below the melting point of most metals. The low melting point of tin, however, is an issue for USW. During the process the tin will melt cooling the interface and preventing full diffusion from occurring. As such, any tin plated terminal must be selectively plated to avoid plating of the weld zone. This selective plating requirement can add significant cost especially for loose plating methods such as rack and barrel plating. This limitation of tin plating is a major disadvantage as compared to nickel plating of terminals or bus components that will be jointed using USW.
When Tin Plating over Nickel Plating is Preferred
Another idea to consider when designing for both corrosion resistance and solderability is to use both nickel and tin plating cooperatively. As noted above, a problem that occurs in tin plating involves the formation of an intermetallic of tin and copper or tin and zinc. The intermetallic layer grows and consumes tin available for soldering and also reduces the ability of the tin to uniformly flow or wet the surface when melting. A great way to prevent an intermetallic from forming is to use a nickel underplate as a barrier between the tin and the substrate. The Face Centered Cubic (FCC) structure of nickel is tightly packed and resists intermetallic diffusion. This prevents the formation of the intermetallic layer thereby improving and preserving solderability.
Another benefit to using both tin and nickel would be to improve corrosion resistance. Good corrosion resistance can be achieved by plating to 0.0005” or greater for a single layer of nickel or tin, however larger deposits may affect the tolerances of a part. Using a duplex or 2-layer system can help limit the porosity and improve the corrosion protection. The tin grain structure differs from the nickel which limits overall porosity to the substrate. This helps prevent corrosion and could lessen the overall thickness of plating required.
Conclusion: Tin Verses Nickel Plating
Nickel and Tin Plating have been used in metal finishing for nearly a century and will continue to be used as further advances are made in industries that require good corrosion protection, conductivity, and solderability. Tin plating provides excellent ductility and solderability for parts that are to be used in moderate temperature and low wear environments. The corrosion resistance and conductivity of nickel plating is best utilized in environments that experience high wear and high temperatures. If a plated component will be ultrasonically welded, nickel is preferred as it is compatible with the USW process without the need for costly selective plating. There are also times where having tin over nickel would improve a parts capability by preventing an intermetallic from forming and promoting overall corrosion performance with a duplex (2-layer) plating system.
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 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 Ryan Kliger, Estimating and Process Engineer with Technical Editing by Matt Lindstedt, President – Advanced Plating Technologies
