How Do You Design an Electrical Contact Rivet That Performs?

You're designing an electrical switching device—a relay, a contactor, or a wall switch. The contact is where the circuit makes or breaks, and its geometry determines whether the device lasts for 100,000 operations or fails after 10,000. An electrical contact rivet design isn't just about picking a shape from a catalog. It's about matching five key dimensions—head diameter, head thickness, shank diameter, shank length, and sphere radius—to your specific application requirements. It's also about choosing the right structure: solid, bimetal, or trimetal. And it's about selecting the right material for your switching environment. This guide walks through each parameter, explains how they interact, and provides a practical framework for selecting the right contact rivet for your switching device. Whether you're designing a new product or optimizing an existing one, understanding these fundamentals will help you make better design decisions.

Head, Shank, and Radius – The Three Core Parameters of Every Contact Rivet

A contact rivet consists of three structural elements: the head, the shank, and the sphere radius at the top of the head. The head is the contact surface that makes and breaks the electrical circuit. The shank is the portion that fits into the terminal or component, providing mechanical attachment and electrical connection. The sphere radius at the top of the head determines the contact area and influences arc behavior during switching.

Head Diameter (D) – The Contact Surface

The head diameter determines the size of the contact surface. A larger head provides more area for current flow, reducing contact resistance and heat generation. However, a larger head also requires more space within the switching device and may increase material cost. The relationship between head diameter and current rating is straightforward: higher currents require larger head diameters to keep contact resistance and temperature rise within acceptable limits. The standard series for head diameter ranges from 2.0mm to 10.0mm.

Head Thickness (T) – Structural Integrity

The head thickness affects both mechanical strength and thermal performance. A thicker head provides more material to dissipate heat generated during switching and resists deformation under mechanical impact. However, excessive thickness adds material cost and may interfere with the mechanical clearance within the switching device. The standard thickness ranges from 0.5mm to 2.0mm, with specific values paired to each head diameter.

Shank Diameter (d) and Length (L) – The Mechanical Anchor

The shank diameter determines the mechanical strength of the rivet and the integrity of the crimp or weld joint. A shank that is too small may deform under mechanical stress or fail to provide adequate current-carrying capacity. A shank that is too large may crack the terminal during riveting. The shank length must be sufficient to pass through the terminal and provide material for the rivet head to form, but not so long that it interferes with other components. The standard shank diameter ranges from 1.0mm to 5.0mm, with shank lengths from 0.8mm to 5.0mm.

Sphere Radius (SR) – Contact Geometry and Arc Behavior

The sphere radius at the top of the head determines the initial contact area and influences arc behavior during switching. A larger radius provides a flatter contact surface, increasing the contact area and reducing contact resistance. A smaller radius concentrates the contact force into a smaller area, which can improve contact reliability in low-force applications but may increase wear. The standard sphere radius ranges from 5mm to 40mm, with specific values paired to each head diameter.

D and T – How Head Diameter and Thickness Define Contact Performance

The relationship between head diameter (D) and head thickness (T) is the first decision in contact design. Larger diameters require greater thickness to maintain structural integrity under mechanical impact. The standard pairing of D and T values reflects this relationship: for a 2.0mm head diameter, thickness options are 0.5mm and 0.7mm; for a 10.0mm head diameter, the thickness is 2.0mm.

How to Choose Between Thickness Options

When multiple thickness options are available for a given head diameter, the choice depends on mechanical load and thermal requirements. For applications with high mechanical impact—such as contactors and circuit breakers—choose the thicker option to resist deformation. For applications where space is limited, choose the thinner option.

The Relationship Between D and Current Rating

A larger head diameter provides a larger contact area, which reduces contact resistance and temperature rise. The AgSnO₂In₂O₃ material used in Saijin's contact rivets has a resistivity of ≤3.50 µΩ·cm, ensuring that even with smaller contact areas, resistance remains low and stable over the service life of the component. This material stability, combined with proper head sizing, ensures consistent performance across millions of operations.

d and L – Shank Dimensions That Determine Mechanical Integrity

The shank diameter (d) and shank length (L) determine how well the rivet anchors into the terminal. The shank must be large enough to provide mechanical strength but small enough to fit through the terminal hole without excessive clearance. For head diameters up to 3.0mm, shank diameters typically range from 1.0mm to 1.5mm. For head diameters between 3.5mm and 5.5mm, shank diameters range from 2.0mm to 3.0mm. For larger heads (6.0mm and above), shank diameters range from 3.0mm to 5.0mm. The shank length must be sufficient to pass through the terminal and allow for proper rivet head formation, typically ranging from 0.8mm to 5.0mm depending on the application.

Shank Diameter and Current-Carrying Capacity

The shank diameter directly affects current-carrying capacity. A shank that is too small creates a current bottleneck, increasing resistance and heat generation at the junction. The shank must be sized to carry the same current as the head. The AgSnO₂In₂O₃ material offers high electrical conductivity, but the cross-sectional area of the shank still determines the maximum current the rivet can carry without excessive heating.

Shank Length and Assembly Process

The shank length must account for the thickness of the terminal and the material required to form a reliable rivet head. Too short, and the rivet won't hold. Too long, and the excess material may interfere with other components or require additional trimming. The standard length ranges provided in the table above (0.8–5.0mm) cover most terminal thicknesses used in relays, switches, and contactors.

Solid, Bimetal, or Trimetal – How Contact Structure Affects Cost and Performance

The structure of a contact rivet determines how precious metal is used—and that directly affects cost and performance.

Solid Contact Rivets – Maximum Performance, Higher Cost

A solid contact rivet is made from a single, homogeneous material throughout. This provides superior conductivity, consistent performance, and maximum durability. Solid rivets are the preferred choice for demanding applications where performance cannot be compromised—high-current contactors, circuit breakers, and critical switching devices.

Bimetal Contact Rivets – Cost-Effective Performance

A bimetal contact rivet combines a precious metal head (typically silver alloy) with a copper or brass shank. The precious metal is used only where it's needed—on the contact surface—while the shank provides mechanical strength at a lower cost. Bimetal rivets offer a cost-effective solution for applications where the contact surface requires silver's conductivity but the shank doesn't.

Trimetal Contact Rivets – Double-Side Contact Capability

A trimetal contact rivet has silver alloy cladding on both the head and the shank end, with a copper core. This configuration is used for double side contact applications where both ends of the rivet serve as contact surfaces. Trimetal rivets are commonly found in relays and switches where the contact needs to function on both ends.

How to Choose the Right Structure

The choice depends on your application: solid for maximum performance and durability; bimetal for cost-effective single-side contacts; trimetal for double-side contact applications where both ends need silver's performance.

AgSnO₂In₂O₃ – The Material That Combines Arc Resistance with Low Contact Resistance

The material of the contact rivet is as important as its dimensions. Saijin's contact rivets are made from AgSnO₂In₂O₃—a silver-tin oxide-indium oxide composite that offers a unique combination of properties for demanding switching applications. The material provides high arc erosion resistance and good resistance against contact welding. By introducing additives, the AgSnO₂ material can be modified to maintain low and stable contact resistance over the service life. Material transfer in DC operation is low, and the material is non-toxic and environmentally friendly. The AgSnO₂In₂O₃ material is widely used in contactors, relays, circuit breakers, and switches. The powder metallurgy technique allows precise control of the material properties, with resistivity as low as ≤2.15 µΩ·cm and hardness up to 950 HV.

Arc Erosion Resistance

During switching, electrical arcs erode the contact surface, gradually changing its geometry and increasing contact resistance. AgSnO₂In₂O₃'s high arc erosion resistance means the contact maintains its shape and performance over millions of operations, extending the service life of the switching device.

Low and Stable Contact Resistance

The material's resistivity ensures that contact resistance remains low and stable over the service life. The material's high hardness also ensures that it resists deformation under mechanical impact.

Environmental Compliance

AgSnO₂In₂O₃ is non-toxic and environmentally friendly, meeting RoHS and REACH requirements. This makes it suitable for products sold in markets with strict environmental regulations.

From Specification to Selection – A Step-by-Step Sizing Example

Designing a contact rivet for a specific application follows a straightforward process: identify the current rating of the switching device, which determines the minimum head diameter and shank diameter required; determine the mechanical clearance available in the device, which sets the maximum head diameter and thickness; select the shank length based on terminal thickness and riveting process; choose the sphere radius based on contact force and arc requirements; and verify the material selection against the application environment—AgSnO₂In₂O₃ for demanding applications with high arc stress, AgSnO₂ for general-purpose switching.

Example: Designing a Contact for a 16A Wall Switch

For a 16A wall switch application, the minimum head diameter would be 4.0mm with a thickness of 1.2mm. The shank diameter would be 2.0mm with a length of 2.0mm. The sphere radius would be 10mm. The material would be AgSnO₂In₂O₃ for its superior arc erosion resistance. The chosen dimensions would fall within the standard series: D=4.0mm, T=1.2mm, d=2.0mm, L=2.0mm, SR=10mm.

Example: Designing a Contact for a High-Voltage Contactor

For a high-voltage contactor application with higher mechanical impact and arc stress, the design would shift to a larger geometry: a 6.0mm head diameter with 2.0mm thickness, a 3.5mm shank diameter with 3.0mm length, and a 20mm sphere radius. The AgSnO₂In₂O₃ material would provide the arc resistance and low contact resistance required for the application.

What Design Engineers Ask About Electrical Contact Rivet Sizing

What are the five key dimensions of a contact rivet?
The five key dimensions are head diameter (D) , head thickness (T) , shank diameter (d) , shank length (L) , and sphere radius (SR) . Each parameter must be matched to the application's current rating, mechanical clearance, and assembly process.

What is the difference between solid, bimetal, and trimetal contact rivets?
A solid rivet is made from a single material throughout—maximum performance, higher cost. A bimetal rivet combines a precious metal head with a copper shank—cost-effective for single-side contacts. A trimetal rivet has precious metal on both head and shank end—for double-side contact applications.

How do I choose the right head diameter for my application?
The head diameter must be large enough to carry the required current without excessive heating and small enough to fit within the mechanical clearance of the switching device. The standard series ranges from 2.0mm to 10.0mm.

What is the sphere radius, and why does it matter?
The sphere radius (SR) determines the contact geometry at the top of the head. It influences the contact area, contact force distribution, and arc behavior during switching. The standard range is 5mm to 40mm.

What material is used for contact rivets?
Saijin's contact rivets use AgSnO₂In₂O₃ material, which offers high arc erosion resistance, low and stable contact resistance, and environmental compliance. The material is widely used in contactors, relays, circuit breakers, and switches.

Let's Match the Right Contact Rivet to Your Application

Selecting the right contact rivet comes down to four factors: current rating (determines D and d), mechanical clearance (determines T and SR), assembly requirements (determine L), and structure (solid, bimetal, or trimetal). Contact Saijin for engineering support and samples → —their team can help you select the right dimensions, structure, and material for your specific switching application.

Saijin (Wenzhou Saijin Electrical Alloy Co., Ltd.) manufactures high-performance electrical contact rivets using AgSnO₂In₂O₃ material with high arc erosion resistance, low and stable contact resistance, and environmental compliance. The product line includes solid, bimetal, and trimetal contact rivets for a wide range of switching applications. Applications include contactors, relays, circuit breakers, and switches in power distribution, communication equipment, and high-power electronic equipment. Saijin provides engineering support, sample verification, and production flexibility. Connect with Saijin to discuss your contact dimensions, structure type, material requirements, and application conditions.