How Does Contact Welding Occur in Automotive Relays and How Is It Prevented?
2026-10-21
A failure analysis engineer receives a returned Automotive Relays from a vehicle manufacturer. The complaint is that the cooling fan runs continuously, even when the ignition is off. The engineer opens the relay and finds that the normally open contacts are welded together. The contact surfaces show a rough, melted area with a metallic bridge between the two pads. The relay passed the bench test at 10,000 cycles with a resistive load. But in the vehicle, the relay switches a cooling fan motor, which is an inductive load with a high inrush current. The bench test did not replicate the inrush current. The welding occurred because the contact area was too small to handle the surge. This guide explains how contact welding occurs and how it is prevented.
1. What Causes Contact Welding in Automotive Relays?
Contact welding occurs when the contact surfaces reach the melting point and fuse together. The heat is generated by the arc that forms when the contacts open or close. The arc is most severe during the closing operation, when the contacts are very close and the voltage is still high. The inrush current of the load amplifies the arc energy. The three primary causes of welding are excessive inrush current, insufficient contact force, and contact material that is prone to welding. The table below shows the relationship between load type and inrush current multiplier.
| Load type | Inrush current multiplier (vs. steady state) | Arc energy | Welding risk |
| Resistive (heater, lamp) | 1 – 2x | Low | Low |
| Inductive (motor, solenoid) | 3 – 6x | Moderate | Moderate |
| Capacitive (LED driver, capacitor bank) | 10 – 50x | High | High |
| Incandescent lamp (cold filament) | 10 – 15x | High | High |
| Motor with high inertia | 6 – 10x | Very high | Very high |
In our factory, we test Automotive Relays with different load types to verify the welding resistance. The test uses a load bank that can simulate resistive, inductive, and capacitive loads. We also measure the contact resistance before and after the test to detect any welding or degradation. Ningbo Huaguan Electronics Co., Ltd. has been manufacturing automotive relays for over 12 years.
2. How Does Contact Material Affect Welding Resistance?
The contact material determines the melting point, the electrical conductivity, and the tendency to form intermetallic compounds. Silver alloys are the most common contact materials. Pure silver has high conductivity but a low melting point and a high tendency to weld. Silver-tin oxide (AgSnO2) has a higher melting point and better welding resistance. Silver-nickel (AgNi) has good conductivity and moderate welding resistance. The table below compares the properties of common contact materials.
| Contact material | Melting point (°C) | Electrical conductivity (% IACS) | Welding resistance | Typical application |
| Pure silver (Ag) | 961 | 105 | Low | Low-power signal relays |
| Silver-nickel (AgNi) | 950 – 1,000 | 80 – 90 | Moderate | General purpose relays |
| Silver-tin oxide (AgSnO2) | 1,100 – 1,300 | 70 – 80 | High | High inrush loads |
| Silver-tungsten (AgW) | 1,500 – 2,000 | 50 – 60 | Very high | High power, DC loads |
In our factory, we use AgSnO2 for Automotive Relays that switch inductive or capacitive loads, such as cooling fans, wipers, and LED drivers. AgSnO2 has a higher melting point than pure silver, which increases the energy required to weld the contacts. We also use a fine-grain microstructure that distributes the arc energy over a larger area, which reduces the localized heating. NHG provides contact material certificates for all of our relays.
3. How Does Contact Force and Gap Affect Welding?
The contact force and the contact gap determine the arc duration and the contact bounce. A higher contact force reduces the contact resistance and increases the force required to weld the contacts. A larger contact gap reduces the arc duration because the arc is extinguished more quickly. The table below shows the relationship between contact force, gap, and welding resistance.
| Contact force (cN) | Contact gap (mm) | Bounce time (ms) | Welding resistance |
| 50 | 0.3 | 2.5 | Low |
| 100 | 0.3 | 1.5 | Moderate |
| 150 | 0.5 | 1.0 | High |
| 200 | 0.5 | 0.8 | Very high |
| 250 | 0.8 | 0.5 | Excellent |
Failure analysis note: In a welded relay, the contact force is often below the design specification because the contact spring has lost its tension or the armature has worn. The contact gap may also be reduced because the contacts have eroded. In our factory, we measure the contact force and gap on every relay before shipment. We also recommend that the customer inspect the relay after any abnormal event, such as a short circuit or a severe voltage spike, because these events can change the contact force and gap.
4. What Arc Suppression Methods Prevent Welding?
Arc suppression reduces the energy that reaches the contacts during switching. The most common methods are a flyback diode, a varistor, an RC snubber, and a magnetic blowout. The flyback diode is used for DC inductive loads. It provides a path for the current to circulate when the relay opens, which prevents the voltage spike. The varistor clamps the voltage at a safe level. The RC snubber absorbs the high-frequency energy. The magnetic blowout uses a magnetic field to push the arc away from the contacts. The table below shows the recommended suppression method for different load types.
| Load type | Recommended suppression | Effect on welding |
| DC motor | Flyback diode + varistor | Reduces arcing at opening |
| AC motor | RC snubber | Reduces arcing at opening |
| Incandescent lamp | NTC thermistor in series | Limits inrush current |
| Capacitive load | Series resistor | Limits inrush current |
| High-power DC | Magnetic blowout | Extends arc and extinguishes it |
In our factory, we design Automotive Relays with built-in arc suppression for specific applications. We also provide application notes that recommend the external suppression components for each load type. Ningbo Huaguan Electronics Co., Ltd. has a testing laboratory that can verify the suppression performance with the customer's actual load.
Frequently Asked Questions About Contact Welding in Automotive Relays
Summary for Failure Analysis Engineers and Designers
Contact welding in Automotive Relays is caused by excessive arc energy, insufficient contact force, and contact materials that are prone to welding. The load type determines the inrush current and the arc energy. The contact material, force, and gap determine the welding resistance. Arc suppression methods reduce the energy that reaches the contacts. By selecting the right relay for the load and using the appropriate suppression, designers can prevent contact welding and ensure reliable switching. Ningbo Huaguan Electronics Co., Ltd. has been manufacturing Automotive Relays for over 12 years and provides full failure analysis and application support for our customers.
Ningbo Huaguan Electronics Co., Ltd. manufactures Automotive Relays with AgSnO2 and AgNi contacts, high contact force, and optional built-in arc suppression. We provide load compatibility charts, failure analysis reports, and custom designs for high inrush applications.