Wetting Current

Key Takeaways

• Wetting current is the small current that flows after a mechanical switch or relay is first opened or closed. This happens when conducting elements (such as metal contacts) within a switch or relay make or break physical contact.

• If two metal surfaces make contact after sitting idle or not in contact for a while, there may be a thin layer of oxide or other contaminants on their surfaces. A wetting current is needed to overcome this resistance

• Wetting current can affect PCB design, especially for circuits involving mechanical switches, relays, connectors, and other components that depend on good electrical contact.

Wetting current is needed to overcome resistance from an oxide layer on metal components

Wetting current is the small current that flows when a mechanical switch or relay is first opened or closed. This happens when conducting elements (such as metal contacts) within a switch or relay make or break physical contact.

If two metal surfaces make contact after sitting idle or not in contact for a while, there may be a thin layer of oxide or other contaminants on their surfaces. Current can't flow through this layer because it can be an insulator. A small initial current is needed to overcome this resistance and establish a conductive path through the contact surface when the switch is closed or opened.

The wetting current helps break through or displace any oxide layers or contaminants, ensuring a low-resistance electrical connection. After the initial current flows, the resistance of the connection decreases, and the subsequent current can flow with minimal interference. The wetting current is usually well below the nominal maximum current rating.

Wetting current is crucial when designing and using switches, relays, and other electrical contacts to ensure reliability and longevity. Insufficient wetting current can cause increased resistance, heating, or switch failure due to arcing and contact surface deterioration.

How Does Wetting Current Affect PCB Design?

Wetting current can affect PCB (Printed Circuit Board) design, especially for circuits involving mechanical switches, relays, connectors, and other components that depend on good electrical contact. Wetting current can affect PCB design in these ways:

• Component Selection: It's essential to consider wetting current specs when choosing switches, relays, or connectors. Components with appropriate contact materials and designs can ensure wetting current is sufficient to establish a reliable connection and prevent problems like high resistance, arcing, and signal degradation.

• Layout and Trace Design: Layout of traces and pads on the PCB affects the wetting current. Wetting current performance can be improved with the right pad size, trace thickness, and spacing. For reliable wetting, ensure components are correctly seated on the pads during assembly.

• Reliability: Low wetting currents can make components unreliable. The circuit can act unpredictably if the wetting current isn't adequate, resulting in intermittent connections or even complete failures. It's essential in applications like industrial controls, medical devices, and automotive electronics, where reliability is critical.

• Contact Resistance: Wetting current breaks through oxide layers or contaminants on the contact surfaces, reducing contact resistance. High contact resistance can cause voltage drops, signal loss, and heat generation, which hampers the circuit's performance. The right contacts can help keep contact resistance low by allowing adequate wetting current.

• Switching Frequency: How fast switches or relays open and close can affect the wetting current. Frequent switching can help maintain clean contact surfaces by preventing the buildup of oxide layers or contaminants. Conversely, infrequent switching might need a higher initial wetting current.

• Signal Integrity: Wetting current can significantly affect signal quality in circuits where signal integrity is critical, like high-speed data transmission lines. High contact resistance caused by inadequate wetting current can lead to signal reflections, attenuation, and jitter, compromising the overall performance of the circuit.

• Environmental Factors: Wetting current can be affected by the environment. Corrosive or harsh environments can make oxide layers or contaminants form faster on contact surfaces. You can mitigate these effects by using conformal coatings or selecting corrosion-resistant components.

The wetting current is important in PCB design, especially for circuits with mechanical contacts. A PCB layout that facilitates sufficient wetting current and components with appropriate wetting characteristics can help make circuits reliable and high-performance.

Cadence Tools Help You Ensure Sufficient Wetting Current in Circuits

You can use Cadence’s PCB Design and Analysis Software and OrCAD PCB Designer to ensure sufficient wetting current for your circuits. OrCAD is the industry’s best PCB design and analysis software with utilities covering schematic capture, PCB layout and routing, and manufacturing.

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