Resistors Noise and Corrosion

In many applications reliability, stability and long lifetime of resistors are of critical importance. Lets learn something about resistors’ noise and corrosion.

In precision and long‑lifetime electronics, resistors are often the hidden limiters of performance and reliability. This guide expands basic concepts of resistor noise and electrical corrosion into practical design rules, examples, and selection guidance for real‑world applications.

1. Fundamentals of Noise in Resistors

Noise in resistors appears as a randomly fluctuating voltage or current superimposed on the ideal DC or AC signal. In practice, we work with time‑averaged quantities, typically as root mean square (RMS) values over a defined bandwidth.

Key points:

• Noise is characterized statistically (mean‑square or RMS), not by instantaneous values.
• The relevant quantity for design is usually noise spectral density (per Hz) integrated over a bandwidth of interest.
• Different physical mechanisms dominate in different technologies and frequency ranges.

2. Thermal Noise (Johnson‑Nyquist Noise)

Thermal noise is caused by the thermally agitated charge carriers in the resistive material. It exists in every resistor, regardless of technology, and depends only on absolute temperature, resistance value, and measurement bandwidth.

The RMS noise voltage across a resistor is given by$$E_{\text{n,th}} = \sqrt{4 k T R \Delta f}$$where $k$ is Boltzmann’s constant, $T$ absolute temperature, $R$ resistance, and $\Delta f$ the bandwidth.

Implications for design:

• Thermal noise is independent of resistor type: thick film, thin film, wirewound, or carbon composition all share the same thermal‑noise level for the same $R$, $T$ and $\Delta f$
• To reduce thermal noise: decrease resistance, reduce bandwidth, or lower temperature where feasible.
• Thermal noise typically dominates at high frequencies where excess/current noise becomes negligible.

3. Excess / Current Noise

When a DC current flows through a resistor, additional noise appears beyond the thermal component. This is commonly called current noise, excess noise, or $1/f$‑like noise depending on context.

3.1 Physical Origin

Current noise is especially pronounced in resistive structures formed by granular or non‑uniform conduction paths, such as carbon composition or certain film resistors. Local changes in contact area and conductivity at grain boundaries and defects create random fluctuations in resistance under bias, which translate into noise voltage.

Even in metal wires and films, current noise exists but is usually negligible for most applications compared to granular materials.

3.2 Frequency and Voltage Dependence

For many resistor technologies, current noise:

• Is approximately proportional to applied DC voltage.
• Shows an approximate $1/f$ dependence on frequency.
• Becomes negligible above roughly 10 kHz in many practical applications.

As a rule of thumb, each frequency decade contains roughly the same contribution to total current noise in $1/f$‑dominated regimes.

4. Noise Metrics and Standards

Because noise is a statistical quantity, standardized measurement conditions are essential. Typical standards specify bias, resistance range, and measurement bandwidth.

4.1 Noise Index

One widely used figure of merit is the noise index (NI):

• Defined as noise voltage in microvolts per volt of applied DC, per frequency decade, often expressed in dB.
• Lower NI indicates a lower‑noise resistor technology for the same operating conditions.

You can treat NI as a comparative selection metric across resistor series and technologies within the same resistance value range.

4.2 Practical Interpretation

Typical qualitative trends:

• Carbon composition and standard thick‑film resistors generally have higher noise index.
• Thin‑film, metal‑foil, and good wirewound resistors exhibit very low excess noise, often negligible compared to thermal noise in many applications.

5. Typical Noise Performance by Technology

The table below gives a qualitative view of relative excess‑noise behavior of common resistor technologies at low frequencies under DC bias.

Resistor technology	Excess noise level (qualitative)	Typical use cases
Carbon composition	High	Legacy designs, surge‑tolerant but noisy applications
Carbon film	Medium–high	General purpose where noise is not critical
Thick film (SMD)	Medium	General electronics, digital, non‑critical analog
Metal film (axial)	Low	Precision analog, audio, instrumentation
Thin film (SMD)	Very low	Precision, low‑noise front ends, A/D references
Metal foil	Extremely low	Ultra‑low noise and high stability applications
Wirewound	Very low	Precision, power shunts, low‑frequency analog

This table is qualitative and exact values depend on manufacturer and series.

6. Design Guidelines for Low‑Noise Circuits

To design for low noise, treat the resistor as part of the entire signal chain rather than in isolation.

Key guidelines:

• Use the lowest resistance value consistent with signal level and loading constraints to reduce thermal noise.
• Prefer thin‑film, metal‑film, or foil resistors in low‑level analog front ends, audio inputs, and sensor interfaces.
• Keep bandwidth limited to what the application truly needs using proper filtering.
• Avoid high‑value carbon or thick‑film resistors in the first stages of precision amplifiers when DC bias is present.

Example: In a low‑level audio preamplifier input, replacing a high‑value thick‑film SMD bias resistor with a lower‑value thin‑film part often yields a measurable improvement in hiss and background noise.

7. Layout and Measurement Considerations

Resistor noise interacts with layout‑dependent parasitics and external interference. Good PCB practice is essential to realize the theoretical performance of chosen components.

7.1 Layout

• Keep high‑impedance nodes short and shielded to minimize capacitive coupling and induced noise.
• Use ground planes and proper star‑grounding in sensitive analog areas.
• Avoid routing digital or high‑current traces parallel and close to low‑level analog resistors.

7.2 Measurement

Measuring low noise requires instrumentation whose own noise floor is well below the device under test.

• Use low‑noise amplifiers and spectrum analyzers with known input‑referred noise.
• Calibrate using known reference resistors and compare against calculated thermal noise.
• Follow standardized bandwidth definitions when reporting noise measurements.

8. Electrical Corrosion in Resistors

Electrical corrosion in resistors arises when moisture and ionic contamination create conductive electrolyte paths between metallic and resistive structures under electrical bias. This can cause resistance drift, increased leakage, or complete open‑circuit failures over time.

Mechanism

In the presence of a thin film of moisture containing dissolved ions:

• DC voltage drives electrochemical reactions at metal interfaces.
• Anodic areas can corrode or dissolve, while cathodic areas may see deposition or hydrogen evolution.
• Localized material loss and chemical changes modify the conduction path and eventually lead to failure.

High‑ohmic and thin‑film structures are particularly sensitive because their current paths are narrow and any local material loss can significantly increase resistance.

9. Environmental and Application Factors

The likelihood and speed of corrosion‑related failures depend strongly on environmental conditions.

Major factors:

• Relative humidity and condensation, especially cyclic humidity.
• Temperature, which accelerates chemical reactions and diffusion.
• Contaminants such as flux residues, salts, industrial pollutants, and sulfur‑bearing gases.
• Applied DC bias and its polarity relative to sensitive structures.

Applications in industrial atmospheres, coastal environments, or under‑hood automotive positions experience significantly harsher conditions than controlled indoor equipment.

10. Construction, Materials, and Corrosion Robustness

Resistor construction has a strong influence on corrosion behavior.

Important aspects:

• Resistive element type (carbon film, metal film, thick film, metal‑glaze, wirewound).
• Termination materials (Cu, Ni, Ag, Ag‑Pd, Sn) and their galvanic potential differences.
• Protective coatings and encapsulation, from simple lacquer to molded epoxy or glass.
• Package style (SMD chip vs. leaded) and accessibility of terminations and resistive paths to the environment.

Metal‑glaze and properly protected metal‑film resistors are generally more resistant to aqueous corrosion than unprotected granular or porous structures exposed to moisture.

11. Test Methods and Reliability Metrics

Corrosion and moisture sensitivity are commonly evaluated using biased humidity tests and related accelerated methods.

Typical approaches:

• Temperature‑humidity‑bias tests, where resistors are powered under high humidity and elevated temperature for extended durations.
• Highly accelerated stress tests for faster screening of weak constructions.
• Sulfur exposure tests for environments containing sulfur compounds, relevant for some thick‑film and silver‑containing terminations.

Results are usually expressed as resistance change distributions, number of failures, and time to specified drift thresholds.

12. Design Guidelines Against Corrosion and Leakage

Corrosion and moisture‑induced leakage can be mitigated by proper component selection, board design, and manufacturing processes.

Key guidelines:

• Choose corrosion‑resistant resistor technologies (for example, metal‑glaze, high‑quality metal film, sulfur‑resistant series) for harsh environments.
• Avoid unnecessarily high resistance values in locations exposed to humidity and contamination; high‑ohmic paths are more affected by leakage and surface films.
• Use conformal coating or encapsulation in high‑humidity or polluted environments to minimize moisture access.
• Control board cleanliness: use appropriate fluxes, thoroughly wash if required, and verify ionic contamination levels.
• Maintain sufficient creepage and clearance distances and avoid sharp edges and contamination traps in PCB layout.

13. Application Mapping Table

The table below gives a qualitative mapping from typical environment to recommended resistor technologies and protective measures.​

Environment / application	Typical stressors	Recommended resistor types	Extra measures
Consumer indoor electronics	Mild temperature, low humidity	Thick film SMD, metal film axial	Standard cleaning, no special coating
Industrial control (indoor)	Elevated temp, moderate humidity, pollution	Metal film, robust thick film, metal‑glaze	Good cleaning, optional coating
Outdoor telecom / base stations	Humidity, condensation, pollution, salt spray	Metal‑glaze, high‑reliability metal film	Conformal coating, spacing, sealing
Automotive cabin	Temp cycling, moderate humidity	Thick film SMD, metal film, automotive‑grade	Automotive‑qualified parts
Automotive under‑hood	High temp, humidity, splash, pollutants	Metal‑glaze, robust thick film, wirewound	Coating, sealing, validated THB tests
Industrial sulfurous atmosphere	Sulfur gases, humidity	Sulfur‑resistant thick film or metal‑glaze	Use sulfur‑resistant series, coating

This mapping is indicative only and should be refined using specific component series data and qualification results.

14. Conclusion

Resistors are more than simple ohmic elements; their intrinsic noise and vulnerability to electrical corrosion can limit both the performance and lifetime of modern electronics. By understanding the underlying noise mechanisms, choosing an appropriate resistor technology, and applying sound layout and environmental protection practices, designers can significantly improve system robustness and precision.

A structured selection process that considers noise, environment, and construction simultaneously is essential, especially in high‑reliability and low‑signal‑level applications. Complementing this with proper testing and qualification closes the loop between theoretical design and long‑term field performance.

FAQ – Resistors Noise and Corrosion