Exploring Transistor In Circuit Board: Material Grades, Properties, and Uses

Types of Circuit Boards Suitable for Transistors

Circuit boards serve as the foundational platform for electronic components like transistors, enabling them to connect, communicate, and perform essential functions in modern electronics. The choice of circuit board significantly impacts performance, reliability, and scalability of transistor-based circuits. There are three primary types of circuit boards used in transistor applications, each offering distinct advantages depending on complexity, space constraints, and application requirements.

Printed Circuit Board (PCB)

The most widely used platform in consumer and industrial electronics, PCBs consist of an insulating substrate—typically fiberglass (FR-4)—coated with a thin layer of conductive copper. This copper is etched into precise pathways, known as traces, which interconnect various components including transistors.

Advantages

• High reliability and repeatability
• Supports both through-hole and surface-mount transistors
• Excellent signal integrity with proper layout
• Cost-effective for mass production

Limitations

• Design changes require new board fabrication
• Prototyping can be time-consuming
• Limited flexibility for experimental circuits

Best for: Commercial electronics, amplifiers, power supplies, and integrated transistor circuits

Single-Sided PCB

A simplified variant of the standard PCB where the copper layer exists on only one side of the insulating base. This design limits routing options but provides sufficient functionality for basic transistor applications with minimal interconnections.

Advantages

• Low manufacturing cost
• Easy to design and assemble
• Ideal for simple switching and amplification circuits
• Excellent for educational and beginner projects

Limitations

• Limited routing space restricts circuit complexity
• Not suitable for high-density transistor arrays
• Manual jumpers often needed for complex connections

Best for: Basic transistor switches, LED drivers, low-frequency amplifiers, DIY kits

Multi-Layer PCB

These advanced PCBs consist of three or more conductive copper layers separated by insulating dielectric materials and bonded under heat and pressure. Internal layers often serve as power planes or ground planes, improving electrical performance and thermal management for transistor-heavy designs.

Advantages

• High component density in compact form factors
• Superior signal integrity and reduced electromagnetic interference
• Improved thermal distribution across layers
• Supports complex transistor networks and high-speed circuits

Limitations

• Higher fabrication and repair costs
• Longer production lead times
• Requires advanced design software and expertise

Best for: High-performance computing, RF circuits, audio amplifiers, and multi-transistor control systems

Type	Complexity	Cost	Density	Best Use Case
Standard PCB	Medium	Low to Medium	Medium	General-purpose transistor circuits, consumer devices
Single-Sided PCB	Low	Low	Low	Simple transistor applications, educational projects
Multi-Layer PCB	High	High	Very High	Advanced electronics, high-frequency and power circuits

Expert Tip: When designing PCBs for high-power transistors, ensure adequate copper pour and thermal vias to dissipate heat effectively and prevent premature component failure.

Key Transistor Features in a Circuit Board: Functionality, Efficiency, and Reliability

Transistors are foundational semiconductor devices in modern circuit boards, serving as the backbone of electronic signal control and power management. Their integration into printed circuit boards (PCBs) enables compact, high-performance electronics across industries—from consumer gadgets to industrial automation. Understanding their core features helps engineers optimize circuit design for efficiency, durability, and functionality.

Core Features and Their Impact on Circuit Performance

Miniaturization and Space Optimization

Modern transistors are engineered for extreme miniaturization, allowing them to adapt seamlessly to shrinking PCB footprints. This scalability is critical in multi-layer circuit boards where vertical stacking limits available surface area for components. Advanced packaging technologies—such as Ball Grid Arrays (BGAs) and Chip-Scale Packages (CSPs)—enable transistors to occupy minimal space while maintaining high electrical performance.

This feature is especially vital in portable electronics like smartphones, wearables, and IoT devices, where every millimeter counts. Miniaturized transistors also reduce parasitic capacitance and inductance, improving signal integrity and enabling faster switching speeds in high-frequency applications.

Efficient Heat Dissipation

Transistors generate heat during operation due to current flow and switching losses, particularly in power amplification or high-current switching roles. To prevent thermal runaway and ensure stable performance, modern PCB designs incorporate advanced heat dissipation techniques such as thermal vias, copper pours, heat sinks, and thermally conductive pads.

Effective thermal management not only prolongs transistor lifespan but also maintains consistent electrical characteristics under load. For example, MOSFETs used in power supplies rely on proper heatsinking to avoid threshold voltage drift and maintain efficiency. In high-density boards, strategic placement near edges or airflow zones further enhances cooling.

Multi-Functionality: Switching and Amplification

One of the most powerful attributes of transistors is their dual capability to function as both switches and amplifiers. As a switch, a transistor can turn current flow on or off in digital circuits—enabling logic gates, microcontroller I/O control, and power regulation (e.g., in DC-DC converters).

In analog applications, transistors act as signal amplifiers, boosting weak input signals from sensors or audio sources to usable levels. Bipolar Junction Transistors (BJTs) and Field-Effect Transistors (FETs) are commonly used in amplifier configurations like common-emitter or common-source setups. This versatility makes transistors indispensable in mixed-signal systems, communication modules, and sensor interfaces.

Durability and Long-Term Reliability

The longevity of a transistor within a circuit board depends on both intrinsic component quality and external design factors. High-quality materials, robust doping processes, and protective encapsulation (e.g., epoxy coatings or conformal sealing) shield transistors from moisture, dust, and mechanical stress.

Additionally, PCB design elements such as proper trace width, via placement, and strain relief contribute to mechanical stability. Transistors in automotive or industrial environments often undergo rigorous qualification testing (e.g., AEC-Q101) to ensure resilience under temperature extremes, vibration, and electrical noise. These enhancements collectively reduce failure rates and support extended operational life, minimizing maintenance and replacement costs.

Feature	Primary Benefit	Design Considerations
Miniaturization	Enables compact, high-density PCB layouts	Use surface-mount technology (SMT); consider thermal and electrical isolation in tight spaces
Heat Dissipation	Prevents overheating and ensures stable operation	Incorporate thermal vias, heatsinks, and adequate copper areas; monitor junction temperature
Switching & Amplification	Supports both digital and analog circuit functions	Select appropriate transistor type (BJT, MOSFET, IGBT) based on application needs
Durability & Longevity	Reduces failure risk and maintenance frequency	Use conformal coatings, derate components, and follow IPC design standards

Best Practices for Maximizing Transistor Performance

Thermal Management: Always evaluate power dissipation and implement adequate cooling solutions. Use thermal simulation tools during PCB layout to identify hotspots.

Proper Biasing: Ensure correct voltage and current biasing to prevent saturation or cutoff in amplifier circuits and avoid shoot-through in switching applications.

Signal Integrity: Minimize trace lengths and avoid sharp bends to reduce noise and crosstalk, especially in high-frequency designs.

Component Selection: Choose transistors with appropriate voltage ratings, gain (hFE or β), and switching speed for your specific use case.

Environmental Protection: Apply conformal coatings in humid or corrosive environments to protect transistor leads and solder joints from degradation.

Important: Overlooking thermal design or misapplying transistors beyond their rated specifications can lead to premature failure, system instability, or safety hazards. Always consult datasheets and adhere to manufacturer guidelines for operating conditions, derating curves, and mounting recommendations. Proper design and maintenance ensure optimal performance and extend the service life of your circuit board.

Uses of Transistors in a Circuit Board

Transistors are fundamental building blocks in modern electronics, serving as both switches and amplifiers in circuit boards across countless applications. Their ability to control current flow with precision makes them essential in managing power, processing signals, and enabling intelligent functionality in electronic systems. Below are the major applications of transistors, expanded with technical insights and real-world examples to illustrate their critical roles.

Residential Devices

In household appliances, transistors act as intelligent control elements that manage power delivery, timing, and operational sequences. By functioning as high-speed electronic switches, they replace mechanical relays, offering quieter, more reliable, and energy-efficient operation.

• In washing machines, microcontroller-driven transistors regulate motor speed and direction during different wash cycles (e.g., gentle, heavy-duty, spin), enabling precise control without manual intervention.
• Refrigerators use transistors in temperature feedback loops, where sensor signals are amplified and processed to modulate compressor activity, maintaining consistent internal temperatures while minimizing energy consumption.
• Dishwashers and ovens leverage transistors for timer circuits, heating element control, and safety interlocks, enhancing user safety and operational efficiency.

Key Benefit: Enables automation, energy savings, and longer appliance lifespan through precise electronic control.

Industrial Machines

Transistors form the backbone of industrial automation and control systems, where reliability, speed, and precision are paramount. They are used extensively in programmable logic controllers (PLCs), motor drives, and power regulation units.

• Motor control circuits use power transistors (such as MOSFETs and IGBTs) to implement variable frequency drives (VFDs), allowing smooth acceleration and deceleration of motors in conveyor systems, pumps, and robotics.
• Voltage regulators and DC-DC converters rely on switching transistors to maintain stable power supplies despite fluctuating input voltages, ensuring consistent performance in harsh environments.
• Safety interlocks and emergency stop systems use transistor-based logic gates to detect faults and instantly cut power, preventing equipment damage and protecting personnel.

Key Benefit: Enhances system responsiveness, reduces downtime, and improves operational safety in high-demand environments.

Response Systems

Emergency and security systems depend on transistors for rapid signal detection, amplification, and actuation. These systems must respond within milliseconds to threats such as fire, intrusion, or gas leaks.

• Smoke and motion detectors use transistors to amplify weak sensor signals (e.g., from photodiodes or piezoelectric elements) to trigger audible alarms or send alerts to monitoring stations.
• In fire alarm panels, transistor-based logic circuits process inputs from multiple sensors and activate sirens, strobes, or building-wide notification systems.
• Automated lockdown systems in secure facilities use transistors to control electromagnetic locks, ensuring doors engage instantly upon detection of unauthorized access.

Key Benefit: Provides fast, reliable response to critical events, improving safety and minimizing risk.

Entertainment Systems

From televisions to high-fidelity audio equipment, transistors play a vital role in processing and amplifying analog and digital signals to deliver superior visual and auditory experiences.

• In audio amplifiers, bipolar junction transistors (BJTs) and MOSFETs are used in preamplifier and power amplifier stages to boost weak audio signals without introducing distortion.
• Modern flat-screen TVs use millions of thin-film transistors (TFTs) in LCD and OLED displays, with each transistor controlling an individual pixel’s brightness and color, enabling high-resolution imaging.
• Digital signal processors (DSPs) and audio/video codecs rely on integrated transistor arrays to decode compressed media formats (e.g., MP3, H.264) in real time.

Key Benefit: Delivers high-fidelity sound, sharp image quality, and seamless multimedia performance.

IoT and Smart Devices

The Internet of Things (IoT) revolution has dramatically increased the demand for compact, low-power transistors capable of continuous operation in wireless, battery-powered devices.

• Smart thermostats, wearables, and home sensors use CMOS transistors due to their ultra-low power consumption in standby mode, enabling months or even years of battery life.
• Wireless communication modules (Wi-Fi, Bluetooth, Zigbee) employ RF transistors to modulate and amplify signals for data transmission and reception.
• Edge computing in smart devices relies on microcontrollers with embedded transistor-based logic to process sensor data locally, reducing latency and cloud dependency.

Key Benefit: Enables always-on connectivity, real-time monitoring, and energy-efficient operation in compact smart devices.

Technical Insight: Modern circuit boards often integrate billions of transistors within microprocessors and memory chips. Even simple PCBs use discrete transistors for tasks like level shifting, load switching, and noise filtering. Understanding the dual role of transistors—as both switches (digital logic) and amplifiers (analog circuits)—is key to designing and troubleshooting electronic systems effectively.

Application Area	Transistor Type	Primary Function	Performance Requirement
Residential Devices	Bipolar Junction Transistor (BJT), MOSFET	Motor and compressor control	Moderate power handling, reliability
Industrial Machines	IGBT, Power MOSFET	High-power switching, VFDs	High current/voltage tolerance, thermal stability
Response Systems	Small-signal BJT, FET	Signal amplification, triggering	Fast response, low noise
Entertainment Systems	Audio MOSFET, TFT, RF Transistor	Audio/video amplification, pixel control	High fidelity, frequency response
IoT and Smart Devices	CMOS, RF Transistor	Sensing, wireless communication	Ultra-low power, miniaturization

Additional Considerations

• Miniaturization: Advancements in semiconductor technology allow transistors to be scaled down to nanometer levels, enabling powerful computing in small devices.
• Heat Management: Power transistors generate heat during operation; proper heat sinking and PCB layout are crucial to prevent thermal runaway.
• Integration: Most modern systems use integrated circuits (ICs) containing thousands to billions of transistors, but discrete transistors remain vital for specific switching and amplification tasks.
• Reliability: Transistors have no moving parts, making them highly durable and resistant to shock and vibration compared to mechanical switches.
• Cost Efficiency: Mass production has made transistors extremely inexpensive, contributing to the affordability of consumer electronics.

Choosing the Right Transistor for Your Circuit Board: A Comprehensive Guide

Selecting the appropriate transistor is a critical step in designing reliable and efficient electronic circuits. Transistors serve as the backbone of modern electronics, functioning as switches or amplifiers in countless applications. Making the right choice involves understanding key parameters such as type, package, power handling, electrical ratings, and thermal performance. This guide breaks down each factor to help you confidently choose the best transistor for your circuit board design, whether for prototyping, production, or repair.

Important Note: Always verify transistor specifications under real-world operating conditions. Relying solely on datasheet values without considering environmental factors like ambient temperature, airflow, and load variations can lead to premature failure or unstable circuit behavior.

Key Factors When Selecting a Transistor

1. Transistor Type: Matching Functionality to Application

   Transistors are primarily categorized by their internal structure and operation. The two most common types are Bipolar Junction Transistors (BJTs) and Field-Effect Transistors (FETs), each with distinct advantages:

   • Bipolar Junction Transistors (BJTs): These current-controlled devices are excellent for analog signal amplification due to their linear response and low noise characteristics. Commonly used in audio amplifiers, sensor interfaces, and linear regulators. Available in NPN and PNP configurations.
   • Field-Effect Transistors (FETs): Voltage-controlled transistors with very high input impedance, making them ideal for digital switching and low-power applications. Among FETs, MOSFETs (Metal-Oxide-Semiconductor FETs) dominate in power electronics, motor control, and switching power supplies due to their fast switching speeds and efficiency.
   • When to Choose Which: Use BJTs when precise analog control and gain are needed. Opt for MOSFETs in high-frequency switching, power regulation, or when minimizing input loading is crucial.
2. Form Factor (Package Type): Balancing Space and Thermal Needs

   The physical package of a transistor affects not only board layout but also thermal management and ease of assembly. Common packages include:

   • Through-Hole Packages (e.g., TO-92, TO-220): Ideal for prototyping and low-volume production. TO-220 packages often include a metal tab for mounting to heat sinks, making them suitable for medium-power applications.
   • Surface-Mount Devices (SMD) (e.g., SOT-23, SOT-223, DPAK): Enable compact, high-density PCB designs. SOT-23 is common for small-signal transistors, while DPAK and similar packages offer better thermal performance for power SMD applications.
   • Considerations: Choose through-hole for hand-soldering and better mechanical stability. Use SMD for automated manufacturing and space-constrained designs. Always check PCB footprint compatibility and thermal pad requirements.
3. Power Rating: Ensuring Long-Term Reliability

   The power rating (usually specified in watts) indicates the maximum power the transistor can dissipate without overheating. Exceeding this limit can cause thermal runaway and permanent damage.

   • Select a transistor with a power rating at least 1.5 to 2 times higher than your expected maximum power dissipation to provide a safety margin.
   • For high-power applications (e.g., motor drivers, power supplies), consider transistors designed with integrated heat spreaders or those compatible with external heat sinks.
   • Use power derating curves from the datasheet to adjust maximum allowable power at elevated temperatures.
4. Current and Voltage Ratings: Matching Circuit Requirements

   These are among the most critical specifications to verify:

   • Collector/Drain Current (Ic/Id): Must exceed the peak current demands of your load. Include surge currents (e.g., motor startup) in your calculations.
   • Collector-Emitter/Drain-Source Voltage (Vce/Vds): Should be significantly higher than the maximum voltage in your circuit. A common rule of thumb is to use a transistor rated for at least 1.5× the supply voltage.
   • Gate/Base Voltage (Vgs/Vbe): Ensure compatibility with your control signal (e.g., 3.3V or 5V microcontroller output).
   • Safety Margin: Design with headroom to accommodate voltage spikes, load transients, and component tolerances.
5. Heat Dissipation: Managing Thermal Performance

   Efficient heat management is essential for stable operation and longevity:

   • Use heat sinks for transistors dissipating more than 1W, especially in enclosed or high-temperature environments.
   • Apply thermal paste or pads between the transistor and heat sink to improve thermal conductivity.
   • Ensure adequate PCB copper area (thermal vias and planes) for SMD transistors to act as heat spreaders.
   • Monitor operating temperature during testing using an infrared thermometer or thermal camera.
   • Consider active cooling (e.g., small fans) in high-power or sealed enclosures.

Selection Factor	Key Questions to Ask	Common Mistakes to Avoid	Recommended Tools/References
Type (BJT vs FET)	Is the application analog or digital? Is input impedance critical?	Using a BJT in a high-speed switching circuit, leading to inefficiency	Datasheets, application notes, SPICE simulation tools
Form Factor	What are the space constraints? Is hand assembly required?	Mismatched footprint, poor solderability, or inadequate thermal design	PCB design software, manufacturer footprint libraries
Power Rating	What is the max power dissipation under load?	Ignoring derating at high temperatures	Power calculators, thermal resistance (Rθ) values
Current/Voltage Ratings	Are peak and transient conditions accounted for?	Selecting a transistor just above nominal values without margin	Multimeter, oscilloscope for transient analysis
Heat Dissipation	Can the transistor stay within safe operating temperature?	Omitting heat sinks or thermal interface materials	Thermal paste, heat sinks, IR thermometer

Expert Tip: Always download and study the full datasheet for any transistor you plan to use. Pay close attention to graphs like Safe Operating Area (SOA), thermal resistance, and switching characteristics. When in doubt, simulate the circuit using tools like LTspice to validate performance before committing to hardware.

Additional Best Practices

• Keep spare transistors on hand for common types (e.g., 2N3904, 2N2222, IRF540N) for quick prototyping and troubleshooting.
• Use protective components like flyback diodes with inductive loads (e.g., relays, motors) to prevent voltage spikes from damaging the transistor.
• Label transistors clearly on schematics and PCBs to avoid confusion during assembly and debugging.
• Consider availability and lead times, especially for SMD components, to avoid production delays.
• For high-reliability designs, choose industrial-grade or automotive-qualified transistors with extended temperature ranges.

Choosing the right transistor is more than just matching specs—it's about understanding how the component will perform in your specific circuit and environment. By carefully evaluating type, package, power, electrical ratings, and thermal needs, you can ensure robust, efficient, and long-lasting circuit performance. When uncertain, consult engineering forums, manufacturer support, or collaborate with experienced designers to validate your selection.

Frequently Asked Questions About Transistors and PCBs

Q1: What role do transistors play in home appliances?

Transistors are fundamental semiconductor devices that act as electronic switches or amplifiers in modern home appliances. In devices like washing machines, refrigerators, air conditioners, and microwave ovens, transistors regulate motor speed, control temperature settings, manage power delivery, and process sensor inputs with high precision.

• Motor Control: Transistors enable variable speed control in washing machine drums and refrigerator compressors by modulating current flow using pulse-width modulation (PWM) techniques.
• Signal Processing: They amplify low-level signals from sensors (e.g., temperature, humidity, door position) so microcontrollers can interpret them accurately.
• Energy Efficiency: By allowing precise control over electrical loads, transistors reduce energy waste and improve appliance efficiency, contributing to lower electricity bills and longer device lifespan.
• Smart Features: Modern smart appliances rely on transistor-based circuits for Wi-Fi connectivity, touch interfaces, and automated functions—features that would be impossible with older electromechanical relays alone.

Without transistors, home appliances would be bulkier, less reliable, slower to respond, and significantly less efficient, limiting both functionality and user experience.

Q2: What factors should be considered when selecting a transistor for a circuit board?

Selecting the right transistor is critical for ensuring circuit reliability, performance, and longevity. While the 1N4148 is actually a switching diode (not a transistor), the principles for choosing a suitable transistor substitute or replacement remain consistent across designs:

• Voltage and Current Ratings: Ensure the transistor’s maximum collector-emitter voltage (V_CE) and collector current (I_C) exceed the circuit’s operating requirements with a safety margin (typically 20–30%).
• Power Dissipation: Check the device’s power rating (in watts) and ensure adequate heat dissipation via heatsinks or PCB copper pours, especially in power amplifiers or motor drivers.
• Gain (h_FE or β): For BJTs, sufficient current gain ensures efficient switching or amplification without overloading the driving stage.
• Switching Speed: High-frequency applications (e.g., switch-mode power supplies) require fast turn-on/turn-off times, making MOSFETs preferable over standard BJTs.
• Form Factor & Package: Match the footprint (e.g., TO-92, SOT-23, DPAK) to your PCB layout—surface-mount (SMD) for compact boards, through-hole for prototyping or high-power needs.
• Thermal Management: Consider junction-to-ambient thermal resistance and ambient operating temperature, especially in enclosed spaces.
• Cost and Availability: Optimize for components that are readily available and cost-effective without compromising performance.

Always consult the transistor’s datasheet and simulate its behavior in your circuit when possible to avoid compatibility issues.

Q3: Which types of transistors are there?

Transistors come in several types, each suited to specific applications based on structure, material, and operational characteristics. The main categories include:

Type	Subtypes	Primary Applications	Key Characteristics
Bipolar Junction Transistors (BJTs)	NPN, PNP	Analog amplification, linear regulators, low-power switching	Current-controlled, moderate speed, good gain, requires base current
Field-Effect Transistors (FETs)	MOSFET, JFET	Digital circuits, power switching, amplifiers	Voltage-controlled, high input impedance, fast switching, energy-efficient
Insulated Gate Bipolar Transistors (IGBTs)	—	High-power systems (inverters, motor drives, EVs)	Combines BJT and MOSFET advantages; handles high voltage/current efficiently
Integrated Transistor Arrays	Darlington pairs, transistor arrays	Compact circuits, LED drivers, relay control	Multiple transistors in one package; saves space and simplifies design

With advancements in miniaturization and integration, multi-transistor modules and system-on-chip (SoC) solutions are increasingly popular in consumer electronics, enabling smaller, smarter, and more efficient devices.

Q4: How do transistors help in signal amplification?

Transistors are essential for signal amplification because they can use a small input signal to control a much larger output current or voltage. This process is known as gain.

• Basic Principle: In a common-emitter BJT configuration or common-source MOSFET setup, a weak input signal applied to the base (or gate) modulates the flow of current between the collector and emitter (or drain and source), effectively replicating the input waveform at a higher power level.
• Amplifier Stages: Multiple transistor stages can be cascaded to achieve high overall gain—for example, pre-amplifiers boost tiny microphone signals, while power amplifiers drive speakers.
• Applications:
  • Audio Systems: Amplify music or voice signals in speakers, headphones, and public address systems.
  • Radios & TVs: Boost weak RF signals received by antennas for demodulation and playback.
  • Telecommunications: Enable long-distance data transmission by amplifying signals in fiber-optic networks and cellular base stations.
  • Medical Devices: Amplify bio-signals such as ECG, EEG, and EMG for accurate monitoring and diagnosis.
• Linearity & Fidelity: Proper biasing ensures the transistor operates in its linear region, minimizing distortion and preserving signal quality.

Without transistor-based amplification, modern communication, entertainment, and diagnostic technologies would not be feasible.

Q5: What are PCBs made of?

Printed Circuit Boards (PCBs) are engineered composites designed to provide mechanical support and electrical connectivity for electronic components. The most common base material is fiberglass-reinforced epoxy resin, known as FR-4. This material offers excellent insulation, durability, and heat resistance.

• Substrate Layer: FR-4 is the standard dielectric material—it prevents short circuits by insulating the conductive copper traces while withstanding temperatures up to 130–140°C.
• Copper Cladding: Thin layers of copper are laminated onto the substrate and etched to form precise conductive pathways (traces) that connect components like transistors, resistors, and ICs.
• Solder Mask: A protective polymer layer (usually green) covers the copper to prevent oxidation and accidental shorts during assembly.
• Silkscreen: Adds labels, part numbers, and polarity indicators to aid in manufacturing and repair.
• Alternative Materials:
  • Flexible PCBs: Use polyimide or polyester films for bendable circuits in wearables and compact devices.
  • High-Frequency PCBs: Employ specialized laminates like Rogers material for minimal signal loss in RF and microwave applications.
  • Aluminum-backed PCBs: Used in LED lighting and power electronics for superior thermal management.

Modern PCBs often feature multiple layers (4, 6, or more) to accommodate complex routing in smartphones, computers, and industrial controllers. Their robust construction ensures long-term reliability even under thermal cycling and mechanical stress.