Electronic products must be reliable and durable, especially when they are designed to operate in harsh environments. Building printed circuit boards (PCBs) to work efficiently in extreme environmental conditions like extensive temperature, moisture, vibration, and pressure is quite challenging. There are multiple industry standards established to validate PCB design and deployment for critical applications like automobiles, military, aerospace, and sub-sea electronics.
Extreme temperatures impact not only PCB materials but also the geometry of the PCB assembly. A pressure difference can induce physical stress on electronic products. Moisture in the working atmosphere can corrode the PCB assembly and break down the entire electronic device. Vibration fatigue in automotive applications is a serious concern for PCB manufacturers.
For pressure-tolerant electronics, the enclosure should be designed to withstand very high or cold temperatures, continuous motion, vibration, and pressure. The PCB design should use components and materials rated to operate in extreme conditions. Meeting the requirements and acceptability standards of rugged electronics will ensure consistency in product performance. Building electronics to sustain harsh environments demands an optimal convergence of PCB design, assembly, and testing processes.
Designing for the Operating Environment of Electronic Products
A detailed understanding of the product’s operating environment is the first step in building rugged electronics. PCBs may be exposed to various types of circumstances:
- Extreme and varying temperatures like space applications
- Shock and vibrations in automotive applications
- Moisture and dirt in marine and military applications
- High pressure in aerospace and sub-sea applications
- Power surges in medical applications
Based on the particular work environment, PCB designers have to capture the necessary information like product deployment location and associated environmental parameters such as:
- Operating temperature range
- Chances of water, air, or dust exposure and the tolerable ranges
- Expected pressure or vibration
- Power surges and electromagnetic interference
Extreme environments can significantly reduce the performance and lifespan of electronic devices. Unless the product is designed for harsh conditions, the extreme temperature can randomly expand the PCB layers along with the copper traces. Varying temperatures also impact the solder joints and hence the signal connectivity. PCB assembly components like transistors, ICs, and discrete parts (resistors, capacitors, inductors, etc.) have parameters dependent on the operating temperature, which may affect the circuit functionality. High temperatures can outgas the PCB material into the enclosure, causing corrosion.
Pressure and vibration can cause the external cabinet to explode, exposing the electronic circuitry to the outside environment. Any pressure difference can impact the PCB and its components. It can quickly diffuse PCB material into the environment. During the chip manufacturing process, voids may be created inside the components and filled with air. Such components mounted on PCBs can be ruptured with any pressure difference, triggering component and product failure.
Moisture or dust on the PCB can cause electrical malfunctions like signal attenuations. Excess humidity can corrode the PCB. It can cause short circuits leading to fire hazards in extreme cases. Power surges due to thunderstorms or electrostatic discharges (ESD) can completely damage the electronic product. Excessive electromagnetic interference from surrounding equipment or work setup can impede the board’s performance.
Design Recommendations for Pressure-Tolerant PCB Assemblies
1. Printed circuit board material selection
The substrate material and copper foil should be chosen according to the work environment of the electronic product.
Polyimide and Rogers materials (hydrocarbon ceramic laminates) are suitable for extremely high temperatures. Aluminum for cryogenic temperature and FR4 (flame retardant PCB material) for low-temperature applications are recommended. In a high-humidity environment, FR4 or low-temperature cofired ceramics (LTCC) materials are better choices. Polyimide and polytetrafluorethylene (PTFE) are examples of corrosion-resistant PCB materials and are suitable for humid environments.
It is required to match the dielectric constant (DK) of different substrates and cores in the PCB stack-up. Also, the coefficient of thermal expansion (CTE) of adjacent substrates should match for uniform expansion or contraction of the PCB layers in harsh conditions.
2. Choose pressure-tolerant components
Component selection is crucial while designing pressure-tolerant electronics. The internal structure and construction of passives, ICs, and other electronic parts can significantly influence board performance under pressure.
Resistors with carbon and metal film through-hole parts and surface mount resistors with thick and thin films are all void-free and preferred in pressure-tolerant designs. Thin-film types have the minimum dependency on pressure and are ideal for high-pressure applications.
Polymer film capacitors are void-free and are quite stable in a high-pressure environment. Ceramic capacitors have good compressive strength and are durable. With soft termination types, ceramic capacitors are more suitable for high-vibration applications like automotive electronics.
Transformers and inductors display the least changes in their ferromagnetic features under high pressure. When choosing isolators, the latest on-chip magnetically and inductively coupled isolators are preferred as they are constructed with no free spaces inside the component. Silicon oscillators without mechanical resonators are preferred in sub-sea electronics to sustain fluid pressure.
CMOS ICs are usually not affected by high pressure but the analog devices like op-amps’ accuracy may degrade with higher compressive forces in the environment. Regular re-calibration and strategic placement of ICs can assist in dealing with die-stress induced due to high environmental pressure. Generally, leaded packages offer higher consistency and durability than surface mount parts in harsh environments. Epoxy-packaged ICs demonstrate linear compression features and are suitable for pressure-tolerant designs.
3. Implement thermal management schemes
Electronic components usually dissipate heat during circuit operation. During the component placement on the PCB, it is necessary to estimate the thermal dissipation and the power budget of the electronic equipment. To operate electronics in extreme environments, an efficient thermal design and implementation of heat dissipation strategies are mandatory.
PCB designers include thermal vias on the circuit board to dissipate heat from the high-current components. Uniform soldering thickness of device joints can reduce heat accumulation at the component pins. If using heat sinks is not sufficient for effective thermal dissipation, then additional cooling circuits like fans are necessary for the PCB assembly.
4. Add extra support to build rugged PCBs
Using through-hole parts (connectors, resistors, capacitors, etc.) is recommended in vibration and pressure-sensitive applications, as SMT PCB assembly does not go well in such requirements. An electromagnetic shield for critical components on the PCB can mitigate external noise issues. To avoid any damage due to dust or moisture, conformal coating on the PCB surface or sealing the components with a resin is advised.
Several types of conformal coating can protect the PCB from environmental damage. Epoxy and polyurethane resins are great insulators against aggressive environments and are used to protect PCBs from moisture, vibration, and thermal and mechanical shocks. The coating thickness supports the immersion of the board in water, solvents, and gases.
Silicon resin coating is made with silicon resin and can resist chemicals, moisture, and vibration. They can protect PCBs over a wider temperature range as compared to other coatings. This feature has made silicon resin coating more popular in automotive applications.
5. Considerations for high-voltage designs under high pressure
An important electrical aspect of pressure-tolerant electronics is to understand the breakdown field of air (also gases and fluids) at high pressure. If two conductors are close together at a high voltage, then electrostatic discharge and dielectric breakdown are possible. Further, if the board is operated under high pressure, then the breakdown field also increases linearly with the applied pressure (Paschen’s law). IPC – 2221B standard specifies the conductor clearance requirement to address the possible dielectric breakdown at a high voltage. Adhering to the standard is critical for pressure-tolerant electronics to operate at high electric field strength.
6. Perform rigorous structural and performance testing
Pressure-tolerant electronics have to undergo additional structural integrity testing before deployment. IPC 6013 is the standard for the qualification and performance of rigid and rigid-flex PCBs.
For Class 3 and Class 3A boards (as defined by IPC 6011) used for critical applications in a harsh environment, the inspection grade is quite stringent to ensure error-free performance. Adhering to these standards ensures the integrity of the PCB product.
Regression tests like the Highly Accelerated Life Test (HALT) are used to evaluate the board’s reliability by simulating extreme conditions. Other tests like the Burn-In Test and Electrical Safety Test (EST) are also recommended. Refer to IPC 9592 standard for details on design for reliability.
For sub-sea electronics, different test methodologies are adopted for components and PCB assemblies. To ensure the long-term reliability of pressure-tolerant electronics, cycle testing of the final production design is mandatory. During this test, the PCB assembly is subjected to repeated pressurization and depressurization at defined rates. The hold time and endpoints are also controlled during the test to identify any performance degradation of the product.
For rugged electronics, a complete PCB development process involves choosing PCB substrates and components carefully and employing appropriate design and assembly procedures. Ensuring regular maintenance of the products can reduce defects and avoid overall system breakdown. Following these guidelines could yield electronic designs that can operate efficiently under pressure in harsh conditions.