Best temperature-tolerant materials to build Class-3 PCBs

PCB requirements in each of these applications differ and are outlined in IPC 6012. This IPC standard describes the requirements and the acceptability specifications for each class of PCBs. Based on the end functionality and required performance, the PCB manufacturing process varies during the design, fabrication, and inspection stages.

While Class 1 PCBs are designed for general electronics with shorter lifecycles and average quality, Class 2 PCBs are used in service electronics. They are intended to operate for a longer duration with moderate quality. But they are not considered performance-critical applications. Class 3 PCBs are built to run in harsh environments like high temperatures, pressure, humid atmosphere, etc. Typical applications for class 3 PCBs are medical devices, aerospace equipment PCBs, and military apparatus.

Requirements to build Class 3 PCBs

Class 3 PCBs are designed for strict specifications and tight tolerance standards. Choosing temperature-tolerant materials and components is mandatory. There are several parameters to be considered while selecting the PCB substrate.  Components used in Class 3 PCBs are of industrial or military grade that is fabricated to perform in extreme conditions.

PCB layout is designed to include the best-known heat management schemes. Thermal design guidelines are included along with DFT and DFM recommendations. Sufficient time should be invested in the design stage to ensure the high quality and robust performance, expected from the Class 3 PCBs.

The fabrication process should be accurate to build flawless and rugged PCBs.  Additional support is necessary while assembling the components to sustain harsh operating conditions. Using through-hole parts to withstand vibrations, shields for electromagnetic compliance, and sealants to make the board waterproof or airtight are some of the requirements in Class 3 PCB assembly. Also, a thorough cleaning process has to follow after the PCB fabrication and assembly stages.

A detailed inspection and testing process has to be included during the manufacturing of Class 3 PCBs to detect any possible faults in the early production stages. This assures the quality standards expected from the functionality-critical Class 3 PCBs.

PCB material parameters influenced by extreme temperature

Extreme temperatures can affect both the material properties and physical form of the PCB. The board performance can deteriorate due to various conditions like thermal cycling, outgassing, or thermal shock. It is necessary to choose the PCB substrate based on critical parameters like:

Thermal parameter

• Glass transition temperature – Tg

It is the temperature range at which the PCB substrate transforms from a glassy, rigid state to a deformed, soft material due to external high temperature. PCB material with Tg greater than 170°C is recommended for high-temperature applications. It is also advised to choose a material whose Tg is at least 25°C higher than the maximum operating temperature of the PCB.

• Coefficient of thermal expansion – CTE

This is the rate of expansion of the PCB material and is measured in parts per million (ppm). CTE of a substrate increases if the PCB is exposed to a temperature higher than its Tg value. A difference in CTE of the substrate and copper layers can induce interconnection issues. During thermal cycling, a disproportionate CTE in the z-axis can damage the Plated Through Holes (PTH) on the PCB. Thus, a material with a CTE<70ppm/°C is recommended for high-temperature applications.  

• Thermal conductivity – K

The amount of heat conducted by the substrate is known as its thermal conductivity, measured in watts per meter at kelvin (W/MK).  To handle high temperatures, the PCB material should have higher thermal conductivity to quickly dissipate the excessive heat into the atmosphere.

• Decomposition temperature – Td

The temperature at which the PCB material decomposes chemically is known as the decomposition temperature and the change is permanent. Hence, a material with Td > max PCB operating temperature has to be selected.

Mechanical parameters

• Peel strength

In a PCB, the bonding ability between the dielectric and copper layers is known as its peel strength. Under thermal stress, higher peel strength material ensures better reliability of the PCB performance.

• Time to delamination

When PCBs are exposed to thermal shocks, the layers get delaminated. The time till the dielectric material can endure this delamination is called the time to delamination.

Materials that are best suited for Class 3 PCB applications

Class 3 PCB products are deployed in harsh environments. The material used should be temperature-tolerant and hence the above-discussed parameters play a vital role. In high temperatures, FR4 with high Tg values can be used. But for extremely high temperatures, Polyimide, Rogers 3000, or Glass-ceramics are recommended. High lead content solder is suitable for high-temperature applications.

FR4 can be used in low temperatures and aluminium or polyimide are suggested for cryogenic temperatures. Indium-based solder is better in low-temperature applications. Also, an ENIG surface finish for the PCB is advised to avoid corrosion due to moisture in a low operating environment.

In applications like satellites, aircraft, and automobiles, PCBs undergo multiple thermal cycling leading to stress and breakdown of the circuit. It is necessary to match the CTE value of each layer in the PCB stack up to avoid uneven expansion or contraction of the PCB layers. Thermal shocks can affect the board strength and result in cracking or bending of the PCB. Choosing low-CTE materials like polyimide can reduce the thermal shock effects on the PCB.

Materials with high thermal conductivity like alumina (28-35 W/m K), aluminium nitride (140-180 W/m K), and Beryllium oxide (170-280 W/m K) are ideal for quick heat dissipation in high-temperature applications. Ceramic substrates with thermal conductivity of 25 W/m K can reliably operate in high temperatures up to 350°C. For example, Rogers RO4000 series are ceramic laminates with Tg=250°C.

Using an aluminium core with FR4 material can also be a good choice for high-temperature applications as the metal substrate can rapidly dissipate heat. FR4 with high Tg is also available, like Nelco N4000 -6 FC.

Conclusion

Building Class 3 PCBs requires a thorough understanding of the performance requirements and associated industry standards. As the end applications are critical, following the design recommendations and collaborating with an experienced contract manufacturer (CM) are the key factors. There are umpteen PCB materials available in the market for temperature-tolerant applications. By understanding the essential parameters discussed in this article, you will be better equipped to make the right choice of material for your Class 3 PCB design.