Part One: What to look out for when making PCBs for extreme environments?

Moisture, steam, dust? No problem! PCB manufacturing can handle these environments too.
All modern electronic devices need printed circuit boards to operate. In printed circuit boards, the circuit is printed on a non-conductive material.

Generally, boards are made from waterproof fiberglass materials that can withstand high temperatures and provide insulation between the copper layers, as well as minimize interference and promote good signal integrity.

These properties make fiberglass an ideal material for universal printed circuit boards. And that is why they are widely used in most consumer electronics products.

What do you mean by extreme conditions?

It does not necessarily have to be a space environment. Even in our everyday surroundings, there are many environments that can be considered extreme for the operation of electronic devices.

These include:

• Environments with extreme temperatures, both hot and cold, which can affect the performance and reliability of electronic devices.
• Environments with significant fluctuations in temperature or humidity, which can place additional stress on electronic components and circuit boards.
• Rainy or highly humid environments, where moisture can penetrate the electronic device and cause corrosion or short circuits.
• Industrial and dusty environments, where dust and other contaminants can accumulate on electronic devices and affect their operation.
• Electrical surges, whether caused by natural phenomena such as lightning or by man-made sources, which can damage sensitive electronic components.
• Locations exposed to electrostatic discharge (ESD) or electromagnetic interference (EMI), which can interfere with the operation of electronic devices.

Designing circuit boards to withstand harsh conditions requires special skills and knowledge to ensure that the final product performs reliably and lasts as long as possible in extreme environments.

Let’s take a look at the challenges faced when manufacturing circuit boards for extreme environments.

Temperature of Electronic Devices

Every electronic device generates a certain amount of heat during operation. For standard circuits, this amount of heat is not enough to damage the entire circuit board. However, in an environment with extremely high temperature, the board would already be problematic.

A way to dissipate the heat from the circuit board is to add a copper surface layer in the non-conductive part of the circuit. For example, FR-4 fiberglass material is not an excellent conductor of heat, and pouring copper in areas outside the circuit helps cool the circuit.

But beware, this technique only works up to a point. If the heat produced is too great, use metal-clad printed circuit boards. These are boards that have an electrically insulated aluminum sheet as a base plate. Aluminum absorbs heat much faster than FR-4 material, protecting the circuit board components from overheating and extending their life. Aluminum circuit boards are generally used, for example, in the manufacture of LED luminaires, especially in flat panel lighting.

 And if even the integration of aluminum boards is not sufficient, a ceramic circuit board can be a suitable choice for demanding electronic devices. Ceramic printed circuit boards are based on highly thermally conductive materials such as aluminium oxide, aluminium nitride and beryllium oxide. These materials enable heat to be removed from hot spots more quickly and dissipated across the surface of the board and into the surrounding environment.

Ceramics offer a significant advantage over traditional materials such as FR-4 and metal-clad boards when it comes to heat dissipation.

Ceramics have a very low coefficient of thermal expansion, providing additional compatibility options in PCB design.

Heat transfer through the board is significantly more efficient, as the components can be mounted directly on the ceramic substrate without an insulating layer.

Generally, when electronic devices are used in environments with extreme temperatures, it is not always necessary to modify the PCB itself. Instead, external cooling solutions may be used to maintain the required operating temperature. The same principle applies to the assembly.

Moisture and dust of Electronic Devices

If the circuit board is used outdoors or exposed to the natural environment, the electronic device must be protected from weather conditions, including water, dirt and dust. Without proper surface treatment, the circuit board or the electronic device may be damaged.

To prevent this, the circuit board should be coated with a protective conformal coating to keep the circuitry dry and free from dust and moisture.If the circuit board will be placed outdoors or exposed to a natural environment, it must be protected from the weather, including water, dirt and dust. Without proper surface treatment, damage to the circuit or equipment will occur. To prevent this, the circuit board must be coated with a surface varnish to keep the circuitry dry and dust free.

There are basic types of lacquers, each with their own advantages and disadvantages.

• Acrylic resin
• Urethanes
• Epoxy resin
• Silicone resin
• Parylene

Acrylic resins: popular for their simple application, minimal thermal effects during drying and easily adjustable viscosity. In addition, they do not shrink further after the solvent evaporates and cover the same surface area as after application. If a solder joint on a PCB needs to be repaired or a component in an electronic device needs to be replaced, the coating can be easily removed using solvents after curing.

Unfortunately, this is due to the low chemical resistance of the coating. And also with this type of coating, what form of curing occurs, if by elevated temperature, then it must be taken into account that acrylic coatings have a low glass transition temperature Tg. If this temperature is exceeded, the coating may stretch. Conversely, if UV is used to cure, imperfect curing may occur in shaded areas. This treatment produces a pungent odour with the side effect of skin irritation.

Urethanes: Coatings are heat curable within three hours but have a shorter life. When curing by evaporation of the solvent, increased humidity, which adversely affects the speed of the process and the desired properties, must be avoided. After curing, they retain their dielectric properties best when loaded in the ambient environment.

Epoxies: can withstand higher glass transition temperatures. They also provide good resistance to mechanical damage. They are resistant to moisture and chemical environmental influences, which means they are almost chemically non-removable for possible repairs.

Silicones: silicone coatings find use in applications where electronic assemblies must withstand extreme temperatures and high dielectric strength is required. Silicone resins are flexible, resistant to moisture and solar UV radiation. They are most commonly used in the automotive industry. Their disadvantage is their reduced handling time before curing and against mechanical stress in the form of abrasion.

Parylene: It works best against environmental chemicals and moisture. It has excellent dielectric properties, mechanical resistance and a low coefficient of thermal expansion. The disadvantages of parylene are the need for vacuum masking of unpainted areas, more technologically demanding repairs and lower UV resistance.

The choice of paint is based on the application and the functional requirements within the application. Some components on the circuit board cannot be covered with a protective varnish. These include, for example, contact systems, trimmers and similar components.

These components should be covered or painted with a removable varnish before application to prevent a protective coating from forming.

High performance printed circuit boards

Circuit boards are made of thin copper traces. These copper traces are typically designed for low-power applications, as the current-carrying capacity of a trace depends on the width and thickness of its cross-sectional area. For standard circuit boards, a common copper thickness is 1 oz, which corresponds to approximately 35 micrometres. Therefore, when an electronic device requires a higher current load, we can increase the width of the trace or use multiple copper layers to increase the cross-sectional area and allow the board to carry more current.

Other challenges include interference caused by electrostatic discharges and electromagnetic waves. However, these issues will be discussed separately in the next section. Designing printed circuit boards for electronic devices that can withstand such demanding conditions is far from simple and involves numerous design constraints.