What PCBs Work Well in Harsh Environments?

Designers at RUSH PCB try to maximize the performance of printed circuit boards (PCBs) they make. Applications demand higher power densities and operating temperatures that can literally cook conductors and dielectrics. Apart from outright failure, elevated temperatures affect electrical and thermal performance of PCBs, causing systems to behave erratically. Cracking and connection failure can result from unequal expansion and shrinking of different material when subjected to cyclic heating or cooling. The dielectric may lose its structural integrity altogether if subjected to high-enough temperature.

Heat has always been a major factor in the performance of a PCB assembly, and designers are familiar with the use of heat sinks on heat producing components on the PCB. However, the high power-density requirements of today’s equipment can overwhelm traditional methods of dealing with heat management.

It is not easy mitigating the effects of high temperatures, as this affects many other factors other than the reliability and performance of PCB working at elevated temperatures. Some factors are linked to power requirements, others to application size, system weight, and cost.

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High Temperature Circuit Board

RUSH PCB typically defines a high temperature circuit board as one with a dielectric having a glass transition temperature (Tg) of higher than 170°C. A simple rule of thumb recommended by RUSH PCB is to allow an operating temperature of ~25°C below the Tg for a PCB with a continuous thermal load. Therefore, any application that has to operate in the vicinity of 130°C or higher, needs a PCB made with high Tg material.

Some common high Tg materials available in the market are:

• ARLON 85N
• ITEQ IT-180A
• Shengyi S1000-2
• ISOLA G200
• ISOLA IS420
• ISOLA IS410

RUSH PCB has some recommendations for design methods and technologies that designers need to follow during PCB fabrication and assembly to cope with applications using such PCBs when operating in high temperatures.

Methods of Heat Dissipation

From high school Physics, it is well-known that there are three basic methods by which heat travels from a region of higher temperature (source) to another region at a lower temperature (sink). These methods are—conduction, convection, and radiation.

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Heat Dissipation by Conduction

Heat dissipation by conduction happens when there is direct physical contact between the heat source and sink. This mechanism is analogous to flow of electric current. The temperature difference between the source and sink is similar to voltage difference, while the opposition the heat faces when flowing through the conductor is similar to electrical resistance, and the amount of heat transfer per unit time is similar to amperage.

In fact, the factors making a good electrical conductor tend also to make a good thermal conductor as well. This is understandable because both mechanisms represent forms of atomic or molecular motion. For instance, Aluminum and Copper are excellent conductors of electricity, and therefore of heat as well.

Following the above logic, large conductor cross-sections are better for electrical conductivity and likewise for heat, while long, narrow flow paths are equally detrimental to a conductor’s electrical and thermal performance.

Heat Dissipation by Convection

Heat dissipation by convection happens when the heat sink is in the form of a fluid— liquid or gas—typically known as the coolant. As the coolant absorbs heat from the source and heats up, it becomes less dense, rises upwards and moves away from the heat source, allowing cold coolant to replace it. As the hot coolant moves away, it cools, regains its density and flows down. while the coolant that had replaced it heats up and moves away. This rotational cycle repeats until the entire coolant is at the elevated temperature, or, if another heat sink is available for the coolant to transfer its heat, operates continuously.

Such removal of heat by convection may be forced by the use of a fan to assist the movement of the coolant, in this case hot air, to be replaced by cold air. The major factors affecting convection are the temperature difference between the source and the coolant, the surface area over which the heat is transferred, the coolant’s flow rate, and the ease with which the coolant absorbs the heat. Liquids are more useful as coolants, as liquids absorb heat more readily than gases do.

Heat dissipation by Radiation

Heat can also be transferred in the form of electromagnetic waves, and this method of transfer is known as heat dissipation by radiation. In fact, any object at a temperature of above Absolute Zero is a radiator of thermal energy. On a PCB, heat dissipation by radiation mostly happens from the copper surface on the top layer or the surface of a metal heat sink attached to it or to the heat producing component.

The basic requirement for efficient heat transfer by radiation requires a relatively clear path away from the source, with key factors affecting the transfer being the absolute temperature of the source, raised to its fourth power, the surface area available for radiation, and the thermal emissivity of the material of the source.

Mechanism of Heat Removal from a PCB

The primary mechanism of heat removal from a PCB involves conducting the heat away from the hot component. If the hot component is mounted on the top side of the PCB and has a large-enough surface, a metal heat sink may be fitted on it serving to remove heat from the component by conduction, and thereafter by convection from the surface of the heat sink itself to the colder air surrounding it.

However, sometimes the hot component may be mounted on the underside of the PCB and it may not be possible to mount a heat sink directly on the component. The technique designers usually follow is to use a large number of heat vias or thermal vias to conduct heat away from the hot component to a copper layer on the top side of the PCB, from where it can be transferred further to a proper heat sink.

Normally, heat sinks fitted to PCBs are large, highly emissive, with surfaces finned or corrugated to increase the surface area. Fans are often added to aid the cooling by forced convection as against cooling by natural convection.

Traditional options available to the designer to manage heat from a PCB can be listed as:

• Isolating or removing the heat source
• Reducing power density
• providing better cooling mechanisms
• Increasing the size of the heat sink
• Using larger conductors
• Using exotic dielectric materials capable of withstanding higher temperatures

All the above impact the weight, size, and cost of the entire system. Designers must consider Implementing them in the earliest stages of design and concept development.

New Technologies Allowing PCBs to Work in Harsh Environments

Eminent PCB manufacturers such as RUSH PCB are aware of the limitations of traditional fabrication practices. Therefore, they always strive to keep abreast of current design challenges by offering newer PCB technologies specifically for applications involving higher temperatures. One of the methods employed is to use heavy copper in their PCBs. This increases the current carrying capacity, and because of lower resistance lowers the heat losses. There are different ways to accomplish this.

Etching Heavy Copper

RUSH PCB classifies these as heavy copper or extreme copper boards. As the name implies, these boards use heavier and thicker copper layers than do standard PCBs. However, heavy copper is required only for circuits carrying higher currents, and a PCB may combine heavy copper and standard copper to allow higher currents and signal currents to be present on a single board.

While the same process applies for fabricating heavy copper PCBs, as that required for the regular boards, some special etching and plating techniques apply. The presence of heavy copper has the advantage of lower resistance, allowing higher currents to flow at lower heat losses in the conductors. This technique allows RUSH PCB to incorporate such features as on-board heat sinks operating at high efficiencies. This not only reduces product size but also allows incorporation of on-board planar transformers. Furthermore, as there is no need for fabrication, manual assembly and bonding of standard heat sinks, assembly costs are relatively lower.

Embedded Heavy Copper

Rather than use a thicker copper layer and plating on a PCB, RUSH PCB recommends another approach—embedding heavy rectangular copper wires where necessary. While the advantages of this technique are the same as that when using heavy copper PCBs, the added advantage is in the fabrication, where special etching and plating requirement is no longer required. Depending on individual applications, soldering a wire-embedded board may be easier than soldering on a thick-copper board.

PCBs for Other Harsh Environments

Other than temperature extremes, PCBs may have to face other harsh environments such as an extreme buildup of moisture, the presence of aggressive chemicals, and the presence of toxic vapors in the atmosphere. Harsh environments may also include intense mechanical vibrations and the presence of elements such as salt spray and fungus. The presence of any of the above or combinations could damage sensitive electronic parts mounted on the PCB. Such harsh environments, together with extremes of temperature are often found in the automotive industry, aerospace, and defense applications.

Encapsulation

One of the most common methods of protecting a PCB and its components from harsh environments such as toxic gases and corrosive fluids is to encapsulate the assembly in a material the toxic or corrosive element cannot penetrate. This material must also be electrically insulating and thermally conductive.

A variety of resins are suitable for encapsulation of a PCB assembly, and each has its advantages and disadvantages:

Resin Type	Advantages	Disadvantages
Urethane Resin (UR)	Very good resistance to chemicals Good resistance to humidity Resistant to mechanical wear	Difficult to remove Long cure times Risk of peeling
Acrylic Resin (AR)	Easy to apply and remove Easy to rework and repair No shrinkage during curing Affordable	Low resistance to chemicals Low resistance to abrasion Not suitable for harsh environments Not good for high temperatures
Epoxy Resin (ER)	Great resistance to moisture and abrasion Great resistance to chemicals and humidity Excellent performance in harsh environments	Difficult to remove Shrinks during curing
Silicone Resin (SR)	Good performance in extreme temperatures Excellent resistance to humidity and corrosion Good resistance to chemicals Adheres well to most PCB components/materials	Most difficult to remove Strong chemicals required for removal Allows localized repairs only
Parylene (XY)	Best resistance to solvents and extreme temperatures High dielectric strength No curing time required Coating forms at room temperatures Transparent and colorless coating	Difficult to remove, must be abraded off Chemical vapor deposition requires specialized equipment Not good for long-term exposure outdoors.

Encapsulation works best for PCBs with components of similar heights. PCBs that have components with highly dissimilar heights are difficult to encapsulate. Heat transfer out of the PCB is another factor that concerns designers who have to encapsulate PCBs. Heat sinks and surfaces that heat sinks attach to must remain free of encapsulation for better heat transfer.

For protecting electronics from extreme mechanical vibrations, the usual technique designers follow is ruggedization. This involves encasing or mounting the PCB assembly or anything that the vibrations may damage in a separate casing with adequate cushioning to prevent the parts from moving in case of an impact or during vibrations.

Conclusion

RUSH PCB recommends gathering as much information as possible about the harsh environment the PCB will be working in before deciding on the steps necessary to mitigate their effects and keep the PCB working.