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Designing and Building PCBs for Harsh Environments
By Akber Roy, CEO, Rush PCB, Inc. D
esigners of PCB assemblies are constantly trying to maximize their performance. Ap- plications demand higher power densities
and operating temperatures that can literally cook conductors and dielectrics. Apart from outright failure, elevated temperatures affect the electrical and thermal performance of PCBs, causing sys- tems to behave erratically. Cracking and connection failure
can result from unequal expansion and shrinking of different materials when subjected to cyclic heating or cooling. The dielectric may lose its structural integrity altogether, if sub- jected to high-enough temperatures. Heat has always been a major
issue in the performance of a PCB as- sembly, and designers are familiar with the use of heat sinks attached to heat-producing components on the PCB. However, the high power density requirements of today’s equipment can overwhelm traditional methods of heat management. It is not easy to mitigate the ef-
fects of high temperatures, since doing so impacts many other factors beyond the reliability and performance of a PCB working at elevated tempera- tures. Some factors are linked to power require- ments, others to application size, system weight and cost.
erating temperature of roughly 45°F (25°C) below the Tg for a PCB with a continuous thermal load. Therefore, any application that has to operate in the vicinity of 266°F (130°C) or higher, needs a PCB made with high-Tg material. Some common high-Tg materials available include: ARLON 85N, ITEQ IT-180A, Shengyi S1000-2, and ISOLA G200, IS420 and IS410.
electrical conductor tend to make a good thermal conductor as well. Following this logic, large conduc- tor cross-sections are better for electrical conductivi- ty and likewise for heat, while long, narrow flow paths are equally detrimental to a conductor’s elec- trical and thermal performance. Heat dissipation by convection occurs 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 tem- perature, or, if another heat sink is available for the coolant to transfer its heat, operates continuously. Heat can be transferred in the
Along with temperature extremes, PCBs often have to face other
harsh environments, such as extreme moisture, aggressive chemicals and toxic vapors.
There are three basic methods by which heat
High-Temperature Circuit Boards We typically define a high-temperature cir-
cuit board as one with a dielectric having a glass transition temperature (Tg) of higher than 338°F (170°C). A simple rule of thumb is to allow an op-
travels from a region of higher temperature (source) to another region at a lower temperature (sink). These methods are conduction, convection and radiation. Heat dissipation by conduction happens when
there is direct physical contact between the heat source and sink. This mechanism is analogous to the flow of electric current. The factors making a good
form of electromagnetic waves, which is known as heat dissipation by radia- tion. In fact, any object at a tempera-
ture of above Absolute Zero is a radia- tor of thermal energy. On a PCB, heat dissipation by radiation mostly hap-
pens 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.
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
Continued on next page
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August, 2019
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