Getting the heat out of PCBs demands intelligent simulation

August 26, 2015 // By Tom Gregory, 6SigmaET
The drive for miniaturisation means that more heat is being generated in very restricted spaces inside electronic and electrical equipment. Accurate thermal simulation of system behaviour can accelerate the design of products that perform better, are smaller, and prove to be more reliable.

Until recently, thermal simulation of PCB assemblies was relatively crude, using approximations of questionable accuracy to predict real-world thermal performance. That’s beginning to change. In this short article, I will examine the key issues around the thermal design of PCBs, and how some of the latest simulation tools are making good thermal design easy to achieve.

Like any aspect of design, creating the optimal printed circuit board (PCB) involves trade-offs. Packaging designers dictate mechanical constraints; electronics engineers set electrical constraints, ranging from signal integrity to EMC compliance; and production engineers demand that the final design can be manufactured as easily and economically as possible. PCB design tools recognise this situation: such tools often incorporate a wide range of constraints data before the design work gets started. Then the tools guide you through the endless compromises and choices you need to make.

In addition, you need to consider the issue of thermal design – something that becomes ever more critical with product miniaturisation. The heat generated by active components often has few places to go, and the build-up of this heat has implications for product performance, reliability, size, and cost.

To complicate things further, the best PCB layout for electrical performance is usually the worst for thermal performance. A wide swath of copper can be great for conductivity, as it can minimise the heating effect of currents passing through it. However, it also creates unwanted capacitive coupling with conductors on other layers of the board, and it takes up too much space at a time when board layouts are becoming ever more complex and dense.

PCBs can carry currents of 100A or more when powerful processors, heavy-duty semiconductor switches, or amplifiers are components of the design. The efficiency of components falls as signal frequencies rise, so high-frequency components may be some of the hottest. However, these components must be connected by short traces to ensure signal integrity, minimise inductive reactance and capacitive coupling, and prevent unwanted RF radiation or pick-up. Additionally, when you bring these components close together for best electrical performance, the heat becomes more concentrated and problematic.

Equipment that will operate in harsh circumstances is particularly susceptible to thermal problems. Most electronic products need to be capable of functioning in ambients up to 50°C, many of them without fans.

Here are just a few of the unwanted effects of elevated temperatures on circuit components:

- Class III (Z5U) ceramic capacitors, used for coupling and decoupling, can lose as much as 56% of their capacitance before they reach their operating limit of 85°C ambient.

- Tantalum and aluminium electrolytic capacitors have to be derated as temperatures rise, and high temperatures dramatically affect reliability. A rule of thumb for aluminium electrolytic capacitors is that the Mean Time Between Failures (MTBF) is halved for every 10°C rise in temperature above 20°C.

- The maximum operating voltage of capacitors declines with temperature. High temperatures may lead to a requirement for capacitors with a higher nominal operating voltage, making these components larger and more expensive.

- High temperatures can change the characteristics of semiconductor devices. For example, some power rectifiers do not operate correctly above 75°C.

- Quartz crystals shift frequency with temperature. The most popular type, AT-cut crystals, have an inflection point of 25°C. Low-cost crystals may shift frequency by 50 ppm at 85°C. You can specify more stable parts, but costs increase significantly.

- Temperature affects the power output of batteries, and some types (Li-Ion included) are prone to catastrophic failure if they overheat.