Components and currents heat up the PCB - but how hot will it get? Will you keep the temperature limits? How long will it take to get too hot? How will a mix of high and low load oscillate?

Neither power dissipation nor electrical current are reliable indicators for the expected temperature. Each printed circuit board is a special case with special losses, layer structure, conductor geometry and cooling environment. Not a single data sheet or internet calculator can tell you how high the expected temperature will be.

PCBI physics is not limited to a maximum number of layers, traces, holes or components. In the first calculation projects, some users will be surprised that their rooted ideas about the flow of electricity and heat won´t be reflected in the results. But you can be sure that the calculation is correct, if the entries and assumptions match. On closer inspection, the result can be explained and understood. Experience has shown that calculation and infrared measurement match very well in a flawless model structure. 

What is PCBI Physics?

PCBI physics is more than only a calculator. It is an electrical and thermal 3D field solver.

• The electric field solver calculates the potential and voltage distribution in traces, planes and vias based on ampere in the detailed layout geometry. Voltage drop and resistance are a by-product.
• As soon as the voltage distribution is available, the detailed spacial thermal power distribution in traces and layers is alsow known.
• The thermal field solver adds the electrical power distribution in conductor paths, planes, vias and the thermal power loss (Watt) of components and calculates a detailed temperature field. The heat spreading in copper layers and FR4 is no longer an assumption, but a result.
• Both solvers can be started either individually or together. The combination of heat and current predicts the track resistance in hot operating conditions.
• The temperature field can also be solved time-dependent (transient). Either with fixed current and component heating or with different operating states. With only one operation you get a heating curve (when the final temperature becomes stable orwhen a component or a trace exceeds its critical temperature limit). If you use several operation states, you can calculate the temperature of a load profile. Each operation state defines certain networks and components and thus you simulate a load sequence.

Allthough current and temperature are based on similar physical descriptions and equations, the technical treatment and the required data are different. For example, voltage calculations require electrical material parameters, sinks and sources in the networks of interest. The electricity is confined to the traces of the board. Thermal calculations require the thermal conducitivity of the material and some information about how the heat is dissipated from the surface to the plate and how it is transported to the surrounding medium. This is the basic effect of surface cooling. But there is also a "volume cooling". The temperature depends on, how layers and prepregs shift the heat within the PCB volume, how thick an seperated they are, how tightly the heating tracks and components are packed and, where the heat can be transported. There are no hard and fast rules for predicting the right temperature. Therefore, a field simulation is indispensable.

Which data do you need?

For the electrical calculation, current or voltage values must be defined for respective networks. As PCB-I knows all PCB details such as number and thickness of layers, trace shape, holes, nets and connections, you are free from any geometric input. All you have to do is to transfer the necessary electrical information from the schematics into a table with networks and pads.

The selection of components and nets for an operating state is very convenient. PCBI physics shows a graph of all involved networks, linked pads and components and automatically fills in current values from the point of current supply to the point of extract. Component losses can also be added here.

As mentioned above, heat must be extracted from the plate. If it is released from the surface into the ambient air, the mechanism will be controlled by the so-called heat transfer coefficient. A built-in wizard calculates this value. If heat is extracted by metallic parts (screws), the temperature of the bolt must be known. You can also assign the thermal resistance of a heat sink.

How fast is PCBI physics?

Field calculations are more complex than spreadsheets and require much more resources because the language of the field is partial differential equations. The results are not available in a flash and solving a 3D field takes computer time. For faster processing, physics is therefore not only developed for CPUs, but also for GPUs. With a good graphics card, the speed gain can be enormous. The results of free convection (also called "still air") on the laboratory bench can best be compared with high accuracy.