Thermodynamics backgrounder
energetic photons from the environment can arrive at the object and raise the temperature of the object.
The trouble with high temperatures High temperatures accelerate the rate of failure for PCB and components, sometimes in exciting ways. When possible, it’s always best to keep your PCB as close to room temperature as possible. But in the real world it’s not possible, so knowing the failure modes and designing mitigations can save your next design!
Other Methods Engineers have developed
methods to improve these three natural passive methods. For example, thermoelectric coolers take advantage of the Peltier effect to provide localized cooling at the expense of heat generation somewhere else. Heat Pipes absorb energy in one location in a liquid to gas phase change, and then transfer the energy back to the environment elsewhere when the gas changes back to liquid. Perflourinatedpolyethers (PTFE) use bulk liquid → gas phase changes and the subsequent convection to transfer heat from a circuit at the bottom of a tank up to the surface, where the gas quickly returns to liquid form. Application to electronic circuits Joule heating applies to ohmic (Q=I²R) and non-ohmic devices (Q=IV). If the generated heat exceeds the dissipated heat, the part will eventually reach a temperature that allows for the thermal decomposition of the material. Molecules will leave the material and enter the environment
Failure modes
Mechanical Failure of PCB and Components The most recognized PCB
-- often as “magic smoke” that is generated when silicon dies and their bonding wires reach a sufficient temperature to vaporize the epoxy die packages. The high temperatures then permanently damage the bonding wires and silicon fractions of a second later.
substrate material is called FR-4 - a NEMA designation for fire-resistant (FR) fiberglass reinforced epoxy resin (4). FR4 is an entire class of woven-fiberglass materials made with different weave patterns, epoxies, and thicknesses. Many materials fall under the FR4 umbrella, but they all have characteristics important to the discussion of thermal failures: the in-plane-of-weave coefficient-of- thermal-expansion (CTE) and the much greater out-of-plane CTE are both significantly greater than the CTE for copper. That means when the temperature of a PCB increases, FR-4 expands more than the copper. The differential CTE places stresses on the PCB
To keep your PCB from turning into a flaming ball of carcinogens, you have to balance the heat generated in the traces and components with the heat dissipated into the environment.
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