This series of articles discusses electronics thermal and stress analysis, and cover electronic assemblies wherever they are used, whether they be in cell phones, cameras, computers, electronics for high power applications, avionics, missile and defense systems, etc.
A few of the analyses and tests that electronics must tolerate are:
- Temperature
- Shock
- Vibration
- Thermal stress
Generally, analysis and tests concentrate on high power components. The small, low power components – such as small resistors – will dissipate their heat quickly, and are generally not much of a problem.
Conduction Paths
Circuit card assembly details such as number of layers, circuit card material, copper thickness, and percentage of copper on each layer all affect circuit card conductivity. The solder connections to the terminals are excellent conductors of heat. Also, many components have heat sinks on top of them which dissipate heat by convicting to the atmosphere, and may also have heat conducting pads between their lower surface and the circuit board.
Altitude Effects
The effect of altitude on electronics cooling is very significant. Air temperatures drop significantly, but then so does air density, making convective heat transfer very inefficient, and at the same time making radiation heat transfer much more dominant. Of course, this will have little effect on conduction.
Heat Flow Paths Out Of Circuit Card Assembly
Conduction is the best method, but certainly not the only way to remove heat from electronic assemblies. Heat conducted into the circuit card must be dissipated by way of a conducting path to the assembly case and to structures external to the electronic assembly were it is convicted or radiated away.
Experience with Compact Fluorescent Light Bulbs
Compact fluorescent light bulbs were supposed to last upwards of 10 years. Not certain of what other people’s experiences of them have been, but mine have been anything but that. They may last between one and two years maximum. Where I suspect the problem lies is in the electronic components within the base of the lightbulb. These components under some conditions may very well last for several years. However, the heat that they have to tolerate drastically shortens their lifespan. Not only that, but the thermal cycling that they experience may not only cause the component to fail but also the circuit card’s solder connections to fail due to the uneven expansion and contraction between the electronic component, solder, and circuit card.
Next – a look at component specifications…
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org