Inappropriate thick film power resistor mounting can severely compromise the resistor performance. In this post, we cover the key points to consider
Position on System Board
As current passes through a resistor it generates heat that in turn induces stresses in the power resistor device as a result of differential thermal expansions of the resistor materials. This heat must be dissipated or the resistor performance and/or its lifetime will be compromised.
The primary heat dissipation mechanism of a thick film power resistor is through radiation. If placed too close to other components the heating effect can compromise performance. It is therefore important to consider the air temperature, air flow around the power resistor and the ability of the resistor device, and any surrounding components, to dissipate heat
Care With Leaded devices
Although thick film technology is generally robust inappropriate handling and processing can impact on the power resistor performance. A potential weak point is where the lead exits the resistor device. Undue force on the leads can damage the power resistor track and/or the ceramic substrate.
It is important to always provide strain relief when lead forming. Bending the lead too close to the resistor device, twisting the leads or excessively splaying the leads should be avoided. If the power resistor component is inserted manually the leads should be made long enough to enable the operator to insert it easily, but not so long as to impair wave soldering.
Using A Heatsink
As stated above heat must be dissipated from the resistor device as, if this exceeds design limits, performance will be compromised or the resistor destroyed. One standard method used to dissipate excessive heat is to use a heatsink
In the design process, it is important to consider the real heatsink temperature. This is dependent on the thermal resistance (see below) of the heatsink, the ambient temperature, air flow and power dissipation should all be part of the power resistor mounting considerations.
For very high power resistors, 200 W and above, a heatsink may not be sufficient and it may be necessary to immerse the resistor in transformer oil to more effectively remove the heat generated. This may be supplemented by active cooling that keeps the transformer oil at a constant temperature, usually around 50 °C to 60 °C for maximum resistor performance.
Any thick film power resistor component has a thermal resistance determined by its internal design. This value is given on most datasheets and it specifies how much hotter the internal resistor is in relation to its case or backplate
The interface between the power resistor device and its heat sink is never perfect, it always has some thermal resistance depending on the surface finish of the Heatsink (flatness) and the thermal grease or pad that is used to fill any voids.
The total thermal resistance is therefore the thermal resistance of the resistor (resistor element to base or case) which is fixed and the thermal resistance between the resistor base or case and the heatsink which can be managed depending on the power resistor mounting method (and force) and the interface.
Care should be taken when interpreting data sheet parameters. Where there is any doubt the device manufacturer or a power resistor design specialist should be consulted.