Inappropriate thick film power resistor mounting can compromise resistor performance. In this post, we cover the key points to consider.
Position on System Board
The primary heat dissipation mechanism of a thick film power resistor is through radiation. It is therefore important to consider the air temperature, airflow around the power resistor and the ability of the resistor device to dissipate heat. Components in close proximity to the resistor should also be reviewed and their impact on the ambient air temperature considered.
Care With Leaded devices
Although thick film technology is generally robust inappropriate handling and processing can impact on 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.
Using A Heatsink
As current passes through a resistor it generates heat. Differential thermal expansions of the resistor materials can cause mechanical stresses in the resistor device. Heat must be dissipated or the resistor performance and/or its lifetime will be compromised.
One method used to dissipate heat is to use a heatsink. The ability of the heat sink to conduct heat away from the resistor is measured in C/W of power dissipation which in turn depends on the heat sink material and its properties, the size and finish of the heatsink and the cooling method. Resistors for heatsink applications are designed to dissipate their heat primarily in one plane – their rear face.
Heatsinks are generally chosen to match the precise requirements of a particular resistor and application. Materials are chosen to have a high thermal conductivity. Aluminium alloys and copper are common choices with steel used in some applications.
The surface area of the heatsink should be much larger than the resistor. This means heat can be efficiently dissipated by convection (natural or forced), conduction or liquid cooling. For a given material and cooling method the larger the heatsink the more heat it may dissipate. Fins may be used to increase its surface area.
In some applications, the resistor may be immersed in transformer oil or deionised water (non conductive), with these materials kept at a suitable constant operating temperature. However, oil and deionised water does need to be filtered, or even exchanged, at intervals as can become conductive over time.
The thermal resistance of a thick film power resistor component is determined by its internal design. It specifies how much hotter the internal resistor is relative to its case or backplate. A thermal resistance value is given on most datasheets.
Thermal resistance is a measure of heat transfer efficiency between two locations. It is modelled as a series of resistances to heat flow. 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 system board and/or the heatsink.
Thermal resistance issues can be managed by modifying the power resistor mounting method, mounting force and the interface between resistor and substrate/heatsink.
To optimise power resistor performance there are many resistor mounting issues to consider. Care should be taken when interpreting datasheet parameters. Where there is any doubt the device manufacturer or a power resistor design specialist should be consulted.