High Power Chip Resistors and Thermal Management
Stackpole has developed several surface mount resistor series in addition to our current sense resistors, which have had
higher power ratings than standard resistor chips. This has caused some uncertainty and even confusion by users as to
how to reliably use these resistors at the higher power ratings in their designs.
The data sheets for the RHC, RMCP, RNCP, CSR, CSRN, CSRF, CSS, and CSSH state that the rated power assumes
an ambient temperature of no more than 100°C for the CSS / CSSH series and 70°C for all other high power resistor
series. In addition, IPC and UL best practices dictate that the combined temperature on any resistor due to power
dissipated and ambient air shall be no more than 105°C. At first glance this wouldn’t seem too difficult, however the graph
below shows typical heat rise for the CSR ½ 100 milliohm at full rated power. The heat rise for the RMCP and RNCP
would be similar. The RHC with its unique materials, design, and processes would have less heat rise and therefore
would be easier to implement for any given customer.
The 102°C heat rise shown here would indicate there will be additional thermal reduction techniques needed to keep this
part under 105°C total hot spot temperature if this part is to be used at 0.75 watts of power. However, this same part at
the usual power rating for this size would have a heat rise of around 72°C. This additional heat rise may be dealt with
using wider conductor traces, larger solder pads and land patterns under the solder mask, heavier copper in the
conductors, via through PCB, air movement, and heat sinks, among many other techniques. Because of the variety of
methods customers can use to lower the effective heat rise of the circuit, resistor manufacturers simply specify power
ratings with the limitations on ambient air temperature and total hot spot temperatures and leave the details of how to best
accomplish this to the design engineers. Design guidelines for products in various market segments can vary widely so
it would be unnecessarily constraining for a resistor manufacturer to recommend the use of any of these methods over
Note: The final resistance value can be affected by the board layout and assembly process, especially the size of the
mounting pads and the amount of solder used. This is especially notable for resistance values ≤ 50mΩ.
This should be taken into account when designing.