Heat Transfer from the Resistors

INDIANA
UNIVERSITY
PURDUE
UNIVERSITY
INDIANAPOLIS

Computational Mechanics
LABORATORY (CML)

723 West Michigan Street, SL 138
Indianapolis, IN 46202-5132

Resistors in the electronic industry are used to reduce the current voltage by applying a resistance against the current. The decrease in the electrical energy is accompanied by the increase in the heat energy in the resistor. For this reason, resistors act as heat generators and they conduct the heat to heat up the electronic packages they are mounted on.

When the electronic device is on, the packages warm up and they cool down when the device is off. These on/off cycles induce cyclic stresses and strains on the packages. Since the solder joints are the least stiff and the most compliant of all the materials used in the packages, they fail first.

For a reliable prediction of the solder fatigue life due to the heating from the resistor, a reasonable temperature field must be determined. This is hard to achieve with experimental readings, since temperature cannot be read at all locations.

Research at the CAD/CAM Laboratory includes the Heat Transfer analyses to determine the temperature fields in the packages. The calculated temperature fields are then used as loadings for the stress analyses and the fatigue lives are calculated based on the results of stress analyses.

Below is the temperature distribution in the vicinity of a resistor. One quarter of the resistor is modeled due to the assumed symmetry conditions.

The comparison of the experimental thermo-couple measurements and the analysis results is presented below.

The creep strain distribution on the solder joint is shown below.The highest strain location is where the fatigue cracks initiate from. The below location is in agreement with the experimental observations.