New useful information from simple forward I-V measurement of a power diode

Forward I-V measurement is an important method used to test the quality of power diodes. Diode with excessive V/sub F/ can be due to either defects in wafer fabrication or soldering processes. However, it is difficult to isolate either of the causes of failure when a diode is found to have high V/sub F/. We first examine the conventional /spl Delta/V/sub F/ measurement method used for detecting poor quality of solder for power diode in industry, and find that the method is ineffective if the poor quality of solder is not in the form of voids in solder, but rather a change of solder shape due to contamination or solder material properties. Then, a new method is reported to accurately measure the series resistance determined from a simple standard forward I-V measurement. The importance of the new method is that, without extra measurement and opening the package, one can identify if the cause of high VF failure is due to poor solder or the dice itself. One can also determine the true forward I-V curve of a power diode dice and use the true V/sub F/ of the dice as a quality control parameter to check for wafer diffusion quality, and use the series resistance of solder plus lead-frame as a quality control parameter to check for process integrity. Finally, it is demonstrated that the method could be applied to determine the solder coverage/abnormal shape for actual power diode. This result, coupled with the standard /spl Delta/V/sub F/ method, also enables us to identify units with poor solder quality.