The EMPF Helpline received a call requesting analysis of a surface mount fast rectifier diode component soldered to a printed circuit board (PCB) that failed. To resolve the customer’s inquiry, the EMPF performed failureanalysis to identify evidence of the failure site, determine the failure mode and complete a report of the findings.

The customer was asked to provide key pieces of information prior to failure analysis to assist with selection of analytical techniques to save time and to prevent errors. This included the component’s data sheet, electrical test information upon failure (if available), external physical conditions at the time of failure (temperature, etc.), whether removal of the device from the PCB is required, and whether additional good components are available for comparison.

Based on the information provided by the customer, the EMPF determined that the following analysis techniques would provide the most insight:

• De-soldering of the component from the board
• Electrical testing using a curve tracer
• Optical microscope analysis
• X-ray analysis
• Decapsulation

After the diode was de-soldered from the board following IPC standard STD-7711, handheld probes were used to connect the leads to a Tektronix 370A programmable curve tracer. The diode was found to have a dead short between the two external leads. Following normal procedures, a good device was also tested, and it showed typical forward and reverse breakdown I-V (current-voltage) characteristics in comparison to the failed diode. Next, the failed diode and the good unit were inspected using a stereo-zoom microscope to look for any abnormalities. The top of the failed diode showed that the surface of the plastic mold compound encapsulant had melted and degraded (Figure 3-1).

X-ray analysis was performed to both view the internal structure in preparation for the decapsulation procedure and also to look for any anomalies. The x-rays showed differences in the lead frame construction of the good diode versus the failed diode as seen in Figure 3-2. After confirmation with the customer, this was confirmed to be due to the vastly different time frame of manufacture (lot numbers) for these components, showing a slight change in the package design used. An anomaly was also noted in the failed diode as a dark shadowed area in the lower left part of the die in Figure 3-2.

Decapsulation was performed on the failed diode in successive steps to remove the mold compound and expose the die using both fuming nitric and fuming sulfuric acids, in accordance with ACI specification AP0600. The external leads did not remain after the long period required to expose the small diode die, but evidence of failure was found on the die in the form of a black, degraded mold compound spot as shown in Figure 3-3. This reveals a condition of electrical overstress (EOS) due to high current flow through the silicon die.

Conclusion
Failure analysis of the diode showed evidence of electrical overstress (EOS) in the corner of the die. This was most likely due to an over-current electrical stress that was hot enough to melt and degrade the plastic mold compound above the die. EOS typically occurs in the corner of die since the electric field density is highest in this location.

The EMPF recommended a review of the test procedures, PCB application conditions, and circuit design, which could present an over-current condition to the diode. A diode with a higher tolerance for high current could also be substituted if deemed appropriate with respect to the circuit board design and operation.

If you have any questions regarding the diagnosis of failure modes, mechanisms, and failure analysis techniques for component and die fault isolation analysis, please contact the EMPF Helpline at (610) 362-1320. A manufacturing expert will be able to offer technical insight and appropriate advice regarding your concerns.