Failure Analysis: Electronics (Diode Investigation)
- Optical Investigation
- X-Ray Planar
- 3D X-Ray CT
- Cross Sectional Analysis
- A customer submitted surface mount diodes that were exhibiting shorts. The failures were traced back to the diode specifically at their facility and it was determined that the PCB was not at issue.
- The submitted failed diodes were examined using optical microscopy in order to determine if there were any superficial variances in the parts. The parts were examined using planar x-ray and CSAM to determine if there was internal damage or defects in the device without damaging or disturbing the component.
- Based on the orientation and size of the wire harness layout, it was determined that 3D X-Ray CT analysis would be necessary to identify internal structures in the single selected device identified for further analysis.
||Close-up of the active area of the device, showing the metallization of the solder attachment migrating down the corner of the device, approaching the metallization on the other side of the device stack.
Micro-X-ray CT performs computation on thousands of images of the device in order to compute a three dimensional data set representing the internal structure of the device. This three dimensional data is extremely detailed and of high resolution, as well as utilizing the numerous photos to be able to resolve details that are often too small or too close in density to the surrounding materials to be seen easily on planar X-ray analysis. This CT data showed that there was an area where the solder metallization was observed to have been applied down one corner, across the diode die.
Cross Sectional Analysis is performed by mounting the device in epoxy to support and protect the device. It is then ground and polished using diamond polishing compounds to create a cross-sectional view of the targeted area. The area was then examined in the SEM, and EDS analysis was used to identify the materials present in the targeted area of the sample.
SEM image of the cross-section through the area of interest. This corner, across the die from the area where there was suspected improper metallization, shows some solder misplaced along the side, though not in a complete conductive path. There is also a small crack at the top of the device, and a significant amount of delamination between the device and encapsulating polymer.
||Close up view of the material from the solder lapping around the corner in the right side of the image. Note that this solder appears to extend back behind the plane of polish on this device, and appears to contact the lower surface of the die to a thickness equal to the metallization on the left side of the die in the image.
Results & Conclusions:
- Optical examination of the devices did not show any significant signs of shorts or device material failure.
- Planar X-Ray and C-SAM examination of the devices did not show any damage, and did not detect the defect that was eventually found. Traditional Non-Destructive Analysis (NDT) proved insufficient for identifying the failure mode.
- Examination of the failed device by 3D X-Ray CT examination showed that there was what appeared to be a narrow strip of higher density material spreading along one corner of the device internals, bridging between the two segments in the active device.
- While 3D X-Ray CT analysis was able to find the defect, Planar X-ray did not show this material, as it was too fine and too thinly spread to show under traditional x-ray illumination powerful enough to penetrate the harness of the device.
- Cross section inspection confirmed the presence of material consistent with the solder used to attach the die structure to the harness.
- This solder formed a thin, monolayer, down the corner of one portion of the device internal, spreading from the conductive plane on one half of the die down across the active area and almost to the conductive backplane of the opposing die segment.
- While there was no sign of ESD and no identifiable area where there was direct electrical contact between the two metallization layers, the solder was observed to spread down the edge of one corner reaching far further towards the opposite contact that it was designed to.
- SEM/EDS analysis confirmed that the solder was almost entirely comprised of lead, not the specification for the part.
- Both the out of specification solder material and the irregular application of solder show that there were assembly and manufacturing issues with the components.
- The stringer of solder bridging across the side surface of the diode die is not an intended design – but a problem with the application of solder to attach the die structure to the wiring internals.
- Migration of this solder, possible due to the bridging structure and small gap under the die encapsulation could easily cause shorting after a period of deployment, especially in high performance environments.