One Road Runner Power MOSFET was received from Acme Supply, Inc. for failure analysis. The submitted device was manufactured by Wiley Coyote International.
Summary
Electrical testing showed that all leads had early breakdowns. Decapsulation revealed a cracked die. Mechanical stress is the most likely cause of this failure. Placing the die very close or over the mold lock, and having die attach in the mold lock would reduce the plastic mold compound adhesion to the paddle. The delamination from this would cause stress on the die. Vibration, flexing, rapid thermal changes, or excess screw torque would add to the predisposition of expanding the crack.
Discussion
No anomalies were observed from the visual inspection. X-ray analysis showed the die to be on or very close to the mold lock channel. Also, die attach appears to have flowed into the mold lock channel. Scanning Acoustic Microscopy (SAM) showed package delamination near the upper die edges. SAM analysis also showed a lighter colored area on the upper part of the die. This was later found to be the area of the die that was cracked.
Electrical testing showed that all leads had early breakdowns. The gate to source characteristics showed the largest current leakage (70uA at 3v). After decapsulation, a cracked die was observed. Liquid Crystal analysis identified a transition site along the die crack near the source pad. Other than the die crack, no additional surface damage was observed at the liquid crystal site. Deprocessing the device showed a damaged gate area at the liquid crystal site.
Conclusions
Mechanical stress is the most likely cause of this failure. Placing the die very close or over the mold lock, and having die attach in the mold lock would reduce the plastic mold compound adhesion to the paddle. The delamination from this would cause stress on the die. This delamination and stress build-up could occur over time ultimately resulting in a crack. Based on the visual inspection and the SAM results, the crack most likely started on the left edge of the die. Once the crack occurred, a leakage site formed along the crack edge causing the damaged polysilicon gate. This electrical defect was the symptom detected as failure. Vibration, flexing, rapid thermal changes, or excess mounting screw torque would add to the predisposition of expanding the crack. Once the die was cracked, metal shorting (along the crack) most likely caused the gate damage seen at the liquid crystal transition site.
0 Comments