January_February_2022_AMP_Digital

1 9 A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y / F E B R U A R Y 2 0 2 2 die sensor surface. The decap proce- dure completely removes not only the polymer packaging material, but un- fortunately the ball bonds and wedge bonds that are attached to the pads on the die and frame. Thus, bond pad and bond wire issues could only be detected using the CT scan for these sensors. The die surfaces were cleaned using an ul- trasonic bath in acetone to remove res- idue from the acid removal procedure. Die surfaces were examined at high magnification using light microscopy and scanning electron microscopy. The surface of a new die was exam- ined using high magnification micros- copy as shown in Fig. 10. No evidence of defects or damage from EOS or ESD was found. Figure 11 shows an SEM image features such as slag and voids. A comparative macrophotograph of a metallographically prepared cross sec- tion of the weld remnant in the same plane is shown in the etched condition in Fig. 4b. The image clearly depicts the microstructure of multiple weld beads, and multiple heat affected zones (HAZ) that developed in the base steel due to multiple weld passes. This information cannot be determined with the CT scan. Figures 5a and b show that in other locations, a single weld pass and conse- quently a single weld bead microstruc- ture is shown. Examination of the fracture using scanning electron microscopy (SEM) showed evidence of shear overload dimples in the form of directional mi- crovoid coalescence, as depicted in Fig. 6. With the complementary use of CT methodology and traditional met- allurgical techniques, the conclusion drawn from the analysis was that while the fillet weld displayed inadequacies in weld quality and an inconsistent number of weld passes, the fracture of the weld occurred within the weld, and was associated with a single overload event, possibly from impact due to a scissor lift hitting the solar panel post. CASE STUDY 2: FAILURE ANALYSIS OF A TEMPERATURE SENSOR Electronic failure analysis can in- volve high magnification surface anal- ysis of the elements of an electronic package including the die chip, lead frame, bond wires, and traces on the die surface. In this case study, a LM20 temperature sensor was experienc- ing failures related to EOS and/or ESD (electrical over stress or electrostatic discharge conditions). The purpose of the analysis was to examine the sensor to identify the failure defect and failure mechanism that caused the device to malfunction. The sensor was documented using digital photography as shown in Fig. 7. A 3Dmicro-computed tomography scan of the sensor was performed. The po- sition of each ball bond and wire bond was identified for possible lift off con- ditions as shown in Figs. 8 and 9. While the position of the ball bonds on the die surface are resolvable using CT, due to the size of the die surface and use of materials with varying density, being able to fully resolve the die surface with high penetrating energies is difficult. However, the benefit of performing the CT scan prior to destructive testing was being able to identify and analyze the position of the wire bonds. The sensor was mounted for de-encapsulation (a decap procedure) using fuming nitric acid to reveal the (a) (b) Fig. 5 — (a) CT slice as indicated by the yellow line in Fig. 3 shows planar section of weld. Note internal porosity and slag detail. (b) Photomicrograph of cross section showing the HAZ indicates a single pass weld in this location. Fig. 6 — SEM photomicrograph of the weld fracture shows fracture mechanism consistent with shear dimples. Fig. 7 — Photograph of a LM20 temperature sensor in molded electronic package. Fig. 8 — 3D CT rendering shows metallized components within plastic package. Fig. 9 — CT slice planar view of packaging indicates ball bond location on chip die surface.