November_EDFA_Digital

edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 19 NO. 4 8 orientation, a map can be obtained. This technique is not quantitative, but a comparison can bemade between two different devices. Here, the author tried to minimize the stress level on the solder material by increasing the thickness of the joint. EBSD analysis also provides information about grain size, phase identification, and texture, which is the prop- erty of amaterial composed of grains oriented in the same crystalline direction. The following images present the results obtained on a thick solder joint along with a com- parison to a thin solder joint. Thematerial used is SnPbAg. Figure 11 presents the phase mapping for thick and thin solder joints. No aging was applied on the devices prior to this analysis (no thermal cycles and only func- tional tests of the DC/DC converter). An important dif- ference to note is the presence of needle-shaped silver precipitates (Ag 3 Sn) in the thick solder joint, whereas those precipitates are more circular in the thin joint. This form factor modification is an indication of early stages of aging for the solder material. Figures 12 and 13 show the local misorientationmaps for both thick and thin joints. The colormap is represen- tative of the crystalline misorientation from one pixel to the other, which can be interpreted as the strain stored in the material. Thehistogramrepartition shows that both the average and the maximum misorientation values are higher for the thin joint, which confirms that it was more affected by thermomechanical stresses. This quantitative observation is consistent with a more qualitative approach. The strain applied by the potting resin deformation is spread over a thinner joint, which implies more local stress. CONCLUSION Failure analysis of a DC/DC converter was successfully under- taken. When defect localization is tricky—as for 3-D systems—a local probing approach is probably the best way to limit the region of inves- tigation, until the defect is actually detected. This step is quite difficult to undertake. Overmolding resin etching must be nondestructive for both the component and thedefect, so sample-preparation techniques must be adapted from device to device. In this case study, laser ablation provided good results, FAILURE ANALYSIS OF DC/DC CONVERTERS: A CASE STUDY (continued from page 6) Fig. 9 SEM images of the soldermaterial between the pin and the pad. Recrystallization was found for both devices. Fig. 10 Strain-versus-temperature curves obtained by TMA Fig. 11 Phase identification for thick (above) andthin (below) solder joints