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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 19 NO. 4 4 EDFAAO (2017) 4:4-9 1537-0755/$19.00 ©ASM International ® FAILURE ANALYSIS OF DC/DC CONVERTERS: A CASE STUDY Jérémie Dhennin, ELEMCA jeremie.dhennin@elemca.com INTRODUCTION DC/DCconverters arewidelyused inelectronic applica- tions and, in particular, in the aerospace industry. In this case study, a defective part was retrieved froman aircraft following an abnormal system behavior detected by the airline crew. The incriminated component is a DC/DC converter, which has a single 28 V input and two outputs, at 5 and 18 V. The main difficulty in the failure analysis of such a component is its integration. Two printed circuit board assemblies (PCBAs) are assembled in the same package and molded in a resin (Fig. 1). This layout induces addi- tional challenges for the failure investigation, because 3-D techniques must be used to locate the defect. As usual, the first step of the failure analysis process is to electrically test the component and determine the region that will be further analyzed in the next steps. Here, any local probing of the PCBAs is complicated by the presence of the overmolding resin. This obstacle makes the failure analysis difficult from the outset. DEFECT LOCALIZATION ELECTRICAL TESTING The observed failure mode is a bad converter startup. Indeed, the overvoltage/undervoltage protection section seems tobeunduly activated. Theoutputs are correctly set by the component but are trimmed down to 0 V after a few milliseconds. An oscilloscope plot is presented in Fig. 2. The resin was locally opened to access probing areas to investigate the overvoltage/undervoltage region. This function consists basically in a comparator system(Fig. 3). LOCAL PROBING The probing areas were opened with a laser ablation system. To accurately locate the test points, x-ray com- puterized tomography (CT) of the entire converter was performed. It is important to obtain precise localization of the regions to be opened, because the functionality of the converter must be ensured. Indeed, approximately 1 mm backlash is used to place the PCBAs in the converter cap before molding. Consequently, the exact position of the components inside the converter is not known. With x-ray CT, a virtual volume of the device was obtained. The latter has been superimposed with an optical image to correlate the internal structure of the PCBA with the external shape of the converter. The 40 µm scan resolution is accurate enough to determine the exact position of the regions to be opened. Figure 4 presents a virtual slice of the PCBA, obtained by x-ray CT, with the opening points identified in red. Local probing shows that the input of the converter system is not defective, but the output exhibits large oscillations (Fig. 5). X-RAY IMAGING Focused x-ray imaging of the small-outline eight-pin (SO8) component (Fig. 6) confirmed that it is the defective Fig. 1 Photos of the DC/DC converter