edfas.org 5 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 23 NO. 1 was stressed at 48 hrs in the unbiased highly accelerated stress test (UHAST) (85°C/85%RH), and verified to show leakage. A reference sample was prepared in the same manner, but without the deliberately introduced contami- nation. The failed sample and reference samplewere both decapsulated using atmospheric pressure oxygen-only MIP decapsulation, cleanly exposing the middle layer die and bond wires for further analysis. The reference sample showed no apparent abnormali- ties on the surface of the die under optical microscopy and SEM (see Fig. 1). Energy-dispersive x-ray spectroscopy (EDS) carried out on several different sites on the surface of the die further confirmed that no contamination was present (see Fig. 2). The failed sample on the other hand, showed appar- ent abnormalities on the surface of the die under optical microscopy and SEM after decapsulation (see Fig. 3). Further analysis of these abnormal structures revealed them to be a thin layer of foreign material on the die and bond pad, as seen in the images below. EDS revealed traces of sodium and aluminum elements on the surface contaminant (see Fig. 4). The traces of Na are present due to the deliberately introduced NaCl contamination, and the presence of Al is due to the corrosion of Al bond pads by the NaCl contamination. As the fragile original surface contaminant is not removed during the decapsulation process, the following failure analysis on the device will be effective and precise. CASE STUDY II: CORROSION AND METAL MIGRATION  Many sample preparation methods are inadequate when it comes to preserving root causes of failures which may involve a thin residue of contamination at an inter- face within the package. During an evaluation, which was Fig. 1 The reference QFN sample after O 2 -only MIP decapsulation, middle layer die and bond wires are exposed. (a) optical, (b) SEMwith EDSmeasurement Site 1 marked. Fig. 2 EDS of reference sample Site 1 show N, O, Si. Fig. 4 EDS spectra of failed sample Site 1 show traces of N, O, Na, Al, Si. Fig. 3 The failed QFN sample after MIP decapsulation, middle layer die&bondwires areexposed. (a) optical, (b) SEM, (c) magnified SEMwith EDS Site 1 and Site 2 marked.