edfas.org 37 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 28 NO. 2 PHASE 2: SILICON INTEGRITY ASSESSMENT (CSAM) Before proceeding to invasive or optical techniques, it is critical to assess the mechanical integrity of the silicon package. Confocal scanning acoustic microscopy (CSAM) was utilized as a noninvasive, ultrasonic technique. CSAM detects internal flaws by analyzing the reflection of high-frequency sound waves off internal boundaries, such as the interface between the silicon die and package substrate. In this case study, CSAM imaging revealed a clean die with no evident delamination or gross physical damage (Fig. 6). This finding was significant as it suggested a Fig. 7 Lock-in thermography analysis of the failed unit.[2] localized “hard failure” rather than a catastrophic thermal EOS event that would typically cause widespread package damage or cracking. PHASE 3: COARSE LOCALIZATION VIA THERMAL IMAGING With a resistive short confirmed and gross silicon damage ruled out, the objective now is to spatially localize the issue. Lock-in thermal imaging was used to capture Joule heating generated by the short (Fig. 7). Biasing the failing pins at a constant 100 mV generated specific current draws: approximately 33.5 mA for the D+ line and 3.0 mA for the D– line. This provided sufficient energy to generate detectable heat signatures.
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