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following examples illustrate how EOTPR can be used to accurately localize faults in advanced IC packages. 2.5D ARCHITECTURES Figure 2 shows a schematic of a 2.5D package con- sisting of an application-specific IC (ASIC) die and high bandwidthmemorydevices assembledontoan interposer and subsequently mounted on an organic substrate. In such devices, defects such as cracks and delamination can occur at the interfaces due to stress induced during the solder reflow and underfill curing cycles used in the manufacturing environment. In this example, a curve trace performedwhile biasing a signal pin with respect to the ground pin identified an open failure. TDR measurements suggested the open failure lay between the substrate and interposer termina- tions but could not accurately identify the location. 2D real-time x-ray and acoustic imaging also could not reveal any anomalies at the associated signal pin interconnects, µbump, and C4 interface. EOTPR was subsequently used to obtain a more accurate defect location. Figure 3 shows the waveforms obtained for the failed unit (red trace), a KGD (green trace), a bare printed circuit board (PCB) substrate that terminates at the C4 bump pad (black trace), and a PCB substratewith interposer, which terminates at the µbump pad (blue curve). The positive peak in the failed unit

short circuit defects within the IC. These reflections are measured by the detector photoconductive switch and are recorded as a function of time, resulting in the EOTPR waveform. An example waveform for an open, high fre- quency probe is shown in Fig. 1b. As EOTPR utilizes an impulse response rather than the step response of con- ventional TDR, high and low impedance discontinuities in a DUT result in positive peaks and negative troughs, respectively, in themeasuredwaveform. Themagnitudeof these features indicates the size of the impedance change encountered by the pulse and their arrival time gives the location of the fault. EOTPR is, by necessity, a comparative technique; to understand whether a waveform feature arises from a fault in the DUT, it is essential to compare the waveform from a known good device (KGD). To localize a fault within the DUT, the time-domain waveform can be readily converted into the distance- domain. This can be accurately achieved by measuring reference devices that have impedance features, such as opens, at known locations. The effective dielectric constant of the DUT can be determined by measuring the time required for the EOTPR signal to travel between reference features or known locations, which is then used to convert the EOTPR waveform from time to distance. [4] EOTPR is now a well-established technique in failure analysis workflows and has been used to interrogate a wide range of distinctive device architectures. The

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

Fig. 2 Schematic of the 2.5D unit used in this study.

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