Abstract
A current probe was designed to measure ball currents in a Ball-Grid-Array (BGA) package via magnetic induction. This probe, which is based on a Rogowski coil structure, is manufactured in a four-layer flex circuit. Very tiny feature sizes, such as 2 mil buried micro vias and 2 mil trace widths/clearances, push the limit of the flex-circuit fabrication process technology. This design allows the current of a ball in a 1-mm pitch BGA package to be measured. It can also be migrated to package with smaller pitch if the fabrication process allows. This probe is practical for engineering applications as a result of three major advantages. It operates in a broadband from 10 to 3 GHz. The probe can be relocated to different balls and no special printed circuit board is needed. The probe shows acceptable shielding effectiveness of the unwanted fields caused by adjacent balls or sources. In conjunction with both a differential amplifier and an active oscilloscope probe, the output signal is sufficiently amplified to overcome the noise figure of the oscilloscope. This allows time domain measurements. Moreover, a frequency-domain data processing program was developed for correcting the probe's frequency response and reconstructing the time domain waveform. The probe is validated against analytical calculation, and full-wave simulation; it is characterized with the aid of circuit modeling. The probe's functionality is finally demonstrated with a field-programmable-gate-array test board.
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