Fault Isolation of High Resistance Defects Using Comparative Magnetic Field Imaging

Abstract

As integrated circuit packages become more complicated, the localization of defects becomes correspondingly more difficult. One particularly difficult class of defects to localize is high resistance (HR) defects. These defects include cracked traces, delaminated vias, C4 non-wet defects, PTH cracks, and any other package or interconnect structure that results in a signal line resistance change that exceeds the specification of the device. These defects can result in devices that do not run at full speed, are not reliable in the field, or simply do not work at all. The main approach for localizing these defects today is time domain reflectometry (TDR) [1]. TDR sends a short electrical pulse into the device and monitors the time to receive reflections. These reflections can correspond to shorts, opens, bends in a wire, normal interfaces between devices, or high resistance defects. Ultimately anything that produces an electrical impedance change will produce a TDR response. These signals are compared to a good part and require time consuming layer-by-layer deprocessing and comparison to a standard part. When complete, the localization is typically at best to within 200 microns. A new approach to isolating high resistance defects has been recently developed using current imaging. In recent years, current imaging through magnetic field detection has become a main-stream approach for short localization in the package [2] and is also heavily utilized for die level applications [3]. This core technology has been applied to the localization of high resistance defects. This paper will describe the approach, and give examples of test samples as well as results from actual yield failures.