Metrology of No Fault Found Events: Using Microwave Spectroscopy and 4D X-Ray Computed Tomography to Detect Thermal Fatigue-Caused Damage in Solder Joints
Authors: Obeng Yaw , Ganju Eshan , Chawla Nikhilesh , Campbell Jason 
Published: December 2024

The reliability of semiconductor packages is a cornerstone in the advancement of high-performance electronics. As semiconductor components undergo continuous miniaturization and grow in complexity, traditional methods for inspection and analysis, such as serial sectioning and scanning electron microscopy, have become inadequate due their destructive nature, prohibitive time costs, and limited characterization abilities. Furthermore, while evaluating solder joint failures and solder joint reliability by direct current (DC) methods is potentially useful, this technique often does not capture all incipient failures leading to No Fault Found (NFF) scenarios, which occur when the product fails, but upon checking, no fault is detected. High radio frequency (RF) measurements of signal paths are more sensitive to such incipient circuit (or solder joint) failures. RF methods can capture failures due to mechanical changes, which affect the impedance of the devices under test, through return loss, insertion loss or phase angle. Furthermore, RF methods can catch such incipient failures well before complete solder joint failure. In this talk, we compare the fault detection capabilities and detection speeds, of direct current resistance (R-DC) to RF based fault detection measurements and correlate the electrical data with the results from non-destructive 4D X-ray Computed Tomography (XCT).
Historically, non-destructive defect characterization techniques such as optical imaging, radiography, and scanning acoustic microscopy (SAM) have been instrumental in assessing defects in semiconductor packages. However, they lack the resolution and capability to accurately characterize defects in dense packages at the micron scale in 3D. Furthermore, the drive for greater device efficiency and performance has led to more complex thermal stress states in semiconductor packages. Therefore, the detection and analysis of defects goes beyond mere quality control and maintenance; they are fundamental for the chip-package-system co-design itself . Thus, 3D X-ray Computed Tomography (XCT) stands out as an ideal tool for the non-destructive assessment of defects in semiconductor packages.. Figure 1 shows the 3D renderings of preliminary work done on the analyses of defect evolution in thermally cycled packages using X-ray Computed Tomography. By capturing spatial and temporal changes, 4D X-ray CT provides unique perspectives into the package behavior under thermal stress. This enables not only the identification and characterization of defects but advancement of our understanding of their origin and development.
As expected, the impedance of the solder joints (i.e., S parameter data) changed over time and temperature cycles as the connectors suffer from wear, even when there are clearly no actual changes to the circuit. The capacitance and associated capacitive reactance that formed by a partial crack in a solder joint was found to be so much larger than the very small resistance of an even tiny remaining amount of intact solder joint that the ultimate effect on the circuit is unmeasurable until the crack is fully open (as indicated by 4D XCT). The results presented in this paper should benefit emerging advanced packing concepts, and traditional components manufacturers to determine the reliability of their components on test boards.

Figure 1