edfas.org 19 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 3 represent the extent of x-ray inspection compatibility, or in other words, the CMx-ray metric. To delineate the proposed framework in a structural manner, let’s take a closer look at the two main blocks: DFI evaluator and the DFI scorer. DFI EVALUATOR The first stage is the DFI evaluation where the threshold levels of the DFI parameters are determined. There are three main inputs: packaging specifications, imaging specifications, and results from sampling and simulation. Packaging specifications: The first step is to identify the specific co-design parameters or DFI parameters for advanced IC package that can be regulated to improve x-ray compatibility. These parameters define the dimensions and spacings between the different structures in the package like microbumps, RDLs, metal interconnects and TSVs.[8] If the pitches and spacings are too small, it can lead to overlapping of the wires, making it difficult to distinguish between them on x-ray images. On the other hand, larger pitches and spacings can result in higher x-ray transparency and better image quality. It is important to note that the set of DFI parameters for a particular package type will vary depending on the architecture of the packaging technology used. For example, for chip-on-waferon-substrate (CoWoS) technology,[18] Table 1 lists some of the critical design specifications that can be considered as DFI parameters. These parameters determine the noise scattering pattern one can expect to observe when x-ray radiation is passed through a sample. Some examples that we have primarily identified as DFI parameters include distance between bumps and RDLs, inter-RDL separation, bump-to-bump pitch, and through silicon via (TSV) pitch. Figure 7 helps visualize these critical pitches or spacings that are referred to here as DFI parameters in a complex heterogeneous system-in-package IC. Apart from the DFI parameters, other package specifications such as the material composition and geometry of different features in the package affect x-ray attenuation and noise scattering in different ways. Imaging specifications: Another important factor is the resolution, r, of the imaging system available. The resolution directly impacts the ability to detect defects within the IC packages. Higher resolution x-ray machines can capture finer details, so it is imperative that the DFI parameter thresholds depend on the resolution capability the designer possesses. Sampling and Simulation: The idea is to create a model of ideal design specifications that will conform perfectly with x-ray observability requirements. This can be achieved by combining observations from collecting x-ray images of physical samples of the target design with observations from modelling x-ray interactions with a CAD model of the sample. The former involves collecting multiple sets of samples each having a different set of specifications Table 1 Dimensions and spacings of critical internal structures in advanced IC packages Feature name Solder ball C4 bump Microbump Hybrid bonding TSV RDL Typical pitch size, µm >1000 ≈300 ≈40 <300 x ≈2 Density, number/ mm sq. <1 ≈10 ≈400 >10000 x ≈100 Typical diameter, µm x x x x 10-20 x Length, µm x x x x >100 x Fig. 7 Observing DFI parameters in a heterogeneous advanced package using CoWoS technology.
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