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  • Characterizing substrates for yeild improvement
    Date :2014-10-9


    New challenges in fabricating HDI substrates are emerging as substrate technology advances. Embedded passives, filled via and stacked via structures can reduce length of trace on a substrate and help minimize size of electronic devices. Moreover, these technologies also enable us to use substrates in higher frequencies than ever. Such developments bring good news to consumers, but not good news to test engineers because they have to think out new ways to evaluate substrates with higher hurdle to clear.
    Of course, substrates with embedded passives and reduced traces by stack vias etc. cannot be tested with a conventional open/short tester. Even though you have a tester with LCR measurement function or a four-terminal resistance measurement function, the tester may not be adequate for testing your substrates. Low LCR measurement involves high measurement resolution, stabilized atmosphere and so forth. Also, stable probing should be done even for tests in mass production. For interconnecting substrates, differential characteristic impedance measurement with a differential TDR tester is becoming essential.
    Describing how to achieve reliable HDI substrate tests in either mass production or a trial stage by using test equipment is the objective of this paper. Various test techniques needed for modern substrate tests, and their applications will be presented.


    Trend in HDI substrates
    Density of pads of HDI substrates has become higher than ever. According to The International Technology Roadmap for Semiconductors, while chip size of high-performance chips will not beyond 310mm2, I/O count will increase. This means density
    will continue to be higher in the future.
    On the other hand, technologies that enable to reduce electrical length of traces are emerging. All-layer IVH structure, stacked-via and via-on-pad structures are effective ways to provide shorter traces, which decreases resistance and inductance. Figure 2 shows changes in resistance of conventional vias and filled vias used for a stacked-via structure. The filled via structure decreases resistance by 20% or more.
    Differential interfaces are another trend in HDI substrates for interconnecting devices. USB 2.0, for example, uses a differential interface, and differential characteristic impedance tests are required for verifying uniformity of two traces used for the interface.
    The trend in HDI substrates is going toward high density, and high frequency. Therefore, test techniques that meet the trend are required.


    High density interconnection and IVH resistance measurement
    The IVH resistance measurement technique for detecting defective vias in a HDI substrate is now commonly used even in semi-mass production. However, the trend described above drives trace resistance small, and makes resistance measurement hard. One digit higher resolution is required for measuring IVH resistance.
    For example, resistance of a trace on a conventional HDI substrate including conventional vias is typically 100mΩ to 400mΩ, whereas resistance of a trace on an all-layer IVH structure including filled vias sometimes becomes less than 100mΩ. Since 4-digit data are at least required to find defective IVHs, 10μΩ or higher resolution is indispensable for resistance
    Stability is another important factor for IVH resistance measurement. The smaller resistance value becomes, the bigger noise affects measurements. Noise disorders measurements, and as a result, stability of the measurements gets worse. Designing a test
    system with low noise is therefore also required. For evaluating measurement stability of a test system, repeatability study by using capability index is often made. The capability index is calculated as follows


    Differential TDR
    Traces in a HDI substrate used for high frequency should be tested with TDR. This is also applied to traces for differential interfacing, such as USB 2.0. However, single-ended TDR system (Figure 4 (a)) cannot evaluate differential transmission lines. Differential lines should not be considered as two of single-ended lines.
    Figure 5 shows an equivalent circuit of a differential transmission line. As shown as “M”,in the figure, mutual effect exists between lines and it affects differential signal quality. Thus a differential transmission line must be evaluated as a whole. A differential TDR measurement unit is used for this purpose.


    Embedded passives
    Not only do embedded passive technology reduce component costs, assembly costs and so forth, but it is also good for high-frequency application because short interconnection between components can be achieved with it. As this technology becoming popular, demand for testing frequency characteristics in HDI fabrication process is on the rise.
    For evaluating frequency characteristics, the following two methods are typically used:
    1. Individual component test by LCR meter
    2. Functional test for combined components by VNA (Vector Network Analyzer)
    Each method has merits and demerits. The LCR meter method is superior in measurement speed, which is very important for application in production test, but it is not enough to judge combined circuit functionality. Even though each component value of a functional circuit comprised of embedded passives is within a specified allowance, frequency characteristic, such as cut-off frequency, sometimes may not satisfy required specifications due to balance among components.
    The VNA method can functionality of combined passive components. However, it consumes time for a test because a lot of measurements are made in a test, sweeping frequency.
    Of the two test methods, the VNA method should be used for embedded passive evaluation because functional evaluation is more important than individual component evaluation in fabricating embedded passives.


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