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44 DESIGN007 MAGAZINE I JANUARY 2024 dip is inversely proportional to the length of residual barrel, and the depth and bandwidth of the dip depend on its shape. Usually, a nar- rower residual sliver means a narrower dip on the frequency plot. ough this is clearly a manufacturing defect, it does happen occasionally because optical inspection of a high-aspect-ratio small hole is challenging. e length, width, and shape of any remaining barrel are highly sta- tistical in such cases. Correlation can only be successful if we first find the defect and get the dimensions of the remaining barrel by further analysis. e next example (Figure 2) is not the result of manufacturing defect; it can happen in any high-speed board. e case study 1 described the impact of registra- tion tolerances on con- nectorized differential pairs. Coaxial connec- tors today tend to be much bigger than our typical trace width in high-density high- speed boards, requiring us to start with uncou- pled traces until we can bring the two traces closer to form a differ- ential pair. is inevi- tably means that the immediate vicinity of the connector launch— though they may be mirror images of each other—will not be iden- tical. e close-up photo of the launch via shows a case where the man- ufacturing tolerance pushed the via barrel sideways, further away from the exiting trace. We can then expect (as it was verified) that the via barrel in the other launch will get closer to the exiting trace. is minute difference is enough to create the highlighted difference in the TDR response. For such cases, the sug- gested solution of the cited reference is to exit differential launches with short parallel trace sections before the traces take a turn. e third illustration, which is related to assembly, is reproduced 2 and is shown in Fig- ure 3. Many of our complex packages today use a ball grid array (BGA) connection to the printed circuit board. ese tiny solder balls are usually placed on a regular grid with a center-to-center spacing called the pitch. For big chips, 0.8 mm and 1.0 mm pitches may Figure 2: Effect of misregistration on differential escape (reproduced 1 ).