Asymmetric ultra-low-loss stackups shift the neutral axis and concentrate reflow shear strain at copper boundaries, requiring aspect ratios under 10:1 to protect barrel fatigue life.
Spread prepreg styles flatten glass yarns to eliminate dielectric window voids, reducing differential phase skew below 1.5 ps per inch in PAM4 signal lines.
Rotating differential traces or panel cuts off-axis relative to glass weave warp yarns equalizes phase delay and mitigates high-speed intra-pair skew.
Substrate dielectric drift alters high-frequency signal phase velocity and impedance under thermal stress, requiring flat TCDk resin selection to hold timing margins.
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