Spread Glass Prepreg Performance in Ultra Low Skew Multilayer Stackups
Spread glass prepregs eliminate dielectric permittivity gaps across differential traces, suppressing signal skew below 1 ps/inch in 112G PAM4 stackups.
CTE Z-Axis Expansion
Material Deformation
Thermal growth occurring perpendicular to the surface of a printed circuit board describes the physical change in laminate thickness as temperature increases. While the glass reinforcement restricts movement in the horizontal plane, cte z-axis expansion remains largely uncontrolled and proceeds at a rate vastly higher than the copper traces or plated holes. This behavior becomes particularly pronounced once the material surpasses its glass transition temperature.
Reliability Impact
Internal stresses generated by the mismatch in expansion rates threaten the integrity of plated through holes and blind vias. As cte z-axis expansion pulls on the copper barrels, the resulting strain can lead to circumferential cracks or corner cracks at the pad interface. High layer count boards face greater risks because the cumulative vertical movement increases the total load on the plated features.
Repeated thermal cycles during reflow or wave soldering accelerate the onset of these fatigue failures.
Design Mitigation
Selection of low expansion resins and high performance glass styles reduces the total displacement. Engineering specifications often limit the allowable cte z-axis expansion to a fixed percentage between ambient and operating temperatures. Proper material choice ensures that the dielectric withstands the pressure of cte z-axis expansion without internal delamination.