IPC Slash Sheet Laminate Selection Principles
IPC-4101 slash sheets establish mandatory baseline physical, thermal, and electrical limits that override generic trade names to enforce material reliability.
Weave Skew
Signal Divergence
The propagation delay difference that arises between two signals in a differential pair running on a PCB dielectric is a serious signal integrity issue. This weave skew occurs because the woven glass fibers and the resin have different dielectric constants, creating localized speed variations. It becomes a major concern for high speed differential pairs operating above ten gigabits per second.
Dielectric Composition
Fiberglass bundles in standard laminates are unevenly distributed, leaving areas of dense glass and areas of resin-rich pockets. When one trace of a pair runs directly over a glass bundle while the other runs over resin, the signals travel at different speeds. This variation causes the signals to arrive at the destination out of phase, which collapses the eye diagram.
Specifying spread-glass styles ensures a more uniform dielectric environment.
Routing Practice
Routing high speed traces at an angle to the weave direction is a common design strategy to average out these local speed variations. This diagonal routing ensures that both traces in the pair experience an equal distribution of glass and resin along their run. Another option is to use a tighter glass weave style that has smaller resin gaps.
These layout methods help maintain signal phase alignment across the entire board run. In some high frequency boards, the entire trace layout is rotated by ten degrees to achieve this effect.