Optimizing Rigid Flex Stackup Architecture for High Speed Signals
Optimizing rigid-flex stackups for high-speed signals requires adhesiveless polyimide cores, cross-hatched reference alignment, and staggered rigid transitions.
Spread Glass Cloth
Woven Reinforcement
Fabrication of high performance laminates uses a glass fiber mat where the individual bundles are flattened to create a more uniform surface. Spread glass cloth replaces traditional round yarn weaves with a wide and thin ribbon format that minimizes the gaps between the fibers.
Skew Reduction
The primary benefit of this material is the elimination of the glass weave effect on differential signal pairs. When a trace travels over the alternating pattern of glass and resin in a standard weave, the signal speed changes depending on the local density. Using spread glass cloth ensures that both lines in a differential pair see the same material properties, reducing the intra-pair skew.
This consistency is vital for data rates exceeding ten gigabits per second where even a few picoseconds of timing mismatch can cause bit errors.
Process Uniformity
Mechanical drilling and laser ablation are more consistent when the glass fibers are distributed evenly. In a spread glass cloth, the laser energy is absorbed more uniformly, resulting in cleaner via holes and better plating quality. The material also offers improved dimensional stability during the thermal cycles of the lamination process.
However, the tighter weave can make the resin impregnation more difficult, requiring careful control of the pre-preg manufacturing process to avoid voids or delamination.