What is the fibre weave effect?
The fibre weave effect is a phenomenon that occurs when manufacturing printed circuit boards (PCBs). It occurs when the fibres of the substrate material form a pattern or ‘weave’ within the material. This can cause a change in the electrical characteristics of the PCB and can affect signal integrity. The fibre weave effect is typically most pronounced in thicker substrates, where the fibres are more tightly packed together.
The weave pattern is the result of how the substrate material was manufactured, which involves layers of woven glass fabric that are impregnated with resin and then hot-pressed into a solid board. As the board is heated, the fibres in the fabric contract and create the weave pattern. Depending on the resin used and how hot it gets during the process, the weave pattern can be very pronounced or barely visible.
The presence of the weave pattern can cause variations in the electrical characteristics of the substrate due to increased capacitance between the fibres. It can also cause reflections and crosstalk, which can disrupt signal transmission. For this reason, it is important for designers to take into account the fibre weave effect when designing PCBs.
Fibre weave effect
A well-behaved differential pair
Plot of insertion loss is linear, Plot of impedance is in required range.
An Insertion loss failed differential pairs
Resonance must relate to certain electrical length, In PCB, skew is the one of common cause
Definition of differential pair
A differential pair is two conductors in a cable or circuit that are placed next to each other so that their electrical properties interact. The two conductors in a differential pair can be referred to as a ‘twisted pair’, since their physical orientation is often twisted together for better performance.
The two wires of a differential pair act in opposition to each other. For example, if one wire carries a positive current the other will carry an equal but opposite negative current. This ensures that any noise picked up by one wire is cancelled out by the other.
Differential pairs are used in high-speed digital circuits because they offer advantages over single-ended signals. The most important benefit of differential pairs is that they enable much higher transmission rates, because they are less prone to interference and crosstalk.
They also have the advantage of being able to send more data through the same amount of copper wiring. Additionally, differential signals are more immune to ground noise, which helps reduce the possibility of errors due to interference.
Design requirements to mitigate fibre weave effect
When it comes to designing PCBs with the fibre weave effect, there are certain design requirements that must be met. The most important of these is maintaining a constant distance between traces of differential pairs. To do this, designers need to make sure that any component or obstacle does not prevent them from doing so. This includes avoiding having objects too close to each other, such as capacitors, vias and traces. Additionally, it’s important to keep ground plane fills away from differentially paired traces to minimize the amount of interference.
Another important requirement is the use of vias to join together all layers of the board. This helps ensure that signals don’t suffer from any skew caused by mismatched distances between traces in different layers. Additionally, designers must pay attention to component placement and PCB stack up in order to keep all signals as uniform as possible across the entire board.
Finally, designers need to consider power plane design in order to reduce any interference caused by the power plane. In order to do this, they should consider using multiple power planes that are separated and shielded from each other with dedicated ground planes. This helps reduce crosstalk between signals and prevents interference from spreading throughout the boar.
Skew During Propagation due to “fibre weave effect”
One of the key factors that affect signal integrity in a printed circuit board (PCB) is the fibre weave effect. The term “fibre weave effect” refers to the phenomenon in which the signal propagates down an interconnect at a slightly different velocity due to the varying dielectric constants between the fibres of the substrate. This variation in speed causes a delay or “skew” between rising and falling edges of the signal.
The amount of skew created depends on the amount of variation in dielectric constants between different fibres. As the fibres are woven together, they create small variations in the dielectric constant of the substrate. These small variations are enough to cause a significant amount of skew when signals are propagating down the interconnects. The more variation there is in the dielectric constants, the more skew will be created.
In addition, the speed at which signals propagate is also affected by temperature. As temperatures increase, the dielectric constant of the substrate decreases, causing a decrease in propagation speed. This can result in even more skew as temperatures rise, so it is important to take this into account when designing a PCB.
Overall, the fibre weave effect plays an important role in signal integrity and must be taken into account when designing a PCB. Not doing so could result in timing issues or even cause the circuit to malfunction.
Propagation of signal in media
Skew
Len1 is length of trace1
Len2 is length of trace2
C0 is light speed in vacuum
εr1 is local Dk of media around trace1
εr2 is local Dk of media around trace2
Effect of skew on insertion loss
The fibre weave effect can have a significant impact on the insertion loss of a PCB substrate. This is because the woven pattern of the fibres in the substrate causes skew, which is a difference in the timing of signal transmission through the different fibres. This skew can cause interference and cause the signal to become distorted, resulting in decreased signal strength and increased insertion loss.
When this happens, the signal loses power, meaning that it cannot be transmitted as far as it otherwise could. This results in decreased data rates, meaning that data cannot be sent or received quickly, and latency increases. The overall performance of the circuit board is also affected, making it more prone to malfunction.
Skew also affects the impedance of the PCB substrate, which is the resistance of the material to electric current. When the impedance is not consistent, it can lead to voltage drops, making it difficult for current to flow through the material. This means that devices connected to the circuit board will not receive the same amount of power and may not operate properly.
In order to reduce these effects, manufacturers use several techniques such as controlling the pattern of the fibre weave and optimizing the dielectric constant (DK) of the substrate material. These measures help to keep the signal strength and impedance consistent, leading to better overall performance and fewer problems.
Resonant Frequency Vs Skew
Modelling Variables for “fibre weave effect”
Modelling- Variable delta_Dk
Resonance frequency point is near to 12.89GHz
Resonance frequency point is higher than 20GHz, 12.89GHz is still affected.
Solutions to manage Fibre weave effect
Rotation when Penalization
Using spread glass cloth
Using low Dk glass cloth
DO non-orthogonal routing high speed channel when layout
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