Y4T139 glass fiber effect of high-speed PCB

时间2018/5/24 9:35:22
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I heard a word two days ago, glass fiber effect

I heard a sentence two days ago, when 100G optical module PCB layout, pay attention to avoid "glass fiber effect"

Today’s chat order

What is glass fiber?

What is the glass fiber effect?

How to avoid glass effect?

We generally think that the material of PCB FR4 is resin-clad copper foil, such as the commonly used differential line in optical modules.

Control the differential impedance, usually through the dielectric constant of the resin, the height, and the thickness of the copper foil to calculate the line width and line spacing

In fact, there is a layer of fiber inside the resin, which is made of glass material, referred to as glass fiber.

What are the benefits of fiberglass? It is heat-resistant, corrosion-resistant, and mechanically strong. Our PCB circuit can't be broken at the touch of a touch, and it will be soft if it can't see the sun, right?

Therefore, in the resin of PCB, woven glass fiber is added, and the number of the glass fiber cloth is defined according to the width of the weft yarn, the width of the warp yarn, the gap between the weft yarns, and the gap between the warp yarns.

Glass fiber cloth, the influence on the signal, mainly comes from the different dielectric constant


In case, a pair of differential cables, one walking on the glass fiber and the other walking in the fiber gap, will die.

The impedance of the two wires is different, because the dielectric constant of the medium is different.

This leads to two risks: One is the impedance mismatch, and the other is the signal transmission speed.

For 10G signals, it does not matter if there is a little delay between the differential lines, but 25G/50G is different, causing serious signal distortion.

This is the glass fiber effect

what is it now? How to deal with the glass fiber effect? Two methods, one is to reduce the gap between the fiber cloth, which is flattening; the other is to route the fiber and the signal line at a certain angle

Flattened, the general fiber is woven like this between warp and weft

When the warp and weft are flattened, the gap becomes smaller, and the dielectric constant of the glass fiber + resin is more uniform


The second is when wiring, stagger with the fiber direction, stagger at a small angle, to avoid the glass fiber effect

 

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