FR4 - The most common PCB Material.
The most common material used to build printed circuit boards (PCBs) is FR4. FR4 employs significantly higher losses and distortions than other materials designed for high frequencies, but despite this, FR4 continues to be used extensively due to one significant advantage: its price.
In addition, chip manufacturers are incorporating signal conditioning techniques such as pre-/de-emphasis and equalization to enable higher speeds on PCBs while still using inexpensive materials.
Understanding the PCB structure
To understand the advantages or disadvantages of FR4, it is first necessary to understand the structure of the raw material. For example, we are all familiar with the benefits of a printed circuit board over wire-up. The most straightforward FR4 material consists of a "rigid" substrate that forms the skeleton of the raw material. The rigid material undergoes a process of coating with epoxy resin, and there is a substrate for the PCB.
In the past, the "rigid" material was based on paper or fabric. One of the biggest problems with these materials was the inability to drill mechanical holes due to a problem with the material's mechanical strength. The closest thing to a drill was punching holes, but even that was limited in terms of diameter and quality.
As circuits became denser and holes became smaller, the need for a rigid material arose. The solution was the use of fiberglass – fiberglass. The mechanical strength of the material was achieved by weaving the fibers in a crisscross pattern.
Fiberglass is a very inexpensive material due to its widespread use in various industries such as automotive, construction, etc.
Some more info...
The weaving of fiberglass threads can be done in a variety of ways, such as:
Different interweaving spacing.
Using a different number of threads as a bundle.
The type/thickness of the thread used in the horizontal weave (Fill - describes a horizontal weave).
The type/thickness of the thread used in the vertical weave (Warp - describes a vertical weave).
The following figure describes different weaves based on fiberglass. It can be seen that below each image, there is a number that describes the characteristics of the weave. The table summarizes the corresponding parametric data.
Figure 1 - Fiberglass Fabric Types
Fiberweave and its implications:
According to Figure 1, it can be seen that due to the weaving, there are areas with different dielectric constants in the PCB.
Why is this important?
In high-speed circuits, controlled impedance is of paramount importance. If a trace is routed alternately between areas with different dielectric constants Er, its characteristic impedance will vary alternately at each transition from one area to another. These areas are much broader than the routed signals. For example, if we take 1080 weaving style, the area of the "squares" created due to the fiberglass weaving is 21.3 x 16.7 mil, which is undoubtedly much larger than a routed signal width of typically 5 or 4 mil.
It is known that propagation delay also depends on Er. This delay is especially important in routing differential pairs when one of the signals is routed in one area and the other in another, distorting the differential coupling, hence the differential signal.
Figure 2 - Impedance Changes in Differential Pairs
Recommendations for solutions
Research conducted at Intel shows that a tilt of α=1º-2º will solve the fiber weave problem, but in PCB production, measurements have been carried out, and weave tilts have been found at angles of about 5º. Therefore, the recommendation is to route the lines in the edit with a tilt of about α=10º.
Figure 4 - Routing lines with a tilt of 10º
If there are routing problems according to the first method, the fiber weave problem can be addressed by performing penalization at the same tilt angle of 10º. The problem with this method is that it is a bit wasteful of raw material production, but it will provide a high-quality solution to the problem.
Advanced Glass Reinforcement Technology for Improved Signal Integrity by R.Dudek
DesignCon2007: Fiber Weave Effect by Jeff Loyer, Richard Kunze, Xioaoning Ye
Signal Integrity Simplified by Eric Bogatin
PCIe 2.0 Signal Integrity Considerations (Fiberweave Effect) by Jeff Loyer