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Impact of PCB laminates on high-speed data rates (Part 1)

Posted: 25 Sep 2015     Print Version  Bookmark and Share

Keywords:PCB laminates  data rates  differential pair  signal path  skew 

In terms of both path length and signal velocity, the problems that arise are caused by the way in which the glass reinforcement in the resin has been woven. The more common glass weaves have glass bundles that are twisted tightly and leave large open spaces in between that are filled with resin. The width of an average trace in a PCB is small compared to the pitch of the glass fibres (figure 2 for a contrast in weave pitch to trace size) so it's common for one of the traces of a differential pair to be more on glass and less on resin and for the other trace to be the opposite (more on resin than on glass). As a result of these factors, we have seen 14" long differential pairs with as much as 60 ps of weave-induced skew. This amount of skew can have a huge effect on the performance of differential signals.

Some factors to take into account
 • The glass cloth woven for use in the fabrication of PCBs comes in many styles. The most commonly used glass cloth styles are shown in the photographs provided below in figure 2.
 • Glass cloth styles are known by numerical designations such as 106, 1080, 1067, 1086, 2116, 3313 and so on.
 • There are standards that define how many threads per inch and the size of the threads for each of these styles. However, there are no standards for how each bundle of glass is formed.
 • Each bundle may be tightly twisted or spread out while still satisfying the standard. As will be shown, how the bundles are formed can have a large effect on the quality of a high-speed differential pair.

Figure 2: Different Weaves in Glass Cloths.

Figure 3: Impedance variation over 1080 glass.

Of the four different types of weave depicted above, 1080 glass cloth has been the type most commonly used to create a 4 mil core. As shown in figure 3, the centreline is 50Ω. The upper line is plus 10% and the lower line is minus 10%. This wide impedance variation is due to the trace alternately running on top of a glass bundle (high Er, low velocity and low impedance) and between bundles (low Er, high velocity and high impedance) that can result in severe skew with high-speed differential pairs. It should not be used in PCBs with this class of signals.

Figure 4: Impedance variation over 3313 glass.

As shown in figure 4, If 3313 glass is compared to 1080 glass, the impedance variation is radically different. In contrast to the 1080 glass, in the 3313 glass, the impedance is much more uniform as it travels across the glass weave in both the horizontal and vertical directions. This results in uniform er and velocity and very low skew. The weaves that have been proven to cause skew in differential pairs are 106, 1080, 2116 and 7628.

The table is the result of constructing several test PCBs from a variety of materials to assess how various glass weaves affect skew.

Table: Skew Test Results for Seven Materials Made From Two Glass Weave Styles (3313 AND 8313).

The reason there are two entries for I-SPEED Is that the first example was constructed with a single ply of 3313 glass between traces and planes and the second was constructed using two plies of 1035 glass between the traces and the planes.

There are a variety of methods for reducing the effects of weave induced skew. (These will be discussed in part 2 of this article.)

About the author
Lee Ritchey is considered to be one of the industry's premier authorities on high-speed PCB and system design. He conducts on-site private training courses for high technology companies as well as at industry trade shows and technical conferences. He also provides consulting services for high-end networking products. His experience range crosses a broad technology spectrum including high-end super computers, disc drives, routers, switches and hubs. Lee holds a B.S.E.E. degree from California State University, Sacramento where he graduated as outstanding senior.

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