The Impact Of Glass Weave Styles On Blind Vias And Differential Skew

In recent years, whenever we have talked about glass weave styles, the conversation has surrounded the effect those styles have on differential skew. But weave styles and their impact on PCB designs actually have their roots in blind vias. Both blind vias and skew can be compensated for through the mechanical spreading of the various glass weave styles. This article will discuss both elements, the ways in which they can be addressed and the status of standardization efforts within the industry thus far.

Note: micro vias and blind vias are terms that are used interchangeably in the industry. For the purposes of this article, I am using blind vias. (A blind via is a via that starts on one side of a PCB, but does not go all the way through the PCB.)

So Much Technology in Such A Little Space

The good news is that the devices we’ve come to rely upon so much keep getting better, and feature an ever increasing amount of functionality in small form factors. These types of geometries can be found in mobile phones and ultra thin laptops. The challenge is that these geometries only have so much “give” and the result is the predominance of blind vias which are found in these products due to pin spacing of less than 1 mm.

Historically, the choice of a laminate and the glass weave style incorporated into it was based on cost. The one that was always chosen was the one that cost the least. This led to the following “standards” or predominance of choices. For two mil cores, the glass weave of choice was 106; for three mil cores, 1080; and for four mil cores, 2116 or 3313. These choices were fine for electronic products that did not require the use of blind vias. But, once laser-drilled vias came on the scene, we wound up with some very ragged holes such as those shown in Figure 1. 

Figure 1. Cross Section of a Blind Via Drilled in 1080 Glass Weave

(Photo courtesy of Mike Carano)

The causative factor of these ragged holes was the irregular distribution of the glass and resin within the weave. Essentially, when the laser was set strong enough to burn through the glass fiber it burned away too much resin.

Figure 2, which is a picture of 106 and 1080 glass cloth, shows the large voids between the bundles which are comprised of pure resin. When the laser beam power is set high enough to burn through the glass bundles, in the regions of pure resin, the beam drills through the copper pad and undercuts the resin on the hole sides, resulting in an unreliable ragged hole. The solution to this problem has been to spread the glass out so that it is uniformly distributed across the surface. And the impetus for this spreading of the glass came from the mobile phone industry whose primary concern is maintaining a very high quality of laser-drilled blind vias.

Figure 2. 106 and 1080 Glass Cloth

As a result of the foregoing, the mechanically spread replacement for 106 glass is 1067 glass while the one for 1080 glass is 1086 glass. These are depicted in Figure 3. Notice that there are no resin-filled gaps as is the case with 106 and 1080 glass.

Figure 3. 1067 and 1086 Glass Cloth

Spreading of the glass makes developers of mobile phones and ultra thin laptop products happy, because the glass and resin are distributed in such a manner that the drilled holes are very clean. Figure 4 shows the cross section of a really nice looking, clean via.

Figure 4. Cross Section of a Blind Via Drilled in 1086 Glass Cloth

Beyond the mobile phone and thin laptop product sector, the other application area for blind vias is FPGA chip carriers. Actually, in terms of unit volume, this sector is probably larger than the mobile phone market. If you consider the number of products that have one PCB and several high pin count FPGA packages, it’s easy to see why this is such a big market sector.

What’s Skew with You?

So, the foregoing is the history of what led to the mechanical spreading of glass weaves. When it comes to the problem of skew, the picture changes somewhat, as the spreading that is done to accommodate blind vias may not be good enough for minimizing skew.

Just to review, the skew which occurs in differential pairs is caused when a trace runs over a glass fiber for some distance and then between glass fibers. When this happens, the dielectric constant varies substantially, causing significant variations in impedance and velocity along the trace. When the two members of a differential pair travel over the weave pattern these variations may not be equal in both members. This results in skew between the two sides of the pair and degradation of the signal at the receiver.

The two glass styles or weaves that are known to cause skew in high speed differential pairs are 106 and 1080. These are pictured in Figure 1 with a 3.5 mil diameter (89 micron) wire running across the weave to represent a trace.

To better address the issues associated with skew, a uniform weave style was introduced with 3313 glass. It is shown in Figure 5. Early results showed that skew was held to a minimum using this glass weave.

Figure 5. 3313 Glass Weave Cloth

However, it should be noted that not every manufacturer fabricates 3313 glass in the same manner. In 2013 we built two different test boards with 3313 glass from two different manufactures. From the first weaver we got unbelievably good controlled skew, while from the second the skew was terrible. Because there is no standard that specifies the manner in which glass will be mechanically spread, it’s possible to get such disparate results as those we obtained.

And, while either one of these weaves would be good enough for those product developers concerned with drilling blind vias, they aren’t good enough for controlling skew. For the instance cited above, and the two different types of 3313 glass used above, in which we had 14 inch (35 cm) traces, the skew was no more than 4 picoseconds with teh first weave. For the other glass weave, the skew was as bad as 62 picoseconds over the same 14 inches. This is 62% of a bit period of 10 Gbps. This equates to a link that is definitely going to fail.

The Status of the Standardization Effort

Some time back, there was a committee formed within the IPC—the Association Connecting Electronics Industries—to specify the mechanical spreading of glass. Initially, the focus of this effort was on eliminating the ragged holes associated with blind vias. The topic of mechanically spreading glass to control skew was an add-on to this effort.

As cited above, the results from our test boards showed that there has been no standardization for the ways in which glass will be mechanically spread to address skew. Just to be successful, high-speed computer system product developers as well as those developing Internet products have taken the position of putting two plies of 1067 glass together to make a four mil core with the expectation that the non-uniformity of each will average out. So far it’s been true. And, while using two plies of 1067 makes the process a little more expensive it is far cheaper than throwing away PCBs that have a multitude of skew problems.

Summary

Glass weave styles have an impact on the quality of holes drilled for the blind vias that are found in chip carriers, mobile phones and ultra thin laptops. They also have an impact on how well differential skew is  controlled in PCBs. The industry has found ways to address both of these issues and ensure that PCBs containing blind vias and those built to minimize skew can be designed and built  right the first time.

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References

1. Ritchey, Lee W. and Zasio, John J., “Right The First Time, A Practical Handbook on High-Speed PCB and System Design, Volumes 2.”
2. McMorrow, Scott etal, “Impact of PCB Laminate Weave on Electrical Performance”, DesignCon, Fall 2005.
3. Bogatin, Eric, “Skewering Skew, Laminate Weave Induces Skew”, Printed Circuit Design, April 2005.
4. Ritchey, Lee W. “A Way to Address the Problem of Jitter and Skew in Gigabit and Faster SignalsCaused byLaminate Weaves,” Current Source, June 2007