In this paper (originally presented at DesignCon 2010), we show a trace loss mechanism due to the glass-weave. Many PCB laminates are composed of a resin or epoxy material and glass fabric. The electrical properties of these two materials are very different. We examine the impact of the dielectric inhomogeneity on signal loss due to periodic loading of the glass-weave. Periodic loading results in a fundamental resonance where the distance between the discontinuities is one half of a wavelength. Periodic trace loading due to the glass weave and the effects of it have been previously ignored by the literature presumably due to the relatively high half-wave resonance it will establish.
A number of key factors, which are presented in detail, make this effect important to fully characterize and understand. Due to these factors, glass-weave periodic loading can introduce additional insertion loss at midrange frequencies. These additional losses are characterized in the paper using actual glass weave cross-sectional data. Parametric dependencies are explored. The paper also shows how trace route angle and length can set up different secondary resonance patterns. Among other findings, the paper shows that although forty-five degree routes are preferred to reduce or eliminate glass weave effects, this strategy does not avoid the additional losses due to glass-weave periodic loading and in fact it results in the lowest, and therefore potentially most harmful, glass-weave fundamental resonant frequency.
As data rates continue to increase, there is a greater interest in understanding all elements of the signal path that contributes to the degradation of the signal. In this paper, we present on a new trace loss mechanism due to the glass-weave periodic loading the transmission line.
Many PCB laminates are composed of a resin or epoxy material and glass fabric. The electrical properties of these two materials are very different: glass has very low loss and a dielectric constant near 6. On the other hand, the epoxy has a dielectric loss of about 3% and a dielectric constant around or below 3. A number of publications have examined the impact of this inhomogeneity on differential skew [1, 2], common-mode conversion [2], etc. In this paper, we examine the impact of the dielectric inhomogeneity on differential and single-ended signal loss due to periodic loading. Periodic loading from the glass weave is due to the periodic nature of the glass weave, which periodically alters both the loss and the dielectric constant along the trace's length.
Periodic loading on transmission lines is a fairly well understood concept [3]. It results in a fundamental resonance where the distance between the discontinuities is one half of a wavelength. Harmonics of the fundamental exist at higher frequencies as well. A single repeating loading distance will introduce a peak in the reflection profile and a dip in the insertion loss at the corresponding half-wave resonance frequency. The magnitude of the peak or dip depends on the number of discontinuities and the physical size of the discontinuity. Prior studies have examined the effect of periodic loading, such as plane cutouts or voids, on loss [4] and crosstalk [5].
Periodic trace loading due to the glass weave and the effects of it have been previously ignored by the literature presumably due to the relatively high half-wave resonance it will establish; for a glass weave pitch of 60 mils the half wave resonance in FR4 is about 45 GHz. A number of factors make this effect important to fully characterize and understand now. First, as data rates continue to increase we are getting closer to the fundamental of the glass weave resonance frequency. Second, there can be an additional steepening of the loss profile slope, well below this fundamental resonance, due to the resonance dip. This leads to additional losses in the insertion loss profile due to glass-weave periodic loading at midrange frequencies. Finally, depending on the trace routing path relative to the underlying glass-weave pattern (imagine a trace jogging through a device pin field), resonances can be set up that are well below the fundamental resonance that exists due to the glass weave itself. Also, we show that even for straight trace routes, lower frequency resonances can be established due to the glass-weave itself, depending on the trace-to-glass-weave angle.
In the paper we characterize these additional losses using actual glass weave cross-sectional data. Parametric dependencies are explored using 3D field solvers, including the impact of the glass thickness, proximity to the resin surface, thickness of the glass weave, and pitch of the glass bundles. More than this, we show how trace route angle and length can determine and set up different secondary resonance patterns. This is studied by showing the insertion loss and reflection profile as a function of trace angle, routing scenarios and trace length for different glass-weaves.
Download the entire paper in PDF format. Topics include: Glass Weave Periodic Loading, the Impact of Trace Route to Glass Weave Angle, The Impact of Meandering Trace Routes, and Test Structure Measurement.
References
- Scott McMorrow, Chris Heard, “The Impact of PCB Laminate Weave on the Electrical Performance of Differential Signaling at Multi-Gigabit Data Rates," DesignCon 2005, Santa Clara, CA.
- Chris Herrick, Thomas Buck, Ruihua Ding, "Bounding the Glass Weave Effect through Simulation," DesignCon 2009, Santa Clara, CA.
- Fred Gardiol: Lossy Transmission Lines. Artech House, Norwood, MA, 1987.
- Gustavo Blando, Jason R. Miller, Istvan Novak, Jim Delap, Cheryl Preston, "Attenuation in PCB Traces due to Periodic Discontinuities," DesignCon 2006, Santa Clara, CA.
- Jason R. Miller, Gustavo Blando, Istvan Novak, "Crosstalk due to Periodic Plane Cutouts," DesignCon 2007, Santa Clara, CA.
- IPC-4412A, Amendment 1, Specification for Finished Fabric Woven from “E” Glass Printed Boards, March 2008.
- Gustavo Blando, Jason R. Miller, Istvan Novak, "Losses Induced by Asymmetry in Differential Transmission Lines," DesignCon 2007, Santa Clara, CA.
This paper was presented at DesignCon 2010. Download the entire paper here.
