Are you bored with your board? How PCB substrates affect aerospace apps

August 03, 2015 // By Rajan Bedi, Spacechips
The performance required to deliver the next generation of satellite services for operators is stretching the capability of traditional materials used to fabricate space-grade PCBs.

Ka-band, RF carriers, broadband ADCs/DACs, noisy switching regulators, low-voltage high-current FPGAs containing multi-gigabit, high-speed serial links, as well as I/O toggling around 1 GHz, all now reside on the same PCB separated by only a few centimetres. The choice of dielectric now has to optimise for RF, analogue, and digital requirements.

Manufacturers of spacecraft avionics are considering new PCB materials with lower relative dielectric constants (Er or Dk) and dissipation factors (Df) to achieve the performances that will enable future missions for satellite operators. For some designs, careful floor-planning and component placement will allow OEMs to deliver the target requirements using existing, lower-cost materials and less expensive fabrication.

At lower frequencies and data rates, signal loss is caused mainly by impedance mismatches: and less so, by dielectric absorption and conductor losses. At higher frequencies, material loss becomes equally important and controlled substrate construction must be considered during the design process.

At higher frequencies (and faster edges), losses occur due to changes in characteristic impedance (Z0), absorption of some of the signal energy by the dielectric material, and resistive channel losses due to skin effect and copper-surface roughness. Reflections are caused by impedance discontinuities, variations in Z0 resulting from differences in laminate thickness, changes in the dielectric constant of the substrate, and fabrication tolerances in the width of the etched traces.

The dielectric constant (Er/Dk) of nearly all PCB substrates decreases with frequency, which manifests itself in two ways: the speed of signals increases and the characteristic impedance of a transmission line becomes smaller. The former generates phase distortion in bandwidth-rich digital signals, while changes in Z0 cause faster edges to reflect more than slower ones. Edges contain harmonics which can have significant amplitudes up to a frequency of 0.35/T, where T is the smaller of rise or fall time in ns.

Within the dielectric, the fibreglass weave pattern and the ratio of reinforcement to resin cause local variations in Er/Dk. The glass and epoxy each have different relative permittivities, thereby presenting a non-homogenous medium for signal propagation.

The tighter the weave netting, the more uniform the dielectric constant. Loose weaves result in more variation within the laminate, causing variations in trace impedance and propagation skews in tightly matched signals, such as differential pairs, which directly reference the weave. Some patterns are shown below.

Figure 1 Different styles of fibreglass weaves impact the dielectric constant, trace impedance, and propagation skews.

For example, on a sparse weaving... (continues)